View source for Ecology

{{Short description|Study of organisms and their environment}}
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{{Other uses}}
{{Distinguish |ethology |ethnology |etiology |deep ecology}}
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{{infobox
| title            = Ecology
| data1            = [[File:Bufo boreas.jpg|160px]] [[File:Blue Linckia Starfish.JPG|90px]]
| data2            = [[File:Hawk eating prey.jpg|119px]] [[File:European honey bee extracts nectar.jpg|131px]]
| data3            = Ecology addresses the full scale of life, from tiny bacteria to processes that span the entire planet. Ecologists study many diverse and [[interspecific interactions|complex relations]] among species, such as [[predation]] and [[pollination]]. The diversity of life is organized into different [[habitats]], from [[terrestrial ecosystem|terrestrial]] to [[aquatic ecosystem]]s.
}}
{{TopicTOC-Biology}}

'''Ecology''' ({{etymology|grc|''{{Wikt-lang|grc|οἶκος}}'' ({{grc-transl|οἶκος}})|house||''{{Wikt-lang|grc|-λογία}}'' ({{grc-transl|[[-logy|-λογία]]}})|study of}}){{Cref2|A}} is the [[natural science]] of the relationships among living [[organism]]s and their [[Natural environment|environment]]. Ecology considers organisms at the individual, [[population]], [[community (ecology)|community]], [[ecosystem]], and [[biosphere]] levels. Ecology overlaps with the closely related sciences of [[biogeography]], [[evolutionary biology]], [[genetics]], [[ethology]], and [[natural history]].

Ecology is a branch of [[biology]], and is the study of [[abundance (ecology)|abundance]], [[biomass (ecology)|biomass]], and distribution of organisms in the context of the environment. It encompasses life processes, interactions, and [[adaptation]]s; movement of materials and [[energy]] through living communities; [[ecological succession|successional]] development of ecosystems; cooperation, competition, and predation within and between [[species]]; and patterns of [[biodiversity]] and its effect on ecosystem processes.

Ecology has practical applications in fields such as [[conservation biology]], [[wetland]] management, [[natural resource management]], and [[human ecology]].

The word ''ecology'' ({{langx|de|Ökologie}}) was coined in 1866 by the German scientist [[Ernst Haeckel]]. The science of ecology as we know it today began with a group of American botanists in the 1890s.<ref>S. E. Kingsland, "Foundational Papers: Defining Ecology as a Science", in L. A. Real and J. H. Brown, eds., ''Foundations of Ecology: Classic Papers with Commentaries''. Chicago: U of Chicago Press, 1991. pp. 1–2.</ref> [[Evolution]]ary concepts relating to adaptation and [[natural selection]] are cornerstones of modern [[theoretical ecology|ecological theory]].

Ecosystems are dynamically interacting systems of organisms, the communities they make up, and the non-living ([[abiotic component|abiotic]]) components of their environment. Ecosystem processes, such as [[primary production]], [[nutrient cycling]], and [[niche construction]], regulate the flux of energy and matter through an environment. Ecosystems have [[biophysics|biophysical]] feedback mechanisms that moderate processes acting on living ([[biotic component|biotic]]) and abiotic components of the planet. Ecosystems sustain life-supporting functions and provide [[ecosystem service]]s like [[biomass]] production (food, fuel, fiber, and medicine), the regulation of [[climate]], global [[biogeochemical cycle]]s, [[water filtration]], [[soil formation]], [[erosion]] control, flood protection, and many other natural features of scientific, historical, economic, or intrinsic value.

== Levels, scope, and scale of organization ==
{{Main|Outline of ecology}}
The scope of ecology contains a wide array of interacting levels of organization spanning micro-level (e.g., [[cell (biology)|cells]]) to a planetary scale (e.g., [[Earth's spheres|biosphere]]) [[phenomena]]. Ecosystems, for example, contain abiotic [[Resource (biology)|resources]] and interacting life forms (i.e., individual organisms that aggregate into [[population]]s which aggregate into distinct ecological communities). Because ecosystems are dynamic and do not necessarily follow a linear successional route, changes might occur quickly or slowly over thousands of years before specific forest successional stages are brought about by biological processes.

An ecosystem's area can vary greatly, from tiny to vast. A single tree is of little consequence to the classification of a forest ecosystem, but is critically relevant to organisms living in and on it.<ref name="Stadler98" /> Several generations of an [[aphid]] population can exist over the lifespan of a single leaf. Each of those aphids, in turn, supports diverse [[bacteria]]l communities.<ref name="Humphreys97" /> The nature of connections in ecological communities cannot be explained by knowing the details of each species in isolation, because the emergent pattern is neither revealed nor predicted until the ecosystem is studied as an integrated whole.<ref name="Liere2012">{{cite journal|last=Liere|first=Heidi|author2=Jackson, Doug|author3=Vandermeer, John|author4=Wilby, Andrew|title=Ecological Complexity in a Coffee Agroecosystem: Spatial Heterogeneity, Population Persistence and Biological Control|journal=PLOS ONE | date=20 September 2012|volume=7|issue=9 | pages=e45508 | doi=10.1371/journal.pone.0045508|bibcode = 2012PLoSO...745508L |pmid=23029061|pmc=3447771|doi-access=free}}</ref> Some ecological principles, however, do exhibit collective properties where the sum of the components explain the properties of the whole, such as birth rates of a population being equal to the sum of individual births over a designated time frame.<ref name="Odum05" />

The main subdisciplines of ecology, [[population ecology|population]] (or [[community ecology|community]]) ecology and [[ecosystem ecology]], exhibit a difference not only in scale but also in two contrasting paradigms in the field. The former focuses on organisms' distribution and abundance, while the latter focuses on materials and energy fluxes.<ref>{{cite book|author1=Steward T.A. Pickett|author2=Jurek Kolasa|author3=Clive G. Jones|title=Ecological Understanding: The Nature of Theory and the Theory of Nature|url=https://archive.org/details/ecologicalunders0000pick|url-access=registration|year=1994|publisher=Academic Press|location=San Diego|isbn=978-0-12-554720-8}}</ref>

=== Hierarchy ===
{{See also|Biological organisation|Biological classification}}

{{quote box
| quote = System behaviors must first be arrayed into different levels of the organization. Behaviors corresponding to higher levels occur at slow rates. Conversely, lower organizational levels exhibit rapid rates. For example, individual tree leaves respond rapidly to momentary changes in light intensity, CO<sub>2</sub> concentration, and the like. The growth of the tree responds more slowly and integrates these short-term changes.| source = O'Neill et al. (1986)<ref name="O'Neill86" />{{Rp|76}}| width = 25%| align = right}}

The scale of ecological dynamics can operate like a closed system, such as aphids migrating on a single tree, while at the same time remaining open about broader scale influences, such as atmosphere or climate. Hence, ecologists classify [[ecosystems]] hierarchically by analyzing data collected from finer scale units, such as [[vegetation association]]s, climate, and [[soil type]]s, and integrate this information to identify emergent patterns of uniform organization and processes that operate on local to regional, [[landscape]], and chronological scales.

To structure the study of ecology into a conceptually manageable framework, the biological world is organized into a [[Biological organisation|hierarchy]], ranging in scale from (as far as ecology is concerned) [[organism]]s, to [[population ecology|populations]], to [[Guild (ecology)|guilds]], to [[Community (ecology)|communities]], to [[ecosystem]]s, to [[biome]]s, and up to the level of the [[biosphere]].<ref name="Nachtomy01"/> This framework forms a [[panarchy (ecology)|panarchy]]<ref name="Holling01"/> and exhibits [[non-linear]] behaviors; this means that "effect and cause are disproportionate, so that small changes to critical variables, such as the number of [[nitrogen fixation|nitrogen fixers]], can lead to disproportionate, perhaps irreversible, changes in the system properties."<ref name="Levin99"/>{{rp|14}}

=== Biodiversity ===
{{main|Biodiversity}}
{{quote box
| quote = Biodiversity refers to the variety of life and its processes. It includes the variety of living organisms, the genetic differences among them, the communities and ecosystems in which they occur, and the ecological and [[evolution]]ary processes that keep them functioning, yet ever-changing and adapting.| source= Noss & Carpenter (1994)<ref name="Noss94"/>{{Rp|5}}| width = 25%| align = right}}

Biodiversity (an abbreviation of "biological diversity") describes the diversity of life from genes to ecosystems and spans every level of biological organization. The term has several interpretations, and there are many ways to index, measure, characterize, and represent its complex organization.<ref name="Noss90"/><ref name="Scholes08"/><ref name=cardinale2012>{{cite journal |last1=Cardinale |first1=Bradley J. |last2=Duffy |first2=J. Emmett |last3=Gonzalez |first3=Andrew |last4=Hooper |first4=David U. |last5=Perrings |first5=Charles |last6=Venail |first6=Patrick |last7=Narwani |first7=Anita |last8=Mace |first8=Georgina M. |last9=Tilman |first9=David |last10=Wardle |first10=David A. |last11=Kinzig |first11=Ann P. |last12=Daily |first12=Gretchen C. |last13=Loreau |first13=Michel |last14=Grace |first14=James B. |last15=Larigauderie |first15=Anne |last16=Srivastava |first16=Diane S. |last17=Naeem |first17=Shahid |title=Biodiversity loss and its impact on humanity |journal=Nature |date=6 June 2012 |volume=486 |issue=7401 |pages=59–67 |doi=10.1038/nature11148 |bibcode=2012Natur.486...59C |pmid=22678280 |s2cid=4333166 |url=https://pub.epsilon.slu.se/10240/7/wardle_d_etal_130415.pdf |access-date=10 August 2019 |archive-date=21 September 2017 |archive-url=https://web.archive.org/web/20170921233215/http://pub.epsilon.slu.se/10240/7/wardle_d_etal_130415.pdf |url-status=live}}</ref> Biodiversity includes [[species diversity]], [[ecosystem diversity]], and [[genetic diversity]] and scientists are interested in the way that this diversity affects the complex ecological processes operating at and among these respective levels.<ref name="Scholes08" /><ref name="Wilson00b"/><ref name="Purvis00"/> 

Biodiversity plays an important role in [[ecosystem service]]s which by definition maintain and improve human quality of life.<ref name="cardinale2012"/><ref name="Ostfeld09"/><ref name="Tierney09"/> Conservation priorities and management techniques require different approaches and considerations to address the full ecological scope of biodiversity. [[Natural capital]] that supports populations is critical for maintaining [[ecosystem services]]<ref name="Ceballos02"/><ref name="Palumbi09"/> and species [[Animal migration|migration]] (e.g., riverine fish runs and avian insect control) has been implicated as one mechanism by which those service losses are experienced.<ref name="Wilcove08"/> An understanding of biodiversity has practical applications for species and ecosystem-level conservation planners as they make management recommendations to consulting firms, governments, and industry.<ref name="Hammond09"/>

=== Habitat ===
{{Main|Habitat}}
[[File:Blue Linckia Starfish.JPG|thumb|upright|Biodiversity of a [[coral reef]]. [[Coral]]s adapt to and modify their environment by forming [[calcium carbonate]] skeletons. This provides growing conditions for future generations and forms a habitat for many other species.<ref name="Kiessling09" />]]
[[File:Long-tailed Broadbill (Psarisomus dalhousiae).jpg|thumb|[[Long-tailed broadbill]] building its [[Bird nest|nest]]]]
The habitat of a species describes the environment over which a species is known to occur and the type of community that is formed as a result.<ref name="Whittaker73"/> More specifically, "habitats can be defined as regions in environmental space that are composed of multiple dimensions, each representing a biotic or abiotic environmental variable; that is, any component or characteristic of the environment related directly (e.g. forage biomass and quality) or indirectly (e.g. elevation) to the use of a location by the animal."<ref name="Beyer10"/>{{Rp|745}} For example, a habitat might be an aquatic or terrestrial environment that can be further categorized as a [[Montane ecosystem|montane]] or [[alpine tundra|alpine]] ecosystem. 

