Ecology 

(from Ancient Greek οἶκος (oîkos) 'house', and -λογία (-logía) 'study of')[A] is the study of the

relationships among living organisms, including humans, and their physical environment. Ecology
considers organisms at the individual, population, community, ecosystem, and biosphere level.
Ecology overlaps with the closely related sciences of biogeography, evolutionary
biology, genetics, ethology, and natural history. Ecology is a branch of biology, and it is not
synonymous with environmentalism.
Among other things, ecology is the study of:

 The abundance, biomass, and distribution of organisms in the context of the

environment
 Life processes, interactions, and adaptations
 The movement of materials and energy through living communities
 The successional development of ecosystems
 Cooperation, competition, and predation within and between species
 Patterns of biodiversity and its effect on ecosystem processes
Ecology has practical applications in conservation biology, wetland management, natural resource
management (agroecology, agriculture, forestry, agroforestry, fisheries, mining, tourism), urban
planning (urban ecology), community health, economics, basic and applied science, and human
social interaction (human ecology).
The word ecology (German: Ökologie) was coined in 1866 by the German scientist Ernst Haeckel.
The science of ecology as we know it today began with a group of American botanists in the 1890s.
[1]
 Evolutionary concepts relating to adaptation and natural selection are cornerstones of
modern ecological 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 as primary
production, nutrient cycling, and niche construction, regulate the flux of energy and matter through
an environment. Ecosystems have biophysical feedback mechanisms that moderate processes
acting on living (biotic) and abiotic components of the planet. Ecosystems sustain life-supporting
functions and provide ecosystem services like biomass production (food, fuel, fiber, and medicine),
the regulation of climate, global biogeochemical cycles, water filtration, soil
formation, erosion control, flood protection, and many other natural features of scientific, historical,
economic, or intrinsic value.

Levels, scope, and scale of organization[edit]

Main article: Outline of ecology
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
abiotic resources and interacting life forms (i.e., individual organisms that aggregate
into populations which aggregate into distinct ecological communities). Ecosystems are dynamic,
they do not always follow a linear successional path, but they are always changing, sometimes
rapidly and sometimes so slowly that it can take thousands of years for ecological processes to bring
about certain successional stages of a forest. 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 an aphid population can exist over
the lifespan of a single leaf. Each of those aphids, in turn, supports diverse bacterial communities.
[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 (or 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.[6]

Hierarchy[edit]
See also: Biological organisation and Biological classification
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, CO2 concentration, and the like. The growth of the tree responds more slowly and
integrates these short-term changes.
O'Neill et al. (1986)[7]: 76 

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 with regard to broader scale influences, such as
atmosphere or climate. Hence, ecologists classify ecosystems hierarchically by analyzing data
collected from finer scale units, such as vegetation associations, climate, and soil 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 a nested hierarchy, ranging in scale from genes, to cells, to tissues, to organs,
to organisms, to species, to populations, to communities, to ecosystems, to biomes, and up to the
level of the biosphere.[8] This framework forms a panarchy[9] 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 fixers, can lead to disproportionate, perhaps irreversible, changes in the
system properties."[10]: 14 

Biodiversity[edit]
Main article: Biodiversity
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 evolutionary processes that keep them
functioning, yet ever-changing and adapting.
Noss & Carpenter (1994)[11]: 5 

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 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.[13][15][16] Biodiversity plays an important role in ecosystem
services which 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 capital that supports populations is critical for
maintaining ecosystem services[19][20] and species migration (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]

Habitat[edit]
Main article: Habitat

Biodiversity of a coral reef. Corals 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.[23]

Long-tailed broadbill building its 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.[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]: 745  For example, a habitat might be an aquatic or terrestrial environment that can be further
categorized as a montane or 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 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.[24][26][27]

Niche[edit]
Main article: Ecological niche
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.[28][29]

Definitions of the niche date back to 1917,[30] but G. Evelyn Hutchinson made 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]: 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.[30][32][33] The Hutchinsonian niche is defined more technically as a
"Euclidean 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."[34]: 71 
Biogeographical patterns and range distributions are explained or predicted through knowledge of a
species' traits and niche requirements.[35] Species have functional traits that are uniquely adapted to
the ecological niche. A trait is a measurable property, phenotype, or characteristic of 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.
The competitive exclusion principle states that two species cannot coexist indefinitely by living off the
same limiting 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.[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 the ecotope, which is defined as the full range of
environmental and biological variables affecting an entire species.[24]

Niche construction[edit]
Main article: Niche construction
See also: Ecosystem engineering
Organisms are subject to environmental pressures, but they also modify their habitats.
The 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.[39] 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."[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 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.[5][28][29]

Biome[edit]
Main article: Biome
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 include tropical rainforest, temperate broadleaf and mixed forest, temperate deciduous
forest, taiga, tundra, hot desert, and polar desert.[43] Other researchers have recently categorized
other biomes, such as the human and oceanic microbiomes. To a microbe, the human body is a
habitat and a landscape.[44] 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.[45]

Biosphere[edit]
Main article: Biosphere
See also: Earth's spheres
The largest scale of ecological organization is the biosphere: the total sum of ecosystems on the
planet. Ecological relationships regulate the flux of energy, nutrients, and climate all the way up to
the planetary scale. For example, the dynamic history of the planetary atmosphere's CO2 and
O2 composition has been affected by the biogenic flux of gases coming from respiration and
photosynthesis, with levels fluctuating over time in relation to the ecology and evolution of plants and
animals.[46] 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.[47] The Gaia hypothesis states that there is an emergent feedback loop generated by
the metabolism of living organisms that maintains the core temperature of the Earth and atmospheric
conditions within a narrow self-regulating range of tolerance.[48]

Population ecology[edit]
Main article: 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.[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 the Malthusian 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]: 18  Simplified population 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:
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.[50][51]
Using these modeling techniques, Malthus' population principle of growth was later transformed
into a model known as the logistic equation by Pierre Verhulst:
where N(t) is the number of individuals measured as 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  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 () will
grow to approach equilibrium, where (), when the rates of increase and crowding are
balanced, . A common, analogous model fixes the equilibrium,  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 life
history, fecundity, and survivorship, and these are analyzed using mathematical techniques
such as matrix 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 the Akaike information criterion,[53] or use
models that can become mathematically complex as "several competing hypotheses are
simultaneously confronted with the data."[54]

Metapopulations and migration[edit]

Main article: Metapopulation
See also: Animal migration
The concept of metapopulations was defined in 1969[55] as "a population of populations
which go extinct locally and recolonize".[56]: 105  Metapopulation ecology is another statistical
approach that is often used in conservation 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 of 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 studies to explain metapopulation structure.[62][63]

Community ecology[edit]

Interspecific interactions such as predation are a key aspect of community ecology.

