Ecology

Ecology

Ecology addresses the full scale of life, from tiny bacteria to processes that span the entire planet. Ecologists study many diverse andcomplex relationsamong species, such aspredationandpollination. The diversity of life is organized into differenthabitats, fromterrestrialtoaquatic ecosystems.

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Ecology(fromAncient Greekοἶκος(oîkos)'house' and-λογία(-logía)'study of')^[A]is thenatural scienceof the relationships among livingorganisms, includinghumans, and theirphysical environment. Ecology considers organisms at the individual,population,community,ecosystem, andbiospherelevels. Ecology overlaps with the closely related sciences ofbiogeography,evolutionary biology,genetics,ethology, andnatural history.

Ecology is a branch ofbiology, and is the study ofabundance,biomass, and distribution of organisms in the context of the environment. It encompasses life processes, interactions, andadaptations; movement of materials andenergythrough living communities;successionaldevelopment of ecosystems; cooperation, competition, and predation within and betweenspecies; and patterns ofbiodiversityand its effect on ecosystem processes.

Ecology has practical applications inconservation biology,wetlandmanagement,natural resource management(agroecology,agriculture,forestry,agroforestry,fisheries,mining,tourism),urban planning(urban ecology),community health,economics,basicandapplied science, and human social interaction (human ecology).

The wordecology(German:Ökologie) was coined in 1866 by the German scientistErnst Haeckel. The science of ecology as we know it today began with a group of American botanists in the 1890s.^[1]Evolutionaryconcepts relating to adaptation andnatural selectionare cornerstones of modernecological theory.

Ecosystems are dynamically interacting systems of organisms, the communities they make up, and the non-living (abiotic) components of their environment. Ecosystem processes, such asprimary production,nutrient cycling, andniche construction, regulate the flux of energy and matter through an environment. Ecosystems havebiophysicalfeedback mechanisms that moderate processes acting on living (biotic) and abiotic components of the planet. Ecosystems sustain life-supporting functions and provideecosystem serviceslikebiomassproduction (food, fuel, fiber, and medicine), the regulation ofclimate, globalbiogeochemical cycles,water filtration,soil formation,erosioncontrol, flood protection, and many other natural features of scientific, historical, economic, or intrinsic value.

The scope of ecology contains a wide array of interacting levels of organization spanning micro-level (e.g.,cells) to a planetary scale (e.g.,biosphere)phenomena. Ecosystems, for example, contain abioticresourcesand interacting life forms (i.e., individual organisms that aggregate intopopulationswhich 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.^[2]Several generations of anaphidpopulation can exist over the lifespan of a single leaf. Each of those aphids, in turn, supports diversebacterialcommunities.^[3]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.^[4]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.^[5]

The main subdisciplines of ecology,population(orcommunity) ecology andecosystem 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.^[6]

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 classifyecosystemshierarchically by analyzing data collected from finer scale units, such asvegetation associations, climate, andsoil types, 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 anested hierarchy, ranging in scale fromgenes, tocells, totissues, toorgans, toorganisms, tospecies, topopulations, toguilds, tocommunities, toecosystems, tobiomes, and up to the level of thebiosphere.^[8]This framework forms apanarchy^[9]and exhibitsnon-linearbehaviors; this means that "effect and cause are disproportionate, so that small changes to critical variables, such as the number ofnitrogen fixers, can lead to disproportionate, perhaps irreversible, changes in the system properties."^[10]: 14

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.^[12][13][14]Biodiversity includesspecies diversity,ecosystem diversity, andgenetic diversityand scientists are interested in the way that this diversity affects the complex ecological processes operating at and among these respective levels.^[13][15][16]Biodiversity plays an important role inecosystem serviceswhich by definition maintain and improve human quality of life.^[14][17][18]Conservation priorities and management techniques require different approaches and considerations to address the full ecological scope of biodiversity.Natural capitalthat supports populations is critical for maintainingecosystem services^[19][20]and speciesmigration(e.g., riverine fish runs and avian insect control) has been implicated as one mechanism by which those service losses are experienced.^[21]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.^[22]

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.^[24]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."^[25]: 745For example, a habitat might be an aquatic or terrestrial environment that can be further categorized as amontaneoralpineecosystem. 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 developmentallife historyof amphibians, and in insects that transition from aquatic to terrestrial habitats.Biotopeand habitat are sometimes used interchangeably, but the former applies to a community's environment, whereas the latter applies to a species' environment.^[24][26][27]

Definitions of the niche date back to 1917,^[30]butG. Evelyn Hutchinsonmade conceptual advances in 1957^[31][32]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."^[30]: 519The ecological niche is a central concept in the ecology of organisms and is sub-divided into thefundamentaland therealizedniche. 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.^[30][32][33]The Hutchinsonian niche is defined more technically as a "Euclideanhyperspacewhosedimensionsare defined as environmental variables and whosesizeis a function of the number of values that the environmental values may assume for which an organism haspositive fitness."^[34]: 71

Biogeographicalpatterns andrangedistributions are explained or predicted through knowledge of a species'traitsand niche requirements.^[35]Species have functional traits that are uniquely adapted to the ecological niche. A trait is a measurable property,phenotype, orcharacteristicof an organism that may influence its survival. Genes play an important role in the interplay of development and environmental expression of traits.^[36]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. Thecompetitive exclusion principlestates that two species cannot coexist indefinitely by living off the same limitingresource; 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.^[37]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.^[38]The habitat plus the niche is called theecotope, which is defined as the full range of environmental and biological variables affecting an entire species.^[24]

Organisms are subject to environmental pressures, but they also modify their habitats. Theregulatory feedbackbetween organisms and their environment can affect conditions from local (e.g., abeaverpond) to global scales, over time and even after death, such as decaying logs orsilicaskeleton deposits from marine organisms.^[39]The process and concept ofecosystem engineeringare related toniche 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."^[40]: 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.^[28][41]An example of natural selection through ecosystem engineering occurs in the nests ofsocial insects, including ants, bees, wasps, and termites. There is an emergenthomeostasisorhomeorhesisin 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.^[5][28][29]

