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The science of ecology includes everything from global processes (above), the study of various marine and terrestrial habitats (middle) to individual interspecific interactions like predation and pollination (below).

Ecology is the interdisciplinary scientific study of the interactions between organisms and the interactions of these organisms with their environment.


From the from oikos, "house, household, housekeeping, or living relations" ; -λογία, -logia, "study of").


Ecology is often misused as a synonym for environment, but it differs from environmental studies, for example, because it is one of the few academic disciplines dedicated to holism. The environment describes all factors and scales of study that are external to an organism, including abiotic factors such as temperature, radiation, light, chemistry, climate and geology, and biotic factors, including genes, cells, organisms, members of the same species (conspecifics) and other species that share a habitat.

Ecology is concerned with the web or network of relations among organisms at different scales of organization .

Ecology is also distinguished from natural history, which deals primarily with the descriptive study of organisms. Ecologists are concerned with ecosystems (e.g. desert) or they can be conceptually abstract schemes showing the direction and quantified amounts of energy and resources flowing through a system or network of relations.

Like many of the natural sciences, however, a conceptual understanding of ecology is found in the broader details of study, including:
  • life processes explaining adaptations
  • distribution and abundance of organisms
  • the flux of materials and energy through living communities
  • the successional development of ecosystems, and
  • the abundance and distribution of biodiversity in context of the environment.


Ecology has many practical applications in conservation biology, wetland management, natural resource management (agriculture, forestry , fisheries), city planning (urban ecology), community health, economics, basic & applied science and it provides a conceptual framework for understanding and researching human social interaction (human ecology).

Value of ecosystems

Ecosystems also provide a host of goods and services often without market value. Broad examples include:
  • regulating (climate, floods, nutrient balance, water filtration)
  • provisioning (food, medicine, fur)
  • cultural (science, spiritual, ceremonial, recreation, aesthetic)
  • supporting (nutrient cycling, phyosynthesis, soil formation).[1123]

Ecology and the Environment

Origins of Life and Ecology

The environment is external yet interlinked directly with ecology. Chemistry, temperature, pressure, gravity, energy, and sunlight describe the main components of Earth's environment that are importantly relevant to ecological processes. It is easier to learn about environmental and ecological relations into conceptually manageable parts. However, once the effective environmental components are learned they conceptually come together as a holocoenotic[1124] system. The chemistry of planet Earth has three important, independent and autocatalytic processes that work in unison when in living beings. The three chemical reactions include multiplication, variation, and heredity. The only ingredients necessary for creating these kinds of reactions independently are abiotic materials coupled with free energy. These are the environmental components that make the ingredients for the 'primordial soup' theory that proposes that life originated from abiotic autocatalytic chemical reactions in Earth's primitive anoxic atmosphere.

In a series of experiements in the 1950s scientists recreated early conditions of the Earth's environment in a test tube. Using molecules suspected to be present in the primite atmosphere (H2, CH4, and NH3) they recreated a kind of primitive ecosystem containing self generated amino acids, which are organic in nature. These experiments provided useful conceptual information about life processes which demonstrate that the molecular parts and processes of life can be recreated in a lab simulating the chemistry, temperature, pressure, and energetics of Earth's young environment. Henceforth, descendants of the early kinds of autocatalytic molecules replicated the process of life and maintain a metabolic equilibrium between matter and energy. The complex process is now governed by the evolved replicatory molecular machinery of these chemical antecedants, which include enzymes, RNA, and DNA. Since the metabolic processes of life convert energy and matter, ecosystem processes are consistent with the laws of thermodynamics. The classical laws of thermodynamics states first that there is a conservation of energy that can change form, but it cannot be created nor destroyed. The second part to the thermodynamic law is that every system transforms naturally from an organized structured state to a disorganized unstructured state that dissipates in the form of heat in the process. The second transformation describes the entropy of the system that is countered by a sustainable input of energy that comes from the environment and enthalpy, which is the energy stored in existing molecular bonds. Based on these metabolic and thermodynamic principles, a complete accounting of energy flow can be traced through an ecosystem.

Metabolism and the early atmosphere

The Earth's environment has not always remained at a constant temperature and the atmosphere has changed significantly as a result of the gross metabolic activity of life on Earth. There is an evolving feedback loop between the ecological processes of life, geochemistry, and Earth's atmosphere. Proceeding through the early stages of life, major ecological transitions modified the Earth's geochemical cycles. The Earth formed approximately 4.5 billion years ago and environmental conditions were too extreme for life to form for the first 500 million years. During this early Hadean period, the Earth started to cool allowing time for a crust and oceans to form. Environmental conditions were unsuitable for the origins of life until approximately 1 billion years after the Earth formed. The Earth's atmosphere transformed from hydrogen dominant, to one composed mostly of methane, and ammonia. Over the next billion years the metabolic activity of life transformed the atmosphere to higher concentrations 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 in heat. There were untapped sources of free energy within the mixture of reducing and oxidizing gasses that set the stages for primitive ecosystems to evolve and, in turn, the atmosphere also evolved.