Main article: Community ecology

Community ecology examines how interactions among species and their environment affect the abundance,
distribution and diversity of species within communities.
Johnson & Stinchcomb (2007)[64]: 250 

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[edit]
Main article: Ecosystem ecology
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.
Tansley (1935)[65]: 299 
 A riparian forest in the White Mountains, New Hampshire (USA) 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),[66]
The underlying concept of an ecosystem can be traced back to 1864 in the published work
of George 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 as terrestrial, freshwater, atmospheric, or 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.[5]

Food webs[edit]
Main article: Food web
See also: Food chain
 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. The larger interlocking
pattern of food chains in an ecological community creates a complex food web. Food webs
are a type of concept map or a heuristic device that is used to illustrate and study pathways
of energy and material flows.[7][70][71]
Food webs are often limited relative to the real world. Complete empirical measurements
are generally restricted to a specific habitat, such as a cave or a pond, and principles
gleaned from food web microcosm studies are extrapolated to larger systems.[72] Feeding
relations require extensive investigations 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 exhibit principles of ecological emergence through the nature of trophic
relationships: some species have many weak feeding links (e.g., omnivores) while some are
more specialized with fewer stronger feeding links (e.g., primary predators). Theoretical and
empirical studies identify non-random emergent patterns of few strong and many weak
linkages that explain how ecological communities remain stable over time.[75] Food webs are
composed of subgroups where members in a community are linked by strong interactions,
and the weak interactions occur between these subgroups. This increases food web
stability.[76] Step by step lines or relations are drawn until a web of life is illustrated.[71][77][78][79]

Trophic levels[edit]
Main article: Trophic level

A trophic pyramid (a) and a food-web (b) illustrating ecological relationships among creatures that
are typical of a northern 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.[5]: 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 pyramids) nearer the abiotic source."[80]: 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 abundance or
biomass at each level.[81] When the relative abundance or biomass of each species is sorted
into its respective trophic level, they naturally sort into a 'pyramid of numbers'.[82]
Species are broadly categorized as autotrophs (or primary
producers), heterotrophs (or consumers), and Detritivores (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).[5] Heterotrophs can be further sub-
divided into different functional groups, including primary consumers (strict
herbivores), secondary consumers (carnivorous predators that feed exclusively on
herbivores), and tertiary consumers (predators that feed on a mix of herbivores and
predators).[83] 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.[84]
Trophic levels are part of the holistic or complex systems view of ecosystems.[85][86] Each
trophic level contains unrelated species that are grouped together because they share
common ecological functions, giving a macroscopic view of the system.[87] 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."[88]: 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."[89]: 612 

Keystone species[edit]
Main article: Keystone species

Sea otters, 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.[90][91] The term keystone species was coined by
Robert Paine in 1969 and is a reference to the 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.[92]
Sea otters (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.[93] Hunting of sea otters, for example, is thought to have led
indirectly to the extinction of the Steller's sea cow (Hydrodamalis gigas).[94] While the
keystone species concept has been used extensively as a 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.[93][95]

Complexity[edit]
Main article: Complexity
See also: Emergence
Complexity is understood as a large computational effort needed to piece together
numerous interacting parts exceeding the iterative memory capacity of the human mind.
Global patterns of biological diversity are complex. This biocomplexity stems from the
interplay among ecological processes that operate and influence patterns at different scales
that grade into each other, such as transitional areas or ecotones spanning landscapes.
Complexity stems from the interplay among levels of biological organization as energy, and
matter is integrated into larger units that superimpose onto the smaller parts. "What were
wholes on one level become parts on a higher one."[96]: 209  Small scale patterns do not
necessarily explain large scale phenomena, otherwise captured in the expression (coined by
Aristotle) 'the sum is greater than the parts'.[97][98][E]
"Complexity in ecology is of at least six distinct types: spatial, temporal, structural, process,
behavioral, and geometric."[99]: 3  From these principles, ecologists have
identified emergent and self-organizing phenomena that operate at different environmental
scales of influence, ranging from molecular to planetary, and these require different
explanations at each integrative level.[48][100] Ecological complexity relates to the dynamic
resilience of ecosystems that transition to multiple shifting steady-states directed by random
fluctuations of history.[9][101] 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).[102] The longest experiment in existence is the Park Grass
Experiment, which was initiated in 1856.[103] Another example is the Hubbard Brook study,
which has been in operation since 1960.[104]

Holism[edit]
Main article: Holism
Holism remains a critical part of the theoretical foundation in contemporary ecological
studies. Holism addresses the biological organization of life that self-organizes into layers of
emergent whole systems that function according to non-reducible properties. This means
that higher-order patterns of a whole functional system, such as an ecosystem, cannot be
predicted or understood by a simple summation of the parts.[105] "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 to reductionistic.[36][100][106] Holism has
three scientific meanings or uses that identify with ecology: 1) the mechanistic complexity of
ecosystems, 2) the practical description of patterns in quantitative reductionist terms where
correlations may be identified but nothing is understood about the causal relations without
reference to the whole system, which leads to 3) a metaphysical hierarchy whereby the
causal relations of larger systems are understood without reference to the smaller parts.
Scientific holism differs from mysticism that has appropriated the same term. An example of
metaphysical holism is identified in the trend of increased exterior thickness in shells of
different species. The reason for a thickness increase can be understood through reference
to principles of natural selection via predation without the need to reference or understand
the biomolecular properties of the exterior shells.[107]