Biomes are larger units of organization that categorize regions of the Earth's ecosystems, mainly according to the structure and composition of vegetation.^[42]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 includetropical rainforest,temperate broadleaf and mixed forest,temperate deciduous forest,taiga,tundra,hot desert, andpolar desert.^[43]Other researchers have recently categorized other biomes, such as the human and oceanicmicrobiomes. To amicrobe, the human body is a habitat and a landscape.^[44]Microbiomes were discovered largely through advances inmolecular 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.^[45]

The largest scale of ecological organization is the biosphere: the total sum of ecosystems on the planet.Ecological relationshipsregulate 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 CO₂and O₂composition 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.^[46]Ecological theory has also been used to explain self-emergent regulatory phenomena at the planetary scale: for example, theGaia hypothesisis an example ofholismapplied in ecological theory.^[47]The Gaia hypothesis states that there is an emergentfeedback loopgenerated by themetabolismof living organisms that maintains the core temperature of the Earth and atmospheric conditions within a narrow self-regulating range of tolerance.^[48]

Population ecology studies the dynamics of species populations and how these populations interact with the wider environment.^[5]A population consists of individuals of the same species that live, interact, and migrate through the same niche and habitat.^[49]

A primary law of population ecology is theMalthusian growth model^[50]which states, "a population will grow (or decline) exponentially as long as the environment experienced by all individuals in the population remains constant."^[50]: 18Simplified populationmodelsusually starts with four variables: death, birth,immigration, andemigration.

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 thatrandomprocesses create the observed data. In these island models, the rate of population change is described by:

${\displaystyle {\frac {\operatorname {d} N(t)}{\operatorname {d} t}}=bN(t)-dN(t)=(b-d)N(t)=rN(t),}$

whereNis the total number of individuals in the population,banddare the per capita rates of birth and death respectively, andris the per capita rate of population change.^[50][51]

Using these modeling techniques, Malthus' population principle of growth was later transformed into a model known as thelogistic equationbyPierre Verhulst:

${\displaystyle {\frac {\operatorname {d} N(t)}{\operatorname {d} t}}=rN(t)-\alpha N(t)^{2}=rN(t)\left({\frac {K-N(t)}{K}}\right),}$

whereN(t)is the number of individuals measured asbiomassdensity as a function of time,t,ris the maximum per-capita rate of change commonly known as the intrinsic rate of growth, and ${\displaystyle \alpha }$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 ( ${\displaystyle \mathrm {d} N(t)/\mathrm {d} t}$) will grow to approach equilibrium, where ( ${\displaystyle \mathrm {d} N(t)/\mathrm {d} t=0}$), when the rates of increase and crowding are balanced, ${\displaystyle r/\alpha }$. A common, analogous model fixes the equilibrium, ${\displaystyle r/\alpha }$asK, 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 includelife history,fecundity, and survivorship, and these are analyzed using mathematical techniques such asmatrix algebra. The information is used for managing wildlife stocks and setting harvest quotas.^[51][52]In cases where basic models are insufficient, ecologists may adopt different kinds of statistical methods, such as theAkaike information criterion,^[53]or use models that can become mathematically complex as "several competing hypotheses are simultaneously confronted with the data."^[54]

The concept of metapopulations was defined in 1969^[55]as "a population of populations which go extinct locally and recolonize".^[56]: 105Metapopulation ecology is another statistical approach that is often used inconservation research.^[57]Metapopulation models simplify the landscape into patches of varying levels of quality,^[58]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.^[59]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.^[60][61]

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 ofjuvenilesthat 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 andfield studiesto explain metapopulation structure.^[62][63]

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 measurespecies diversityin 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.

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 measureprimary production(g C/m^2) in awetlandin 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.,fungiand bacteria),^[66]

The underlying concept of an ecosystem can be traced back to 1864 in the published work ofGeorge Perkins Marsh("Man and Nature").^[67][68]Within an ecosystem, organisms are linked to the physical and biological components of their environment to which they are adapted.^[65]Ecosystems are complex adaptive systems where the interaction of life processes form self-organizing patterns across different scales of time and space.^[69]Ecosystems are broadly categorized asterrestrial,freshwater, atmospheric, ormarine. Differences stem from the nature of the unique physical environments that shapes the biodiversity within each. A more recent addition to ecosystem ecology aretechnoecosystems, which are affected by or primarily the result of human activity.^[5]

A food web is the archetypalecological network. Plants capturesolar energyand use it to synthesizesimple sugarsduringphotosynthesis. As plants grow, they accumulate nutrients and are eaten by grazingherbivores, and the energy is transferred through a chain of organisms by consumption. The simplified linear feeding pathways that move from a basaltrophic speciesto a top consumer is called thefood chain. Food chains in an ecological community create a complex food web. Food webs are a type ofconcept mapthat is used to illustrate and study pathways of energy and material flows.^[7][70][71]

Empirical measurementsare 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.^[72]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.^[73]Despite these limitations, food webs remain a valuable tool in understanding community ecosystems.^[74]

Food webs illustrate importantprinciples of ecology: some species have many weak feeding links (e.g.,omnivores) while some are more specialized with fewer stronger feeding links (e.g.,primary predators). Such linkages explain how ecological communities remain stable over time^[75][76]and eventually can illustrate a "complete" web of life.^{[71][77][78][79]}

Thedisruption of food websmay have a dramatic impact on the ecology of individual species or whole ecosystems. For instance, the replacement of anantspecies by another (invasive) ant species has been shown to affect howelephantsreduce tree cover and thus the predation oflionsonzebras.^[80][81]

A trophic level (from Greektroph, τροφή, trophē, meaning "food" or "feeding") is "a group of organisms acquiring a considerable majority of its energy from the lower adjacent level (according toecological pyramids) nearer the abiotic source."^[82]: 383Links in food webs primarily connect feeding relations ortrophismamong 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 theabundanceor biomass at each level.^[83]When the relative abundance or biomass of each species is sorted into its respective trophic level, they naturally sort into a 'pyramid of numbers'.^[84]