One of the earliest organisms was likely an anaerobic methanogen microbe that would have converted atmosopheric hydrogen into methane (4H2 + CO2 → CH4 + 2H2O). Anoxygenic photosynthesis converting hydrogen sulfide into other sulfur compounds or water (2H2S + CO2 → hv → CH2O → H2O → + 2S or 2H2 + CO2 + hv → CH2O + H2O), as occurs in deep sea hydrothermal vents today, would have also reduced hydrogen and increased atmospheric methane. Early forms of fermentation would have also been a component of the primitive ecology producing higher levels of atmospheric methane. The transition to an oxygen dominant atmospheric transition did not begin until approximately 2.4-2.3 billion years ago, but photosynthetic processes had started 0.3 to 1 billion years prior. Hence, the transition to an oxygen environment was ecologically latent. The evolution of the Earth's ecosystems demonstrates how smaller scale metabolic processes of life can regulate larger scale environmental phenomena, such as the Earth's atmosphere. This relationship has led to the development of the Gaia hypothesis, which states that there is a feedback process generated by living organisms that maintains the temperature of the Earth and atmospheric conditions within a narrow self-regulating range of tolerance. Hence, the gross ecology of the planet acts as a single regulatory or holistic unit called Gaia.

Radiation: Light, Heat, and Temperature

Almost all aspects of functional ecology is effected indirectly or directly by radiant energy from the sun. There are different wavelengths of electromagnetic energy emanating from the sun that provides inputs into the ecological energy budget of the planet. 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.

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 also relate importantly and differently affects two metabolic divisions in animals, poikilotherms, having a body temperature that is largely regulated and dependent on the temperature of the external environment, and homeotherms, having a body temperature that is internally regulated and maintained by expending metabolic energy.

Light is the primary source of energy on the planet. 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 are responsible for primary production and the assimilation of light energy that becomes metabolically stored as potentional energy in biochemical enthalpic bonds. Heterotrophs feed on autotrophs for their supply of energy and nutrients. Hence, there is a relationship between light, production, and supplies of energy that affects the distribution, composition and structure of ecosystem dynamics across the planet.

Physical environment: Gravity, Pressure, Turbulence

The shape and energy of the land is affected to a large degree by gravitational forces. On a larger scale, the distribution of gravitational forces on the earth are uneven and influence the shape and movement of tectonic plates as well as having an influence on geomorphic processes such as orogeny and erosion. These forces govern some of the geo-physical properties and distributions of biomes across the Earth. On a organism scale, gravitational forces provide directional cues for plant and fungal growth (gravitropism), orientation cues for animal migrations, and influences the biomechanics and size of animals.

Pressure effects the environment and the organism. It acts as a mechanical force with close connections to gravity causing increased levels of pressure moving toward the Earth. Pressure exerts significant influence over the atmosphere, climate, water environments, and on smaller scale there are osmotic forces at work. Organisms are physiologically sensitive and adapted to atmospheric and osmotic water pressures. Climatic and osmotic pressure places physiological constraints on organisms, such as flight and respiration at high altitudes, or diving to deep ocean depths. These constraints influence vertical limits of ecosystems in the biosphere.

Turbulent forces in air and water have significant effects on the environment and ecosystem distribution, form and dynamics. On a planetary scale, ecosystems are affected by circulation patterns in the global trade winds. Locally, wind power and the turbulent forces it creates can influence heat, nutrient, and biochemical profiles of ecosystems. For example, winds running over the surface of lakes winds creates turbulence that stirs the water column and influences the environmental profile to create thermaly layered zones that partially governs how the fish, algae, and other parts of the aquatic ecology are structured.


Minerals, soil, pH, ions, water and atmospheric gases are all considered significant environmental factors in their own right, but like all the other parameters described in this section, they work in combination to influence ecosystem structure and process. The substrate or medium where an organism lives and the nutrient pool of a community are important factors that are governed by the biogeochemical cycles of the Earth. Some of the more prominent cycles include the hydrological, nitrogen, carbon, sulfur, and phosphorus. Ecologists study and measure nutrient budgets to understand the flow of such materials through the environment and ecosystem processes. One of the more important issues of recent times is the global carbon cycle and climate change. The focus has largely been attuned carbon because 60% of human induced climate change is attributed toward CO2 emissions. Ecologists study the source and sink dynamics of carbon flow through ecosystems and the environment on a global to local scale. For example, the ocean is estimated to hold 40,000 Gt carbon, vegetation and soil is estimated to hold 2070 Gt carbon, and fossil fuel emissions are estimated to regulate an annual flux of 6.3 Gt carbon. Hence, the carbon cycle in relation to global climate change is an ecological issue.

Niche construction and ecosystem engineers

Organisms are subject to environmental pressures, but they are also modifiers of the environment. The regulatory feedback relationship between organisms and their environment can significantly modify conditions from a local scale to global scale (e.g., Gaia) and they can also modify conditions over time even after an organism has passed away, such as the remnants of an old beaver dam or silica skeleton deposits from marine organisms. 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."