Relation to evolution[edit]
Main article: Evolutionary ecology
Ecology and evolutionary biology are considered sister disciplines of the life
sciences. Natural selection, life history, development, adaptation, populations,
and inheritance are examples of concepts that thread equally into ecological and
evolutionary theory. Morphological, behavioural, and genetic traits, for example, can be
mapped onto evolutionary trees to study the historical development of a species in relation
to their functions and roles in different ecological circumstances. In this framework, the
analytical tools of ecologists and evolutionists overlap as they organize, classify, and
investigate life through common systematic principles, such as phylogenetics or
the Linnaean system of taxonomy.[108] The two disciplines often appear together, such as in
the title of the journal Trends in Ecology and Evolution.[109] 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,[110][111] and evolution can be rapid, occurring on ecological
timescales as short as one generation.[112]

Behavioural ecology[edit]
Main article: Behavioural ecology

Social display and colour variation in differently adapted species of chameleons (Bradypodion spp.).

Chameleons change their skin colour to match their background as a behavioural defence mechanism
and also use colour to communicate with other members of their species, such as dominant (left)
versus submissive (right) patterns shown in the three species (A-C) above.[113]

All organisms can exhibit behaviours. Even plants express complex behaviour, including
memory and communication.[114] Behavioural ecology is the study of an organism's behaviour
in its environment and its ecological and evolutionary implications. Ethology is the study of
observable movement or behaviour in animals. This could include investigations of
motile sperm of plants, mobile phytoplankton, zooplankton swimming toward the female egg,
the cultivation of fungi by weevils, the mating dance of a salamander, or social gatherings
of amoeba.[115][116][117][118][119]
Adaptation is the central unifying concept in behavioural ecology.[120] 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.[121][122]

Mutualism: Leafhoppers (Eurymela fenestrata) are protected by ants (Iridomyrmex purpureus) in

a mutualistic relationship. The ants protect the leafhoppers from predators and stimulate feeding
in the leafhoppers, and in return, the leafhoppers feeding on plants exude honeydew from their
anus that provides energy and nutrients to tending ants.[123]

Predator-prey interactions are an introductory concept into food-web studies as well as

behavioural ecology.[124] 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][110][125] 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"[126] or "[t]he optimal flight initiation distance occurs where expected postencounter
fitness is maximized, which depends on the prey's initial fitness, benefits obtainable by not
fleeing, energetic escape costs, and expected fitness loss due to predation risk."[127]
Elaborate sexual displays and posturing are encountered in the behavioural ecology of
animals. The birds-of-paradise, for example, sing and display elaborate ornaments
during courtship. These displays serve a dual purpose of signalling healthy or well-adapted
individuals and desirable genes. The displays are driven by sexual selection as an
advertisement of quality of traits among suitors.[128]

Cognitive ecology[edit]
Cognitive ecology integrates theory and observations from evolutionary
ecology and neurobiology, primarily cognitive science, in order to understand the effect that
animal interaction with their habitat has on their cognitive systems and how those systems
restrict behavior within an ecological and evolutionary framework.[129] "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."[130][131] As a study involving the 'coupling' or interactions
between organism and environment, cognitive ecology is closely related to enactivism,[129] a
field based upon the view that "...we must see the organism and environment as bound
together in reciprocal specification and selection...".[132]

Social ecology[edit]
Main article: Social ecology (academic field)
Social-ecological behaviours are notable in the social insects, slime moulds, social
spiders, human society, and naked mole-rats where eusocialism has evolved. Social
behaviours include reciprocally beneficial behaviours among kin and nest mates[117][122][133] and
evolve from kin and group selection. Kin selection explains altruism through genetic
relationships, whereby an altruistic behaviour leading to death is rewarded by the survival of
genetic copies distributed among surviving relatives. The social insects,
including ants, bees, and wasps are most famously studied for this type of relationship
because the male drones are clones that share the same genetic make-up as every other
male in the colony.[122] In contrast, group selectionists find examples of altruism among non-
genetic relatives and explain this through selection acting on the group; whereby, it
becomes selectively advantageous for groups if their members express altruistic behaviours
to one another. Groups with predominantly altruistic members survive better than groups
with predominantly selfish members.[122][134]

Coevolution[edit]
Main article: Coevolution

Bumblebees and the flowers they pollinate have coevolved so that both have become dependent

on each other for survival.

Parasitism: A harvestman arachnid being parasitized by mites. The harvestman is being

consumed, while the mites benefit from traveling on and feeding off of their host.
Ecological interactions can be classified broadly into a host and an associate relationship. A
host is any entity that harbours another that is called the associate.
[135]
 Relationships between species that are mutually or reciprocally beneficial are
called mutualisms. Examples of mutualism include fungus-growing ants employing
agricultural symbiosis, bacteria living in the guts of insects and other organisms, the fig
wasp and yucca moth pollination complex, lichens with fungi and photosynthetic algae,
and corals with photosynthetic algae.[136][137] If there is a physical connection between host and
associate, the relationship is called symbiosis. Approximately 60% of all plants, for example,
have a symbiotic relationship with arbuscular mycorrhizal fungi living in their roots forming
an exchange network of carbohydrates for mineral nutrients.[138]
Indirect mutualisms occur where the organisms live apart. For example, trees living in the
equatorial regions of the planet supply oxygen into the atmosphere that sustains species
living in distant polar regions of the planet. This relationship is
called commensalism because many others receive the benefits of clean air at no cost or
harm to trees supplying the oxygen.[5][139] If the associate benefits while the host suffers, the
relationship is called parasitism. Although parasites impose a cost to their host (e.g., via
damage to their reproductive organs or propagules, denying the services of a beneficial
partner), their net effect on host fitness is not necessarily negative and, thus, becomes
difficult to forecast.[140][141] Co-evolution is also driven by competition among species or among
members of the same species under the banner of reciprocal antagonism, such as grasses
competing for growth space. The Red Queen Hypothesis, for example, posits that parasites
track down and specialize on the locally common genetic defense systems of its host that
drives the evolution of sexual reproduction to diversify the genetic constituency of
populations responding to the antagonistic pressure.[142][143]