Species are broadly categorized asautotrophs(orprimary producers),heterotrophs(orconsumers), andDetritivores(ordecomposers). Autotrophs are organisms that produce their own food (production is greater than respiration) by photosynthesis orchemosynthesis. Heterotrophs are organisms that must feed on others for nourishment and energy (respiration exceeds production).^[5]Heterotrophs can be further sub-divided into different functional groups, includingprimary consumers(strict herbivores),secondary consumers(carnivorouspredators that feed exclusively on herbivores), and tertiary consumers (predators that feed on a mix of herbivores and predators).^[85]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.^[86]

Trophic levels are part of theholisticorcomplex systemsview of ecosystems.^[87][88]Each trophic level contains unrelated species that are grouped together because they share common ecological functions, giving a macroscopic view of the system.^[89]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."^[90]: 815Nonetheless, 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."^[91]: 612

A keystone species is a species that is connected to a disproportionately large number of other species in thefood-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 (termedtrophic cascades) that alters trophic dynamics, other food web connections, and can cause the extinction of other species.^[92][93]The term keystone species was coined by Robert Paine in 1969 and is a reference to thekeystonearchitectural 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.^[94]

Sea otters(Enhydra lutris) are commonly cited as an example of a keystone species because they limit the density ofsea urchinsthat feed onkelp. 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.^[95]Hunting of sea otters, for example, is thought to have led indirectly to the extinction of theSteller's sea cow(Hydrodamalis gigas).^[96]While the keystone species concept has been used extensively as aconservationtool, 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.^[95][97]

Complexity is understood as a large computational effort needed to piece together numerous interacting parts exceeding the iterative memory capacity of the human mind. Global patterns of biological diversity are complex. Thisbiocomplexitystems from the interplay among ecological processes that operate and influence patterns at different scales that grade into each other, such as transitional areas orecotonesspanning landscapes. Complexity stems from the interplay among levels of biological organization as energy, and matter is integrated into larger units that superimpose onto the smaller parts. "What were wholes on one level become parts on a higher one."^[98]: 209Small scale patterns do not necessarily explain large scale phenomena, otherwise captured in the expression (coined by Aristotle) 'the sum is greater than the parts'.^[99][100][E]

"Complexity in ecology is of at least six distinct types: spatial, temporal, structural, process, behavioral, and geometric."^[101]: 3From these principles, ecologists have identifiedemergentandself-organizingphenomena that operate at different environmental scales of influence, ranging from molecular to planetary, and these require different explanations at eachintegrative level.^[48][102]Ecological complexity relates to the dynamic resilience of ecosystems that transition to multiple shifting steady-states directed by random fluctuations of history.^[9][103]Long-term ecological studies provide important track records to better understand the complexity and resilience of ecosystems over longer temporal and broader spatial scales. These studies are managed by the International Long Term Ecological Network (LTER).^[104]The longest experiment in existence is thePark Grass Experiment, which was initiated in 1856.^[105]Another example is theHubbard Brook study, which has been in operation since 1960.^[106]

Holism remains a critical part of the theoretical foundation in contemporary ecological studies. Holism addresses thebiological organizationof life thatself-organizesinto 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 anecosystem, cannot be predicted or understood by a simple summation of the parts.^[107]"New properties emerge because the components interact, not because the basic nature of the components is changed."^[5]: 8

Ecological studies are necessarily holistic as opposed toreductionistic.^{[36][102][108]}Holism has three scientific meanings or uses that identify with ecology: 1) the mechanistic complexity of ecosystems, 2) the practical description of patterns in quantitative reductionist terms where correlations may be identified but nothing is understood about the causal relations without reference to the whole system, which leads to 3) ametaphysicalhierarchy whereby the causal relations of larger systems are understood without reference to the smaller parts. Scientific holism differs frommysticismthat has appropriated the same term. An example of metaphysical holism is identified in the trend of increased exterior thickness in shells of different species. The reason for a thickness increase can be understood through reference to principles of natural selection via predation without the need to reference or understand thebiomolecularproperties of the exterior shells.^[109]

Ecology and evolutionary biology are considered sister disciplines of the life sciences.Natural selection,life history,development,adaptation,populations, andinheritanceare examples of concepts that thread equally into ecological and evolutionary theory. Morphological, behavioural, and genetic traits, for example, can be mapped onto evolutionary trees to study the historical development of a species in relation to their functions and roles in different ecological circumstances. In this framework, the analytical tools of ecologists and evolutionists overlap as they organize, classify, and investigate life through common systematic principles, such asphylogeneticsor theLinnaean system of taxonomy.^[110]The two disciplines often appear together, such as in the title of the journalTrends in Ecology and Evolution.^[111]There is no sharp boundary separating ecology from evolution, and they differ more in their areas of applied focus. Both disciplines discover and explain emergent and unique properties and processes operating across different spatial or temporal scales of organization.^[36][48]While the boundary between ecology and evolution is not always clear, ecologists study the abiotic and biotic factors that influence evolutionary processes,^[112][113]and evolution can be rapid, occurring on ecological timescales as short as one generation.^[114]

All organisms can exhibit behaviours. Even plants express complex behaviour, including memory and communication.^[116]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 motilespermof plants, mobilephytoplankton,zooplanktonswimming toward the female egg, the cultivation of fungi byweevils, the mating dance of asalamander, or social gatherings ofamoeba.^{[117][118][119][120][121]}

Adaptation is the central unifying concept in behavioural ecology.^[122]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.^[123][124]