The process of ecosystem engineeering has also been called niche construction. Although it had long been understood that organisms modify their environment, the ecological engineering concept stimulated a new appreciation for the degree of modification and the influence this has on the ecosystem and evolutionary process. The niche construction model highlights a previously under appreciated feedback mechanism of natural selection imparting forces on the abiotic niche.

For example, many ant and termite species regulate temperature by plugging nest entrances at night or in the cold, by adjusting the height or shape of their mounds to optimize the intake of the sun’s rays, or by carrying their brood around their nest to the place with the optimal temperature and humidity for the brood’s development.

Ecology and evolution

Ecology is closely allied with its sister discipline evolution. The two disciplines often appear together, such as in the title of the popular journal in the field, Trends in Ecology and Evolution. Ecology and evolution are scientifically connected because they both study hierarchies, networks, relations, and kinship among genes, cells, individuals, communities, species, and the biosphere. There is no sharp dichotomous boundary that separates the two disciplines and they differ more in their areas of applied focus than in their shared scientific philosophies on nature. Both disciplines find and explain unique properties and processes operating in different ways according the spatial or temporal scales being considered. Ecological theory is not necessarily invoked in evolutionary research, such as what role it played in the major transitions in the history of life. Evolution is concerned primarily with the nature of change through the guiding principals of natural selection, inheritance, and differential survival. While the boundary between ecology and evolution is not always clear, it is understood that ecology studies the abiotic and biotic factors that influence the evolutionary process.

Ecology and evolution can be studied at a wide range of levels, from large to small scale. Levels of ecological organization, as well as an example of a question ecologists would ask at each level, include (from the broadest to the most specific):
  • Biosphere: "What role does concentration of atmospheric carbon dioxide play in the growth of forests?"
  • Region: "How has geological history influenced regional diversity within certain groups of organisms?"
  • Landscape: "How do vegetated corridors affect the rate of movement by mammals among isolated fragments?"
  • Ecosystem: "How does fire affect nutrient availability in grassland ecosystems?"
  • Community: "How does disturbance influence the number of mammal species in African grasslands?"
  • Interactions: "What evolutionary benefit do zebras gain by allowing birds to remove parasites?"
  • Population: "What factors control zebra populations?"
  • Individual organism: "How do zebras regulate internal water balance?"
  • Gene: "How does genetic diversity relate to the different breeding behaviours of different species?"

Natural selection, life history, development, adaptation, populations, and inheritance all play an prominent conceptual roles in ecological as well as evolutionary theory. The fundamental scale of study for both disciplines is the gene. Connections between ecology and genetics became more prominent after the publication Molecular Ecology starting in 1992. Molecular ecology uses various analytical techniques to study genes in and among organisms. However, there has long been an understanding of the relationship between ecology and the inheritance of acquired characters through genetic inheritance.

New technologies associated with molecular ecology has engendered a new and collaborative research paradigm that investigates and probes questions about life that were otherwise intractable. Genetics and ecology have became partners in testing and solving ecological problems and theory. Genes provide a sampling of organisms. Through genetics, ecologists study previously obscured sexual behaviour in animals, such as secret mate preferences of female pocket gophers or multiple male partners in the socially monogomous tree swallows.

Ecosystem communities: Food webs and Trophic dynamics

Food webs

A schematic illustration of a salamander food-web in a pond.

Early naturalists in the 16th-18th century realized the importance of food and feeding as an agent of transfer for energy and nutrients among species. They also understood that the supply ultimately depended upon plants converting energy from the sun into organic matter that gets eaten by herbivores. The first person to fully elaborate and place the concept of food chains into a scientific framework was Charles Elton in his classical book 'Animal Ecology'. Elton defined ecological relations through food-chains, food-cycles, food-size, and described numerical relations among different functional groups and their relative abundance. Elton's term 'food-cycle' was replaced by 'food-web' in a subsequent ecological text and has been used ever since. Elton's book, however, broke conceptual ground on ecological perspectives and food-web diagrams became more popular as a result. Today, food-webs are found in every introductory text book on ecology and it is also popular in the mainstream reference to ecology. Food-webs are an effective way to conceptually illustrate the interactive links among species in a community.

Food-webs summarize the complexity of communities. There are different dimensions that describe a community food-web, including species composition (type of species), richness (number of species), biomass & productivity, and stability. The food-web diagram illustrates species composition and shows how a change in a single species can influence many others. Microcosm studies simplify food-web research into semi-isolated units such as small springs, decaying logs and cowpats. Principals gleaned from food-web microcosm studies are used to extrapolate smaller dynamic concepts to larger systems. Food-chain length is an important parameter in describing larger food-web dynamics. The food-chain length is defined as: "The number of transfers of energy or nutrients from the base to the top of a food web..."