Biogeography[edit]
Main article: Biogeography
Biogeography (an amalgamation of biology and geography) is the comparative study of the
geographic distribution of organisms and the corresponding evolution of their traits in space
and time.[144] The Journal of Biogeography was established in 1974.[145] Biogeography and
ecology share many of their disciplinary roots. For example, the theory of island
biogeography, published by the Robert MacArthur and Edward O. Wilson in 1967[146] is
considered one of the fundamentals of ecological theory.[147]
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.[144] Biogeographical patterns result from ecological processes that
influence range distributions, such as migration and dispersal.[147] and from historical
processes that split populations or species into different areas. The biogeographic
processes that result in the natural splitting of species explain much of the modern
distribution of the Earth's biota. The splitting of lineages in a species is called vicariance
biogeography and it is a sub-discipline of biogeography.[148] There are also practical
applications in the field of biogeography concerning ecological systems and processes. For
example, the range and distribution of biodiversity and invasive species responding to
climate change is a serious concern and active area of research in the context of global
warming.[149][150]
r/K selection theory[edit]
Main article: r/K selection theory
A population ecology concept is r/K selection theory,[D] one of the first predictive models in
ecology used to explain life-history evolution. The premise behind the r/K selection model is
that natural selection pressures change according to population density. For example, when
an island is first colonized, density of individuals is low. The initial increase in population size
is not limited by competition, leaving an abundance of available resources for rapid
population growth. These early phases of population growth experience density-
independent forces of natural selection, which is called r-selection. As the population
becomes more crowded, it approaches the island's carrying capacity, thus forcing
individuals to compete more heavily for fewer available resources. Under crowded
conditions, the population experiences density-dependent forces of natural selection,
called K-selection.[151]
In the r/K-selection model, the first variable r is the intrinsic rate of natural increase in
population size and the second variable K is the carrying capacity of a population.[33] Different
species evolve different life-history strategies spanning a continuum between these two
selective forces. An r-selected species is one that has high birth rates, low levels of parental
investment, and high rates of mortality before individuals reach maturity. Evolution favours
high rates of fecundity in r-selected species. Many kinds of insects and invasive
species exhibit r-selected characteristics. In contrast, a K-selected species has low rates of
fecundity, high levels of parental investment in the young, and low rates of mortality as
individuals mature. Humans and elephants are examples of species exhibiting K-selected
characteristics, including longevity and efficiency in the conversion of more resources into
fewer offspring.[146][152]

Molecular ecology[edit]
Main article: Molecular ecology
The important relationship between ecology and genetic inheritance predates modern
techniques for molecular analysis. Molecular ecological research became more feasible with
the development of rapid and accessible genetic technologies, such as the polymerase
chain reaction (PCR). The rise of molecular technologies and the influx of research
questions into this new ecological field resulted in the publication Molecular Ecology in 1992.
[153]
 Molecular ecology uses various analytical techniques to study genes in an evolutionary
and ecological context. In 1994, John Avise also played a leading role in this area of science
with the publication of his book, Molecular Markers, Natural History and Evolution.[154] Newer
technologies opened a wave of genetic analysis into organisms once difficult to study from
an ecological or evolutionary standpoint, such as bacteria, fungi, and nematodes. 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.[154] For example, molecular ecology
revealed promiscuous sexual behaviour and multiple male partners in tree
swallows previously thought to be socially monogamous.[155] In a biogeographical context, the
marriage between genetics, ecology, and evolution resulted in a new sub-discipline
called phylogeography.[156]

Human ecology[edit]
Main article: Human ecology
The history of life on Earth has been a history of interaction between living things and their surroundings. To a large
extent, the physical form and the habits of the earth's vegetation and its animal life have been molded by the
environment. Considering the whole span of earthly time, the opposite effect, in which life actually modifies its
surroundings, has been relatively slight. Only within the moment of time represented by the present century has one
species man acquired significant power to alter the nature of his world.
Rachel Carson, "Silent Spring"[157]

Ecology is as much a biological science as it is a human science.[5] Human ecology is

an interdisciplinary investigation into the ecology of our species. "Human ecology may be
defined: (1) from a bioecological standpoint as the study of man as the ecological dominant
in plant and animal communities and systems; (2) from a bioecological standpoint as simply
another animal affecting and being affected by his physical environment; and (3) as a
human being, somehow different from animal life in general, interacting with physical and
modified environments in a distinctive and creative way. A truly interdisciplinary human
ecology will most likely address itself to all three."[158]: 3  The term was formally introduced in
1921, but many sociologists, geographers, psychologists, and other disciplines were
interested in human relations to natural systems centuries prior, especially in the late 19th
century.[158][159]
The ecological complexities human beings are facing through the technological
transformation of the planetary biome has brought on the Anthropocene. The unique set of
circumstances has generated the need for a new unifying science called coupled human
and natural systems that builds upon, but moves beyond the field of human ecology.
[105]
 Ecosystems tie into human societies through the critical and all-encompassing life-
supporting functions they sustain. In recognition of these functions and the incapability of
traditional economic valuation methods to see the value in ecosystems, there has been a
surge of interest in social-natural capital, which provides the means to put a value on the
stock and use of information and materials stemming from ecosystem goods and services.
Ecosystems produce, regulate, maintain, and supply services of critical necessity and
beneficial to human health (cognitive and physiological), economies, and they even provide
an information or reference function as a living library giving opportunities for science and
cognitive development in children engaged in the complexity of the natural world.
Ecosystems relate importantly to human ecology as they are the ultimate base foundation of
global economics as every commodity, and the capacity for exchange ultimately stems from
the ecosystems on Earth.[105][160][161][162]
Main article: Restoration ecology
See also: Natural resource management
Ecosystem management is not just about science nor is it simply an extension of traditional resource management;
it offers a fundamental reframing of how humans may work with nature.
Grumbine (1994)[163]: 27 

