Ecology

{{Short description |Study of organisms and their environment}} {{CS1 config |mode=cs1}} {{Hatnote group | {{Other uses}} {{Distinguish |ethology |ethnology |etiology |deep ecology}} }} {{Good article}} {{Use dmy dates |date=January 2020}} {{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 (biology)|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 term ''ecology'' ({{langx |de |Ökologie}}) was coined in 1866 by the German scientist [[Ernst Haeckel]]. The science of ecology as it is known 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 such as [[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 ==

Ecosystems vary 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 |article-number=e45508 |doi=10.1371/journal.pone.0045508 |bibcode=2012PLoSO...745508L |pmid=23029061 |pmc=3447771 |doi-access=free}}</ref>

The main subdisciplines of ecology, [[population ecology |population]] (or [[community ecology |community]]) ecology and [[ecosystem ecology]], differ in their contrasting paradigms. 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 ===

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 ===

[[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"/>]]

Biodiversity (an abbreviation of "biological diversity") describes the diversity of life from genes to ecosystems and spans every level of [[Biological organisation|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"/> It delivers ecosystem services across heterogeneous real-world landscapes, influenced by human management and environmental conditions.<ref>Qiu, J. & Mitchell, M. (2024). ''Understanding biodiversity–ecosystem service linkages in real landscapes.'' ''Landscape Ecology'', 39, 188. https://doi.org/10.1007/s10980-024-01980-3</ref> 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 ===

[[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 it occurs and the type of community that is formed.<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}}

=== 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 niche date back to 1917.<ref name="Wiens05"/> In 1957, [[G. Evelyn Hutchinson]] introduced "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 niche is a central concept in the ecology of organisms and is sub-divided into ''fundamental'' and ''realized'' niches. 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}}

===Niche construction===

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"/> [[Ecosystem engineering]] is 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 feedback on 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}}

=== 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 ===

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 CO2 and O2 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 ===

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 ===

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"/>

=== 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.<ref name="Johnson07"/>{{rp|250}}

=== 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|A [[riparian forest]] in the [[White Mountains (New Hampshire) |White Mountains, New Hampshire]] (US) is an example of [[ecosystem ecology]]]]

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"/> 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>

=== Food webs ===

[[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]] used to illustrate and study pathways of energy and material flows.<ref name="O'Neill86"/><ref name="Pimm02"/><ref name="Pimm91"/>

=== Trophic levels ===

[[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">{{cite book |last=Elton |first=C. S. |title=Animal Ecology |publisher=Sidgwick and Jackson |place=London |year=1927 |isbn=0-226-20639-4}}</ref>

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 ===

[[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}}</ref> [[Sea otter]]s (''Enhydra lutris'') are commonly cited as an example because they limit the density of [[sea urchins]] that feed on [[kelp]].<ref>{{Cite journal |last=Estes |first=James A. |last2=Burdin |first2=Alexander |last3=Doak |first3=Daniel F. |date=2016-01-26 |title=Sea otters, kelp forests, and the extinction of Steller’s sea cow |url=https://pnas.org/doi/full/10.1073/pnas.1502552112 |journal=Proceedings of the National Academy of Sciences |language=en |volume=113 |issue=4 |pages=880–885 |doi=10.1073/pnas.1502552112 |issn=0027-8424 |pmc=4743786 |pmid=26504217}}</ref> 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"/>

== Complexity ==

Complexity is understood as a large computational effort needed to assemble numerous interacting parts. Global patterns of biological diversity are complex. This [[biocomplexity]] stems from the interplay among ecological processes that influence patterns at different scales, 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.<ref name="Novikoff45"/>{{rp|209}} Small scale patterns do not necessarily explain larger ones, as in Aristotle's expression "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"/>

=== Holism ===

Holism is a critical part of the theory of ecology. 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}}

== Relation to evolution ==

Ecology and evolutionary biology are sister disciplines. [[Natural selection]], [[Biological life cycle |life history]], [[Developmental biology |development]], [[adaptation]], [[Population (biology)|populations]], and [[heredity |inheritance]] thread equally into both. In this framework, the analytical tools of ecologists and evolutionists overlap as they study life through [[phylogenetics]] or [[Linnaean taxonomy]].<ref name="Miles93"/> There is no sharp boundary separating ecology from evolution, and they differ more in their areas of applied focus. Both explain properties and processes across different spatial or temporal scales of organization.<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="Lovelock03"/> 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 ===

[[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 have 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"/>

=== Cognitive ecology ===

Cognitive ecology integrates theory and observations from [[evolutionary ecology]] and [[cognitive science]], to understand the effect of animal interaction with their habitat on their cognitive systems.<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 |hdl=10533/174736 |hdl-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>

=== Social ecology ===

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 ===

[[File:Seney National Wildlife Refuge - Youth (9705383116).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"/>

=== Biogeography ===

Biogeography 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. ''[[Island biogeography]]'', published by Robert MacArthur and [[Edward O. Wilson]] in 1967,<ref name="MacArthur67"/> is 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 ====

r/K selection theory{{Cref2 |D}} is one of the first predictive models in ecology used to explain [[Life history theory |life-history evolution]]. Its premise is that natural selection varies with [[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"/>

=== Molecular ecology ===

The 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 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 analytical techniques to study genes in an evolutionary and ecological context. In 1994, [[John Avise]] played a leading role in this area of science with the publication of his book, ''Molecular Markers, Natural History and Evolution''.<ref name="Avise94"/>