Predator-prey interactions are an introductory concept into food-web studies as well as behavioural ecology.^[126]Prey species can exhibit different kinds of behavioural adaptations to predators, such as avoid, flee, or defend. Many prey species are faced with multiple predators that differ in the degree of danger posed. To be adapted to their environment and face predatory threats, organisms must balance their energy budgets as they invest in different aspects of their life history, such as growth, feeding, mating, socializing, or modifying their habitat. Hypotheses posited in behavioural ecology are generally based on adaptive principles of conservation, optimization, or efficiency.^{[33][112][127]}For example, "[t]he threat-sensitive predator avoidance hypothesis predicts that prey should assess the degree of threat posed by different predators and match their behaviour according to current levels of risk"^[128]or "[t]he optimalflight initiation distanceoccurs where expected postencounter fitness is maximized, which depends on the prey's initial fitness, benefits obtainable by not fleeing, energetic escape costs, and expected fitness loss due to predation risk."^[129]

Elaborate sexualdisplaysand posturing are encountered in the behavioural ecology of animals. Thebirds-of-paradise, for example, sing and display elaborate ornaments duringcourtship. These displays serve a dual purpose of signalling healthy or well-adapted individuals and desirable genes. The displays are driven bysexual selectionas an advertisement of quality of traits amongsuitors.^[130]

Cognitive ecology integrates theory and observations fromevolutionary ecologyandneurobiology, primarilycognitive science, in order to understand the effect that animal interaction with their habitat has on their cognitive systems and how those systems restrict behavior within an ecological and evolutionary framework.^[131]"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."^[132][133]As a study involving the 'coupling' or interactions between organism and environment, cognitive ecology is closely related toenactivism,^[131]a field based upon the view that "...we must see the organism and environment as bound together in reciprocal specification and selection...".^[134]

Social-ecological behaviours are notable in thesocial insects,slime moulds,social spiders,human society, andnaked mole-ratswhereeusocialismhas evolved. Social behaviours include reciprocally beneficial behaviours among kin and nest mates^{[119][124][135]}and evolve from kin and group selection.Kin selectionexplains 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, includingants,bees, andwaspsare most famously studied for this type of relationship because the male drones areclonesthat share the same genetic make-up as every other male in the colony.^[124]In contrast,group selectionistsfind 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.^[124][136]

Ecological interactions can be classified broadly into ahostand an associate relationship. A host is any entity that harbours another that is called the associate.^[137]Relationshipsbetween speciesthat are mutually or reciprocally beneficial are calledmutualisms. Examples of mutualism includefungus-growing antsemploying agricultural symbiosis, bacteria living in the guts of insects and other organisms, thefig waspandyucca mothpollination complex,lichenswith fungi and photosyntheticalgae, andcoralswith photosynthetic algae.^[138][139]If there is a physical connection between host and associate, the relationship is calledsymbiosis. Approximately 60% of all plants, for example, have a symbiotic relationship witharbuscular mycorrhizal fungiliving in their roots forming an exchange network of carbohydrates formineral nutrients.^[140]

Indirect mutualisms occur where the organisms live apart. For example, trees living in the equatorial regions of the planet supply oxygen into the atmosphere that sustains species living in distant polar regions of the planet. This relationship is calledcommensalismbecause many others receive the benefits of clean air at no cost or harm to trees supplying the oxygen.^[5][141]If the associate benefits while the host suffers, the relationship is calledparasitism. Although parasites impose a cost to their host (e.g., via damage to their reproductive organs orpropagules, denying the services of a beneficial partner), their net effect on host fitness is not necessarily negative and, thus, becomes difficult to forecast.^[142][143]Co-evolution is also driven by competition among species or among members of the same species under the banner of reciprocal antagonism, such as grasses competing for growth space. TheRed Queen Hypothesis, for example, posits that parasites track down and specialize on the locally common genetic defense systems of its host that drives the evolution of sexual reproduction to diversify the genetic constituency of populations responding to the antagonistic pressure.^[144][145]

Biogeography (an amalgamation ofbiologyandgeography) is the comparative study of the geographic distribution of organisms and the corresponding evolution of their traits in space and time.^[146]TheJournal of Biogeographywas established in 1974.^[147]Biogeography and ecology share many of their disciplinary roots. For example,the theory of island biogeography, published by the Robert MacArthur andEdward O. Wilsonin 1967^[148]is considered one of the fundamentals of ecological theory.^[149]

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.^[146]Biogeographical patterns result from ecological processes that influence range distributions, such asmigrationanddispersal.^[149]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 calledvicariance biogeographyand it is a sub-discipline of biogeography.^[150]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 ofglobal warming.^[151][152]

A population ecology concept is r/K selection theory,^[D]one of the first predictive models in ecology used to explainlife-history evolution. The premise behind the r/K selection model is that natural selection pressures change according topopulation 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 availableresourcesfor rapid population growth. These early phases ofpopulation growthexperiencedensity-independentforces of natural selection, which is calledr-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, calledK-selection.^[153]

In ther/K-selection model, the first variableris the intrinsic rate of natural increase in population size and the second variableKis the carrying capacity of a population.^[33]Different species evolve different life-history strategies spanning a continuum between these two selective forces. Anr-selected species is one that has high birth rates, low levels of parental investment, and high rates of mortality before individuals reach maturity. Evolution favours high rates offecundityinr-selected species. Many kinds of insects andinvasive speciesexhibitr-selectedcharacteristics. In contrast, aK-selected species has low rates of fecundity, high levels of parental investment in the young, and low rates of mortality as individuals mature. Humans and elephants are examples of species exhibitingK-selected characteristics, including longevity and efficiency in the conversion of more resources into fewer offspring.^[148][154]