Ecology is an employed science of restoration, repairing disturbed sites through human

intervention, in natural resource management, and in environmental impact assessments.
Edward O. Wilson predicted in 1992 that the 21st century "will be the era of restoration in
ecology".[164] Ecological science has boomed in the industrial investment of restoring
ecosystems and their processes in abandoned sites after disturbance. Natural resource
managers, in forestry, for example, employ ecologists to develop, adapt, and
implement ecosystem based methods into the planning, operation, and restoration phases
of land-use. Another example of conservation is seen on the east coast of the United States
in Boston, MA. The city of Boston implemented the Wetland Ordinance,[165] 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][163][166][167]

Relation to the environment[edit]

Main article: Natural environment
The environment of ecosystems includes both physical parameters and biotic attributes. It is
dynamically interlinked and contains resources for organisms at any time throughout their
life cycle.[5][168] 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."[169]: 62  The physical
environment is external to the level of biological organization under investigation,
including abiotic factors such as temperature, radiation, light, chemistry, climate and
geology. The biotic environment includes genes, cells, organisms, members of the same
species (conspecifics) and other species that share a habitat.[170]
The distinction between external and internal environments, however, is an abstraction
parsing life and environment into units or facts that are inseparable in reality. There is an
interpenetration of cause and effect between the environment and life. The laws
of thermodynamics, for example, apply to ecology by means of its physical state. With an
understanding of metabolic and thermodynamic principles, a complete accounting of energy
and material flow can be traced through an ecosystem. In this way, the environmental and
ecological relations are studied through reference to conceptually manageable and
isolated material parts. After the effective environmental components are understood
through reference to their causes; however, they conceptually link back together as an
integrated whole, or holocoenotic system as it was once called. This is known as
the dialectical approach to ecology. The dialectical approach examines the parts but
integrates the organism and the environment into a dynamic whole (or umwelt). Change in
one ecological or environmental factor can concurrently affect the dynamic state of an entire
ecosystem.[36][171]

Disturbance and resilience[edit]

Main article: Resilience (ecology)
Ecosystems are regularly confronted with natural environmental variations and disturbances
over time and geographic space. A disturbance is any process that removes biomass from a
community, such as a fire, flood, drought, or predation.[172] Disturbances occur over vastly
different ranges in terms of magnitudes as well as distances and time periods,[173] and are
both the cause and product of natural fluctuations in death rates, species assemblages, and
biomass densities within an ecological community. These disturbances create places of
renewal where new directions emerge from the patchwork of natural experimentation and
opportunity.[172][174][175] Ecological resilience is a cornerstone theory in ecosystem management.
Biodiversity fuels the resilience of ecosystems acting as a kind of regenerative insurance.[175]

Metabolism and the early atmosphere[edit]

Main article: Early atmosphere
Metabolism – the rate at which energy and material resources are taken up from the environment, transformed
within an organism, and allocated to maintenance, growth and reproduction – is a fundamental physiological trait.
Ernest et al.[176]: 991 

The Earth was formed approximately 4.5 billion years ago.[177] As it cooled and a crust and
oceans formed, its atmosphere transformed from being dominated by hydrogen to one
composed mostly of methane and ammonia. Over the next billion years, the metabolic
activity of life transformed the atmosphere into a mixture of carbon dioxide, nitrogen, and
water vapor. These gases changed the way that light from the sun hit the Earth's surface
and greenhouse effects trapped heat. There were untapped sources of free energy within
the mixture of reducing and oxidizing gasses that set the stage for primitive ecosystems to
evolve and, in turn, the atmosphere also evolved.[178]
The leaf is the primary site of photosynthesis in most plants.

Throughout history, the Earth's atmosphere and biogeochemical cycles have been in

a dynamic equilibrium with planetary ecosystems. The history is characterized by periods of
significant transformation followed by millions of years of stability.[179] The evolution of the
earliest organisms, likely anaerobic methanogen microbes, started the process by
converting atmospheric hydrogen into methane (4H2 + CO2 → CH4 + 2H2O). Anoxygenic
photosynthesis reduced hydrogen concentrations and increased atmospheric methane, by
converting hydrogen sulfide into water or other sulfur compounds (for example, 2H2S +
CO2 + hv → CH2O + H2O + 2S). Early forms of fermentation also increased levels of
atmospheric methane. The transition to an oxygen-dominant atmosphere (the Great
Oxidation) did not begin until approximately 2.4–2.3 billion years ago, but photosynthetic
processes started 0.3 to 1 billion years prior.[179][180]

Radiation: heat, temperature and light[edit]

The biology of life operates within a certain range of temperatures. Heat is a form of energy
that regulates temperature. Heat affects growth rates, activity, behaviour, and primary
production. Temperature is largely dependent on the incidence of solar radiation. The
latitudinal and longitudinal spatial variation of temperature greatly affects climates and
consequently the distribution of biodiversity and levels of primary production in different
ecosystems or biomes across the planet. Heat and temperature relate importantly to
metabolic activity. Poikilotherms, for example, have a body temperature that is largely
regulated and dependent on the temperature of the external environment. In
contrast, homeotherms regulate their internal body temperature by expending metabolic
energy.[110][111][171]
There is a relationship between light, primary production, and ecological energy budgets.
Sunlight is the primary input of energy into the planet's ecosystems. Light is composed
of electromagnetic energy of different wavelengths. Radiant energy from the sun generates
heat, provides photons of light measured as active energy in the chemical reactions of life,
and also acts as a catalyst for genetic mutation.[110][111][171] Plants, algae, and some bacteria
absorb light and assimilate the energy through photosynthesis. Organisms capable of
assimilating energy by photosynthesis or through inorganic fixation of H2S are autotrophs.
Autotrophs—responsible for primary production—assimilate light energy which becomes
metabolically stored as potential energy in the form of biochemical enthalpic bonds.[110][111][171]

Physical environments[edit]
Water[edit]
Main article: Aquatic ecosystem
Wetland conditions such as shallow water, high plant productivity, and anaerobic substrates provide a suitable
environment for important physical, biological, and chemical processes. Because of these processes, wetlands
play a vital role in global nutrient and element cycles.
 Cronk & Fennessy (2001)[181]: 29 