Habitat shifts provide important evidence of competition in nature where one population changes relative to the habitats that most other individuals of the species occupy. For example, one population of a species of tropical lizard (''Tropidurus hispidus'') has a flattened body relative to the main populations that live in open savanna. The population that lives in an isolated rock outcrop hides in crevasses where its flattened body offers a selective advantage. Habitat shifts also occur in the developmental [[life history theory|life history]] of amphibians, and in insects that transition from aquatic to terrestrial habitats. [[Biotope]] and habitat are sometimes used interchangeably, but the former applies to a community's environment, whereas the latter applies to a species' environment.<ref name="Whittaker73"/><ref name="Schoener75"/><ref name="Vitt97"/>

=== Niche ===
{{Main|Ecological niche}}

[[File:Termite mound-Tanzania.jpg|thumb|upright|[[Termite]] mounds with varied heights of chimneys regulate gas exchange, temperature and other environmental parameters that are needed to sustain the internal physiology of the entire colony.<ref name="Laland99" /><ref name="Hughes08"/>]]

Definitions of the niche date back to 1917,<ref name="Wiens05"/> but [[G. Evelyn Hutchinson]] made conceptual advances in 1957<ref name="Hutchinson57"/><ref name="Hutchinson57b"/> by introducing a widely adopted definition: "the set of biotic and abiotic conditions in which a species is able to persist and maintain stable population sizes."<ref name="Wiens05" />{{Rp|519}} The ecological niche is a central concept in the ecology of organisms and is sub-divided into the ''fundamental'' and the ''realized'' niche. The fundamental niche is the set of environmental conditions under which a species is able to persist. The realized niche is the set of environmental plus ecological conditions under which a species persists.<ref name="Wiens05"/><ref name="Hutchinson57b"/><ref name="Begon05"/> The Hutchinsonian niche is defined more technically as a "[[Euclidean space|Euclidean]] [[N-dimensional space|hyperspace]] whose ''dimensions'' are defined as environmental variables and whose ''size'' is a function of the number of values that the environmental values may assume for which an organism has ''positive fitness''."<ref name="Hardesty75"/>{{rp|71}}

[[Biogeography|Biogeographical]] patterns and [[Range (biology)|range]] distributions are explained or predicted through knowledge of a species' [[trait (biology)|traits]] and niche requirements.<ref name="Pearman08"/> Species have functional traits that are uniquely adapted to the ecological niche. A trait is a measurable property, [[phenotype]], or [[Phenotypic trait|characteristic]] of an organism that may influence its survival. Genes play an important role in the interplay of development and environmental expression of traits.<ref name="Levins80" /> Resident species evolve traits that are fitted to the selection pressures of their local environment. This tends to afford them a competitive advantage and discourages similarly adapted species from having an overlapping geographic range. The [[competitive exclusion principle]] states that two species cannot coexist indefinitely by living off the same limiting [[Resource (biology)|resource]]; one will always out-compete the other. When similarly adapted species overlap geographically, closer inspection reveals subtle ecological differences in their habitat or dietary requirements.<ref name="Hardin60"/> Some models and empirical studies, however, suggest that disturbances can stabilize the co-evolution and shared niche occupancy of similar species inhabiting species-rich communities.<ref name="Scheffer06"/> The habitat plus the niche is called the [[ecotope]], which is defined as the full range of environmental and biological variables affecting an entire species.<ref name="Whittaker73" />

===Niche construction===
{{Main|Niche construction}}
{{See also|Ecosystem engineering}}

Organisms are subject to environmental pressures, but they also modify their habitats. The [[negative feedback|regulatory feedback]] between organisms and their environment can affect conditions from local (e.g., a [[beaver]] [[pond]]) to global scales, over time and even after death, such as decaying logs or [[silica]] skeleton deposits from marine organisms.<ref name="Hastings07"/> The process and concept of [[ecosystem engineering]] are related to [[niche construction]], but the former relates only to the physical modifications of the habitat whereas the latter also considers the evolutionary implications of physical changes to the environment and the feedback this causes on the process of natural selection. Ecosystem engineers are defined as: "organisms that directly or indirectly modulate the availability of resources to other species, by causing physical state changes in biotic or abiotic materials. In so doing they modify, maintain and create habitats."<ref name="Jones94"/>{{Rp|373}}

The ecosystem engineering concept has stimulated a new appreciation for the influence that organisms have on the ecosystem and evolutionary process. The term "niche construction" is more often used in reference to the under-appreciated feedback mechanisms of natural selection imparting forces on the abiotic niche.<ref name="Laland99"/><ref name="Write06"/> An example of natural selection through ecosystem engineering occurs in the nests of [[social insects]], including ants, bees, wasps, and termites. There is an emergent [[homeostasis]] or [[homeorhesis]] in the structure of the nest that regulates, maintains and defends the physiology of the entire colony. Termite mounds, for example, maintain a constant internal temperature through the design of air-conditioning chimneys. The structure of the nests themselves is subject to the forces of natural selection. Moreover, a nest can survive over successive generations, so that progeny inherit both genetic material and a legacy niche that was constructed before their time.<ref name="Odum05" /><ref name="Laland99"/><ref name="Hughes08"/>

=== Biome ===
{{main|Biome}}

Biomes are larger units of organization that categorize regions of the Earth's ecosystems, mainly according to the structure and composition of vegetation.<ref name="Palmer94"/> There are different methods to define the continental boundaries of biomes dominated by different functional types of vegetative communities that are limited in distribution by climate, precipitation, weather, and other environmental variables. Biomes include [[tropical rainforest]], [[temperate broadleaf and mixed forest]], [[temperate deciduous forest]], [[taiga]], [[tundra]], [[hot desert]], and [[polar desert]].<ref name="Prentice92"/> Other researchers have recently categorized other biomes, such as the human and oceanic [[microbiome]]s. To a [[microorganism|microbe]], the human body is a habitat and a landscape.<ref name="Turnbaugh07"/> Microbiomes were discovered largely through advances in [[molecular genetics]], which have revealed a hidden richness of microbial diversity on the planet. The oceanic microbiome plays a significant role in the ecological biogeochemistry of the planet's oceans.<ref name="DeLong09"/>

=== Biosphere ===
{{main|Biosphere}}
{{See also|Earth's spheres}}
The largest scale of ecological organization is the biosphere: the total sum of ecosystems on the planet. [[Ecological relationship]]s regulate the flux of energy, nutrients, and climate all the way up to the planetary scale. For example, the dynamic history of the planetary atmosphere's CO<sub>2</sub> and O<sub>2</sub> composition has been affected by the biogenic flux of gases coming from respiration and photosynthesis, with levels fluctuating over time in relation to the ecology and evolution of plants and animals.<ref name="igamberdiev06"/> Ecological theory has also been used to explain self-emergent regulatory phenomena at the planetary scale: for example, the [[Gaia hypothesis]] is an example of [[holism]] applied in ecological theory.<ref name="Lovelock73"/> The Gaia hypothesis states that there is an emergent [[feedback loop]] generated by the [[metabolic theory of ecology|metabolism]] of living organisms that maintains the core temperature of the Earth and atmospheric conditions within a narrow self-regulating range of tolerance.<ref name="Lovelock03"/>

=== Population ecology ===
{{Main|Population ecology}}
{{See also|Lists of organisms by population}}

Population ecology studies the dynamics of species populations and how these populations interact with the wider environment.<ref name="Odum05" /> A population consists of individuals of the same species that live, interact, and migrate through the same niche and habitat.<ref name="Waples06"/>

A primary law of population ecology is the [[Malthusian growth model]]<ref name="Turchin01"/> which states, "a population will grow (or decline) exponentially as long as the environment experienced by all individuals in the population remains constant."<ref name="Turchin01" />{{Rp|18}} Simplified population [[Scientific modelling|models]] usually starts with four variables: death, birth, [[immigration]], and [[emigration]].

An example of an introductory population model describes a closed population, such as on an island, where immigration and emigration does not take place. Hypotheses are evaluated with reference to a null hypothesis which states that [[random]] processes create the observed data. In these island models, the rate of population change is described by:
: <math>\frac{\operatorname{d}N(t)}{\operatorname{d}t} = bN(t) - dN(t) = (b - d)N(t) = rN(t), </math>

where ''N'' is the total number of individuals in the population, ''b'' and ''d'' are the per capita rates of birth and death respectively, and ''r'' is the per capita rate of population change.<ref name="Turchin01"/><ref name="Vandermeer03"/>

Using these modeling techniques, Malthus' population principle of growth was later transformed into a model known as the [[Law of population growth|logistic equation]] by [[Pierre Verhulst]]:
: <math>\frac{\operatorname{d}N(t)}{\operatorname{d}t} = rN(t) - \alpha N(t)^2 = rN(t)\left(\frac{K - N(t)}{K}\right),</math>

where ''N(t)'' is the number of individuals measured as [[biomass (ecology)|biomass]] density as a function of time, ''t'', ''r'' is the maximum per-capita rate of change commonly known as the intrinsic rate of growth, and <math>\alpha</math> is the crowding coefficient, which represents the reduction in population growth rate per individual added. The formula states that the rate of change in population size (<math>\mathrm{d}N(t)/\mathrm{d}t</math>) will grow to approach equilibrium, where (<math>\mathrm{d}N(t)/\mathrm{d}t = 0</math>), when the rates of increase and crowding are balanced, <math>r/\alpha</math>. A common, analogous model fixes the equilibrium, <math>r/\alpha</math> as ''K'', which is known as the "carrying capacity."

Population ecology builds upon these introductory models to further understand demographic processes in real study populations. Commonly used types of data include [[Biological life cycle|life history]], [[fecundity]], and survivorship, and these are analyzed using mathematical techniques such as [[matrix (mathematics)|matrix algebra]]. The information is used for managing wildlife stocks and setting harvest quotas.<ref name="Vandermeer03" /><ref name="Berryman92"/> In cases where basic models are insufficient, ecologists may adopt different kinds of statistical methods, such as the [[Akaike information criterion]],<ref name="Anderson00">{{cite journal | last1=Anderson | first1=D. R. | last2=Burnham | first2=K. P. | last3=Thompson | first3=W. L. | year=2000 | title=Null hypotheses testing: Problems, prevalence, and an alternative | journal=J. Wildl. Manage. | volume=64 | issue=4 | pages=912–923 | url=http://people.nnu.edu/jocossel/BIOL4240/Anderson%20et%20al%202000.pdf | archive-url=https://web.archive.org/web/20130602150107/http://people.nnu.edu/jocossel/BIOL4240/Anderson%20et%20al%202000.pdf | archive-date=2 June 2013| doi=10.2307/3803199 | jstor=3803199 | access-date=4 August 2012}}</ref> or use models that can become mathematically complex as "several competing hypotheses are simultaneously confronted with the data."<ref name="Johnson04"/>

=== Metapopulations and migration ===
{{Main|Metapopulation}}
{{See also|Animal migration}}
The concept of metapopulations was defined in 1969<ref name="Levins69"/> as "a population of populations which go extinct locally and recolonize".<ref name="Levins70"/>{{Rp|105}} Metapopulation ecology is another statistical approach that is often used in [[conservation biology|conservation research]].<ref name="Smith05"/> Metapopulation models simplify the landscape into patches of varying levels of quality,<ref name="Hanski98"/> and metapopulations are linked by the migratory behaviours of organisms. Animal migration is set apart from other kinds of movement because it involves the seasonal departure and return of individuals from a habitat.<ref name="Nebel10"/> Migration is also a population-level phenomenon, as with the migration routes followed by plants as they occupied northern post-glacial environments. Plant ecologists use pollen records that accumulate and stratify in wetlands to reconstruct the timing of plant migration and dispersal relative to historic and contemporary climates. These migration routes involved an expansion of the range as plant populations expanded from one area to another. There is a larger taxonomy of movement, such as commuting, foraging, territorial behavior, stasis, and ranging. Dispersal is usually distinguished from migration because it involves the one-way permanent movement of individuals from their birth population into another population.<ref name="Clark98"/><ref name="Dingle96"/>

In metapopulation terminology, migrating individuals are classed as emigrants (when they leave a region) or immigrants (when they enter a region), and sites are classed either as sources or sinks. A site is a generic term that refers to places where ecologists sample populations, such as ponds or defined sampling areas in a forest. Source patches are productive sites that generate a seasonal supply of [[Juvenile (organism)|juveniles]] that migrate to other patch locations. Sink patches are unproductive sites that only receive migrants; the population at the site will disappear unless rescued by an adjacent source patch or environmental conditions become more favorable. Metapopulation models examine patch dynamics over time to answer potential questions about spatial and demographic ecology. The ecology of metapopulations is a dynamic process of extinction and colonization. Small patches of lower quality (i.e., sinks) are maintained or rescued by a seasonal influx of new immigrants. A dynamic metapopulation structure evolves from year to year, where some patches are sinks in dry years and are sources when conditions are more favorable. Ecologists use a mixture of computer models and [[field study|field studies]] to explain metapopulation structure.<ref name="Hanski04"/><ref name="MacKenzie06"/>

=== Community ecology ===
[[File:Male Lion and Cub Chitwa South Africa Luca Galuzzi 2004 edit1.jpg|thumb|right|Interspecific interactions such as [[predation]] are a key aspect of [[community ecology]].]]

{{Main|Community ecology}}
{{quote box
| quote = Community ecology examines how interactions among species and their environment affect the abundance, distribution and diversity of species within communities.
| source = Johnson & Stinchcomb (2007)<ref name="Johnson07" />{{Rp|250}}
| width = 25%
| align = left}}

Community ecology is the study of the interactions among a collection of species that inhabit the same geographic area. Community ecologists study the determinants of patterns and processes for two or more interacting species. Research in community ecology might measure [[species diversity]] in grasslands in relation to soil fertility. It might also include the analysis of predator-prey dynamics, competition among similar plant species, or mutualistic interactions between crabs and corals.