== Human ecology ==

=== A dual discipline ===

{{quote box |quote=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=right}}

Ecology is both a biological science and 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"/>

=== Restoration Ecology ===

{{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>

== Relation to the 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"/>

=== Disturbance and resilience ===

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"/>

=== Metabolism and the 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 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 (4H2 + CO2 → CH4 + 2H2O). [[Anoxygenic photosynthesis]] reduced hydrogen concentrations and increased [[atmospheric methane]], by converting [[hydrogen sulfide]] into water or other sulfur compounds (for example, 2H2S + CO2 + h''v'' → CH2O + H2O + 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 billion years ago, but photosynthetic processes started 0.3–1 billion years prior.<ref name="Goldblatt06"/><ref name="Catling05"/>

=== Radiation: heat, temperature and light ===

{{Anchor |Sunlight}}

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]] have a body temperature largely 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 |H2S]] 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 ====

{{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 O2 concentration below 2 mg/liter) and eventually completely [[Anoxic waters |anoxic]] where [[anaerobic bacteria]] thrive among the roots. Water influences the intensity and [[Electromagnetic spectrum |spectral composition]] of light as it reflects off the water surface and submerged particles.<ref name="Cronk01"/> 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.<ref name="Cronk01"/> The physiology of fish is 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. These govern many of the geophysical properties and distributions of 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 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 adapted to deal with changes in sound due to water pressure differences.<ref name="Kastak98"/>

==== 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 influence [[evapotranspiration rates]] and energy budgets in plants and animals.<ref name="Cronk01"/><ref name="Wolf96"/> Wind speed, temperature and moisture content 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 to intermix with sister lineages that are segregated to the interior mountain systems.<ref name="Daubenmire75"/><ref name="Steele05"/>

==== Fire ====

{{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 CO2 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"/>

==== Soils ====

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"/> 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.<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>

==== Biogeochemistry and climate ====

Ecologists study 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 of ecological processes amplify and regulate the [[biogeochemical cycle]]s of the Earth.<ref name="Falkowoski08"/>

== History ==

=== 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% |align=right}}

Ecology has a complex origin.<ref name="Egerton01"/> Ancient Greek philosophers such as [[Hippocrates]] and [[Aristotle]] recorded observations on natural history. However, they saw species as unchanging, while varieties were seen as aberrations. Modern ecology sees varieties as the real phenomena, leading to adaptation by [[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> Ecological concepts such as a balance and regulation in nature can be traced to [[Herodotus]] (died ''c''. 425 BC), who described [[mutualism (biology) |mutualism]] in his observation of "natural dentistry". Basking [[Nile crocodile]]s, he noted, opened 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 and his student [[Theophrastus]] observed plant and animal migrations, biogeography, physiology, and their behavior, giving an early analogue to the 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"/> |width=30% |align=right}}

<div class="thumb floatleft" 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 developed in the 1700s, through the 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) recognized ecological gradients, where species are replaced or altered in form along [[environmental gradient]]s. Humboldt drew inspiration from [[Isaac Newton]], as he developed a form of "terrestrial physics".<ref name="Kingsland04"/><ref name="Rosenzweig03"/><ref name="Hawkins01"/> Natural historians, such as Humboldt, [[James Hutton]], and [[Jean-Baptiste Lamarck]] laid the foundations of ecology.<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}}</ref> Haeckel was a zoologist, artist, writer, and later in life a professor of comparative anatomy.<ref name="Stauffer57"/><ref name="Friederichs58"/>

Linnaeus founded an early branch of ecology that he called the economy of nature.<ref name="Egerton07"/> He influenced Charles Darwin, who adopted Linnaeus' phrase in ''[[The Origin of Species]]''.<ref name="Stauffer57"/> Linnaeus was the first to frame the [[balance of nature]] as a testable hypothesis.<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"/>]]

=== Since 1900 ===

Modern ecology first attracted substantial scientific attention toward the end of the 19th century. [[Ellen Swallow Richards]] adopted the term "[[oekology]]" 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 description 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 |url-access=subscription }}</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 launched a debate between ecological holism and individualism that lasted until the 1970s.<ref name="Simberloff80"/<ref name="Gleason26"/>

In 1942, [[Raymond Lindeman]] wrote a landmark paper on the [[Trophic dynamics#Trophic dynamics and multitrophic interactions |trophic dynamics]] of ecology. Trophic dynamics became the foundation for much work on energy and material flow through ecosystems. [[Robert MacArthur]] advanced mathematical theory, predictions, and tests in ecology in the 1950s.<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>

{{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% |align=right}}

Ecology surged in popular and scientific interest during the 1960–1970s [[environmental movement]].<ref name="McIntosh85"/> 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]] (C14H9Cl5), [[Bioaccumulation |bioaccumulating]] in the environment. 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">{{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><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 ==

{{cols |colwidth=16em}} * [[Anoxic microsites]] *[[Carrying capacity]] * [[Ecological death]] * [[Ecological overshoot]] * [[Ecological psychology]] * [[Ecology movement]] * [[Ecopsychology]]

* [[Natural resource]] * [[Philosophy of ecology]] * [[Sustainable development]]  {{colend}}

; Lists {{cols}} * [[Glossary of ecology]] * [[List of ecologists]] * [[: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."}}

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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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