Diffusion of carbon dioxide and oxygen is approximately 10,000 times slower in water than
in air. When soils are flooded, they quickly lose oxygen, becoming hypoxic (an environment
with O2 concentration below 2 mg/liter) and eventually completely anoxic where anaerobic
bacteria thrive among the roots. Water also influences the intensity and spectral
composition of light as it reflects off the water surface and submerged particles.[181] 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, CO2 and O2) used in respiration and photosynthesis. Salt water plants
(halophytes) have additional specialized adaptations, such as the development of special
organs for shedding salt and osmoregulating their internal salt (NaCl) concentrations, to live
in estuarine, brackish, or oceanic environments. Anaerobic soil microorganisms in aquatic
environments use nitrate, manganese ions, ferric ions, sulfate, carbon dioxide, and
some organic compounds; other microorganisms are facultative anaerobes and use oxygen
during respiration when the soil becomes drier. The activity of soil microorganisms and the
chemistry of the water reduces the oxidation-reduction potentials of the water. Carbon
dioxide, for example, is reduced to methane (CH4) by methanogenic bacteria.[181] The
physiology of fish is also specially adapted to compensate for environmental salt levels
through osmoregulation. Their gills form electrochemical gradients that mediate salt
excretion in salt water and uptake in fresh water.[182]
Gravity[edit]
The shape and energy of the land are significantly affected by gravitational forces. On a
large scale, the distribution of gravitational forces on the earth is uneven and influences the
shape and movement of tectonic plates as well as influencing geomorphic processes such
as orogeny and erosion. These forces govern many of the geophysical properties and
distributions of ecological biomes across the Earth. On the organismal scale, gravitational
forces provide directional cues for plant and fungal growth (gravitropism), orientation cues
for animal migrations, and influence the biomechanics and size of animals.[110] 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.
[183]
 The cardiovascular systems of animals are functionally adapted to overcome the
pressure and gravitational forces that change according to the features of organisms (e.g.,
height, size, shape), their behaviour (e.g., diving, running, flying), and the habitat occupied
(e.g., water, hot deserts, cold tundra).[184]
Pressure[edit]
Climatic and osmotic pressure places physiological constraints on organisms, especially
those that fly and respire at high altitudes, or dive to deep ocean depths.[185] These
constraints influence vertical limits of ecosystems in the biosphere, as organisms are
physiologically sensitive and adapted to atmospheric and osmotic water pressure
differences.[110] For example, oxygen levels decrease with decreasing pressure and are a
limiting factor for life at higher altitudes.[186] Water transportation by plants is another
important ecophysiological process affected by osmotic pressure gradients.[187][188][189] Water
pressure in the depths of oceans requires that organisms adapt to these conditions. For
example, diving animals such as whales, dolphins, and seals are specially adapted to deal
with changes in sound due to water pressure differences.[190] Differences
between hagfish species provide another example of adaptation to deep-sea pressure
through specialized protein adaptations.[191]
Wind and turbulence[edit]
 The architecture of the inflorescence in grasses is subject to the physical pressures of wind and
shaped by the forces of natural selection facilitating wind-pollination (anemophily).[192][193]

Turbulent forces in air and water affect the environment and ecosystem distribution, form,
and dynamics. On a planetary scale, ecosystems are affected by circulation patterns in the
global trade winds. Wind power and the turbulent forces it creates can influence heat,
nutrient, and biochemical profiles of ecosystems.[110] For example, wind running over the
surface of a lake creates turbulence, mixing the water column and influencing the
environmental profile to create thermally layered zones, affecting how fish, algae, and other
parts of the aquatic ecosystem are structured.[194][195] Wind speed and turbulence also
influence evapotranspiration rates and energy budgets in plants and animals.[181][196] Wind
speed, temperature and moisture content can vary as winds travel across different land
features and elevations. For example, the westerlies come into contact with the coastal and
interior mountains of western North America to produce a rain shadow on the leeward side
of the mountain. The air expands and moisture condenses as the winds increase in
elevation; this is called orographic lift and can cause precipitation. This environmental
process produces spatial divisions in biodiversity, as species adapted to wetter conditions
are range-restricted to the coastal mountain valleys and unable to migrate across
the xeric ecosystems (e.g., of the Columbia Basin in western North America) to intermix with
sister lineages that are segregated to the interior mountain systems.[197][198]
Fire[edit]
Main article: Fire ecology

