Ecology (from Greek: οἶκος, "house" ; -λογία, "study of") is the interdisciplinary scientific study of the interactions between organisms and their environment.^[1] Ecology is also the study of ecosystems. Ecosystems describe the web or network of relations among organisms at different scales of organization. Since ecology refers to any form of biodiversity, ecologists can conduct research on the smallest bacteria to the the global flux of atmospheric gases that are regulated by photosynthesis and respiration as organisms breath in and out of the biosphere. Ecology is a recent discipline that emerged from the natural sciences in the late 19th century. Ecology is not synonymous with environment, environmentalism, or environmental science.^[1][2][3]

Like many of the natural sciences, a conceptual understanding of ecology is found in the broader details of study, including:

• life processes explaining adaptations
• distribution and abundance of organisms
• the movement of materials and energy through living communities
• the successional development of ecosystems, and
• the abundance and distribution of biodiversity in context of the environment.^[1][2][3]

Ecology is distinguished from natural history, which deals primarily with the descriptive study of organisms. It is a sub-discpline of biology, which is the study of life.

There are many practical applications of ecology 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).^[4][5][6][7]

[edit] Levels of organization and study

Ecology is challenged by a constant analytical problem of how to deal with different scales of pattern in space and time. Ecological processes can take decades and even hundreds of years to mature and cover broad geographic areas. Ecologists study ecosystems by sampling a certain number of individuals that are representative of a population. Long-term ecological studies, such as sites managed by the Long Term Ecological Network [1] including the Hubbard Brook study in operation since 1960 [2], provide important ecological track records; the longest experiment in existence is the 'Park Grass Exeriment' that starte din 1856^[8]. Most studies, however, cover only a fraction of the life-span in the development of an ecosystem, such as the different seral stages leading up to an old-growth forest. Ecology is also complicated by the fact that small scale patterns do not necessarily explain large scale phenomena, otherwise captured in the expression 'the sum is greater than the parts'.^[9] These emergent phenomena operate at different environmental scales of influence, ranging from molecular to galactic spheres, and require different sets of scientific explanation.^[10][11]

To simplify and place the study of ecology into a manageable framework of understanding, the biological world is conceptually organized as a nested hierarchy of individuality, ranging in scale from genes, to cells, to tissues, to organs, to organisms, to species and up to the level of the biosphere.^[12] Ecosystems are primarily researched at (but not restricted to) three key levels of organization, including (1) organisms, (2) populations, and (3) communities. Ecosystems consist of communities containing different species of organisms. Communities consist of organisms living in different populations. ^[13][14] Ecosystem diversity is a part of biodiversity. Biodiversity includes all the varieties and processes of life, including organisms and their genetic differences that are evolutionarily classified into hierarchical, branching and coalescing dimensions.^[15][16][17]

[edit] Ecological niche

The ecological niche is a central concept in ecology. There are many definitions of the niche dating back to 1917^[18], but George Evelyn Hutchinson made conceptual advances on the concept in 1957^[19][20] and introduced the most widely accepted definition:

"The niche is the set of biotic and abiotic conditions in which a species is able to persist and maintain stable population sizes."^[18]:519

There are two differentiated kinds of ecological niche known as the fundamental and the realized niche. The fundamental niche describes the abiotic conditions under which a species is able to persist. The realized niche is the set of conditions under which a species persists in the context of other resource competitors or predators.^[20][18][13] Organisms fit into a particular ecological niche according to their functional traits. A trait is a measurable property of an individual that strongly influences its performance.^[21] Moreover, species become specialized within their niche and competitively exclude other species from living in the same geographic area if they fit into the same ecological niche. This is called the competitive exclusion principle.^[22] Equally important to the concept of niche is habitat. The habitat describes the environment over which a species is known to occur and the type of community that is formed as a result.^[23] For example, habitat might refer to an aquatic versus terrestrial environment that can be further categorized as montane or alpine.