The important relationship between ecology and genetic inheritance predates modern techniques for molecular analysis. Molecular ecological research became more feasible with the development of rapid and accessible genetic technologies, such as thepolymerase chain reaction (PCR). The rise of molecular technologies and the influx of research questions into this new ecological field resulted in the publicationMolecular Ecologyin 1992.^[155]Molecular ecologyuses various analytical techniques to study genes in an evolutionary and ecological context. In 1994,John Avisealso played a leading role in this area of science with the publication of his book,Molecular Markers, Natural History and Evolution.^[156]Newer technologies opened a wave of genetic analysis into organisms once difficult to study from an ecological or evolutionary standpoint, such as bacteria, fungi, andnematodes. Molecular ecology engendered a new research paradigm for investigating ecological questions considered otherwise intractable. Molecular investigations revealed previously obscured details in the tiny intricacies of nature and improved resolution into probing questions about behavioural and biogeographical ecology.^[156]For example, molecular ecology revealedpromiscuoussexual behaviour and multiple male partners intree swallowspreviously thought to be sociallymonogamous.^[157]In a biogeographical context, the marriage between genetics, ecology, and evolution resulted in a new sub-discipline calledphylogeography.^[158]

Ecology is as much a biological science as it is a human science.^[5]Human ecology is aninterdisciplinaryinvestigation 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."^[160]: 3The 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.^[160][161]

The ecological complexities human beings are facing through the technological transformation of the planetary biome has brought on theAnthropocene. The unique set of circumstances has generated the need for a new unifying science calledcoupled human and natural systemsthat builds upon, but moves beyond the field of human ecology.^[107]Ecosystems tie into human societies through the critical and all-encompassing life-supporting functions they sustain. In recognition of these functions and the incapability of traditional economic valuation methods to see the value in ecosystems, there has been a surge of interest insocial-natural capital, which provides the means to put a value on the stock and use of information and materials stemming fromecosystem goods and services. Ecosystems produce, regulate, maintain, and supply services of critical necessity and beneficial to human health (cognitive and physiological), economies, and they even provide an information or reference function as a living library giving opportunities for science and cognitive development in children engaged in the complexity of the natural world. Ecosystems relate importantly to human ecology as they are the ultimate base foundation of global economics as every commodity, and the capacity for exchange ultimately stems from the ecosystems on Earth.^{[107][162][163][164]}

Ecology is an employed science of restoration, repairing disturbed sites through human intervention, in natural resource management, and inenvironmental impact assessments. Edward O. Wilson predicted in 1992 that the 21st century "will be the era of restoration in ecology".^[166]Ecological science has boomed in the industrial investment of restoring ecosystems and their processes in abandoned sites after disturbance. Natural resource managers, inforestry, for example, employ ecologists to develop, adapt, and implementecosystem based methodsinto the planning, operation, and restoration phases of land-use. Another example of conservation is seen on the east coast of the United States in Boston, MA. The city of Boston implemented the Wetland Ordinance,^[167]improving the stability of their wetland environments by implementing soil amendments that will improve groundwater storage and flow, and trimming or removal of vegetation that could cause harm to water quality.^{[citation needed]}Ecological science is used in the methods of sustainable harvesting, disease, and fire outbreak management, in fisheries stock management, for integrating land-use with protected areas and communities, and conservation in complex geo-political landscapes.^{[22][165][168][169]}

The environment of ecosystems includes both physical parameters and biotic attributes. It is dynamically interlinked and containsresourcesfor organisms at any time throughout their life cycle.^[5][170]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."^[171]: 62The physical environment is external to the level of biological organization under investigation, includingabioticfactors such as temperature, radiation, light, chemistry,climateand geology. The biotic environment includes genes, cells, organisms, members of the same species (conspecifics) and other species that share a habitat.^[172]

The distinction between external and internal environments, however, is an abstraction parsing life and environment into units or facts that are inseparable in reality. There is an interpenetration of cause and effect between the environment and life. The laws ofthermodynamics, for example, apply to ecology by means of its physical state. With an understanding of metabolic and thermodynamic principles, a complete accounting of energy and material flow can be traced through an ecosystem. In this way, the environmental and ecological relations are studied through reference to conceptually manageable and isolatedmaterialparts. After the effective environmental components are understood through reference to their causes; however, they conceptually link back together as an integrated whole, orholocoenoticsystem as it was once called. This is known as thedialecticalapproach to ecology. The dialectical approach examines the parts but integrates the organism and the environment into a dynamic whole (orumwelt). Change in one ecological or environmental factor can concurrently affect the dynamic state of an entire ecosystem.^[36][173]

A disturbance is any process that changes or removes biomass from a community, such as a fire, flood, drought, or predation.^[174]Disturbances are both the cause and product of natural fluctuations within an ecological community.^{[175][174][176][177]}Biodiversity can protect ecosystems from disturbances.^[177]

The effect of a disturbance is often hard to predict, but there are numerous examples in which a single species can massively disturb an ecosystem. For example, a single-celledprotozoanhas been able to kill up to 100% ofsea urchinsin somecoral reefsin theRed Seaand WesternIndian Ocean. Sea urchins enable complex reef ecosystems to thrive by eatingalgaethat would otherwise inhibit coral growth.^[178]Similarly,invasive speciescan wreak havoc on ecosystems. For instance, invasiveBurmese pythonshave caused a 98% decline of smallmammalsin theEverglades.^[179]

The Earth was formed approximately 4.5 billion years ago.^[181]As it cooled and a crust and oceans formed, its atmosphere transformed from being dominated byhydrogento one composed mostly ofmethaneandammonia. Over the next billion years, the metabolic activity of life transformed the atmosphere into a mixture ofcarbon 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 ofreducing and oxidizinggasses that set the stage for primitive ecosystems to evolve and, in turn, the atmosphere also evolved.^[182]

Throughout history, the Earth's atmosphere andbiogeochemical cycleshave been in adynamic equilibriumwith planetary ecosystems. The history is characterized by periods of significant transformation followed by millions of years of stability.^[183]The evolution of the earliest organisms, likely anaerobicmethanogenmicrobes, started the process by converting atmospheric hydrogen into methane (4H₂+ CO₂→ CH₄+ 2H₂O).Anoxygenic photosynthesisreduced hydrogen concentrations and increasedatmospheric methane, by convertinghydrogen sulfideinto water or other sulfur compounds (for example, 2H₂S + CO₂+ hv→ CH₂O + H₂O + 2S). Early forms offermentationalso increased levels of atmospheric methane. The transition to an oxygen-dominant atmosphere (theGreat Oxidation) did not begin until approximately 2.4–2.3 billion years ago, but photosynthetic processes started 0.3 to 1 billion years prior.^[183][184]