=== Ecosystem ecology ===
{{main|Ecosystem ecology}}
{{quote box
| quote = These ecosystems, as we may call them, are of the most various kinds and sizes. They form one category of the multitudinous physical systems of the universe, which range from the universe as a whole down to the atom.
| source = Tansley (1935)<ref name="Tansley35"/>{{Rp|299}}
| width = 25%
| align = left}}

[[File:Ecoecolfigure1.jpg|thumb|right|A [[riparian forest]] in the [[White Mountains (New Hampshire)|White Mountains, New Hampshire]] (US) is an example of [[ecosystem ecology]]]]

Ecosystems may be habitats within biomes that form an integrated whole and a dynamically responsive system having both physical and biological complexes. Ecosystem ecology is the science of determining the fluxes of materials (e.g. carbon, phosphorus) between different pools (e.g., tree biomass, soil organic material). Ecosystem ecologists attempt to determine the underlying causes of these fluxes. Research in ecosystem ecology might measure [[primary production]] (g C/m^2) in a [[wetland]] in relation to decomposition and consumption rates (g C/m^2/y). This requires an understanding of the community connections between plants (i.e., primary producers) and the decomposers (e.g., [[fungi]] and bacteria).<ref name="Brinson81">{{cite journal|last1 = Brinson| first1 = M. M.|last2=Lugo|first2=A. E.|last3=Brown|first3=S|title = Primary Productivity, Decomposition and Consumer Activity in Freshwater Wetlands|journal = Annual Review of Ecology and Systematics|volume = 12|pages=123–161|year = 1981| issue = 1|doi = 10.1146/annurev.es.12.110181.001011| bibcode = 1981AnRES..12..123B}}</ref>

The underlying concept of an ecosystem can be traced back to 1864 in the published work of [[George Perkins Marsh]] ("Man and Nature").<ref name="Marsh64"/><ref name="O'Neil01"/> Within an ecosystem, organisms are linked to the physical and biological components of their environment to which they are adapted.<ref name="Tansley35"/> Ecosystems are complex adaptive systems where the interaction of life processes form self-organizing patterns across different scales of time and space.<ref name="Levin98"/> Ecosystems are broadly categorized as [[Terrestrial ecosystem|terrestrial]], [[Freshwater ecosystem|freshwater]], atmospheric, or [[Marine ecosystem|marine]]. Differences stem from the nature of the unique physical environments that shapes the biodiversity within each. A more recent addition to ecosystem ecology are [[technoecosystems]], which are affected by or primarily the result of human activity.<ref name="Odum05" />

=== Food webs ===
{{Main|Food web}}
{{See also|Food chain}}
[[File:Chesapeake Waterbird Food Web.jpg|thumb|upright=1.65|Generalized food web of waterbirds from [[Chesapeake Bay]]]]

A food web is the archetypal [[ecological network]]. Plants capture [[solar energy]] and use it to synthesize [[simple sugars]] during [[photosynthesis]]. As plants grow, they accumulate nutrients and are eaten by grazing [[herbivores]], and the energy is transferred through a chain of organisms by consumption. The simplified linear feeding pathways that move from a basal [[trophic species]] to a top consumer is called the [[food chain]]. Food chains in an ecological community create a complex food web. Food webs are a type of [[concept map]] that is used to illustrate and study pathways of energy and material flows.<ref name="O'Neill86"/><ref name="Pimm02"/><ref name="Pimm91"/>

'''Empirical measurements''' are generally restricted to a specific habitat, such as a cave or a pond, and principles gleaned from small-scale studies are extrapolated to larger systems.<ref name="Worm03"/> Feeding relations require extensive investigations, e.g. into the gut contents of organisms, which can be difficult to decipher, or stable isotopes can be used to trace the flow of nutrient diets and energy through a food web.<ref name="McCann07"/> Despite these limitations, food webs remain a valuable tool in understanding community ecosystems.<ref name="Wilbur97"/>

Food webs illustrate important '''principles of ecology''': some species have many weak feeding links (e.g., [[omnivores]]) while some are more specialized with fewer stronger feeding links (e.g., [[predator|primary predators]]). Such linkages explain how ecological communities remain stable over time<ref name="Emmerson"/><ref name="Kraus03"/> and eventually can illustrate a "complete" web of life.<ref name="Pimm91"/><ref name="Egerton07b"/><ref name="Shurin06"/><ref name="Edwards83"/>

The '''disruption of food webs''' may have a dramatic impact on the ecology of individual species or whole ecosystems. For instance, the replacement of an [[ant]] species by another (invasive) ant species has been shown to affect how [[elephant]]s reduce tree cover and thus the predation of [[lion]]s on [[zebra]]s.<ref>{{Cite journal |last=Gaynor |first=Kaitlyn M. |date=2024-01-26 |title=A big-headed problem drives an ecological chain reaction |url=https://www.science.org/doi/10.1126/science.adn3484 |journal=Science |language=en |volume=383 |issue=6681 |pages=370–371 |doi=10.1126/science.adn3484 |pmid=38271504 |bibcode=2024Sci...383..370G |issn=0036-8075}}</ref><ref>{{Cite journal |last1=Kamaru |first1=Douglas N. |last2=Palmer |first2=Todd M. |last3=Riginos |first3=Corinna |last4=Ford |first4=Adam T. |last5=Belnap |first5=Jayne |last6=Chira |first6=Robert M. |last7=Githaiga |first7=John M. |last8=Gituku |first8=Benard C. |last9=Hays |first9=Brandon R. |last10=Kavwele |first10=Cyrus M. |last11=Kibungei |first11=Alfred K. |last12=Lamb |first12=Clayton T. |last13=Maiyo |first13=Nelly J. |last14=Milligan |first14=Patrick D. |last15=Mutisya |first15=Samuel |date=2024-01-26 |title=Disruption of an ant-plant mutualism shapes interactions between lions and their primary prey |url=https://www.science.org/doi/10.1126/science.adg1464 |journal=Science |language=en |volume=383 |issue=6681 |pages=433–438 |doi=10.1126/science.adg1464 |pmid=38271503 |bibcode=2024Sci...383..433K |issn=0036-8075}}</ref>

=== Trophic levels ===
{{Main|Trophic level}}

[[File:TrophicWeb.jpg|thumb|upright=2.05|A trophic pyramid (a) and a food-web (b) illustrating [[ecological relationship]]s among creatures that are typical of a northern [[boreal ecosystem|boreal]] terrestrial ecosystem. The trophic pyramid roughly represents the biomass (usually measured as total dry-weight) at each level. Plants generally have the greatest biomass. Names of trophic categories are shown to the right of the pyramid. Some ecosystems, such as many wetlands, do not organize as a strict pyramid, because aquatic plants are not as productive as long-lived terrestrial plants such as trees. Ecological trophic pyramids are typically one of three kinds: 1) pyramid of numbers, 2) pyramid of biomass, or 3) pyramid of energy.<ref name="Odum05"/>{{rp|598}}]]

A trophic level (from Greek ''troph'', τροφή, trophē, meaning "food" or "feeding") is "a group of organisms acquiring a considerable majority of its energy from the lower adjacent level (according to [[ecological pyramid]]s) nearer the abiotic source."<ref name="Hariston93"/>{{rp|383}} Links in food webs primarily connect feeding relations or [[trophism]] among species. Biodiversity within ecosystems can be organized into trophic pyramids, in which the vertical dimension represents feeding relations that become further removed from the base of the food chain up toward top predators, and the horizontal dimension represents the [[Relative species abundance|abundance]] or biomass at each level.<ref name="Duffy07"/> When the relative abundance or biomass of each species is sorted into its respective trophic level, they naturally sort into a 'pyramid of numbers'.<ref name="Elton27" />

Species are broadly categorized as [[autotrophs]] (or [[primary producers]]), [[heterotrophs]] (or [[consumer (food chain)|consumer]]s), and [[Detritivore]]s (or [[decomposers]]). Autotrophs are organisms that produce their own food (production is greater than respiration) by photosynthesis or [[chemosynthesis]]. Heterotrophs are organisms that must feed on others for nourishment and energy (respiration exceeds production).<ref name="Odum05" /> Heterotrophs can be further sub-divided into different functional groups, including [[primary consumers]] (strict herbivores), [[Trophic dynamics|secondary consumers]] ([[carnivorous]] predators that feed exclusively on herbivores), and tertiary consumers (predators that feed on a mix of herbivores and predators).<ref name="David03"/> Omnivores do not fit neatly into a functional category because they eat both plant and animal tissues. It has been suggested that omnivores have a greater functional influence as predators because compared to herbivores, they are relatively inefficient at grazing.<ref name="Oksanen91"/>

Trophic levels are part of the [[holistic]] or [[complex systems]] view of ecosystems.<ref name="Loehle88"/><ref name="Ulanowicz79"/> Each trophic level contains unrelated species that are grouped together because they share common ecological functions, giving a macroscopic view of the system.<ref name="Li00"/> While the notion of trophic levels provides insight into energy flow and top-down control within food webs, it is troubled by the prevalence of omnivory in real ecosystems. This has led some ecologists to "reiterate that the notion that species clearly aggregate into discrete, homogeneous trophic levels is fiction."<ref name="Polis96"/>{{rp|815}} Nonetheless, recent studies have shown that real trophic levels do exist, but "above the herbivore trophic level, food webs are better characterized as a tangled web of omnivores."<ref name="Thompson07"/>{{rp|612}}

=== Keystone species ===
{{main|Keystone species}}
[[File:Sea otters holding hands, cropped.jpg|thumb|[[Sea otter]]s, an example of a keystone species]]
A keystone species is a species that is connected to a disproportionately large number of other species in the [[food-web]]. Keystone species have lower levels of biomass in the trophic pyramid relative to the importance of their role. The many connections that a keystone species holds means that it maintains the organization and structure of entire communities. The loss of a keystone species results in a range of dramatic cascading effects (termed ''trophic cascades'') that alters trophic dynamics, other food web connections, and can cause the extinction of other species.<ref name="Fisher06"/><ref name="Libralato06"/> The term keystone species was coined by Robert Paine in 1969 and is a reference to the [[Keystone (architecture)|keystone]] architectural feature as the removal of a keystone species can result in a community collapse just as the removal of the keystone in an arch can result in the arch's loss of stability.<ref>{{Cite journal |last=Paine |first=R. T. |date=January 1969 |title=A Note on Trophic Complexity and Community Stability |journal=The American Naturalist |volume=103 |issue=929 |pages=91–93 |doi=10.1086/282586 |bibcode=1969ANat..103...91P |s2cid=83780992 |issn=0003-0147}}</ref>

[[Sea otter]]s (''Enhydra lutris'') are commonly cited as an example of a keystone species because they limit the density of [[sea urchins]] that feed on [[kelp]]. If sea otters are removed from the system, the urchins graze until the kelp beds disappear, and this has a dramatic effect on community structure.<ref name="Mills93"/> Hunting of sea otters, for example, is thought to have led indirectly to the extinction of the [[Steller's sea cow]] (''Hydrodamalis gigas'').<ref name="Anderson95"/> While the keystone species concept has been used extensively as a [[Conservation biology|conservation]] tool, it has been criticized for being poorly defined from an operational stance. It is difficult to experimentally determine what species may hold a keystone role in each ecosystem. Furthermore, food web theory suggests that keystone species may not be common, so it is unclear how generally the keystone species model can be applied.<ref name="Mills93"/><ref name="Polis00"/>

== Complexity ==
{{Main|Complexity}}
{{See also|Emergence}}

Complexity is understood as a large computational effort needed to piece together numerous interacting parts exceeding the iterative memory capacity of the human mind. Global patterns of biological diversity are complex. This [[biocomplexity]] stems from the interplay among ecological processes that operate and influence patterns at different scales that grade into each other, such as transitional areas or [[ecotones]] spanning landscapes. Complexity stems from the interplay among levels of biological organization as energy, and matter is integrated into larger units that superimpose onto the smaller parts. "What were wholes on one level become parts on a higher one."<ref name="Novikoff45"/>{{rp|209}} Small scale patterns do not necessarily explain large scale phenomena, otherwise captured in the expression (coined by Aristotle) 'the sum is greater than the parts'.<ref name="Schneider01"/><ref name="Molnar04"/>{{Cref2|E}}

"Complexity in ecology is of at least six distinct types: spatial, temporal, structural, process, behavioral, and geometric."<ref name="Loehle04"/>{{rp|3}} From these principles, ecologists have identified [[emergence|emergent]] and [[Self-organization#Biology|self-organizing]] phenomena that operate at different environmental scales of influence, ranging from molecular to planetary, and these require different explanations at each [[integrative level]].<ref name="Lovelock03" /><ref name="Odum1977"/> Ecological complexity relates to the dynamic resilience of ecosystems that transition to multiple shifting steady-states directed by random fluctuations of history.<ref name="Holling01"/><ref name="Carpenter01"/> Long-term ecological studies provide important track records to better understand the complexity and resilience of ecosystems over longer temporal and broader spatial scales. These studies are managed by the International Long Term Ecological Network (LTER).<ref name="urlWelcome to ILTER – ILTER"/> The longest experiment in existence is the [[Park Grass Experiment]], which was initiated in 1856.<ref name="Siverton06"/> Another example is the [[Hubbard Brook Experimental Forest|Hubbard Brook study]], which has been in operation since 1960.<ref>{{cite web |url=http://www.hubbardbrook.org/ |title=Hubbard Brook Ecosystem Study Front Page |access-date=16 March 2010 |url-status=live |archive-url=https://web.archive.org/web/20100324131250/http://www.hubbardbrook.org/ |archive-date=24 March 2010}}</ref>

=== Holism ===
{{Main|Holism}}

Holism remains a critical part of the theoretical foundation in contemporary ecological studies. Holism addresses the [[Biological organisation|biological organization]] of life that [[Systems biology|self-organizes]] into layers of emergent whole systems that function according to non-reducible properties. This means that higher-order patterns of a whole functional system, such as an [[ecosystem]], cannot be predicted or understood by a simple summation of the parts.<ref name="Liu09"/> "New properties emerge because the components interact, not because the basic nature of the components is changed."<ref name="Odum05"/>{{rp|8}}

Ecological studies are necessarily holistic as opposed to [[reductionistic]].<ref name="Levins80" /><ref name="Odum1977"/><ref name="Mikkelson10"/> Holism has three scientific meanings or uses that identify with ecology: 1) the mechanistic complexity of ecosystems, 2) the practical description of patterns in quantitative reductionist terms where correlations may be identified but nothing is understood about the causal relations without reference to the whole system, which leads to 3) a [[metaphysics|metaphysical]] hierarchy whereby the causal relations of larger systems are understood without reference to the smaller parts. Scientific holism differs from [[mysticism]] that has appropriated the same term. An example of metaphysical holism is identified in the trend of increased exterior thickness in shells of different species. The reason for a thickness increase can be understood through reference to principles of natural selection via predation without the need to reference or understand the [[biomolecular]] properties of the exterior shells.<ref name="Wilson88"/>