Some of the biodiversity of a coral reef

Organisms are subject to environmental pressures, but they are also modifiers of their habitats. The regulatory feedback relationship between organisms and their environment can significantly modify conditions from a local scale (e.g., a pond) 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.^[24] This process of ecosystem engineering has also been called niche construction. 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." ^[25]:373

Although it has long been understood that organisms modify their environment, the ecological engineering concept has stimulated a new appreciation for the degree of modification and the influence organisms have on the ecosystem and evolutionary process.^[26][27] The niche construction concept 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."^[27]:10242

[edit] Population ecology

The first journal publication of the Society of Population Ecology, titled Population Ecology (originally called Researches on Population Ecology), was released in 1952.[3] Population ecology is concerned with the study of groups of organisms that live together in time and space. One of the first laws of population ecology is the Thomas Malthus' exponential law of population growth.^[28] This law states that:

"...a population will grow (or decline) exponentially as long as the environment experienced by all individuals in the population remains constant."^[28]:18

This simplified premise in population ecology provides the basis for formulating predictive theories and tests that follow. Simplified models in population ecology usually start with four key variables including death, birth, immigration, and emigration. The ecology of populations are simplified in the mathematical models that calculate changes in population demographics and evolution under the assumption (or null hypothesis) of no external influence. Some models can become more mathematically complex where "...several competing hypotheses are simultaneously confronted with the data."^[30] For example, in a closed system where immigration and emigration does not take place, the per capita rates of change in a population can be mathematically described as:

dN / dT = B − D = bN − dN = (b − d)N = rN,

where N is the total number of individuals in the population, B is the number of births, D is the number of deaths, b and d are the per capita rates of birth and death respectively, and r is the per capita rate of population change. In simple terms, this formula can be understood as the rate of change in the population (dN/dT) is equal to births minus deaths (B - D).^[28][29]

Using these techniques, Malthus' population principal of growth was later transformed into a mathematical model known as the logistic equation:

dN / dT = aN(1 − N / K),

where N is the biomass density, a is the maximum per-capita rate of change, and K is the carrying capacity of the population. The formula can be read as follows, the rate of change in the population (dN/dT) is equal to growth (aN) that is limited by carrying capacity (1-N/K). From these basic mathematical principals the discipline of population ecology expands into a field of investigation that queries the demographics of real populations and tests these results against those of various statistical models. Beyond these, the field of population ecology often uses data on life history and matrix algebra to develop projection matrices on fecundity and survivorship. This kind of information can be used for managing wildlife stocks and harvest quotas ^[31][29]

These mathematical models introduce two important variables that are commonly invoked in population ecology, namely r (intrinsic rate of natural increase in population size, density independent) and K (carrying capacity of a population, density dependent).^[13] These two variables where used in development of the concept of r and K selection. An r-selected species (e.g., many kinds of insects, such as aphids^[32]) is one that has high rates of fecundity, low levels of parental investment in the young, and high rates of mortality before individuals reach maturity. In r-selected species evolution favors productivity. In contrast, a K-selected species (such as humans) has low rates of fecundity, high levels of parental investment in the young, and low rates of mortality as individuals develop toward maturity. Evolution in K-selected species favors efficiency in the conversion of resources into fewer offspring.^[33][34]

[edit] Community ecology

Ecosystems are most generally studied at the local or effective community scale, such as measuring primary production in a wetland in relation to decomposition and consumption rates^[36] or the analysis of predator-prey dynamics affecting amphibian biomass^[37]. The vast majority of research into community ecology examines population dynamics of pairs of species to understand how entire communities function. Two conceptual models that have been used in understanding community ecology include food webs and trophic levels.^[38][39]

[edit] Food webs

Food webs are a type of concept map that are used to understand and map out real pathways in the series of ecological events usually starting with solar energy being photosynthesized in plants. Plants grow and accumulate nutrients that are in turn eaten by grazing herbivores and step by step the lines are drawn and until the web of life is illustrated.^[41][42]

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'.^[43] Elton^[43] defined ecological relations using concepts of 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^[44]. Elton's book broke conceptual ground by illustrating complex ecological relations through simpler food-web diagrams.^[41] Food-webs are an effective way to conceptually illustrate and teach about the interactive links among species in a community.^[45][46]