There are different ways of calculating food-chain length depending on what parameters of the food-web dynamic are being considered: connectance, energy, or interaction. Hence, in a simple example of connectance a deer is one step removed from the plants it eats (chain length = 1) and a wolf that eats the dear is two steps removed (chain length = 2). The relative amount or strength of influence that these parameters have on the food-web are used to address questions about:
  • the identity or existence of a few dominant species (called strong interactors or keystone species)
  • the total number of species and food-chain length (including many weak interactors) and
  • how community structure, function and stability is determined.

Trophic dynamics

List of ecological functional groups, definitions and examples
Functional Group Definition and Examples
Producers or Autotrophs Usually plants or cyanobacteria that are capable of photosynthesis but could be other organisms such as the bacteria near ocean vents that are capable of chemosynthesis.
Consumers or Heterotrophs Animals, which can be primary consumers (herbivorous), or secondary or tertiary consumers (carnivorous and omnivores).
Decomposers or Detritivores Bacteria, fungi, and insects which degrade organic matter of all types and restore nutrients to the environment. The producers will then consume the nutrients, completing the cycle.

Links in food-webs relate of primary importance to feeding relations or trophism (The Greek root of the word τροφή, trophē, means food or feeding). Elton noted how important and influence the feeding relations had on ecosystem structure. He proposed that ecosystems naturally sort into a ‘pyramid of numbers’ when the relative abundance of each functional group is stacked into their respective trophic levels. Functional groups are broadly categorized as autotrophs (e.g., plants), heterotrophs (e.g., deer, wolves), and detrivores (e.g., bacteria, fungi). It is not always entirely clear what creatures belong in what group. Some organisms are omnivores, meaning they eat both plant and animal tissues and don't fit neatly into a category. However, it has been suggested that omnivores have a greater functional ecosystem influence as predators because relative to herbivores they are comparatively inefficient at grazing. Every species fits into an ecosystem in a particular way that is called its ecological niche. The ecological niche of a species describes their unique habitat requirements and influence or roles within the ecosystem.

Trophic levels stem from the systems view of ecology that groups smaller components of the system to formulate a macroscopic image of the larger functional design. Trophic levels are abstractions of the system, but they explain real phenomena. For example, it is understood that functional trophic groups sort out hierarchically into trophic levels because it requires specialized adaptations to become a photosynthesizer or a predator, but rarely a skillful combination of both. These functional adaptations to trophism (feeding) solidifies emergent ecosystem principals of functional groups. Moreover, species become so specialized at their respective ecological niche that they competitively exclude other competitor species from living in the same geographic area if they strongly share the same ecological niche. This is called the competitive exclusion principle.

A trophic pyramid

Functional groups are usually depicted in hierarchical schemes with three or more trophic levels including primary producers (autotrophs) and levels of heterotrophic consumers including the herbivores (primary consumers), predators (secondary consumers), predators that eat predators (tertiary consumers), and ultimately ending at the detrivores in the soil ecosystems. The pyramidal arrangement of trophic levels is a consistent feature across ecosystems with the primary producers having the larger base and consumer densities and amounts of energy decreasing as species become further removed from the photosynthetic source of production. The size of each level in the pyramid generally represents biomass, which is often measured as the dry weight of an organism. Trophic levels and food webs can be used to depict and calculate mathematical and statistical parameters such as those used in other kinds of network analysis, including graph theory.

Food-web links point to direct trophic relationships among species, but there are also indirect effects that can alter the abundance, distribution, or biomass in the trophic levels. For example, predators eating herbivores indirectly influence the control and regulation of primary production in plants. Although the predators do not eat the plants directly, they regulate the population of herbibores that are directly linked to plant trophism. The net effect of direct and indirect relations is called trophic cascades. Trophic cascades are separated into species-level cascades, where only a subset of the food-web dynamic is impacted by a change in population numbers, and community-level cascades, where a change in population numbers has a dramatic effect on the entire food-web, such as the distribution of plant biomass. The keystone species concept is closely aligned to species-level cascades, where a single species occupies a particularly strong node in the food-web and its removal results in the collapse of the food-web structure and extinction of other species. Sea otters (Enhydra lutris) are the classical example of a keystone species because they limit the density of 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. Hunting of sea otters, for example, is thought to have lead to the extinction of the Steller's Sea Cow (Hydrodamalis gigas). While the keystone species concept has been used extensively as a conservation tool, it has been criticized for being poorly defined. Different ecosystems express different complexities and so it is unclear how applicable and general the keystone species model can be applied. To better understand the keystone species and trophic cascade models, ecologists conduct removal experiments to measure the relative impact, strength and influence of interaction among different species on community dynamics.

Behavioral Ecology

Adaptation is the central unifying concept in behavioral ecology.[1125] Behaviors can be recorded as traits and inherited in much the same way that eye and hair color can. As such, behaviors are subject to the forces of natural selection. Hence, behaviors can be adaptive in nature, meaning that they evolved and serve a functional utility such as enhancing ones opportunity to successfully reproduce and increase fitness. Fitness is measured in terms of reproductive success. An animal with behaviors that afford it some degree of leverage in the struggle for existence such that it survives to pass on its heritable traits to its offspring is considered fit if the adaptation succeeds and propagates more of its kind in subsequent generations. A measure of fitness is the numerical differential and representation in frequency of a trait over subsequent generations.