== Relation to evolution ==
{{main|Evolutionary ecology}}

Ecology and evolutionary biology are considered sister disciplines of the life sciences. [[Natural selection]], [[Biological life cycle|life history]], [[Developmental biology|development]], [[adaptation]], [[populations]], and [[heredity|inheritance]] are examples of concepts that thread equally into ecological and evolutionary theory. Morphological, behavioural, and genetic traits, for example, can be mapped onto evolutionary trees to study the historical development of a species in relation to their functions and roles in different ecological circumstances. In this framework, the analytical tools of ecologists and evolutionists overlap as they organize, classify, and investigate life through common systematic principles, such as [[phylogenetics]] or the [[Linnaean taxonomy|Linnaean system of taxonomy]].<ref name="Miles93"/> The two disciplines often appear together, such as in the title of the journal ''[[Trends in Ecology and Evolution]]''.<ref name="TREE">{{cite web | editor-last=Craze | editor-first=P. | title=Trends in Ecology and Evolution | publisher=Cell Press, Elsevier, Inc | url=http://www.cell.com/trends/ecology-evolution/home | date=2 August 2012 | archive-url=http://arquivo.pt/wayback/20090724103422/http://www.cell.com/trends/ecology-evolution/home | archive-date=24 July 2009 | access-date=9 December 2009 }}</ref> There is no sharp boundary separating ecology from evolution, and they differ more in their areas of applied focus. Both disciplines discover and explain emergent and unique properties and processes operating across different spatial or temporal scales of organization.<ref name="Levins80" /><ref name="Lovelock03" /> While the boundary between ecology and evolution is not always clear, ecologists study the abiotic and biotic factors that influence evolutionary processes,<ref name="Allee49"/><ref name="Ricklefs96"/> and evolution can be rapid, occurring on ecological timescales as short as one generation.<ref>{{cite journal | last=Yoshida | first=T | title=Rapid evolution drives ecological dynamics in a predator–prey system | journal=Nature | volume=424 | issue=6946 | pages=303–306 | publisher=Nature Publishing Group| doi=10.1038/nature01767 | pmid=12867979 | year=2003 | bibcode=2003Natur.424..303Y| s2cid=4425455 }}</ref>

=== Behavioural ecology ===
{{Main|Behavioural ecology}}
[[File:Chameleon spectra.jpg|left|upright=1.6|thumb|Social display and colour variation in differently adapted species of [[chameleons]] (''Bradypodion'' spp.). Chameleons change their skin colour to match their background as a behavioural defence mechanism and also use colour to communicate with other members of their species, such as dominant (left) versus submissive (right) patterns shown in the three species (A-C) above.<ref name="Stuart-Fox08"/>]]

All organisms can exhibit behaviours. Even plants express complex behaviour, including memory and communication.<ref name="Karban08"/> Behavioural ecology is the study of an organism's behaviour in its environment and its ecological and evolutionary implications. Ethology is the study of observable movement or behaviour in animals. This could include investigations of motile [[sperm]] of plants, mobile [[phytoplankton]], [[zooplankton]] swimming toward the female egg, the cultivation of fungi by [[weevils]], the mating dance of a [[salamander]], or social gatherings of [[amoeba]].<ref name="Tinbergen63"/><ref name="Hamner85"/><ref name="Strassmann00"/><ref name="Sakurai85"/><ref name="Anderson61"/>

Adaptation is the central unifying concept in behavioural ecology.<ref>{{cite web|url=http://www.behavecol.com/pages/society/welcome.html |title=Behavioral Ecology |publisher=International Society for Behavioral Ecology |access-date=15 April 2011 |archive-url=https://web.archive.org/web/20110410105207/http://www.behavecol.com/pages/society/welcome.html |archive-date=10 April 2011}}</ref> Behaviours can be recorded as traits and inherited in much the same way that eye and hair colour can. Behaviours can evolve by means of natural selection as adaptive traits conferring functional utilities that increases reproductive fitness.<ref name="Gould82"/><ref name="Wilson00"/>

[[File:Common jassid nymphs and ants02.jpg|thumb|upright|'''Mutualism:''' [[Leafhopper]]s (''Eurymela fenestrata'') are protected by [[meat ant|ants]] (''Iridomyrmex purpureus'') in a [[Mutualism (biology)|mutualistic]] relationship. The ants protect the leafhoppers from predators and stimulate feeding in the leafhoppers, and in return, the leafhoppers feeding on plants exude honeydew from their anus that provides energy and nutrients to tending ants.<ref name="Eastwood04"/>]]

Predator-prey interactions are an introductory concept into food-web studies as well as behavioural ecology.<ref name="Ives04"/> Prey species can exhibit different kinds of behavioural adaptations to predators, such as avoid, flee, or defend. Many prey species are faced with multiple predators that differ in the degree of danger posed. To be adapted to their environment and face predatory threats, organisms must balance their energy budgets as they invest in different aspects of their life history, such as growth, feeding, mating, socializing, or modifying their habitat. 

Hypotheses posited in behavioural ecology are generally based on adaptive principles of conservation, optimization, or efficiency.<ref name="Begon05"/><ref name="Allee49"/><ref name="Krebs93"/> For example, "[t]he threat-sensitive predator avoidance hypothesis predicts that prey should assess the degree of threat posed by different predators and match their behaviour according to current levels of risk"<ref name="Webb10"/> or "[t]he optimal [[Escape distance|flight initiation distance]] occurs where expected postencounter fitness is maximized, which depends on the prey's initial fitness, benefits obtainable by not fleeing, energetic escape costs, and expected fitness loss due to predation risk."<ref name="Cooper10"/>

Elaborate sexual [[display (zoology)|displays]] and posturing are encountered in the behavioural ecology of animals. The [[birds-of-paradise]], for example, sing and display elaborate ornaments during [[courtship]]. These displays serve a dual purpose of signalling healthy or well-adapted individuals and desirable genes. The displays are driven by [[sexual selection]] as an advertisement of quality of traits among [[suitors]].<ref name="Kodric-Brown84"/>

=== Cognitive ecology ===

Cognitive ecology integrates theory and observations from [[evolutionary ecology]] and [[neurobiology]], primarily [[cognitive science]], in order to understand the effect that animal interaction with their habitat has on their cognitive systems and how those systems restrict behavior within an ecological and evolutionary framework.<ref name=Palacios>{{cite journal |journal=Biology Research |volume=36 |issue=1 |pages=95–99 |year=2003 |title=An "enactive" approach to integrative and comparative biology: Thoughts on the table |author=Adrian G Palacios, [[Francisco Bozinovic]] |doi=10.4067/S0716-97602003000100008 |pmid=12795209 |last2=Bozinovic |doi-access=free }}</ref> "Until recently, however, cognitive scientists have not paid sufficient attention to the fundamental fact that cognitive traits evolved under particular natural settings. With consideration of the selection pressure on cognition, cognitive ecology can contribute intellectual coherence to the multidisciplinary study of cognition."<ref name=Dukas>{{cite book |title=Cognitive Ecology: The Evolutionary Ecology of Information Processing and Decision Making |author=Reuven Dukas |editor=Reuven Dukas |isbn=978-0-226-16932-3 |year=1998 |chapter=§1.3 Why study cognitive ecology? |publisher=University of Chicago Press |chapter-url=https://books.google.com/books?id=nNRXQM7_R0UC&pg=PA4 |page=4 |access-date=27 June 2015 |archive-date=18 March 2015 |archive-url=https://web.archive.org/web/20150318140450/http://books.google.com/books?id=nNRXQM7_R0UC&pg=PA4 |url-status=live }}</ref><ref name=Dukas2>{{cite book |title=Cognitive Ecology II |chapter=Introduction |pages=1 ''ff'' |author1=Reuven Dukas |author2=John M. Ratcliffe |editor1=Reuven Dukas |editor2=John M. Ratcliffe |isbn=978-0-226-16937-8 |publisher=University of Chicago Press |year=2009 |quote=Cognitive ecology focuses on the ecology and evolution of "cognition" defined as the neuronal processes concerned with the acquisition, retention, and use of information....we ought to rely on ecological and evolutionary knowledge for studying cognition. |chapter-url=https://books.google.com/books?id=TAiAcZ0Q9LQC&pg=PA1 |access-date=27 June 2015 |archive-date=18 March 2015 |archive-url=https://web.archive.org/web/20150318133553/http://books.google.com/books?id=TAiAcZ0Q9LQC&pg=PA1 |url-status=live }}</ref> As a study involving the 'coupling' or interactions between organism and environment, cognitive ecology is closely related to [[enactivism]],<ref name=Palacios/> a field based upon the view that "...we must see the organism and environment as bound together in reciprocal specification and selection...".<ref name=Varela>{{cite book |author1=Francisco J Varela |author2=Evan Thompson |author3=Eleanor Rosch |year=1993 |title=The Embodied Mind: Cognitive Science and Human Experience |publisher=MIT Press |url=https://books.google.com/books?id=QY4RoH2z5DoC |isbn=978-0-262-26123-4 |page=174 |access-date=27 June 2015 |archive-date=1 August 2020 |archive-url=https://web.archive.org/web/20200801065309/https://books.google.com/books?id=QY4RoH2z5DoC |url-status=live }}</ref>

=== Social ecology ===
{{main|Social ecology (academic field)}}

Social-ecological behaviours are notable in the [[social insects]], [[slime moulds]], [[social spider]]s, [[human society]], and [[naked mole-rat]]s where [[eusocialism]] has evolved. Social behaviours include reciprocally beneficial behaviours among kin and nest mates<ref name="Strassmann00" /><ref name="Wilson00" /><ref name="Sherman95"/> and evolve from kin and group selection. [[Kin selection]] explains altruism through genetic relationships, whereby an altruistic behaviour leading to death is rewarded by the survival of genetic copies distributed among surviving relatives. The social insects, including [[ant]]s, [[bee]]s, and [[wasp]]s are most famously studied for this type of relationship because the male drones are [[Cloning|clones]] that share the same genetic make-up as every other male in the colony.<ref name="Wilson00" /> In contrast, [[group selection]]ists find examples of altruism among non-genetic relatives and explain this through selection acting on the group; whereby, it becomes selectively advantageous for groups if their members express altruistic behaviours to one another. Groups with predominantly altruistic members survive better than groups with predominantly selfish members.<ref name="Wilson00" /><ref name="Wilson07"/>

=== Coevolution ===
{{main|Coevolution}}
[[File:Bombus 6867.JPG|thumb|right|[[Bumblebee]]s and the [[flower]]s they [[pollination|pollinate]] have coevolved so that both have become dependent on each other for survival.]]
[[File:Parasitismus.jpg|thumb|upright|'''Parasitism:''' A harvestman [[arachnid]] being parasitized by [[mite]]s. The harvestman is being consumed, while the mites benefit from traveling on and feeding off of their host.]]

Ecological interactions can be classified broadly into a [[host (biology)|host]] and an associate relationship. A host is any entity that harbours another that is called the associate.<ref name="Page91"/> Relationships [[Interspecific interaction|between species]] that are mutually or reciprocally beneficial are called [[mutualisms]]. Examples of mutualism include [[fungus-growing ants]] employing agricultural symbiosis, bacteria living in the guts of insects and other organisms, the [[fig wasp]] and [[Prodoxidae|yucca moth]] pollination complex, [[lichen]]s with fungi and photosynthetic [[algae]], and [[coral]]s with photosynthetic algae.<ref name="Herre99"/><ref name="Gilbert90"/> If there is a physical connection between host and associate, the relationship is called [[symbiosis]]. Approximately 60% of all plants, for example, have a symbiotic relationship with [[arbuscular mycorrhizal fungi]] living in their roots forming an exchange network of carbohydrates for [[nutrients|mineral nutrients]].<ref name="Kiers06"/>

Indirect mutualisms occur where the organisms live apart. For example, trees living in the equatorial regions of the planet supply oxygen into the atmosphere that sustains species living in distant polar regions of the planet. This relationship is called [[commensalism]] because many others receive the benefits of clean air at no cost or harm to trees supplying the oxygen.<ref name="Odum05" /><ref>{{cite journal|last1=Strain | first1=B. R.|year=1985|title=Physiological and ecological controls on carbon sequestering in terrestrial ecosystems|journal=Biogeochemistry|volume=1|issue=3|pages=219–232|doi=10.1007/BF02187200| bibcode=1985Biogc...1..219S| s2cid=98479424}}</ref> If the associate benefits while the host suffers, the relationship is called [[parasitism]]. Although parasites impose a cost to their host (e.g., via damage to their reproductive organs or [[propagule]]s, denying the services of a beneficial partner), their net effect on host fitness is not necessarily negative and, thus, becomes difficult to forecast.<ref name="Bronstein01"/><ref name="Irwin10"/> Co-evolution is also driven by competition among species or among members of the same species under the banner of reciprocal antagonism, such as grasses competing for growth space. The [[Red Queen Hypothesis]], for example, posits that parasites track down and specialize on the locally common genetic defense systems of its host that drives the evolution of sexual reproduction to diversify the genetic constituency of populations responding to the antagonistic pressure.<ref name="Boucher82"/><ref name="King09">{{cite journal | last1=King | first1=K. C. | last2=Delph | first2=L. F. | last3=Jokela | first3=J. | last4=Lively | first4=C. M. | title=The geographic mosaic of sex and the Red Queen | journal=Current Biology | volume=19 | issue=17 | pages=1438–1441 | doi=10.1016/j.cub.2009.06.062| year=2009 | pmid=19631541| s2cid=12027050 | doi-access=free | bibcode=2009CBio...19.1438K }}</ref>

=== Biogeography ===
{{Main|Biogeography}}

Biogeography (an amalgamation of ''biology'' and ''geography'') is the comparative study of the geographic distribution of organisms and the corresponding evolution of their traits in space and time.<ref name="Parenti90"/> The ''[[Journal of Biogeography]]'' was established in 1974.<ref name="JBiog">{{cite web | title = Journal of Biogeography – Overview | publisher = Wiley | url = http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1365-2699/homepage/ProductInformation.html | access-date = 16 March 2018 | url-status=live | archive-url = https://archive.today/20130209095040/http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1365-2699/homepage/ProductInformation.html | archive-date = 9 February 2013| doi = 10.1111/(ISSN)1365-2699}}</ref> Biogeography and ecology share many of their disciplinary roots. For example, [[Island biogeography|the theory of island biogeography]], published by the Robert MacArthur and [[Edward O. Wilson]] in 1967<ref name="MacArthur67" /> is considered one of the fundamentals of ecological theory.<ref name="Wiens04"/>