There are different dimensions in ecological communities that can be used to create more complicated food-webs, including: species composition (type of species), richness (number of species), biomass (the dry weight of plants and animals), productivity (rates of conversion of energy and nutrients into growth), and stability (food-webs over time). A food-web diagram illustrating species composition shows how a change in one single species can directly and indirectly influence many others. Microcosm studies are used to simplify food-web research into semi-isolated units such as small springs, decaying logs and cowpats. Principals gleaned from these food-web microcosm studies are used to extrapolate smaller dynamic concepts to larger systems.^[47] Food-chain length is an important parameter in describing larger food-web dynamics and is defined as:

"The number of transfers of energy or nutrients from the base to the top of a food web..."^[48]:269

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.^[48] Hence, in a simple predator-prey example a deer is one step removed from the plants it eats (chain length = 1) and a wolf that eats the deer 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.^[47]

[edit] Trophic dynamics

Links in food-webs relate of primary importance to feeding relations or trophism (The Greek root of the word troph, τροφή, trophē, means food or feeding). Elton^[43] noted how important an 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.

A trophic pyramid

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.^[49]

Trophic levels are part of the holistic or complex systems view of ecosystems. Each trophic level contains different species that share common ecological functions. Different species, such as ferns and lillys, are grouped very differently from an evolutionary view of their ecology, but functionally they both photosynthesize the sun's energy and classified as autotrophs. Grouping these functionaly similar species into a trophic system gives a macroscopic image of the larger functional design. Trophic levels are abstractions of the system, but they explain real phenomena. For example, the autotrophs have the highest global proportion of biomass, followed closely behind by microbes (prokaryotes - decomposers), then heterotrophs.^[50][51] Functional trophic groups sort out hierarchically into pyramidic trophic levels because it requires specialized adaptations to become a photosynthesizer or a predator, but rarely are their organisms having a skillful combination of both functional abilities. This is why functional adaptations to trophism (feeding) organizes different species into emergent functional groups.^[49]

Paleoecology is also an active field of study as shown in these reconstructions of fossilized food-web ecosystems also illustrating trophic levels sorted vertically. Primary producers are at the base as red spheres, predator's are at the top as yellow spheres, and the lines represent feeding links. Original food-webs (on the left) are simplified on the right panels by aggregating groups that feed on the same foods into trophic species. S: number of species (nodes). L: number of trophic links. C: connectance; L/S2. MaxTL: maximum trophic level of a species in the web.^[52]

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.^[53] 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.^[54] The size of each level in the pyramid generally represents biomass, which is often measured as the dry weight of an organism.^[53] 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.^[55]

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.^[56]

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.^[57] Hunting of sea otters, for example, is thought to have indirectly lead to the extinction of the Steller's Sea Cow (Hydrodamalis gigas).^[58] 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.^[56][57]

[edit] Biosphere

The largest scale of ecological organization is the total sum of every ecosystem on the planet and the atmosphere it regulates, which is called the biosphere. Ecological relations regulate the flux of energy, nutrients, and climate all the way up to the planetary scale. For example, the dynamic history of the planetary CO₂ and O₂ composition of the atmosphere has been largely regulated by the biogenic flux of gases coming from respiration and photosynthesis with levels fluctuating in time and in relation to the ecology and evolution of plants and animals.^[59] When sub-component parts, such as the full variety of ecosystems diversifying the planet, are organized into a whole there are oftentimes identifiable properties or characteristics that describe the nature of the system under investigation. Ecological theory has been used to explain self emergent regulatory phenomena at the planetary scale. This is known as the Gaia hypothesis^[11]. 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.^[60]

[edit] Ecology and evolution

Ecology and evolution are considered sister disciplines. Ecology and evolution are academic branches of the life sciences. Natural selection, life history, development, adaptation, populations, and inheritance all play an prominent conceptual roles in ecological as well as evolutionary theory. Both disciplines also employ genetics in their investigations. For example, morphological, behavioural and/or genetic traits can be mapped onto evolutionary trees to study principals of inheritance that relates back to the ecology of adaptations. Ecology and evolution are scientifically connected because they both study hierarchies, networks, relations, and kinship among genes, cells, individuals, communities, species, and the biosphere.^[61] The two disciplines often appear together, such as in the title of the journal Trends in Ecology and Evolution.^[62] There is no sharp dichotomous boundary that separates ecology from evolution and 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.^[63][11][64] While the boundary between ecology and evolution is not always clear, it is understood that ecologists study the abiotic and biotic factors that influence the evolutionary process.^[2][53]