Ecology is usually considered as a branch of biology, the general science that studies living organisms. It is associated with the highest levels of biological organization, including the individual organism, the population, the ecological community, the ecosystem and the biosphere as a whole. When referring to the study of a single species, a distinction is often made between its "ecology" and its "biology". For example, "polar bear biology" might include the study of the polar bear's physiology, morphology, pathology and ontogeny, whereas "polar bear ecology" would include a study of its prey species, its population and metapopulation status, distribution, dependence on environmental conditions, etc.

Because of its focus on the interrelations between organisms and their environment, ecology is a multidisciplinary science that draws on many other branches, including geology and geography, meteorology, soil science, genetics, chemistry, physics, mathematics and statistics. Due to its breadth of scope, ecology is considered by some to be a holistic science, one that over-arches older disciplines such as biology which in this view become sub-disciplines contributing to ecological knowledge. It has been argued that the mechanistic models which have driven the development of most other sciences are inappropriate for unraveling the complex interactions in most ecosystems, and that progress in ecology is better served by a central paradigm driven by information theory and complexity theory.

Ecology is also a highly applied science, especially with respect to issues of natural resource management. Efforts related to wildlife conservation, habitat management, mitigation of ecological impacts of environmental pollution, ecosystem restoration, species reintroductions, fisheries, forestry and game management are often the direct domain of applied ecology. Urban development, agricultural and public health issues are also often informed by ecological perspectives and analysis.


Ecology is a broad discipline comprising many sub-disciplines. A common, broad classification, moving from lowest to highest complexity, where complexity is defined as the number of entities and processes in the system under study, is:
  • Ecophysiology examines how the physiological functions of organisms influence the way they interact with the environment, both biotic and abiotic.
  • Ecomechanics uses physics and engineering principles to examine the interaction of organisms with their environment and with other species.
  • Behavioral ecology examines the roles of behavior in enabling an animal to adapt to its environment.
  • Population ecology studies the dynamics of populations of a single species.
  • Community ecology (or synecology) focuses on the interactions between species within an ecological community.
  • Ecosystem ecology studies the flows of energy and matter through the biotic and abiotic components of ecosystems.
  • Systems ecology is an interdisciplinary field focusing on the study, development, and organization of ecological systems from a holistic perspective.
  • Landscape ecology examines processes and relationship in a spatially explicit manner, often across multiple ecosystems or very large geographic areas.
  • Evolutionary ecology studies ecology in a way that explicitly considers the evolutionary histories of species and their interactions.
  • Political ecology connects politics and economy to problems of environmental control and ecological change.

Ecology can also be sub-divided according to the species of interest into fields such as animal ecology, plant ecology, insect ecology, and so on. Another frequent method of subdivision is by biome studied, e.g., Arctic ecology (or polar ecology), tropical ecology, desert ecology, marine ecology, etc. The primary technique used for investigation is often used to subdivide the discipline into groups such as chemical ecology, molecular ecology, field ecology, quantitative ecology, theoretical ecology, and so forth.

Subdivisions of ecology are not mutually exclusive; indeed, very few exist in isolation. Many of them overlap, complement and inform each other. For example, the population ecology of an organism is a consequence of its behavioral ecology and intimately tied to its community ecology. Methods from molecular ecology might inform the study of the population, and all kinds of data are modeled and analyzed using quantitative ecology techniques, often motivated by basic results in theoretical ecology.

Levels of organization

One of the unique and complex aspects to ecology is that there are emergent phenomena operating at different environmental scales of influence, ranging from molecular to galactic spheres. These scaled phenomena require different sets of scientific explanation, which is otherwise captured in the expression 'the sum is greater than the parts'. An understanding of these emergent phenomena operating at different scales requires a conceptual distinction between ecology and the environment. While the environment of an organism includes everything in the universe that is external to it, only some levels are more evidently of direct importance and this is called the effective environment.

Ecosystems are most commonly studied at the local or effective community scale, such as measurements of primary production in a wetland or the analysis of predator-prey dynamics affecting amphibian biomass. Ecological relations also regulate the flux of energy, nutrients, and climate all the way up to the planetary scale. The global sum of ecosystems is known as the biosphere where ecological theory has been used to explain self emergent regulatory phenomena at the planetary scale. This is known as the Gaia hypothesis. The Gaia hypothesis is an example of holism in ecology because it tests for principals relating to an evolving and self regulating planetary ecosystem that requires different explanations than those governing ecosystems at a smaller scale, such as a single wetland.


For modern ecologists, ecology can be studied at several levels, as defined in the biological organisation of life: population level (individuals of the same species in the same or similar environment), biocoenosis level (or community of species), ecosystem level, and biosphere level.