Biogeography has a long history in the natural sciences concerning the spatial distribution of plants and animals. Ecology and evolution provide the explanatory context for biogeographical studies.<ref name="Parenti90" /> Biogeographical patterns result from ecological processes that influence range distributions, such as [[Animal migration|migration]] and [[Biological dispersal|dispersal]].<ref name="Wiens04" /> and from historical processes that split populations or species into different areas. The biogeographic processes that result in the natural splitting of species explain much of the modern distribution of the Earth's biota. The splitting of lineages in a species is called [[Allopatric speciation|vicariance biogeography]] and it is a sub-discipline of biogeography.<ref name="Morrone95"/> There are also practical applications in the field of biogeography concerning ecological systems and processes. For example, the range and distribution of biodiversity and invasive species responding to climate change is a serious concern and active area of research in the context of [[global warming]].<ref name="Svennin08" /><ref name="Landhäusser09"/>

==== r/K selection theory ====
{{Main|r/K selection theory}}

A population ecology concept is r/K selection theory,{{Cref2|D}} one of the first predictive models in ecology used to explain [[Life history theory|life-history evolution]]. The premise behind the r/K selection model is that natural selection pressures change according to [[population densities|population density]]. For example, when an island is first colonized, density of individuals is low. The initial increase in population size is not limited by competition, leaving an abundance of available [[Resource (biology)|resources]] for rapid population growth. These early phases of [[population growth]] experience ''density-independent'' forces of natural selection, which is called ''r''-selection. As the population becomes more crowded, it approaches the island's carrying capacity, thus forcing individuals to compete more heavily for fewer available resources. Under crowded conditions, the population experiences density-dependent forces of natural selection, called ''K''-selection.<ref name="Reznick02"/>

In the ''r/K''-selection model, the first variable ''r'' is the intrinsic rate of natural increase in population size and the second variable ''K'' is the carrying capacity of a population.<ref name="Begon05" /> Different species evolve different life-history strategies spanning a continuum between these two selective forces. An ''r''-selected species is one that has high birth rates, low levels of parental investment, and high rates of mortality before individuals reach maturity. Evolution favours high rates of [[fecundity]] in ''r''-selected species. Many kinds of insects and [[invasive species]] exhibit ''r''-selected [[Phenotypic trait|characteristics]]. In contrast, a ''K''-selected species has low rates of fecundity, high levels of parental investment in the young, and low rates of mortality as individuals mature. Humans and elephants are examples of species exhibiting ''K''-selected characteristics, including longevity and efficiency in the conversion of more resources into fewer offspring.<ref name="MacArthur67"/><ref name="Pianka72"/>

=== Molecular ecology ===
{{Main|Molecular ecology}}

The important relationship between ecology and genetic inheritance predates modern techniques for molecular analysis. Molecular ecological research became more feasible with the development of rapid and accessible genetic technologies, such as the [[Polymerase chain reaction|polymerase chain reaction (PCR)]]. The rise of molecular technologies and the influx of research questions into this new ecological field resulted in the publication ''[[Molecular Ecology]]'' in 1992.<ref name="MolEcol">{{Cite journal| title = Molecular Ecology| editor-last=Rieseberg| editor-first= L.| publisher = Wiley| doi = 10.1111/(ISSN)1365-294X| journal = Molecular Ecology}}</ref> [[Molecular ecology]] uses various analytical techniques to study genes in an evolutionary and ecological context. In 1994, [[John Avise]] also played a leading role in this area of science with the publication of his book, ''Molecular Markers, Natural History and Evolution''.<ref name="Avise94"/> 

Newer technologies opened a wave of genetic analysis into organisms once difficult to study from an ecological or evolutionary standpoint, such as bacteria, fungi, and [[nematode]]s. Molecular ecology engendered a new research paradigm for investigating ecological questions considered otherwise intractable. Molecular investigations revealed previously obscured details in the tiny intricacies of nature and improved resolution into probing questions about behavioural and biogeographical ecology.<ref name="Avise94"/> For example, molecular ecology revealed [[Promiscuity#Other animals|promiscuous]] sexual behaviour and multiple male partners in [[tree swallow]]s previously thought to be socially [[Monogamy in animals|monogamous]].<ref name="Obryan07"/> In a biogeographical context, the marriage between genetics, ecology, and evolution resulted in a new sub-discipline called [[phylogeography]].<ref name="Avise00"/>

== Human ecology ==
{{Main|Human ecology}}
{{quote box
| quote = The history of life on Earth has been a history of interaction between living things and their surroundings. To a large extent, the physical form and the habits of the earth's vegetation and its animal life have been molded by the environment. Considering the whole span of earthly time, the opposite effect, in which life actually modifies its surroundings, has been relatively slight. Only within the moment of time represented by the present century has one species man acquired significant power to alter the nature of his world.
| source = Rachel Carson, "Silent Spring"<ref name=Carson/>
| width = 25%
| align = left}}

Ecology is as much a biological science as it is a human science.<ref name="Odum05" /> Human ecology is an [[interdisciplinary]] investigation into the ecology of our species. "Human ecology may be defined: (1) from a bioecological standpoint as the study of man as the ecological dominant in plant and animal communities and systems; (2) from a bioecological standpoint as simply another animal affecting and being affected by his physical environment; and (3) as a human being, somehow different from animal life in general, interacting with physical and modified environments in a distinctive and creative way. A truly interdisciplinary human ecology will most likely address itself to all three."<ref name="Young74"/>{{rp|3}} The term was formally introduced in 1921, but many sociologists, geographers, psychologists, and other disciplines were interested in human relations to natural systems centuries prior, especially in the late 19th century.<ref name="Young74"/><ref name="Gross04"/>

The ecological complexities human beings are facing through the technological transformation of the planetary biome has brought on the [[Anthropocene]]. The unique set of circumstances has generated the need for a new unifying science called [[coupled human and natural systems]] that builds upon, but moves beyond the field of human ecology.<ref name="Liu09" /> Ecosystems tie into human societies through the critical and all-encompassing life-supporting functions they sustain. In recognition of these functions and the incapability of traditional economic valuation methods to see the value in ecosystems, there has been a surge of interest in [[social capital|social]]-[[natural capital]], which provides the means to put a value on the stock and use of information and materials stemming from [[ecosystem services|ecosystem goods and services]]. Ecosystems produce, regulate, maintain, and supply services of critical necessity and beneficial to human health (cognitive and physiological), economies, and they even provide an information or reference function as a living library giving opportunities for science and cognitive development in children engaged in the complexity of the natural world. Ecosystems relate importantly to human ecology as they are the ultimate base foundation of global economics as every commodity, and the capacity for exchange ultimately stems from the ecosystems on Earth.<ref name="Liu09" /><ref name="MEA05"/><ref name="de Groot02"/><ref name="Aguirre09"/>

=== Restoration Ecology ===
{{main|Restoration ecology}}
{{see also|Natural resource management}}

{{quote box
| quote = Ecosystem management is not just about science nor is it simply an extension of traditional resource management; it offers a fundamental reframing of how humans may work with nature.
| source = Grumbine (1994)<ref name="Grumbine94">{{cite journal|last1=Grumbine|first1=R. E.|year=1994|title=What is ecosystem management?|journal=Conservation Biology|volume=8|issue=1|pages=27–38|url=http://www.pelagicos.net/MARS6920_spring2010/readings/Grumbine_1994.pdf|doi=10.1046/j.1523-1739.1994.08010027.x|bibcode=1994ConBi...8...27G |archive-url=https://web.archive.org/web/20130502051519/http://www.pelagicos.net/MARS6920_spring2010/readings/Grumbine_1994.pdf|archive-date=2 May 2013}}</ref>{{Rp|27}}
| width = 25%
| align = right}}

Ecology is an employed science of restoration, repairing disturbed sites through human intervention, in natural resource management, and in [[environmental impact assessment]]s. Edward O. Wilson predicted in 1992 that the 21st century "will be the era of restoration in ecology".<ref name="Wilson92">{{cite book| last1=Wilson| first1=E. O.| year=1992| title=The Diversity of Life| publisher=Harvard University Press| isbn=978-0-674-05817-0| page=[https://archive.org/details/diversityoflife000wils/page/n448 440]| url=https://archive.org/details/diversityoflife000wils| url-access=registration}}</ref> Ecological science has boomed in the industrial investment of restoring ecosystems and their processes in abandoned sites after disturbance. Natural resource managers, in [[silviculture|forestry]], for example, employ ecologists to develop, adapt, and implement [[Ecosystem management|ecosystem based methods]] into the planning, operation, and restoration phases of land-use. 

Another example of conservation is seen on the east coast of the United States in Boston, MA. The city of Boston implemented the Wetland Ordinance,<ref>{{cite web |title=Boston Wetlands Ordinance |url=https://www.boston.gov/departments/environment/boston-conservation-commission |website=City of Boston |date=17 July 2016 |access-date=5 December 2022 |archive-date=5 December 2022 |archive-url=https://web.archive.org/web/20221205224924/https://www.boston.gov/departments/environment/boston-conservation-commission |url-status=live }}</ref> improving the stability of their wetland environments by implementing soil amendments that will improve groundwater storage and flow, and trimming or removal of vegetation that could cause harm to water quality.{{citation needed|date=December 2022}} Ecological science is used in the methods of sustainable harvesting, disease, and fire outbreak management, in fisheries stock management, for integrating land-use with protected areas and communities, and conservation in complex geo-political landscapes.<ref name="Hammond09" /><ref name="Grumbine94" /><ref name="Slocombe93">{{cite journal| last1=Slocombe| first1=D. S.| title=Implementing ecosystem-based management| volume=43| issue=9| pages=612–622| year=1993| jstor=1312148| doi=10.2307/1312148| journal=BioScience}}</ref><ref name="Hobbs01">{{cite journal| last1=Hobss| first1=R. J.| last2=Harris| first2=J. A.| title=Restoration ecology: Repairing the Earth's ecosystems in the new millennium| journal=Restoration Ecology| volume=9| issue=2| pages=239–246|doi=10.1046/j.1526-100x.2001.009002239.x| url=http://planet.botany.uwc.ac.za/nisl/invasives/assignment1/hobbsandharris.pdf| year=2001| bibcode=2001ResEc...9..239H| s2cid=908668| url-status=live| archive-url=https://web.archive.org/web/20130512223703/http://planet.botany.uwc.ac.za/nisl/invasives/assignment1/hobbsandharris.pdf| archive-date=12 May 2013}}</ref>

== Relation to the environment ==
{{main|Natural environment}}

The environment of ecosystems includes both physical parameters and biotic attributes. It is dynamically interlinked and contains [[Resource (biology)|resources]] for organisms at any time throughout their life cycle.<ref name="Odum05" /><ref name="Mason57"/> Like ecology, the term environment has different conceptual meanings and overlaps with the concept of nature. Environment "includes the physical world, the social world of human relations and the built world of human creation."<ref name="Kleese01"/>{{Rp|62}} The physical environment is external to the level of biological organization under investigation, including [[abiotic]] factors such as temperature, radiation, light, chemistry, [[climate]] and geology. The biotic environment includes genes, cells, organisms, members of the same species ([[conspecific]]s) and other species that share a habitat.<ref name="Campbell06"/>

The distinction between external and internal environments, however, is an abstraction parsing life and environment into units or facts that are inseparable in reality. There is an interpenetration of cause and effect between the environment and life. The laws of [[thermodynamics]], for example, apply to ecology by means of its physical state. With an understanding of metabolic and thermodynamic principles, a complete accounting of energy and material flow can be traced through an ecosystem. In this way, the environmental and ecological relations are studied through reference to conceptually manageable and isolated [[materialism|material]] parts. After the effective environmental components are understood through reference to their causes; however, they conceptually link back together as an integrated whole, or ''holocoenotic'' system as it was once called. This is known as the [[dialectical]] approach to ecology. The dialectical approach examines the parts but integrates the organism and the environment into a dynamic whole (or [[umwelt]]). Change in one ecological or environmental factor can concurrently affect the dynamic state of an entire ecosystem.<ref name="Levins80" /><ref name="Kormondy95"/>

=== Disturbance and resilience ===
{{Main|Resilience (ecology)}}

A disturbance is any process that changes or removes biomass from a community, such as a fire, flood, drought, or predation.<ref name="Hughes10"/> Disturbances are both the cause and product of natural fluctuations within an ecological community.<ref name="Levin92"/><ref name="Hughes10"/><ref name="Holling73" /><ref name="Folke04"/> Biodiversity can protect ecosystems from disturbances.<ref name="Folke04"/>

The effect of a disturbance is often hard to predict, but there are numerous examples in which a single species can massively disturb an ecosystem. For example, a single-celled [[protozoa]]n has been able to kill up to 100% of [[sea urchin]]s in some [[coral reef]]s in the [[Red Sea]] and Western [[Indian Ocean]]. Sea urchins enable complex reef ecosystems to thrive by eating [[algae]] that would otherwise inhibit coral growth.<ref>{{Cite journal |last1=Roth |first1=Lachan |last2=Eviatar |first2=Gal |last3=Schmidt |first3=Lisa-Maria |last4=Bonomo |first4=Mai |last5=Feldstein-Farkash |first5=Tamar |last6=Schubert |first6=Patrick |last7=Ziegler |first7=Maren |last8=Al-Sawalmih |first8=Ali |last9=Abdallah |first9=Ibrahim Souleiman |last10=Quod |first10=Jean-Pascal |last11=Bronstein |first11=Omri |date=May 2024 |title=Mass mortality of diadematoid sea urchins in the Red Sea and Western Indian Ocean |url=https://linkinghub.elsevier.com/retrieve/pii/S0960982224005311 |journal=Current Biology |volume=34 |issue=12 |pages=2693–2701.e4 |language=en |doi=10.1016/j.cub.2024.04.057|pmid=38788707 |bibcode=2024CBio...34.2693R }}</ref> Similarly, [[invasive species]] can wreak havoc on ecosystems. For instance, invasive [[Burmese python]]s have caused a 98% decline of small [[mammal]]s in the [[Everglades]].<ref>{{Cite journal |last=Zimmer |first=Katarina |date=2024-05-30 |title=Invasive species are transforming the Everglades |url=https://knowablemagazine.org/content/article/food-environment/2024/how-to-save-the-everglades |journal=Knowable Magazine |language=en |doi=10.1146/knowable-053024-2|doi-access=free }}</ref>

=== Metabolism and the early atmosphere ===
{{main|Early atmosphere}}

{{quote box
| quote = Metabolism – the rate at which energy and material resources are taken up from the environment, transformed within an organism, and allocated to maintenance, growth and reproduction – is a fundamental physiological trait.
| source = Ernest et al.<ref name="Ernest03"/>{{Rp|991}}
| width=25%
| align=right
}}

The Earth was formed approximately 4.5&nbsp;billion years ago.<ref name="Allègre95"/> As it cooled and a crust and oceans formed, its atmosphere transformed from being dominated by [[hydrogen]] to one composed mostly of [[methane]] and [[ammonia]]. Over the next billion years, the metabolic activity of life transformed the atmosphere into a mixture of [[carbon dioxide]], [[nitrogen]], and water vapor. These gases changed the way that light from the sun hit the Earth's surface and greenhouse effects trapped heat. There were untapped sources of free energy within the mixture of [[Redox|reducing and oxidizing]] gasses that set the stage for primitive ecosystems to evolve and, in turn, the atmosphere also evolved.<ref name="Wills01"/>

[[File:Leaf 1 web.jpg|left|thumb|The [[leaf]] is the primary site of [[photosynthesis]] in many higher plants.]]