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, andprimary production. Temperature is largely dependent on the incidence ofsolar radiation. The latitudinal and longitudinal spatial variation oftemperaturegreatly affects climates and consequently the distribution ofbiodiversityand levels of primary production in different ecosystems or biomes across the planet. Heat and temperature relate importantly to metabolic activity.Poikilotherms, for example, have a body temperature that is largely regulated and dependent on the temperature of the external environment. In contrast,homeothermsregulate their internal body temperature by expendingmetabolic energy.^{[112][113][173]}

There is a relationship between light, primary production, and ecologicalenergy budgets. Sunlight is the primary input of energy into the planet's ecosystems. Light is composed ofelectromagnetic energyof differentwavelengths.Radiant energyfrom the sun generates heat, provides photons of light measured as active energy in the chemical reactions of life, and also acts as a catalyst forgenetic mutation.^{[112][113][173]}Plants, algae, and some bacteria absorb light and assimilate the energy throughphotosynthesis. Organisms capable of assimilating energy by photosynthesis or through inorganic fixation ofH₂Sareautotrophs. Autotrophs—responsible for primary production—assimilate light energy which becomes metabolically stored aspotential energyin the form of biochemicalenthalpicbonds.^{[112][113][173]}

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, becominghypoxic(an environment with O₂concentration below 2 mg/liter) and eventually completelyanoxicwhereanaerobic bacteriathrive among the roots. Water also influences the intensity andspectral compositionof light as it reflects off the water surface and submerged particles.^[185]Aquatic plants exhibit a wide variety of morphological and physiological adaptations that allow them to survive, compete, and diversify in these environments. For example, their roots and stems contain large air spaces (aerenchyma) that regulate the efficient transportation of gases (for example, CO₂and O₂) used in respiration and photosynthesis. Salt water plants (halophytes) have additional specialized adaptations, such as the development of special organs for shedding salt andosmoregulatingtheir internal salt (NaCl) concentrations, to live inestuarine,brackish, oroceanicenvironments. Anaerobic soilmicroorganismsin aquatic environments usenitrate,manganese ions,ferric ions,sulfate,carbon dioxide, and someorganic compounds; other microorganisms arefacultative anaerobesand use oxygen during respiration when the soil becomes drier. The activity of soil microorganisms and the chemistry of the water reduces theoxidation-reductionpotentials of the water. Carbon dioxide, for example, is reduced to methane (CH₄) by methanogenic bacteria.^[185]The physiology of fish is also specially adapted to compensate for environmental salt levels through osmoregulation. Their gills formelectrochemical gradientsthat mediate salt excretion in salt water and uptake in fresh water.^[186]

The shape and energy of the land are significantly affected by gravitational forces. On a large scale, the distribution of gravitational forces on the earth is uneven and influences the shape and movement oftectonic platesas well as influencinggeomorphicprocesses such asorogenyanderosion. These forces govern many of the geophysical properties and distributions of ecological biomes across the Earth. On the organismal scale, gravitational forces provide directional cues for plant and fungal growth (gravitropism), orientation cues for animal migrations, and influence thebiomechanicsand size of animals.^[112]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.^[187]Thecardiovascular systemsof 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).^[188]

Climatic andosmotic pressureplacesphysiologicalconstraints on organisms, especially those that fly and respire at high altitudes, or dive to deep ocean depths.^[189]These constraints influence vertical limits of ecosystems in the biosphere, as organisms are physiologically sensitive and adapted to atmospheric and osmotic water pressure differences.^[112]For example, oxygen levels decrease with decreasing pressure and are a limiting factor for life at higher altitudes.^[190]Water transportationby plants is another importantecophysiologicalprocess affected by osmotic pressure gradients.^{[191][192][193]}Water pressurein the depths of oceans requires that organisms adapt to these conditions. For example, diving animals such aswhales,dolphins, andsealsare specially adapted to deal with changes in sound due to water pressure differences.^[194]Differences betweenhagfishspecies provide another example of adaptation to deep-sea pressure through specialized protein adaptations.^[195]

Turbulent forcesin air and water affect the environment and ecosystem distribution, form, and dynamics. On a planetary scale, ecosystems are affected by circulation patterns in the globaltrade winds. Wind power and the turbulent forces it creates can influence heat, nutrient, and biochemical profiles of ecosystems.^[112]For example, wind running over the surface of a lake creates turbulence, mixing thewater columnand influencing the environmental profile to createthermally layered zones, affecting how fish, algae, and other parts of theaquatic ecosystemare structured.^[198][199]Wind speed and turbulence also influenceevapotranspiration ratesand energy budgets in plants and animals.^[185][200]Wind speed, temperature and moisture content can vary as winds travel across different land features and elevations. For example, thewesterliescome into contact with thecoastaland interior mountains of western North America to produce arain shadowon the leeward side of the mountain. The air expands and moisture condenses as the winds increase in elevation; this is calledorographic liftand 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 thexericecosystems (e.g., of theColumbia Basinin western North America) to intermix with sister lineages that are segregated to the interior mountain systems.^[201][202]

Plants convert carbon dioxide into biomass and emit oxygen into the atmosphere. By approximately 350 million years ago (the end of theDevonian period), photosynthesis had brought the concentration of atmospheric oxygen above 17%, which allowed combustion to occur.^[203]Fire releases CO₂and converts fuel into ash and tar. Fire is a significant ecological parameter that raises many issues pertaining to its control and suppression.^[204]While the issue of fire in relation to ecology and plants has been recognized for a long time,^[205]Charles Cooperbrought attention to the issue of forest fires in relation to the ecology of forest fire suppression and management in the 1960s.^[206][207]