The outer layer of the planet Earth can be divided into several compartments: the hydrosphere (or sphere of water), the lithosphere (or sphere of soils and rocks), and the atmosphere (or sphere of the air). The biosphere (or sphere of life), sometimes described as "the fourth envelope", is all living matter on the planet or that portion of the planet occupied by life. It reaches well into the other three spheres, although there are no permanent inhabitants of the atmosphere. Relative to the volume of the Earth, the biosphere is only the very thin surface layer that extends from 11,000 meters below sea level to 15,000 meters above.

It is thought that life first developed in the hydrosphere, at shallow depths, in the photic zone. (Recently, though, a competing theory has emerged, that life originated around hydrothermal vents in the deeper ocean. See Origin of life.) Multicellular organisms then appeared and colonized benthic zones. Photosynthetic organisms gradually produced the chemically unstable oxygen-rich atmosphere that characterizes our planet. Terrestrial life developed later, protected from UV rays by the ozone layer. Diversification of terrestrial species is thought to be increased by the continents drifting apart, or alternately, colliding. Biodiversity is expressed at the ecological level (ecosystem), population level (intraspecific diversity), species level (specific diversity), and genetic level.

The biosphere contains great quantities of elements such as carbon, nitrogen, hydrogen, and oxygen. Other elements, such as phosphorus, calcium, and potassium, are also essential to life, yet are present in smaller amounts. At the ecosystem and biosphere levels, there is a continual recycling of all these elements, which alternate between the mineral and organic states.

Although there is a slight input of geothermal energy, the bulk of the functioning of the ecosystem is based on the input of solar energy. Plants and photosynthetic microorganisms convert light into chemical energy by the process of photosynthesis, which creates glucose (a simple sugar) and releases free oxygen. Glucose thus becomes the secondary energy source that drives the ecosystem. Some of this glucose is used directly by other organisms for energy. Other sugar molecules can be converted to molecules such as amino acids. Plants use some of this sugar, concentrated in nectar, to entice pollinators to aid them in reproduction.

Cellular respiration is the process by which organisms (like mammals) break the glucose back down into its constituents, water and carbon dioxide, thus regaining the stored energy the sun originally gave to the plants. The proportion of photosynthetic activity of plants and other photosynthesizers to the respiration of other organisms determines the specific composition of the Earth's atmosphere, particularly its oxygen level. Global air currents mix the atmosphere and maintain nearly the same balance of elements in areas of intense biological activity and areas of slight biological activity.

Water is also exchanged between the hydrosphere, lithosphere, atmosphere, and biosphere in regular cycles. The oceans are large tanks that store water, ensure thermal and climatic stability, and facilitate the transport of chemical elements thanks to large oceanic currents.

For a better understanding of how the biosphere works, and various dysfunctions related to human activity, American scientists attempted to simulate the biosphere in a small-scale model, called Biosphere 2marker.


A central principle of ecology is that each living organism has an ongoing and continual relationship with every other element that makes up its environment. The sum total of interacting living organisms (the biocoenosis) and their non-living environment (the biotope) in an area is termed an ecosystem. Studies of ecosystems usually focus on the movement of energy and matter through the system.

Almost all ecosystems run on energy captured from the sun by primary producers via photosynthesis. This energy then flows through the food chains to primary consumers (herbivores who eat and digest the plants), and on to secondary and tertiary consumers (either carnivores or omnivores). Energy is lost to living organisms when it is used by the organisms to do work, or is lost as waste heat.

Matter is incorporated into living organisms by the primary producers. Photosynthetic plants fix carbon from carbon dioxide and nitrogen from atmospheric nitrogen or nitrates present in the soil to produce amino acids. Much of the carbon and nitrogen contained in ecosystems is created by such plants, and is then consumed by secondary and tertiary consumers and incorporated into themselves. Nutrients are usually returned to the ecosystem via decomposition. The entire movement of chemicals in an ecosystem is termed a biogeochemical cycle, and includes the carbon and nitrogen cycle.

Ecosystems of any size can be studied; for example, a rock and the plant life growing on it might be considered an ecosystem. This rock might be within a plain, with many such rocks, small grass, and grazing animals—also an ecosystem. This plain might be in the tundra, which is also an ecosystem (although once they are of this size, they are generally termed ecozones or biomes). In fact, the entire terrestrial surface of the earth, all the matter which composes it, the air that is directly above it, and all the living organisms living within it can be considered as one, large ecosystem.

Ecosystems can be roughly divided into terrestrial ecosystems (including forest ecosystems, steppes, savannas, and so on), freshwater ecosystems (lakes, ponds and rivers), and marine ecosystems, depending on the dominant biotope.

Dynamics and stability

Ecological factors that affect dynamic change in a population or species in a given ecology or environment are usually divided into two groups: abiotic and biotic.