[edit] Behavioral ecology

Behavioural ecology is the field of study concerned with ethology and its implications to broader ecological theory. Adaptation is the central unifying concept in behavioral ecology.[4] 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.^[65] 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.^[66] 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.^[65]

Social display and color variation in differently adapted species of chameleons (Bradypodion spp.). Chameleons can change their skin color to match their background as a behavioral defense mechanism and can also use color to signal to other members of their species, such as males signalling females during courtship.^[67]

Predator-prey interactions are a fundamental and introductory concept in food-web studies as well as behavioural ecology.^[68] 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 predatory threats, organisms must balance their energy budgets as they invest in different aspects of their life history, such as feeding, mating, socializing, or modifying their habitat. Hypotheses posited in behavioural ecology are generally based on adaptive principals of conservation or efficiency. For example,

"The threat-sensitive predator avoidance hypothesis predicts that prey should assess the degree of threat posed by different predators and match their behavior according to current levels of risk."^[69]

"The 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."^[70]

The behaviour of long-toed salamanders (Ambystoma macrodactylum) present another example in this context. When threatened, the long-toed salamander will defend itself by waving its tail and secreting a white milky fluid.^[71][72] The excreted fluid is distasteful, toxic and adhesive, but it is also used for nutrient and energy storage during hibernation. Hence, salamanders subjected to frequent predatory attack will be energetically compromised as they use up their energy stores.^{[73] [74]} Some species are also able to avoid predators altogether, such as small velvet gecko's (Oedura lesueurii). This species is specially adapted to smell the body chemicals of snakes that linger after they pass through an area, even though snakes rarely pose a significant danger.^[69]

Leafhoppers are protected by an army of meat ants in a symbiotic relationship.

Many creatures also form mutually beneficial relationships called mutualisms.^[75] Approximately 60% of all plants, for example, form a symbiotic relationship with arbuscular mycorrhizal fungi. The plants and fungi exchange carbohydrates for mineral nutrients.^[76] The range of relationships that can form in ecology can be simplfied down to the host and the associate. A host is any entity that harbors another that is called the associate. A genome is the host for the associate genes in the same way that parasites are associates on host organisms.^[77] The are different types of ecological interactions that can reciprocally develop from the host and associate relationship. If the relationship is mutually beneficial it is called mutualism, one species benefits and the other suffers, such as in parasitism or predation. Competition is reciprocal antagonism among species. Symbiosis is a form of mutualism where the organisms are physically connected, whereas there are mutualisms that form out of indirect relations.^[78]

A grid of type of relations that can evolve among species, where antagonistic relations are denoted by a minus sign, beneficial relations are denoted by a plus sign, and neutral relations are denoted by the zero's.^[79]

There are many examples in all corners of life of interspecific mutualistic relations. Famous ecological study systems where mutualism occurs include, fungus-growing ants with agricultural like societies, bacteria living in the guts of insects and other organisms, the fig wasp and yucca moth polination complex, and corals with their photosynthetic algae.^[80][81]

Intraspecific behaviours are most 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.^[65][82] Social behaviours 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.^[65] 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 a group if its members express altruistic behaviours to one another. Groups that are predominantely altruists beat groups that are predominantely selfish.^[65][83]

A often quoted hypothesis in behavioural ecology is known as Lack's brood reduction hypothesis, which posits an evolutionary and ecological explanation as to why birds often lay a series of eggs with an asynchronous delay such that the young are of mixed age and weights. According to Lack, this brood-reducation behaviour is a sort of ecological insurance that allows some birds to survive in poor years and all birds to survive when food is plentiful.^[84][85]

"The clutch size of each species of bird is characteristic, and in general seems adapted to correspond with the largest number of young which can be successfully raised. Probably there is a small hereditary variation, which is explicable through rather larger clutches being favoured in some years and rather smaller clutches in other years."^[86]:333