Throughout history, the Earth's atmosphere and [[biogeochemical cycles]] have been in a [[dynamic equilibrium]] with planetary ecosystems. The history is characterized by periods of significant transformation followed by millions of years of stability.<ref name="Goldblatt06"/> The evolution of the earliest organisms, likely anaerobic [[methanogen]] microbes, started the process by converting atmospheric hydrogen into methane (4H<sub>2</sub> + CO<sub>2</sub> → CH<sub>4</sub> + 2H<sub>2</sub>O). [[Anoxygenic photosynthesis]] reduced hydrogen concentrations and increased [[atmospheric methane]], by converting [[hydrogen sulfide]] into water or other sulfur compounds (for example, 2H<sub>2</sub>S + CO<sub>2</sub> + h''v'' → CH<sub>2</sub>O + H<sub>2</sub>O + 2S). Early forms of [[Fermentation (biochemistry)|fermentation]] also increased levels of atmospheric methane. The transition to an oxygen-dominant atmosphere (the ''[[Great Oxygenation Event|Great Oxidation]]'') did not begin until approximately 2.4–2.3&nbsp;billion years ago, but photosynthetic processes started 0.3–1&nbsp;billion years prior.<ref name="Goldblatt06"/><ref name="Catling05"/>

=== Radiation: heat, temperature and light ===
{{Anchor|Sunlight}}<!-- E.g. in [[Neritic zone]] -->
The biology of life operates within a certain range of temperatures. Heat is a form of energy that regulates temperature. Heat affects growth rates, activity, behaviour, and [[primary production]]. Temperature is largely dependent on the incidence of [[solar radiation]]. The latitudinal and longitudinal spatial variation of [[temperature]] greatly affects climates and consequently the distribution of [[biodiversity]] and levels of primary production in different ecosystems or biomes across the planet. Heat and temperature relate importantly to metabolic activity. [[Poikilotherms]], for example, have a body temperature that is largely regulated and dependent on the temperature of the external environment. In contrast, [[homeotherms]] regulate their internal body temperature by expending [[food energy|metabolic energy]].<ref name="Allee49"/><ref name="Ricklefs96"/><ref name="Kormondy95"/>

There is a relationship between light, primary production, and ecological [[energy budget]]s. Sunlight is the primary input of energy into the planet's ecosystems. Light is composed of [[electromagnetic energy]] of different [[wavelength]]s. [[Radiant energy]] from the sun generates heat, provides photons of light measured as active energy in the chemical reactions of life, and also acts as a catalyst for [[genetic mutation]].<ref name="Allee49"/><ref name="Ricklefs96"/><ref name="Kormondy95"/> Plants, algae, and some bacteria absorb light and assimilate the energy through [[photosynthesis]]. Organisms capable of assimilating energy by photosynthesis or through inorganic fixation of [[hydrogen sulfide|H<sub>2</sub>S]] are [[autotrophs]]. Autotrophs—responsible for primary production—assimilate light energy which becomes metabolically stored as [[potential energy]] in the form of biochemical [[Enthalpy|enthalpic]] bonds.<ref name="Allee49"/><ref name="Ricklefs96"/><ref name="Kormondy95"/>

=== Physical environments ===

==== Water ====
{{Main|Aquatic ecosystem}}
{{quote box
| quote = Wetland conditions such as shallow water, high plant productivity, and anaerobic substrates provide a suitable environment for important physical, biological, and chemical processes. Because of these processes, wetlands play a vital role in global nutrient and element cycles.
| source = Cronk & Fennessy (2001)<ref name="Cronk01" />{{Rp|29}}
| width = 25%
| align = right}}
Diffusion of carbon dioxide and oxygen is approximately 10,000 times slower in water than in air. When soils are flooded, they quickly lose oxygen, becoming [[Hypoxia (environmental)|hypoxic]] (an environment with O<sub>2</sub> concentration below 2&nbsp;mg/liter) and eventually completely [[Anoxic waters|anoxic]] where [[anaerobic bacteria]] thrive among the roots. Water also influences the intensity and [[Electromagnetic spectrum|spectral composition]] of light as it reflects off the water surface and submerged particles.<ref name="Cronk01"/> Aquatic plants exhibit a wide variety of morphological and physiological adaptations that allow them to survive, compete, and diversify in these environments. For example, their roots and stems contain large air spaces ([[aerenchyma]]) that regulate the efficient transportation of gases (for example, CO<sub>2</sub> and O<sub>2</sub>) used in respiration and photosynthesis. 

Salt water plants ([[halophyte]]s) have additional specialized adaptations, such as the development of special organs for shedding salt and [[osmoregulation|osmoregulating]] their internal salt (NaCl) concentrations, to live in [[Estuary|estuarine]], [[brackish]], or [[ocean]]ic environments. Anaerobic soil [[microorganism]]s in aquatic environments use [[nitrate]], [[Manganese|manganese ions]], [[ferric|ferric ions]], [[sulfate]], [[carbon dioxide]], and some [[organic compounds]]; other microorganisms are [[facultative anaerobes]] and use oxygen during respiration when the soil becomes drier. The activity of soil microorganisms and the chemistry of the water reduces the [[Reduction potential|oxidation-reduction]] potentials of the water. Carbon dioxide, for example, is reduced to methane (CH<sub>4</sub>) by methanogenic bacteria.<ref name="Cronk01"/> The physiology of fish is also specially adapted to compensate for environmental salt levels through osmoregulation. Their gills form [[electrochemical gradient]]s that mediate salt excretion in salt water and uptake in fresh water.<ref name="Evans99"/>

==== Gravity ====

The shape and energy of the land are significantly affected by gravitational forces. On a large scale, the distribution of gravitational forces on the earth is uneven and influences the shape and movement of [[tectonic plates]] as well as influencing [[geomorphic]] processes such as [[orogeny]] and [[erosion]]. These forces govern many of the geophysical properties and distributions of ecological biomes across the Earth. 

On the organismal scale, gravitational forces provide directional cues for plant and fungal growth ([[gravitropism]]), orientation cues for animal migrations, and influence the [[biomechanics]] and size of animals.<ref name="Allee49"/> Ecological traits, such as allocation of biomass in trees during growth are subject to mechanical failure as gravitational forces influence the position and structure of branches and leaves.<ref name="Swenson08"/> The [[Circulatory system|cardiovascular systems]] of animals are functionally adapted to overcome the pressure and gravitational forces that change according to the features of organisms (e.g., height, size, shape), their behaviour (e.g., diving, running, flying), and the habitat occupied (e.g., water, hot deserts, cold tundra).<ref name="Garnter10"/>

==== Pressure ====

Climatic and [[osmotic pressure]] places [[physiological]] constraints on organisms, especially those that fly and respire at high altitudes, or dive to deep ocean depths.<ref name="Neri" /> These constraints influence vertical limits of ecosystems in the biosphere, as organisms are physiologically sensitive and adapted to atmospheric and osmotic water pressure differences.<ref name="Allee49"/> For example, oxygen levels decrease with decreasing pressure and are a limiting factor for life at higher altitudes.<ref name="Jacobsen08"/> 

[[Xylem|Water transportation]] by plants is another important [[ecophysiology|ecophysiological]] process affected by osmotic pressure gradients.<ref name="Strook08"/><ref name="Pockman95"/><ref name="Zimmermann02"/> [[Fluid pressure|Water pressure]] in the depths of oceans requires that organisms adapt to these conditions. For example, diving animals such as [[whale]]s, [[dolphin]]s, and [[seal (animal)|seals]] are specially adapted to deal with changes in sound due to water pressure differences.<ref name="Kastak98"/> Differences between [[hagfish]] species provide another example of adaptation to deep-sea pressure through specialized protein adaptations.<ref name="Nishiguchi10"/>

==== Wind and turbulence ====

[[File:Grassflowers.jpg|thumb|The architecture of the [[inflorescence]] in grasses is subject to the physical pressures of wind and shaped by the forces of natural selection facilitating wind-pollination ([[anemophily]]).<ref name="Friedman04"/><ref name="Harder09"/>]]

[[Turbulent forces]] in air and water affect the environment and ecosystem distribution, form, and dynamics. On a planetary scale, ecosystems are affected by circulation patterns in the global [[trade winds]]. Wind power and the turbulent forces it creates can influence heat, nutrient, and biochemical profiles of ecosystems.<ref name="Allee49"/> For example, wind running over the surface of a lake creates turbulence, mixing the [[water column]] and influencing the environmental profile to create [[thermally layered zones]], affecting how fish, algae, and other parts of the [[aquatic ecosystem]] are structured.<ref name="Shimeta95"/><ref name="Etemad01"/> 

Wind speed and turbulence also influence [[evapotranspiration rates]] and energy budgets in plants and animals.<ref name="Cronk01" /><ref name="Wolf96"/> Wind speed, temperature and moisture content can vary as winds travel across different land features and elevations. For example, the [[westerlies]] come into contact with the [[coastal]] and interior mountains of western North America to produce a [[rain shadow]] on the leeward side of the mountain. The air expands and moisture condenses as the winds increase in elevation; this is called [[orographic lift]] and can cause precipitation. This environmental process produces spatial divisions in biodiversity, as species adapted to wetter conditions are range-restricted to the coastal mountain valleys and unable to migrate across the [[xeric]] ecosystems (e.g., of the [[Columbia River Drainage Basin|Columbia Basin]] in western North America) to intermix with sister lineages that are segregated to the interior mountain systems.<ref name="Daubenmire75"/><ref name="Steele05"/>

==== Fire ====
{{Main|Fire ecology}}

{{multiple image
| footer = Forest fires modify the land by leaving behind an environmental mosaic that diversifies the landscape into different [[seral community|seral]] stages and habitats of varied quality (left). Some species are adapted to forest fires, such as pine trees that open their cones only after fire exposure (right).
| align = left
| image1 = Mosaic fire burn.jpg
| width1 = {{#expr: (100 * 1999 / 1277) round 0}}
| image2 = Lodgepole pine cone after fire.jpg
| width2 = {{#expr: (100 * 1999 / 1355) round 0}}
}}

Plants convert carbon dioxide into biomass and emit oxygen into the atmosphere. By approximately 350 million years ago (the end of the [[Devonian period]]), photosynthesis had brought the concentration of atmospheric oxygen above 17%, which allowed combustion to occur.<ref name="Lenton00"/> Fire releases CO<sub>2</sub> and converts fuel into ash and tar. Fire is a significant ecological parameter that raises many issues pertaining to its control and suppression.<ref name="Lobert93"/> While the issue of fire in relation to ecology and plants has been recognized for a long time,<ref name="Garren43"/> [[Charles F. Cooper (ecologist)|Charles Cooper]] brought attention to the issue of forest fires in relation to the ecology of forest fire suppression and management in the 1960s.<ref name="Cooper60"/><ref name="Cooper61"/>

[[Indigenous peoples of the Americas|Native North Americans]] were among the first to influence fire regimes by controlling their spread near their homes or by lighting fires to stimulate the production of herbaceous foods and basketry materials.<ref name="Wagtendonk07"/> Fire creates a heterogeneous ecosystem age and canopy structure, and the altered soil nutrient supply and cleared canopy structure opens new ecological niches for seedling establishment.<ref name="Boerner82"/><ref name="Goubitz03"/> Most ecosystems are adapted to natural fire cycles. Plants, for example, are equipped with a variety of adaptations to deal with forest fires. Some species (e.g., ''[[Pinus halepensis]]'') cannot [[germination|germinate]] until after their seeds have lived through a fire or been exposed to certain compounds from smoke. Environmentally triggered germination of seeds is called [[serotiny]].<ref name="Neeman04"/><ref name="Flematti04"/> Fire plays a major role in the persistence and [[Resilience (ecology)|resilience]] of ecosystems.<ref name="Holling73"/>

==== Soils ====
{{main|Soil ecology}}

Soil is the living top layer of mineral and organic dirt that covers the surface of the planet. It is the chief organizing centre of most ecosystem functions, and it is of critical importance in agricultural science and ecology. The [[decomposition]] of dead organic matter (for example, leaves on the forest floor), results in soils containing [[minerals]] and nutrients that feed into plant production. The whole of the planet's soil ecosystems is called the [[pedosphere]] where a large biomass of the Earth's biodiversity organizes into trophic levels. Invertebrates that feed and shred larger leaves, for example, create smaller bits for smaller organisms in the feeding chain. Collectively, these organisms are the [[detritivore]]s that regulate soil formation.<ref name="Coleman04"/><ref name="Wilkinson09"/> Tree roots, fungi, bacteria, worms, ants, beetles, centipedes, spiders, mammals, birds, reptiles, amphibians, and other less familiar creatures all work to create the trophic web of life in soil ecosystems. 

