Editing Ecology (section)

== 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"/>