Elaborate sexual displays and posturing are often encountered in the behavioural ecology of animals. Many birds, for example, display elaborate ornaments during courtship. These displays serve a dual purpose of signalling healthy or well-adapted individuals and good genes. The elaborate displays are driven by sexual selection as the displays serve as an advertisment of quality traits in sexual partners.^[87]

[edit] Biogeography

As the name implies, biogeography is an amalgamation of the words biology and geography. The word was first coined by the German geographer, Friedrich Ratzel in 1891.^[88] The Journal of Biogeography was established in 1974 and publishes "...papers dealing with all aspects of spatial, ecological and historical biogeography."[5]Biogeography and ecology share much of the same disciplinary roots. For example, the theory of island biogeography, elucidated by Robert MacArthur and E. O. Wilson in 1967^[33] is considered one of the fundamentals of ecological theory.^[89]

Biogeography has a long and rich history in the natural sciences where questions arise concerning the spatial distribution of plants and animals. Ecology and evolution provides the explanatory context for biogeographical studies.^[88] Biogeographical patterns result from ecological processes that influences dispersal (or dispersion)^[89] and from historical processes that split populations or species into different areas.^[90] The biogeographic processes that result in the natural splitting of species explains much of the modern distribution of the Earth's biota. This area of focus is called vicariance biogeography and it is a sub-discipline of biogeography. It is a separate discipline because it specifically studies the branching, phylogenetic, or speciation process in evolutionary studies and explains much of the patterns in biodiversity across the globe.^[90][91][92]

There are many applications in the field of biogeography that concern 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 context of global warming. ^{[93] [94]}

[edit] Molecular Ecology

There has long been an understanding of the important relationship between ecology and genetic inheritance.^[2] This branch of research became more feasible with the development of genetic technologies, such as the polymerase chain reaction (PCR), and through the publication Molecular Ecology starting in 1992.^[95] Molecular ecology uses various analytical techniques to study genes in evolutionary and ecological context. In 1994, professor John Avise played a leading role in popularizing this field of study through the publication of his book, Molecular Markers, Natural History and Evolution .^[96] Newer genetic technologies made genetic sampling of organisms simpler and engendered a new and collaborative research paradigm that investigates and probes ecological questions that were otherwise intractable. Molecular ecology revealed previously obscured details in the intricacies of nature and improved resolution into probing questions about behavioural and biogeographical ecology. For example, molecular ecology revealed promiscuous sexual behaviour that is driven by female choice in pocket gophers ^[97] and multiple male partners in tree swallows previously thought to be socially monogamous.^[98] In a biogeographical context, the marriage between genetics, ecology and evolution created a new sub-discipline called phylogeography.^[99]

[edit] Ecology and the environment

The environment is external yet interlinked directly with ecology. Chemistry, temperature, pressure, gravity, energy, and sunlight are properties of Earth's environment that are relevant to ecology. Environmental and ecological relations are often studied through conceptually and practically manageable parts. However, once the effective environmental components are understood they conceptually link as a holocoenotic[6] system.

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.^[10] 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.^[100] In contrast, ecology focuses on biological relations and studies how these relate to the environment.^[10] Ecosystem processes are consistent with the laws of thermodynamics. Armed with an understanding of metabolic and thermodynamic principles, a complete accounting of energy and material flow can be traced through an ecosystem.^[101]

[edit] 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^[103] 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.^[104]

The leaf is the primary site of photosynthesis in plants.

One of the earliest organisms was likely an anaerobic methanogen microbe that would have converted atmospheric 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.^[105] 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.^[11]

[edit] 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.^[2][53][101]

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.^[2][53][101]

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.^[2][53][101]