Soils form composite phenotypes where inorganic matter is enveloped into the physiology of a whole community. As organisms feed and migrate through soils they physically displace materials, an ecological process called [[bioturbation]]. This aerates soils and stimulates heterotrophic growth and production. Soil [[microorganisms]] are influenced by and are fed back into the trophic dynamics of the ecosystem. No single axis of causality can be discerned to segregate the biological from geomorphological systems in soils.<ref name="Phillips09">{{cite journal| last1=Phillips| first1=J. D.| year=2009| title=Soils as extended composite phenotypes| volume=149| issue=1–2| pages=143–151| journal=Geoderma| doi=10.1016/j.geoderma.2008.11.028| bibcode=2009Geode.149..143P}}</ref><ref name="Reinhard10">{{cite journal| last1=Reinhardt| first1=L.| last2=Jerolmack| first2=D.| last3=Cardinale| first3=B. J.| last4=Vanacker| first4=V.| last5=Wright| first5=J.| title=Dynamic interactions of life and its landscape: Feedbacks at the interface of geomorphology and ecology| journal=Earth Surface Processes and Landforms| volume=35| issue=1| pages=78–101| doi=10.1002/esp.1912| url=http://snre.umich.edu/cardinale/wp-content/uploads/2012/04/reinhardt_earthsur_2010.pdf| bibcode=2010ESPL...35...78R| archive-url=https://web.archive.org/web/20150317140954/http://snre.umich.edu/cardinale/wp-content/uploads/2012/04/reinhardt_earthsur_2010.pdf| archive-date=17 March 2015| year=2010| s2cid=14924423| access-date=2 January 2015}}</ref> [[Paleoecology|Paleoecological]] studies of soils places the origin for bioturbation to a time before the Cambrian period. Other events, such as the [[Tree#Evolutionary history|evolution of trees]] and the [[colonization of land]] in the Devonian period played a significant role in the early development of ecological trophism in soils.<ref name="Wilkinson09"/><ref name="Davic04"/><ref name="Hasiotis03"/>

==== Biogeochemistry and climate ====
{{Main|Biogeochemistry}}
{{See also|Nutrient cycle|Climate}}

Ecologists study and measure nutrient budgets to understand how these materials are regulated, flow, and [[recycling (ecological)|recycled]] through the environment.<ref name="Allee49"/><ref name="Ricklefs96"/><ref name="Kormondy95"/> This research has led to an understanding that there is global feedback between ecosystems and the physical parameters of this planet, including minerals, soil, pH, ions, water, and atmospheric gases. Six major elements ([[hydrogen]], [[carbon]], [[nitrogen]], [[oxygen]], [[sulfur]], and [[phosphorus]]; H, C, N, O, S, and P) form the constitution of all biological macromolecules and feed into the Earth's geochemical processes. From the smallest scale of biology, the combined effect of billions upon billions of ecological processes amplify and ultimately regulate the [[biogeochemical cycle]]s of the Earth. Understanding the relations and cycles mediated between these elements and their ecological pathways has significant bearing toward understanding global biogeochemistry.<ref name="Falkowoski08"/>

The ecology of global carbon budgets gives one example of the linkage between biodiversity and biogeochemistry. It is estimated that the Earth's oceans hold 40,000 gigatonnes (Gt) of carbon, that vegetation and soil hold 2,070 Gt, and that fossil fuel emissions are 6.3 Gt carbon per year.<ref name="Grace04"/> There have been major restructurings in these global carbon budgets during the Earth's history, regulated to a large extent by the ecology of the land. For example, through the early-mid Eocene volcanic [[outgassing]], the oxidation of methane stored in wetlands, and seafloor gases increased atmospheric CO<sub>2</sub> (carbon dioxide) concentrations to levels as high as 3500&nbsp;[[Parts per million|ppm]].<ref name="Pearson00"/>

In the [[Oligocene]], from twenty-five to thirty-two million years ago, there was another significant restructuring of the global [[carbon cycle]] as grasses evolved a new mechanism of photosynthesis, [[C4 carbon fixation|C<sub>4</sub> photosynthesis]], and expanded their ranges. This new pathway evolved in response to the drop in atmospheric CO<sub>2</sub> concentrations below 550 ppm.<ref name="Pagani05"/> The relative abundance and distribution of biodiversity alters the dynamics between organisms and their environment such that ecosystems can be both cause and effect in relation to climate change. 

Human-driven modifications to the planet's ecosystems (e.g., disturbance, [[biodiversity loss]], agriculture) contributes to rising atmospheric greenhouse gas levels. Transformation of the global carbon cycle in the next century is projected to raise planetary temperatures, lead to more extreme fluctuations in weather, alter species distributions, and increase extinction rates. The effect of global warming is already being registered in melting glaciers, melting mountain ice caps, and rising sea levels. Consequently, species distributions are changing along waterfronts and in continental areas where migration patterns and breeding grounds are tracking the prevailing shifts in climate. 

Large sections of [[permafrost]] are also melting to create a new mosaic of flooded areas having increased rates of soil decomposition activity that raises methane (CH<sub>4</sub>) emissions. There is concern over increases in atmospheric methane in the context of the global carbon cycle, because methane is a [[greenhouse gas]] that is 23 times more effective at absorbing long-wave radiation than CO<sub>2</sub> on a 100-year time scale.<ref name="Zhuan07"/> Hence, there is a relationship between global warming, decomposition and respiration in soils and wetlands producing significant climate feedbacks and globally altered biogeochemical cycles.<ref name="Liu09"/><ref name="Cox00"/><ref name="Erwin09">{{cite journal| last1=Erwin| first1=D. H.| year=2009 | title=Climate as a driver of evolutionary change| journal=Current Biology| volume=19|issue=14| pages=R575–R583| doi=10.1016/j.cub.2009.05.047| pmid=19640496| s2cid=6913670| doi-access=free| bibcode=2009CBio...19.R575E}}</ref><ref name="Bamber12">{{cite journal|last1=Bamber|first1=J.|year=2012|title=Shrinking glaciers under scrutiny|journal=Nature|volume=482|pages=482–483|url=ftp://cola.gmu.edu/pub/klinger/CLIM690/onjacobetal12.pdf|doi=10.1038/nature10948|bibcode=2012Natur.482..482B|issue=7386|pmid=22318516|s2cid=7311971|access-date=12 June 2017}}</ref><ref name="Heiman08"/><ref name="Davidson06"/>

== History ==
{{Main|History of ecology}}

=== Early beginnings ===
{{quote box
| quote = By ecology, we mean the whole science of the relations of the organism to the environment including, in the broad sense, all the "conditions of existence". Thus, the theory of evolution explains the housekeeping relations of organisms mechanistically as the necessary consequences of effectual causes; and so forms the [[monism|monistic]] groundwork of ecology.
| source = Ernst Haeckel (1866)<ref name="Stauffer57" />{{Rp|140}} {{Cref2|B}}
| width = 30%
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Ecology has a complex origin, due in large part to its interdisciplinary nature.<ref name="Egerton01"/> Ancient Greek philosophers such as [[Hippocrates]] and [[Aristotle]] were among the first to record observations on natural history. However, they viewed life in terms of [[essentialism]], where species were conceptualized as static unchanging things while varieties were seen as aberrations of an [[Idealism|idealized type]]. This contrasts against the modern understanding of [[Theoretical ecology|ecological theory]] where varieties are viewed as the real phenomena of interest and having a role in the origins of adaptations by means of [[natural selection]].<ref name="Odum05" /><ref name="Benson00"/><ref name="Sober80">{{cite journal| last1=Sober| first1=E.| title=Evolution, population thinking, and essentialism| journal=Philosophy of Science| volume=47| issue=3| pages=350–383| jstor=186950| doi=10.1086/288942| year=1980| s2cid=170129617}}</ref> Early conceptions of ecology, such as a balance and regulation in nature can be traced to [[Herodotus]] (died ''c''. 425 BC), who described one of the earliest accounts of [[mutualism (biology)|mutualism]] in his observation of "natural dentistry". Basking [[Nile crocodile]]s, he noted, would open their mouths to give [[sandpiper]]s safe access to pluck [[leech]]es out, giving nutrition to the sandpiper and oral hygiene for the crocodile.<ref name="Egerton01" /> Aristotle was an early influence on the philosophical development of ecology. He and his student [[Theophrastus]] made extensive observations on plant and animal migrations, biogeography, physiology, and their behavior, giving an early analogue to the modern concept of an ecological niche.<ref name="Hughes85"/><ref name="Hughes75"/>

{{quote box
| quote = Nowhere can one see more clearly illustrated what may be called the sensibility of such an organic complex, – expressed by the fact that whatever affects any species belonging to it, must speedily have its influence of some sort upon the whole assemblage. He will thus be made to see the impossibility of studying any form completely, out of relation to the other forms, – the necessity for taking a comprehensive survey of the whole as a condition to a satisfactory understanding of any part.
| source = Stephen Forbes (1887)<ref name="Forbes1887"/>
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<div class="thumb tleft" style="background:#f9f9f9; padding:5px; border:1px solid gray; margin:0em 2em 0em 0em; font-size:11px;">
[[File:Nicola Perscheid - Ernst Haeckel.jpg|90px]] [[File:Warming,Eugen-c1900.jpg|90px]]
<div style="border: none; width:180px;"><div class="thumbcaption"> [[Ernst Haeckel]] (left) and [[Eugenius Warming]] (right), two founders of ecology</div></div></div>

Ecological concepts such as food chains, population regulation, and productivity were first developed in the 1700s, through the published works of microscopist [[Antonie van Leeuwenhoek]] (1632–1723) and botanist [[Richard Bradley (botanist)|Richard Bradley]] (1688?–1732).<ref name="Odum05" /> Biogeographer [[Alexander von Humboldt]] (1769–1859) was an early pioneer in ecological thinking and was among the first to recognize ecological gradients, where species are replaced or altered in form along [[environmental gradient]]s, such as a [[cline (biology)|cline]] forming along a rise in elevation. Humboldt drew inspiration from [[Isaac Newton]], as he developed a form of "terrestrial physics". In Newtonian fashion, he brought a scientific exactitude for measurement into natural history and even alluded to concepts that are the foundation of a modern ecological law on species-to-area relationships.<ref name="Kingsland04" /><ref name="Rosenzweig03"/><ref name="Hawkins01"/> Natural historians, such as Humboldt, [[James Hutton]], and [[Jean-Baptiste Lamarck]] (among others) laid the foundations of the modern ecological sciences.<ref name="McIntosh85">{{cite book| last1=McIntosh| first1=R. P.| title=The Background of Ecology: Concept and Theory| publisher=Cambridge University Press| year=1985| isbn=0-521-27087-1| page=400| url=https://archive.org/details/backgroundofecol0000mcin| url-access=registration}}</ref> The term "ecology" ({{langx|de|Oekologie, Ökologie}}) was coined by [[Ernst Haeckel]] in his book ''Generelle Morphologie der Organismen'' (1866).<ref>{{cite book|last1=Haeckel|first1=Ernst|title=Generelle Morphologie der Organismen|trans-title=The General Morphology of Organisms|date=1866|publisher=Georg Reimer|location=Berlin, (Germany)|volume=2|page=286|url=https://www.biodiversitylibrary.org/item/52177#page/454/mode/1up|language=de|access-date=27 February 2019|archive-date=18 June 2019|archive-url=https://web.archive.org/web/20190618151420/https://www.biodiversitylibrary.org/item/52177#page/454/mode/1up|url-status=live}} From p. 286: ''"Unter Oecologie verstehen wir die gesammte Wissenschaft von den Beziehungen des Organismus zur umgebenden Aussenwelt, wohin wir im weiteren Sinne alle "Existenz-Bedingungen" rechnen können."'' (By "ecology" we understand the comprehensive science of the relationships of the organism to its surrounding environment, where we can include, in the broader sense, all "conditions of existence".)</ref> Haeckel was a zoologist, artist, writer, and later in life a professor of comparative anatomy.<ref name="Stauffer57"/><ref name="Friederichs58"/>

Opinions differ on who was the founder of modern ecological theory. Some mark Haeckel's definition as the beginning;<ref name="Hinchman07"/> others say it was [[Eugenius Warming]] with the writing of [[Plantesamfund|Oecology of Plants: An Introduction to the Study of Plant Communities]] (1895),<ref name="Goodland75"/> or [[Carl Linnaeus]]' principles on the economy of nature that matured in the early 18th century.<ref name="Egerton07"/><ref name="Kormandy78"/> Linnaeus founded an early branch of ecology that he called the economy of nature.<ref name="Egerton07"/> His works influenced Charles Darwin, who adopted Linnaeus' phrase on the ''economy or polity of nature'' in ''[[The Origin of Species]]''.<ref name="Stauffer57" /> Linnaeus was the first to frame the [[balance of nature]] as a testable hypothesis. Haeckel, who admired Darwin's work, defined ecology in reference to the economy of nature, which has led some to question whether ecology and the economy of nature are synonymous.<ref name="Kormandy78" />

[[File:Darwin EcoExperiment.JPG|thumb|The layout of the first ecological experiment, carried out in a grass garden at [[Woburn Abbey]] in 1816, was noted by Charles Darwin in ''The Origin of Species''. The experiment studied the performance of different mixtures of species planted in different kinds of soils.<ref name="Hector02" /><ref name="Sinclair26" />]]