Forest fires modify the land by leaving behind an environmental mosaic that diversifies the landscape
into different seral stages and habitats of varied quality (left). Some species are adapted to forest
fires, such as pine trees that open their cones only after fire exposure (right).
Plants convert carbon dioxide into biomass and emit oxygen into the atmosphere. By
approximately 350 million years ago (the end of the Devonian period), photosynthesis had
brought the concentration of atmospheric oxygen above 17%, which allowed combustion to
occur.[199] Fire releases CO2 and converts fuel into ash and tar. Fire is a significant ecological
parameter that raises many issues pertaining to its control and suppression.[200] While the
issue of fire in relation to ecology and plants has been recognized for a long time,[201] Charles
Cooper brought attention to the issue of forest fires in relation to the ecology of forest fire
suppression and management in the 1960s.[202][203]
Native North Americans were among the first to influence fire regimes by controlling their
spread near their homes or by lighting fires to stimulate the production of herbaceous foods
and basketry materials.[204] 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.[205][206] Most ecosystems are adapted to natural
fire cycles. Plants, for example, are equipped with a variety of adaptations to deal with forest
fires. Some species (e.g., Pinus halepensis) cannot germinate until after their seeds have
lived through a fire or been exposed to certain compounds from smoke. Environmentally
triggered germination of seeds is called serotiny.[207][208] Fire plays a major role in the
persistence and resilience of ecosystems.[174]
Soils[edit]
Main article: Soil ecology
Soil is the living top layer of mineral and organic dirt that covers the surface of the planet. It
is the chief organizing centre of most ecosystem functions, and it is of critical importance in
agricultural science and ecology. The decomposition of dead organic matter (for example,
leaves on the forest floor), results in soils containing minerals and nutrients that feed into
plant production. The whole of the planet's soil ecosystems is called the pedosphere where
a large biomass of the Earth's biodiversity organizes into trophic levels. Invertebrates that
feed and shred larger leaves, for example, create smaller bits for smaller organisms in the
feeding chain. Collectively, these organisms are the detritivores that regulate soil formation.
[209][210]
 Tree roots, fungi, bacteria, worms, ants, beetles, centipedes, spiders, mammals, birds,
reptiles, amphibians, and other less familiar creatures all work to create the trophic web of
life in soil ecosystems. Soils form composite phenotypes where inorganic matter is
enveloped into the physiology of a whole community. As organisms feed and migrate
through soils they physically displace materials, an ecological process called bioturbation.
This aerates soils and stimulates heterotrophic growth and production.
Soil microorganisms are influenced by and are fed back into the trophic dynamics of the
ecosystem. No single axis of causality can be discerned to segregate the biological from
geomorphological systems in soils.[211][212] Paleoecological studies of soils places the origin for
bioturbation to a time before the Cambrian period. Other events, such as the evolution of
trees and the colonization of land in the Devonian period played a significant role in the early
development of ecological trophism in soils.[210][213][214]
Biogeochemistry and climate[edit]
Main article: Biogeochemistry
See also: Nutrient cycle and Climate
Ecologists study and measure nutrient budgets to understand how these materials are
regulated, flow, and recycled through the environment.[110][111][171] This research has led to an
understanding that there is global feedback between ecosystems and the physical
parameters of this planet, including minerals, soil, pH, ions, water, and atmospheric gases.
Six major elements (hydrogen, carbon, nitrogen, oxygen, sulfur, and phosphorus; H, C, N,
O, S, and P) form the constitution of all biological macromolecules and feed into the Earth's
geochemical processes. From the smallest scale of biology, the combined effect of billions
upon billions of ecological processes amplify and ultimately regulate the biogeochemical
cycles of the Earth. Understanding the relations and cycles mediated between these
elements and their ecological pathways has significant bearing toward understanding global
biogeochemistry.[215]
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.[216] There have been major restructurings in these global carbon budgets
during the Earth's history, regulated to a large extent by the ecology of the land. For
example, through the early-mid Eocene volcanic outgassing, the oxidation of methane
stored in wetlands, and seafloor gases increased atmospheric CO2 (carbon dioxide)
concentrations to levels as high as 3500 ppm.[217]
In the Oligocene, from twenty-five to thirty-two million years ago, there was another
significant restructuring of the global carbon cycle as grasses evolved a new mechanism of
photosynthesis, C4 photosynthesis, and expanded their ranges. This new pathway evolved
in response to the drop in atmospheric CO2 concentrations below 550 ppm.[218] The relative
abundance and distribution of biodiversity alters the dynamics between organisms and their
environment such that ecosystems can be both cause and effect in relation to climate
change. Human-driven modifications to the planet's ecosystems (e.g.,
disturbance, biodiversity loss, agriculture) contributes to rising atmospheric greenhouse gas
levels. Transformation of the global carbon cycle in the next century is projected to raise
planetary temperatures, lead to more extreme fluctuations in weather, alter species
distributions, and increase extinction rates. The effect of global warming is already being
registered in melting glaciers, melting mountain ice caps, and rising sea levels.
Consequently, species distributions are changing along waterfronts and in continental areas
where migration patterns and breeding grounds are tracking the prevailing shifts in climate.
Large sections of permafrost are also melting to create a new mosaic of flooded areas
having increased rates of soil decomposition activity that raises methane (CH4) emissions.
There is concern over increases in atmospheric methane in the context of the global carbon
cycle, because methane is a greenhouse gas that is 23 times more effective at absorbing
long-wave radiation than CO2 on a 100-year time scale.[219] Hence, there is a relationship
between global warming, decomposition and respiration in soils and wetlands producing
significant climate feedbacks and globally altered biogeochemical cycles.[105][220][221][222][223][224]

History[edit]
Main article: History of ecology

Early beginnings[edit]
By ecology, we mean the whole science of the relations of the organism to the environment including, in the
broad sense, all the "conditions of existence". Thus, the theory of evolution explains the housekeeping relations of
organisms mechanistically as the necessary consequences of effectual causes; and so forms
the monistic groundwork of ecology.
Ernst Haeckel (1866)[225]: 140  [B]

Ecology has a complex origin, due in large part to its interdisciplinary nature.[226] Ancient
Greek philosophers such as Hippocrates and Aristotle were among the first to record
observations on natural history. However, they viewed life in terms of essentialism, where
species were conceptualized as static unchanging things while varieties were seen as
aberrations of an idealized type. This contrasts against the modern understanding
of ecological theory where varieties are viewed as the real phenomena of interest and
having a role in the origins of adaptations by means of natural selection.[5][227][228] Early
conceptions of ecology, such as a balance and regulation in nature can be traced
to Herodotus (died c. 425 BC), who described one of the earliest accounts of mutualism in
his observation of "natural dentistry". Basking Nile crocodiles, he noted, would open their
mouths to give sandpipers safe access to pluck leeches out, giving nutrition to the sandpiper
and oral hygiene for the crocodile.[226] Aristotle was an early influence on the philosophical
development of ecology. He and his student Theophrastus made extensive observations on
plant and animal migrations, biogeography, physiology, and their behavior, giving an early
analogue to the modern concept of an ecological niche.[229][230]
Nowhere can one see more clearly illustrated what may be called the sensibility of such an organic complex, –
expressed by the fact that whatever affects any species belonging to it, must speedily have its influence of some
sort upon the whole assemblage. He will thus be made to see the impossibility of studying any form completely,
out of relation to the other forms, – the necessity for taking a comprehensive survey of the whole as a condition to
a satisfactory understanding of any part.
Stephen Forbes (1887)[231]