[edit] Physical environments

[edit] Water

The rate of diffusion of carbon dioxide and oxygen is approximately 10,000 times slower in water than it is in air. When soils become flooded, they quickly loose oxygen from low-concentration (hypoxic) to an (anoxic) environment where anaerobic bacteria thrive among the roots^[106]. Aquatic plants exhibit a wide variety of morphological and physiological adaptations that allow them to survive, compete and diversify these environments. For example, the roots and stems develop large cellular air spaces to allow for the efficient transportation gases (for example, CO₂ and O₂) used in respiration and photosynthesis. In drained soil, microorganisms use oxygen during respiration. In aquatic environments, anaerobic soil microorganisms use nitrate, manganic ions, ferric ions, sulfate, carbon dioxide and some organic compounds. 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 methanogen bacteria. Salt water also requires special physiological adaptations to deal with water loss. Salt water plants (or halophytes) are able to osmo-regulate their internal salt (NaCl) concentrations or develop special organs for shedding salt away.^[106]. The physiology of fish is also specially adapted to deal with high levels of salt through osmoregulation. Their gills form electrochemical gradients that mediate salt excrusion in salt water and uptake in fresh water.^[107]

[edit] Gravity

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.^[2]

[edit] Pressure

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.^[2] Water transportation through trees, for example, is an important eco-physiological parameter.^[108][109] Water pressure in the depths of oceans requires adaptations to deal with the different living conditions. Mammals, such as whales, dolphins and seals require special adaptations to deal with the change in sound due to water pressure differences.^[110] 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.^[2]

[edit] Wind and turbulence

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.^[2] For example, winds running over the surface of lakes winds creates turbulence that stirs the water column and influences the environmental profile to create thermally layered zones that partially governs how the fish, algae, and other parts of the aquatic ecology are structured.^[111][112] Wind speed and turbulence also exert influence on rates of evapotranspiration rates and energy budgets in plants and animals ^[106][113]

[edit] Fire

Plants spew oxygen into the atmosphere. Approximately 350 million years ago (near the Devonian period) the photosynthetic process brought atmospheric oxygen levels above 17% in concentration, which allowed for the combustion of fire.^[114] 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 in management.^[115] While the issue of fire in relation to ecology and plants has been recognized for a long time^[116], Charles Cooper brought attention to the issue of forest fires in relation to the ecology of forest fire suppression and management in the 1960s.^[117][118] The association for fire ecology launched a journal, "Fire Ecology", in 2005 that is specifically devoted to the study of fire ecology and management.[7].

Fire creates environmental mosaics and a patchiness to ecosystem age and canopy structure. 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.^[119] Most ecosystem are adapted to some level of natural fire cycles. Plants, for example, are equipped with a variety of special adaptations to deal with forest fires. The altered state of soil nutrient supply and cleared canopy structure creates a new niche for seedling establishment.^[120][121] Some species (e.g., Pinus halepensis) cannot germinate until after their seeds have lived through a fire. This environmental trigger for seedlings is called serotiny.^[122] Some compounds from smoke also promote seed germination.^[123]

[edit] Biogeochemistry

Ecologists study and measure nutrient budgets to understand how these materials are regulated and flow through the environment.^[2][53][101] This research has led to an understanding that there is a global feedback between ecosystems and the physical parameters of this planet including minerals, soil, pH, ions, water and atmospheric gases. There are six major elements, including H (hydrogen), C (carbon), N (nitrogen), O (oxygen), S (sulfur), and P (phosphorus) that 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.^[124]

The ecology of global carbon budgets gives one example of the linkage between biodiversity and biogeochemistry. For starters, 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 emit an annual flux of 6.3 Gt carbon.^[125] At different times in the Earth's history there has been major restructuring in these global carbon budgets that was regulated to a large extent by the ecology of the land. For example, through the early-mid Eocene volcanic out gassing, the oxidation of methane stored in wetlands, and seafloor gases increased atmospheric CO₂ concentrations to levels as high as 3500 ppm.^[126] In the Oligocene, from 25 to 32 million years ago, there was another significant restructuring in the global carbon cycle as grasses evolved a special type of C4 photosynthesis and expanded their ranges. This new photosynthetic pathway evolved in response to the drop in atmospheric CO₂ concentrations below 550 ppm.^[127] Ecosystem functions such as these feed back significantly into global atmospheric models for carbon cycling. Loss in the abundance and distribution of biodiversity causes global carbon cycle feedbacks that are expected to increase rates global warming in the next century.^[128] Global warming melting large sections of permafrost creates a new mosaic of flooded areas where decomposition emits methane (CH₄). Hence, there is a relationship between global warming, decomposition and respiration in soils and wetlands producing significant climate feedbacks and alters global biogeochemical cycles.^[129][130] There is concern over methane increases in the atmosphere in context of the carbon cycle, because methane is also a greenhouse gas that is 23 times more effective at absorbing long-wave radiation on a 100 year time scale.^[131]