From Aristotle until Darwin, the natural world was predominantly considered static and unchanging. Prior to ''The Origin of Species'', there was little appreciation or understanding of the dynamic and reciprocal relations between organisms, their adaptations, and the environment.<ref name="Benson00"/> An exception is the 1789 publication ''Natural History of Selborne'' by [[Gilbert White]] (1720–1793), considered by some to be one of the earliest texts on ecology.<ref name="May99"/> While [[Charles Darwin]] is mainly noted for his treatise on evolution,<ref name=Darwin/> he was one of the founders of [[soil ecology]],<ref name="Meysman06"/> and he made note of the first ecological experiment in ''The Origin of Species''.<ref name="Hector02"/> Evolutionary theory changed the way that researchers approached the ecological sciences.<ref name="Acot97"/>

=== Since 1900 ===
Modern ecology is a young science that first attracted substantial scientific attention toward the end of the 19th century (around the same time that evolutionary studies were gaining scientific interest). The scientist [[Ellen Swallow Richards]] adopted the term "[[oekology]]" (which eventually morphed into [[home economics]]) in the U.S. as early as 1892.<ref name="Hunt">{{cite book| last = Hunt| first = Caroline Louisa| title = The life of Ellen H. Richards| publisher = [[Whitcomb & Barrows]]| location = Boston| year = 1912 | url = https://archive.org/details/lifeofellenhrich00huntrich}}</ref>

In the early 20th century, ecology transitioned from a more [[metaphysics|descriptive form]] of [[natural history]] to a more [[scientific method|analytical form]] of ''scientific natural history''.<ref name="Kingsland04"/><ref name="McIntosh85" /><ref>{{Cite journal |last=Jones |first=Madison |date=2021-08-08 |title=A Counterhistory of Rhetorical Ecologies |url=https://www.tandfonline.com/doi/full/10.1080/02773945.2021.1947517 |journal=Rhetoric Society Quarterly |language=en |volume=51 |issue=4 |pages=336–352 |doi=10.1080/02773945.2021.1947517 |s2cid=238358762 |issn=0277-3945}}</ref> [[Frederic Clements]] published the first American ecology book, ''Research Methods in Ecology'' in 1905,<ref name="Clements05">{{cite book | last1=Clements | first1=F. E. | title=Research methods in ecology | publisher=University Pub. Comp. | place=Lincoln, Neb. | year=1905 | isbn=0-405-10381-6 | url=https://books.google.com/books?id=vRy-VJctJjcC | access-date=6 January 2020 | archive-date=1 August 2020 | archive-url=https://web.archive.org/web/20200801074030/https://books.google.com/books?id=vRy-VJctJjcC | url-status=live }}</ref> presenting the idea of plant communities as a [[superorganism]]. This publication launched a debate between ecological holism and individualism that lasted until the 1970s. Clements' superorganism concept proposed that ecosystems progress through regular and determined stages of [[seral development]] that are analogous to the developmental stages of an organism. The Clementsian paradigm was challenged by [[Henry Gleason]],<ref name="Simberloff80"/> who stated that ecological communities develop from the unique and coincidental association of individual organisms. This perceptual shift placed the focus back onto the life histories of individual organisms and how this relates to the development of community associations.<ref name="Gleason26"/>

The Clementsian superorganism theory was an overextended application of an [[idealism|idealistic form]] of holism.<ref name="Levins80">{{cite journal|last1=Levins |first1=R. |last2=Lewontin |first2=R. |title=Dialectics and reductionism in ecology |journal=Synthese |volume=43 |pages=47–78 |year=1980 |url=http://www.ecologia.unam.mx/laboratorios/comunidades/pdf/pdf%20curso%20posgrado%20Elena/Tema%201/LevinsLewontinSynthese1980.pdf |doi=10.1007/bf00413856 |s2cid=46984334 |archive-url=https://web.archive.org/web/20130510183920/http://www.ecologia.unam.mx/laboratorios/comunidades/pdf/pdf%20curso%20posgrado%20Elena/Tema%201/LevinsLewontinSynthese1980.pdf |archive-date=10 May 2013}}</ref><ref name="Wilson88" /> The term "holism" was coined in 1926 by [[Jan Smuts|Jan Christiaan Smuts]], a South African general and polarizing historical figure who was inspired by Clements' superorganism concept.<ref name="Foster08"/>{{Cref2|C}} Around the same time, [[Charles Sutherland Elton|Charles Elton]] pioneered the concept of food chains in his classical book ''Animal Ecology''.<ref name="Elton27">{{cite book|last=Elton|first=C. S.|title=Animal Ecology|publisher=Sidgwick and Jackson|place=London|year=1927|isbn=0-226-20639-4}}</ref> Elton<ref name="Elton27" /> defined ecological relations using concepts of food chains, food cycles, and food size, and described numerical relations among different functional groups and their relative abundance. Elton's 'food cycle' was replaced by 'food web' in a subsequent ecological text.<ref name="Allee32"/> [[Alfred J. Lotka]] brought in many theoretical concepts applying thermodynamic principles to ecology.

In 1942, [[Raymond Lindeman]] wrote a landmark paper on the [[Trophic dynamics#Trophic dynamics and multitrophic interactions|trophic dynamics]] of ecology, which was published posthumously after initially being rejected for its theoretical emphasis. Trophic dynamics became the foundation for much of the work to follow on energy and material flow through ecosystems. [[Robert MacArthur]] advanced mathematical theory, predictions, and tests in ecology in the 1950s, which inspired a resurgent school of theoretical mathematical ecologists.<ref name="McIntosh85" /><ref name="Cook77">{{cite journal | last1=Cook | first1=R. E. | title=Raymond Lindeman and the trophic-dynamic concept in ecology | journal=Science | volume=198 | issue=4312 | pages=22–26 | year=1977 | doi=10.1126/science.198.4312.22 | url=http://www.esf.edu/efb/schulz/Seminars/Cook.pdf | bibcode=1977Sci...198...22C | pmid=17741875 | s2cid=30340899 | url-status=live | archive-url=https://web.archive.org/web/20121005002320/http://www.esf.edu/efb/schulz/Seminars/Cook.pdf | archive-date=5 October 2012}}</ref><ref name="Odum68">{{cite journal | last1=Odum | first1=E. P. | year=1968 | title=Energy flow in ecosystems: A historical review | journal=American Zoologist | volume=8 | issue=1 | pages=11–18 | jstor=3881528 | doi=10.1093/icb/8.1.11| doi-access=free }}</ref> Ecology has also developed through contributions from other nations, including Russia's [[Vladimir Vernadsky]] and his founding of the biosphere concept in the 1920s<ref name="Ghilarov95"/> and Japan's [[Kinji Imanishi]] and his concepts of harmony in nature and habitat segregation in the 1950s.<ref name="Itô91"/> Scientific recognition of contributions to ecology from non-English-speaking cultures is hampered by language and translation barriers.<ref name="Ghilarov95" />

{{quote box
| quote = This whole chain of poisoning, then, seems to rest on a base of minute plants which must have been the original concentrators. But what of the opposite end of the food chain—the human being who, in probable ignorance of all this sequence of events, has rigged his fishing tackle, caught a string of fish from the waters of Clear Lake, and taken them home to fry for his supper?
| source = Rachel Carson (1962)<ref name="Carson62">{{cite book | last1=Carson | first1=R. | title=Silent Spring | publisher=Houghton Mifflin Company | page=[https://archive.org/details/silentspring00cars_0/page/348 348] | url=https://archive.org/details/silentspring00cars_0| url-access=registration | isbn=0-618-24906-0 | year=2002}}</ref>{{Rp|48}}
| width = 30%
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Ecology surged in popular and scientific interest during the 1960–1970s [[environmental movement]]. There are strong historical and scientific ties between ecology, environmental management, and protection.<ref name="McIntosh85" /> The historical emphasis and poetic naturalistic writings advocating the protection of wild places by notable ecologists in the history of [[conservation biology]], such as [[Aldo Leopold]] and [[Arthur Tansley]], have been seen as far removed from urban centres where, it is claimed, the concentration of pollution and [[environmental degradation]] is located.<ref name="McIntosh85" /><ref name="Palamar08">{{cite journal | last1=Palamar | first1=C. R. | year=2008 | title=The justice of ecological restoration: Environmental history, health, ecology, and justice in the United States | journal=Human Ecology Review | volume=15 | issue=1 | pages=82–94 | url=http://www.humanecologyreview.org/pastissues/her151/palamar.pdf | archive-url=https://web.archive.org/web/20110726161950/http://www.humanecologyreview.org/pastissues/her151/palamar.pdf | archive-date=26 July 2011 | access-date=8 August 2012 }}</ref> Palamar (2008)<ref name="Palamar08" /> notes an overshadowing by mainstream environmentalism of pioneering women in the early 1900s who fought for urban health ecology (then called [[euthenics]])<ref name="Hunt" /> and brought about changes in environmental legislation. Women such as [[Ellen Swallow Richards]] and [[Julia Lathrop]], among others, were precursors to the more popularized environmental movements after the 1950s.

In 1962, marine biologist and ecologist [[Rachel Carson]]'s book ''[[Silent Spring]]'' helped to mobilize the environmental movement by alerting the public to toxic [[pesticide]]s, such as [[DDT]] (C<sub>14</sub>H<sub>9</sub>Cl<sub>5</sub>), [[Bioaccumulation|bioaccumulating]] in the environment. Carson used ecological science to link the release of environmental toxins to human and [[ecosystem health]]. Since then, ecologists have worked to bridge their understanding of the degradation of the planet's ecosystems with environmental politics, law, restoration, and natural resources management.<ref name="Hammond09" /><ref name="McIntosh85" /><ref name="Palamar08" /><ref name="Krebs99">{{cite journal|last1=Krebs |first1=J. R. |last2=Wilson |first2=J. D. |last3=Bradbury |first3=R. B. |last4=Siriwardena |first4=G. M. |title=The second Silent Spring |journal=Nature |volume=400 |year=1999 |pages=611–612 |doi=10.1038/23127 |url=http://www.hillnet.com/silent_spring.pdf |bibcode=1999Natur.400..611K |issue=6745 |s2cid=9929695 |archive-url=https://web.archive.org/web/20130331100623/http://www.hillnet.com/silent_spring.pdf |archive-date=31 March 2013}}</ref>

== See also ==
{{Portal|Biology|Ecology|Plants|Trees|Water}}
{{cols|colwidth=16em}}
* [[Carrying capacity]]
* [[Chemical ecology]]
* [[Climate justice]]
* [[Circles of Sustainability]]
* [[Cultural ecology]]
* [[Dialectical naturalism]]
* [[Ecological death]]
* [[Ecological empathy]]
* [[Ecological overshoot]]
* [[Ecological psychology]]
* [[Ecology movement]]
* [[Ecosophy]]
* [[Ecopsychology]]
* [[Human ecology]]
* [[Industrial ecology]]
* [[Information ecology]]
* [[Landscape ecology]]
* [[Natural resource]]
* [[Normative science]]
* [[Philosophy of ecology]]
* [[Political ecology]]
* [[Theoretical ecology]]
* [[Sensory ecology]]
* [[Sexecology]]
* [[Spiritual ecology]]
* [[Sustainable development]]
{{colend}}

; Lists
{{cols}}
* [[Glossary of ecology]]
* [[Index of biology articles]]
* [[List of ecologists]]
* [[Outline of biology]]
* [[:Category:Ecology terminology|Terminology of ecology]]
{{colend}}

== Notes ==
{{Cnote2 Begin|list-style=upper-alpha|colwidth=30em}}

{{Cnote2|A|In Ernst Haeckel's (1866) footnote where the term ecology originates, he also gives attribute to {{langx|grc|χώρας|khōrā |χωρα}}, meaning "dwelling place, distributional area" —quoted from Stauffer (1957).}}
{{Cnote2|B|This is a copy of Haeckel's original definition (Original: Haeckel, E. (1866) Generelle Morphologie der Organismen. Allgemeine Grundzige der organischen Formen- Wissenschaft, mechanisch begriindet durch die von Charles Darwin reformirte Descendenz-Theorie. 2 vols. Reimer, Berlin.) translated and quoted from Stauffer (1957).}}
{{Cnote2|C|Foster & Clark (2008) note how Smut's holism contrasts starkly against his racial political views as the father of [[apartheid]].}}
{{Cnote2|D|First introduced in MacArthur & Wilson's (1967) book of notable mention in the history and theoretical science of ecology, ''[[The Theory of Island Biogeography]]''.}}
{{Cnote2|E|Aristotle wrote about this concept in ''[[Metaphysics (Aristotle)|Metaphysics]]'' (Quoted from [http://classics.mit.edu/Aristotle/metaphysics.mb.txt The Internet Classics Archive] translation by [[W. D. Ross]]. Book VIII, Part 6): "To return to the difficulty which has been stated with respect both to definitions and to numbers, what is the cause of their unity? In the case of all things which have several parts and in which the totality is not, as it were, a mere heap, but the whole is something besides the parts, there is a cause; for even in bodies contact is the cause of unity in some cases and in others viscosity or some other such quality."}}

{{Cnote2 End}}

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<ref name=Carson>{{Cite web |url=http://core.ecu.edu/soci/juskaa/SOCI3222/carson.html |title="Silent Spring" (excerpt) |author=Rachel Carson |publisher=Houghton Mifflin |year=1962 |access-date=4 October 2012 |url-status=live |archive-url=https://web.archive.org/web/20121014113101/http://core.ecu.edu/soci/juskaa/SOCI3222/carson.html |archive-date=14 October 2012}}</ref>

<ref name=Neri>{{Cite book |title= Geography, structural Change and Economic Development: Theory and Empirics |author=Neri Salvadori |author2=Pasquale Commendatore |author3=Massimo Tamberi |publisher= Edward Elgar Publishing |date=14 May 2014}}</ref>
}}

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