Ernst Haeckel (left) and Eugenius Warming (right), two founders of ecology

Ecological concepts such as food chains, population regulation, and productivity were first
developed in the 1700s, through the published works of microscopist Antoni van
Leeuwenhoek (1632–1723) and botanist Richard Bradley (1688?–1732).
[5]
 Biogeographer Alexander von Humboldt (1769–1859) was an early pioneer in ecological
thinking and was among the first to recognize ecological gradients, where species are
replaced or altered in form along environmental gradients, such as a cline forming along a
rise in elevation. Humboldt drew inspiration from Isaac Newton, as he developed a form of
"terrestrial physics". In Newtonian fashion, he brought a scientific exactitude for
measurement into natural history and even alluded to concepts that are the foundation of a
modern ecological law on species-to-area relationships.[232][233][234] Natural historians, such as
Humboldt, James Hutton, and Jean-Baptiste Lamarck (among others) laid the foundations of
the modern ecological sciences.[235] The term "ecology" (German: Oekologie, Ökologie) was
coined by Ernst Haeckel in his book Generelle Morphologie der Organismen (1866).
[236]
 Haeckel was a zoologist, artist, writer, and later in life a professor of comparative
anatomy.[225][237]
Opinions differ on who was the founder of modern ecological theory. Some mark Haeckel's
definition as the beginning;[238] others say it was Eugenius Warming with the writing
of Oecology of Plants: An Introduction to the Study of Plant Communities (1895),[239] or Carl
Linnaeus' principles on the economy of nature that matured in the early 18th century.[240]
[241]
 Linnaeus founded an early branch of ecology that he called the economy of nature.[240] His
works influenced Charles Darwin, who adopted Linnaeus' phrase on the economy or polity
of nature in The Origin of Species.[225] Linnaeus was the first to frame the balance of
nature as a testable hypothesis. Haeckel, who admired Darwin's work, defined ecology in
reference to the economy of nature, which has led some to question whether ecology and
the economy of nature are synonymous.[241]
 The layout of the first ecological experiment, carried out in a grass garden at Woburn Abbey in
1816, was noted by Charles Darwin in The Origin of Species. The experiment studied the
performance of different mixtures of species planted in different kinds of soils.[242][243]

From Aristotle until Darwin, the natural world was predominantly considered static and
unchanging. Prior to The Origin of Species, there was little appreciation or understanding of
the dynamic and reciprocal relations between organisms, their adaptations, and the
environment.[227] An exception is the 1789 publication Natural History of Selborne by Gilbert
White (1720–1793), considered by some to be one of the earliest texts on ecology.
[244]
 While Charles Darwin is mainly noted for his treatise on evolution,[245] he was one of the
founders of soil ecology,[246] and he made note of the first ecological experiment in The Origin
of Species.[242] Evolutionary theory changed the way that researchers approached the
ecological sciences.[247]

Since 1900[edit]
Modern ecology is a young science that first attracted substantial scientific attention toward
the end of the 19th century (around the same time that evolutionary studies were gaining
scientific interest). The scientist Ellen Swallow Richards adopted the term "oekology" (which
eventually morphed into home economics) in the U.S. as early as 1892.[248]
In the early 20th century, ecology transitioned from a more descriptive form of natural
history to a more analytical form of scientific natural history.[232][235] Frederic
Clements published the first American ecology book in 1905,[249] presenting the idea of plant
communities as a superorganism. This publication launched a debate between ecological
holism and individualism that lasted until the 1970s. Clements' superorganism concept
proposed that ecosystems progress through regular and determined stages of seral
development that are analogous to the developmental stages of an organism. The
Clementsian paradigm was challenged by Henry Gleason,[250] 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.[251]
The Clementsian superorganism theory was an overextended application of an idealistic
form of holism.[36][107] The term "holism" was coined in 1926 by Jan Christiaan Smuts, a South
African general and polarizing historical figure who was inspired by Clements'
superorganism concept.[252][C] Around the same time, Charles Elton pioneered the concept of
food chains in his classical book Animal Ecology.[82] Elton[82] 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.[253] Alfred J. Lotka brought in many
theoretical concepts applying thermodynamic principles to ecology.
In 1942, Raymond Lindeman wrote a landmark paper on the trophic dynamics of ecology,
which was published posthumously after initially being rejected for its theoretical emphasis.
Trophic dynamics became the foundation for much of the work to follow on energy and
material flow through ecosystems. Robert MacArthur advanced mathematical theory,
predictions, and tests in ecology in the 1950s, which inspired a resurgent school of
theoretical mathematical ecologists.[235][254][255] Ecology also has developed through
contributions from other nations, including Russia's Vladimir Vernadsky and his founding of
the biosphere concept in the 1920s[256] and Japan's Kinji Imanishi and his concepts of
harmony in nature and habitat segregation in the 1950s.[257] Scientific recognition of
contributions to ecology from non-English-speaking cultures is hampered by language and
translation barriers.[256]
This whole chain of poisoning, then, seems to rest on a base of minute plants which must have been the original
concentrators. But what of the opposite end of the food chain—the human being who, in probable ignorance of all
this sequence of events, has rigged his fishing tackle, caught a string of fish from the waters of Clear Lake, and
taken them home to fry for his supper?
Rachel Carson (1962)[258]: 48 

Ecology surged in popular and scientific interest during the 1960–1970s environmental

movement. There are strong historical and scientific ties between ecology, environmental
management, and protection.[235] The historical emphasis and poetic naturalistic writings
advocating the protection of wild places by notable ecologists in the history of conservation
biology, such as Aldo Leopold and Arthur Tansley, have been seen as far removed from
urban centres where, it is claimed, the concentration of pollution and environmental
degradation is located.[235][259] Palamar (2008)[259] notes an overshadowing by mainstream
environmentalism of pioneering women in the early 1900s who fought for urban health
ecology (then called euthenics)[248] and brought about changes in environmental legislation.
Women such as Ellen Swallow Richards and Julia Lathrop, among others, were precursors
to the more popularized environmental movements after the 1950s.
In 1962, marine biologist and ecologist Rachel Carson's book Silent Spring helped to
mobilize the environmental movement by alerting the public to toxic pesticides, such
as DDT, bioaccumulating in the environment. Carson used ecological science to link the
release of environmental toxins to human and ecosystem health. Since then, ecologists
have worked to bridge their understanding of the degradation of the planet's ecosystems
with environmental politics, law, restoration, and natural resources management.[22][235][259][260]