[edit] Historical roots of ecology

In the early 20th century, ecology was called scientific natural history and was influenced by the analytical precision of Newtonian sciences.^[132] A comprehensive historical account of ecology is a complicated task because ecology is one of the most diverse of the scientific disciplines.^[133] Several published books provide extenstive coverage of the classics.^[134][135] The term "ecology" (German: Oekologie) is a more recent scientific development and was first coined by the German biologist Ernst Haeckel in his book Generelle Morpologie der Organismen (1866). The definition offered by Haeckel appeared in the frontispiece of the classical text Principles of Animal Ecology.^[2]

Oppinions differ on who was the founder of modern ecological theory. Some some mark Haeckel's definition as the beginning^[137], others suggest it started with the Greeks, such as Aristotle and his student Theoprastis^[138], some say it was Eugen Warming^[139] and others posit that the science of ecology began with Carl Linnaeus' research principals on the economy of nature that matured in the early 18th century.^[140][41] The works of Carl Linnaeus influenced Darwin, for example, as evidenced by his reference to ecology through his adopted usage of Linnaeus' phrase economy or polity of nature in The Origin of Species.^[141] Ernst Haeckel was strongly influenced by Darwin's work. He defined ecology in reference to the economy of nature and this has lead some to question if ecology is synonymous with Linnaeus' concepts for the economy of nature.^[140] Some have suggested that ecology started with Alexander von Humbolt (1809-1882), who was also admired by Charles Darwin.^[132] Baron Humbolt was among the first to recognize ecological gradients and alluded to the modern law of species to area relationships in ecology.^[142][143]

The modern synthesis of ecology is a young science that flourished and attracted much research attention around the same time as evolutionary studies at the end of the 19th century. However, many observations, interpretations and discoveries relating to ecology extend back to much earlier studies in natural history. For example, the concept on the balance or regulation of nature can be traced back to Herodotos (died c. 425 BC) who described an early account of mutualism along the Nile river where crocodiles open their mouths to beneficially allow sandpipers safe access to pluck leaches away.^[133]. In the broader contributions to the historical development of the ecological sciences, Aristotle is considered one of the earliest naturalists who had a highly influential role in the philosophical development of ecological sciences. One of Aristotle's students, Theophrastus, made astute ecological observations about plants and posited a philosophical stance about the autonomous relations between plants and their environment that is more in line with modern ecological thought. Both Aristotle and Theophrastus made extensive observations on plant and animal migrations, biogeography, physiology, and their habits in what might be considered a modern analog of the ecological niche.^[144][138]

Carl Linnaeus (1707–1778), a well known naturalist also holds a prominent place in the history of ecological sciences as he invented the first branch of ecological study he called the economy of nature.^[41] Linnaeus was one of the first to attempt to define on the balance of nature, which had previously been held as an assumption rather than formulated as a testable hypothesis. From Aristotle to Darwin, however, the natural world was predominantly considered static and unchanged since its original creation. Hence, there was little appreciation and understanding for the dynamic and reciprocal relations between organisms, their adaptations and modifications to the environment.^[145][136]

While Charles Darwin is most notable for his treatise on evolution^[147], he was also a notable and astute ecologist as he meticuously researched earthworms in relation to soil ecology^[148] and in The Origin of Species he made note of the very first ecological experiment that was published in 1816.^[149][150] In the science leading up to Darwin the notion of evolving species was gaining popular support. This scientific paradigm changed the way that researchers approached the ecological sciences.