Abiotic factors are geological, geographical, hydrological, and climatological parameters. A biotope is an environmentally uniform region characterized by a particular set of abiotic ecological factors. Specific abiotic factors include:
  • Water, which is at the same time an essential element to life and a milieu
  • Air, which provides oxygen, nitrogen, and carbon dioxide to living species and allows the dissemination of pollen and spores
  • Soil, at the same time a source of nutriment and physical support
    • Soil pH, salinity, nitrogen and phosphorus content, ability to retain water, and density are all influential
  • Temperature, which should not exceed certain extremes, even if tolerance to heat is significant for some species
  • Light, which provides energy to the ecosystem through photosynthesis
  • Natural disasters can also be considered abiotic

Biocenose, or community, is a group of populations of plants, animals, microorganisms. Each population is the result of procreations between individuals of the same species and cohabitation in a given place and for a given time. When a population consists of an insufficient number of individuals, that population is threatened with extinction; the extinction of a species can approach when all biocenoses composed of individuals of the species are in decline. In small populations, consanguinity can result in reduced genetic diversity, which can further weaken the biocenose.

Biotic ecological factors also influence biocenose viability; these factors are considered as either intraspecific or interspecific relations.

Intraspecific relations are those that are established between individuals of the same species, forming a population. They are relations of cooperation or competition, with division of the territory, and sometimes organization in hierarchical societies.

Interspecific relationsinteractions between different species—are numerous, and usually described according to their beneficial, detrimental, or neutral effect (for example, mutualism (relation ++) or competition (relation --). The most significant relation is the relation of predation (to eat or to be eaten), which leads to the essential concepts in ecology of food chains (for example, the grass is consumed by the herbivore, itself consumed by a carnivore, itself consumed by a carnivore of larger size). A high predator to prey ratio can have a negative influence on both the predator and prey biocenoses in that low availability of food and high death rate prior to sexual maturity can decrease (or prevent the increase of) populations of each, respectively. Selective hunting of species by humans that leads to population decline is one example of a high predator to prey ratio in action. Other interspecific relations include parasitism, infectious disease, and competition for limited resources, which can occur when two species share the same ecological niche.

The existing interactions between the various living beings go along with a permanent mixing of mineral and organic substances, absorbed by organisms for their growth, their maintenance, and their reproduction, to be finally rejected as waste. These permanent recycling of the elements (in particular carbon, oxygen, and nitrogen) as well as the water are called biogeochemical cycles. They guarantee a durable stability of the biosphere (at least when unchecked human influence and extreme weather or geological phenomena are left aside). This self-regulation, supported by negative feedback controls, ensures the perenniality of the ecosystems. It is shown by the very stable concentrations of most elements of each compartment. This is referred to as homeostasis. The ecosystem also tends to evolve to a state of ideal balance, called the climax, which is reached after a succession of events (for example a pond can become a peat bog).

Spatial relationships and subdivisions of land

Ecosystems are not isolated from each other, but are interrelated. For example, water may circulate between ecosystems by means of a river or ocean current. Water itself, as a liquid medium, even defines ecosystems. Some species, such as salmon or freshwater eels, move between marine systems and fresh-water systems. These relationships between the ecosystems lead to the concept of a biome.

A biome is a homogeneous ecological formation that exists over a large region, such as tundra or steppes. The biosphere comprises all of the Earth's biomes—the entirety of places where life is possible—from the highest mountains to the depths of the oceans.

Biomes correspond rather well to subdivisions distributed along the latitudes, from the equator towards the pole, with differences based on the physical environment (for example, oceans or mountain ranges) and the climate. Their variation is generally related to the distribution of species according to their ability to tolerate temperature, dryness, or both. For example, one may find photosynthetic algae only in the photic part of the ocean (where light penetrates), whereas conifers are mostly found in mountains.

Though this is a simplification of a more complicated scheme, latitude and altitude approximate a good representation of the distribution of biodiversity within the biosphere. Very generally, the richness of biodiversity (as well for animal as for plant species) is decreasing most rapidly near the equator and less rapidly as one approach the poles.

The biosphere may also be divided into ecozones, which are very well defined today and primarily follow the continental borders. The ecozones are themselves divided into ecoregions, though there is not agreement on their limits.

Ecological crisis

Generally, an ecological crisis occurs with the loss of adaptive capacity when the resilience of an environment or of a species or a population evolves in a way unfavourable to coping with perturbation that interfere with that ecosystem, landscape or species survival (Note: The concept of resilience is not universally accepted in ecology, and moreso represents a contingent within the field that take a holist view of the environment. There are also many ecologists that take a reductionistic perspective and that believe that the environment, at base, is indeterministic). It may be that the environment quality degrades compared to the species needs, after a change in an abiotic ecological factor (for example, an increase of temperature, less significant rainfalls) . It may be that the environment becomes unfavourable for the survival of a species (or a population) due to an increased pressure of predation (for example overfishing). Lastly, it may be that the situation becomes unfavourable to the quality of life of the species (or the population) due to a rise in the number of individuals (overpopulation).

Ecological crises vary in length and severity, occurring within a few months or taking as long as a few million years. They can also be of natural or anthropic origin. They may relate to one unique species or to many species, as in an Extinction event. Lastly, an ecological crisis may be local (as an oil spill) or global (a rise in the sea level due to global warming).