Native North Americanswere among the first to influence fire regimes by controlling their spread near their homes or by lighting fires to stimulate the production of herbaceous foods and basketry materials.^[208]Fire creates a heterogeneous ecosystem age and canopy structure, and the altered soil nutrient supply and cleared canopy structure opens new ecological niches for seedling establishment.^[209][210]Most ecosystems are adapted to natural fire cycles. Plants, for example, are equipped with a variety of adaptations to deal with forest fires. Some species (e.g.,Pinus halepensis) cannotgerminateuntil after their seeds have lived through a fire or been exposed to certain compounds from smoke. Environmentally triggered germination of seeds is calledserotiny.^[211][212]Fire plays a major role in the persistence andresilienceof ecosystems.^[176]

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. Thedecompositionof dead organic matter (for example, leaves on the forest floor), results in soils containingmineralsand nutrients that feed into plant production. The whole of the planet's soil ecosystems is called thepedospherewhere 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 thedetritivoresthat regulate soil formation.^[213][214]Tree roots, fungi, bacteria, worms, ants, beetles, centipedes, spiders, mammals, birds, reptiles, amphibians, and other less familiar creatures all work to create the trophic web of life in soil ecosystems. Soils form composite phenotypes where inorganic matter is enveloped into the physiology of a whole community. As organisms feed and migrate through soils they physically displace materials, an ecological process calledbioturbation. This aerates soils and stimulates heterotrophic growth and production. Soilmicroorganismsare influenced by and are fed back into the trophic dynamics of the ecosystem. No single axis of causality can be discerned to segregate the biological from geomorphological systems in soils.^[215][216]Paleoecologicalstudies of soils places the origin for bioturbation to a time before the Cambrian period. Other events, such as theevolution of treesand thecolonization of landin the Devonian period played a significant role in the early development of ecological trophism in soils.^{[214][217][218]}

Ecologists study and measure nutrient budgets to understand how these materials are regulated, flow, andrecycledthrough the environment.^{[112][113][173]}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, andphosphorus; H, C, N, O, S, and P) form the constitution of all biological macromolecules and feed into the Earth's geochemical processes. From the smallest scale of biology, the combined effect of billions upon billions of ecological processes amplify and ultimately regulate thebiogeochemical cyclesof the Earth. Understanding the relations and cycles mediated between these elements and their ecological pathways has significant bearing toward understanding global biogeochemistry.^[219]

The ecology of global carbon budgets gives one example of the linkage between biodiversity and biogeochemistry. It is estimated that the Earth's oceans hold 40,000 gigatonnes (Gt) of carbon, that vegetation and soil hold 2070 Gt, and that fossil fuel emissions are 6.3 Gt carbon per year.^[220]There have been major restructurings in these global carbon budgets during the Earth's history, regulated to a large extent by the ecology of the land. For example, through the early-mid Eocene volcanicoutgassing, the oxidation of methane stored in wetlands, and seafloor gases increased atmospheric CO₂(carbon dioxide) concentrations to levels as high as 3500ppm.^[221]

In theOligocene, from twenty-five to thirty-two million years ago, there was another significant restructuring of the globalcarbon cycleas grasses evolved a new mechanism of photosynthesis,C₄photosynthesis, and expanded their ranges. This new pathway evolved in response to the drop in atmospheric CO₂concentrations below 550 ppm.^[222]The relative abundance and distribution of biodiversity alters the dynamics between organisms and their environment such that ecosystems can be both cause and effect in relation to climate change. Human-driven modifications to the planet's ecosystems (e.g., disturbance,biodiversity loss, agriculture) contributes to rising atmospheric greenhouse gas levels. Transformation of the global carbon cycle in the next century is projected to raise planetary temperatures, lead to more extreme fluctuations in weather, alter species distributions, and increase extinction rates. The effect of global warming is already being registered in melting glaciers, melting mountain ice caps, and rising sea levels. Consequently, species distributions are changing along waterfronts and in continental areas where migration patterns and breeding grounds are tracking the prevailing shifts in climate. Large sections ofpermafrostare also melting to create a new mosaic of flooded areas having increased rates of soil decomposition activity that raises methane (CH₄) emissions. There is concern over increases in atmospheric methane in the context of the global carbon cycle, because methane is agreenhouse gasthat is 23 times more effective at absorbing long-wave radiation than CO₂on a 100-year time scale.^[223]Hence, there is a relationship between global warming, decomposition and respiration in soils and wetlands producing significant climate feedbacks and globally altered biogeochemical cycles.^{[107][224][225][226][227][228]}

Ecology has a complex origin, due in large part to its interdisciplinary nature.^[230]Ancient Greek philosophers such asHippocratesandAristotlewere among the first to record observations on natural history. However, they viewed life in terms ofessentialism, where species were conceptualized as static unchanging things while varieties were seen as aberrations of anidealized type. This contrasts against the modern understanding ofecological theorywhere varieties are viewed as the real phenomena of interest and having a role in the origins of adaptations by means ofnatural selection.^{[5][231][232]}Early conceptions of ecology, such as a balance and regulation in nature can be traced toHerodotus(diedc. 425 BC), who described one of the earliest accounts ofmutualismin his observation of "natural dentistry". BaskingNile crocodiles, he noted, would open their mouths to givesandpiperssafe access to pluckleechesout, giving nutrition to the sandpiper and oral hygiene for the crocodile.^[230]Aristotle was an early influence on the philosophical development of ecology. He and his studentTheophrastusmade extensive observations on plant and animal migrations, biogeography, physiology, and their behavior, giving an early analogue to the modern concept of an ecological niche.^[233][234]