According to its degree of endemism, a local crisis will have more or less significant consequences, from the death of many individuals to the total extinction of a species. Whatever its origin, disappearance of one or several species often will involve a rupture in the food chain, further impacting the survival of other species.

In the case of a global crisis, the consequences can be much more significant; some extinction events showed the disappearance of more than 90% of existing species at that time. However, it should be noted that the disappearance of certain species, such as the dinosaurs, by freeing an ecological niche, allowed the development and the diversification of the mammals. An ecological crisis thus paradoxically favoured biodiversity.

Sometimes, an ecological crisis can be a specific and reversible phenomenon at the ecosystem scale. But more generally, the crises impact will last. Indeed, it rather is a connected series of events, that occur till a final point. From this stage, no return to the previous stable state is possible, and a new stable state will be set up gradually (see homeorhesy).

Lastly, if an ecological crisis can cause extinction, it can also more simply reduce the quality of life of the remaining individuals. Thus, even if the diversity of the human population is sometimes considered threatened (see in particular indigenous people), few people envision human disappearance at short span. However, epidemic diseases, famines, impact on health of reduction of air quality, food crises, reduction of living space, accumulation of toxic or non degradable wastes, threats on keystone species (great apes, panda, whales) are also factors influencing the well-being of people.

Due to the increases in technology and a rapidly increasing population, humans have more influence on their own environment than any other ecosystem engineer.

Historical roots of ecology

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

Ecology as a scientific discipline is relatively young, reaching prominence mostly in the second half of the 20th century. However, systematic ecological studies can trace roots to ancient times, with Aristotle and Theophrastus, for example, making early observations on animal migrations and plant biogeography respectively. Several notable 19th century scientists such as Alexander Humboldt (1769 – 1859), Charles Darwin (1809 – 1882), Alfred Russel Wallace (1823 – 1913) and Karl Möbius (1825 – 1908) made many important contributions, from laying down the foundation of biogeography to identifying an interacting groups of organisms as a functionally connected community (biocoenosis).

The term "ecology" itself ( ) was first coined by the German biologist Ernst Haeckel in 1866, who defined it as "the comprehensive science of the relationship of the organism to the environment." The first significant textbook on the subject (together with the first university course) was written by the Danishmarker botanist, Eugenius Warming. For this early work, Warming is sometimes identified as the founder of ecology.

See also



  1. Millennium Ecosystem Assessment, 2005. Ecosystems and Human Well-being: Biodiversity Synthesis. World Resources Institute, Washington, DC.
  2. Piglucci, M. (2007). Do we need an extended evolutionary synthesis? Evolution 61(12): 2743–2749
  3. Ecology: Concepts & Applications. Fourth Edition Manuel C. Molles Jr. U of New Mexico. 2008 McGraw Hill Publishing. ISBN 978-0-07-305082-9
  4. [1]
  5. Patton, J. L., and Smith, M. F. (1993). Molecular evidence for mating asymmetry and female choice in a pocket gopher (Thomomys) hybrid zone. Molecular Ecology, 2, 3-8.
  6. R. Ulanowicz, Ecology: The Ascendent Perspective, Columbia (1997)
  7. Goodland, R.J. (1975) The tropical origin of ecology: Eugen Warming’s jubilee. Oikos 26, 240-245.


  • Brinson, M. M., Lugo, A. E. and Brown, S. (1984). Primary Productivity, Decomposition and Consumer Activity in Freshwater Wetlands. Annual Review of Ecology and Systematics, 12, 123-161.
  • David, R. D. and Welsh, H. H. (2004). On the ecological role of salamanders. Annual Review of Ecology and Systematics, 35, 405-434
  • Gould, S. J. and Lloyd, E. A. (1999). Individuality and adaptation across levels of selection: How shall we name and generalize the unit of Darwinism? Proceedings of the National Academy of Science, 96(21), 11904-11909.[1126]
  • Haeckel, E. (1866) General Morphology of Organisms; General Outlines of the Science of Organic Forms based on Mechanical Principles through the Theory of Descent as reformed by Charles Darwin. Berlin
  • Lovelock, J. (2003). Gaia: The living earth. Nature, 426, 769-770 [1127]
  • Odum, E. P. (1971) General Principles of Ecology, Third Edition W. B. Suanders Company. pp 17–20
  • Odum, E. P. (1977) The emergence of ecology as a new integrative discipline. Science, 195, 1289-1293.
  • Warming, E. (1909) Oecology of Plants - an introduction to the study of plant-communities. Clarendon Press, Oxford.
  • Whiles, M. R., Lips, K. R., Pringle, C. M., Kilham, S. S., Bixby, R. J., Brenes, R., Connelly, S., Colon-Gaud, J. C., Hunte-Browjn, M., Huryn, A. D., Montgomery, C., and Peterson, S. 2006. The effects of amphibian population declines on the structure and function of Neotropical stream ecosystems. Front Ecol Environ, 4(1), 27–34

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