Ecological concepts such as food chains, population regulation, and productivity were first developed in the 1700s, through the published works of microscopistAntonie van Leeuwenhoek(1632–1723) and botanistRichard Bradley(1688?–1732).^[5]BiogeographerAlexander von Humboldt(1769–1859) was an early pioneer in ecological thinking and was among the first to recognize ecological gradients, where species are replaced or altered in form alongenvironmental gradients, such as aclineforming along a rise in elevation. Humboldt drew inspiration fromIsaac Newton, as he developed a form of "terrestrial physics". In Newtonian fashion, he brought a scientific exactitude for measurement into natural history and even alluded to concepts that are the foundation of a modern ecological law on species-to-area relationships.^{[236][237][238]}Natural historians, such as Humboldt,James Hutton, andJean-Baptiste Lamarck(among others) laid the foundations of the modern ecological sciences.^[239]The term "ecology" (German:Oekologie, Ökologie) was coined byErnst Haeckelin his bookGenerelle Morphologie der Organismen(1866).^[240]Haeckel was a zoologist, artist, writer, and later in life a professor of comparative anatomy.^[229][241]

Opinions differ on who was the founder of modern ecological theory. Some mark Haeckel's definition as the beginning;^[242]others say it wasEugenius Warmingwith the writing ofOecology of Plants: An Introduction to the Study of Plant Communities(1895),^[243]orCarl Linnaeus' principles on the economy of nature that matured in the early 18th century.^[244][245]Linnaeus founded an early branch of ecology that he called the economy of nature.^[244]His works influenced Charles Darwin, who adopted Linnaeus' phrase on theeconomy or polity of natureinThe Origin of Species.^[229]Linnaeus was the first to frame thebalance of natureas a testable hypothesis. Haeckel, who admired Darwin's work, defined ecology in reference to the economy of nature, which has led some to question whether ecology and the economy of nature are synonymous.^[245]

From Aristotle until Darwin, the natural world was predominantly considered static and unchanging. Prior toThe Origin of Species, there was little appreciation or understanding of the dynamic and reciprocal relations between organisms, their adaptations, and the environment.^[231]An exception is the 1789 publicationNatural History of SelbornebyGilbert White(1720–1793), considered by some to be one of the earliest texts on ecology.^[248]WhileCharles Darwinis mainly noted for his treatise on evolution,^[249]he was one of the founders ofsoil ecology,^[250]and he made note of the first ecological experiment inThe Origin of Species.^[246]Evolutionary theory changed the way that researchers approached the ecological sciences.^[251]

Modern ecology is a young science that first attracted substantial scientific attention toward the end of the 19th century (around the same time that evolutionary studies were gaining scientific interest). The scientistEllen Swallow Richardsadopted the term "oekology" (which eventually morphed intohome economics) in the U.S. as early as 1892.^[252]

In the early 20th century, ecology transitioned from a moredescriptive formofnatural historyto a moreanalytical formofscientific natural history.^{[236][239][253]}Frederic Clementspublished the first American ecology book in 1905,^[254]presenting the idea of plant communities as asuperorganism. This publication launched a debate between ecological holism and individualism that lasted until the 1970s. Clements' superorganism concept proposed that ecosystems progress through regular and determined stages ofseral developmentthat are analogous to the developmental stages of an organism. The Clementsian paradigm was challenged byHenry Gleason,^[255]who stated that ecological communities develop from the unique and coincidental association of individual organisms. This perceptual shift placed the focus back onto the life histories of individual organisms and how this relates to the development of community associations.^[256]

The Clementsian superorganism theory was an overextended application of anidealistic formof holism.^[36][109]The term "holism" was coined in 1926 byJan Christiaan Smuts, a South African general and polarizing historical figure who was inspired by Clements' superorganism concept.^[257][C]Around the same time,Charles Eltonpioneered the concept of food chains in his classical bookAnimal Ecology.^[84]Elton^[84]defined ecological relations using concepts of food chains, food cycles, and food size, and described numerical relations among different functional groups and their relative abundance. Elton's 'food cycle' was replaced by 'food web' in a subsequent ecological text.^[258]Alfred J. Lotkabrought in many theoretical concepts applying thermodynamic principles to ecology.

In 1942,Raymond Lindemanwrote a landmark paper on thetrophic dynamicsof ecology, which was published posthumously after initially being rejected for its theoretical emphasis. Trophic dynamics became the foundation for much of the work to follow on energy and material flow through ecosystems.Robert MacArthuradvanced mathematical theory, predictions, and tests in ecology in the 1950s, which inspired a resurgent school of theoretical mathematical ecologists.^{[239][259][260]}Ecology also has developed through contributions from other nations, including Russia'sVladimir Vernadskyand his founding of the biosphere concept in the 1920s^[261]and Japan'sKinji Imanishiand his concepts of harmony in nature and habitat segregation in the 1950s.^[262]Scientific recognition of contributions to ecology from non-English-speaking cultures is hampered by language and translation barriers.^[261]

Ecology surged in popular and scientific interest during the 1960–1970senvironmental movement. There are strong historical and scientific ties between ecology, environmental management, and protection.^[239]The historical emphasis and poetic naturalistic writings advocating the protection of wild places by notable ecologists in the history ofconservation biology, such asAldo LeopoldandArthur Tansley, have been seen as far removed from urban centres where, it is claimed, the concentration of pollution andenvironmental degradationis located.^[239][264]Palamar (2008)^[264]notes an overshadowing by mainstream environmentalism of pioneering women in the early 1900s who fought for urban health ecology (then calledeuthenics)^[252]and brought about changes in environmental legislation. Women such asEllen Swallow RichardsandJulia Lathrop, among others, were precursors to the more popularized environmental movements after the 1950s.

In 1962, marine biologist and ecologistRachel Carson's bookSilent Springhelped to mobilize the environmental movement by alerting the public to toxicpesticides, such asDDT,bioaccumulatingin the environment. Carson used ecological science to link the release of environmental toxins to human andecosystem health. Since then, ecologists have worked to bridge their understanding of the degradation of the planet's ecosystems with environmental politics, law, restoration, and natural resources management.^{[22][239][264][265]}

Lists

• "Ecology"entry by Alkistis Elliott-Graves in theStanford Encyclopedia of Philosophy
• The Nature Education Knowledge Project: Ecology