The first American ecology book was published in 1905 by Frederic Clements.^[151] Frederic Clements forwarded the idea of plant communities as a superorganism. According to this premise, single species populations could be classified, using identifiable plant associations, into larger superorganism entities that were believed to progress through regular and determined stages of seral development. Seral stages of an ecosystem were thought to be analogous to developmental stages of an organism. Not until the 1970's had the Clementsian paradigm been overthrown by the Gleasonian paradigm ^[152] which emphasized the overriding role of individual organisms and their life histories in the development of community associations. Hence, seral stages of ecosystems were not a predictable and reproduceabe outcome. Each ecological community develops from the unique and coincidental juxtopostion of organism. The Gleasonian paradigm suggests that a precise structural uniformity to ecosystems does not exist because the different community associations are historically and geographically context dependent.^[153] According to Gleason^[153], there is much variation among sites that is driven by extrinsic, not internal factors.

The number of authors publishing on the topic of ecology has grown considerably since the turn of 20th century.^[154] The explosion of information that is now available to the modern researcher of ecology makes it is an impossible task for any individual to sift through the entire history. The identification of classics in the history of ecology is a difficult designation to make.^[155]

Ellen Swallow Richards, the first female student and instructor at MIT.

"Vito Volterra's "Variations and Fluctuations of the Number of Individuals in Animal Species Living Together" would be so recognized by population ecologists. R. L. Lindeman's, "The Trophic-Dynamic Aspect of Ecology"; A. S. Watt's "Pattern and Process in the Plant Community"; G. E. Hutchinson's ambigouusly titled "Concluding Remarks" (concealing his multidimensional formulation of the niche concept), and Robert MacArthur's "On the Relative Abundance of Bird Species" (including his broken-stick model) have all been described as classic, and many persons would agree. The designation of an article as a classic of ecology, however, may or may not represent a consensus and may be attributable to the idiosyncrasy of the designator."^[155]:32

Political persuation, subjectivity and other overiding social factors are also encountered in the citation history of ecological journals.^[156] Feminist researchers, such as Ellen Swallow Richards, might not be as prominently noted as their male counterparts.^[157] Researchers are studying what factors contribute to the citation rates of certain papers and have published selected lists of papers that identify significant contributions to ecology.^[156][155]

"...social factors, such as the professional standing of the cited author, play a significant role in citation decisions in ecology. Furthermore, the dependence of the citation rates of ecological papers on the direction of study outcome with respect to the hypothesis tested suggests that citations in ecological papers are used as rhetorical devices to convince the readers of the validity of the study claims rather than as simple acknowledgements of the sources of background information."^[156]:31

Ecology has also developed in other nations, including Russia's Vladimir Vernadsky and his development of the biosphere concept in the 1920's^[158] or Japan's Kinji Imanishi and his concepts of harmony in nature and habitat segregration in the 1950's^[159] where recognition or scientific importance of the contributions to ecology is hampered by language and translation barriers.^[158] The history of ecology remains an active area of study.

[edit] Ecosystem services and the biodiversity crisis

Pollination by a bumblebee, a type of ecosystem service

Ecosystems regulate the global geo-physical cycles of energy, climate, soil nutrients, and water that in turn support and grow natural capital in the environmental, physiological, cognitive, cultural, and spiritual dimensions of life. Ecosystems are considered common-pool resources because ecosystems do not exclude beneficiaries and they can be depleted or degraded.^[184] For example, green space within communities provides common-pool health services. Research shows that people who are more engaged with regular access to natural areas have lower rates of diabetes, heart disease and psychological disorders.^[185] These ecological health services are regularly depleted through urban development projects that do not factor in the common-pool value of ecosystems.^[186][187] The ecological commons delivers a diverse supply of community services that sustains the well-being of human society.^[188][189] The Millineum Ecosystem Assessment[10], an international UN initiative involving more than 1,360 experts worldwide, identifies four main ecosystem service types having 30 sub-categories stemming from natural capital. The ecological commons includes provisioning (e.g., food, raw materials, medicine, water supplies), regulating (e.g., climate, water, soil retention, flood retention), cultural (e.g., science and education, artistic, spiritual), and supporting (e.g., soil formation, nutrient cycling, water cycling) services.

Ecology is an economic science that uses many of the same terms and methods that are used in accounting.^[191] There is a journal called Ecological Economics[[11]] and the International Society of Ecological Economics [[12]] that researches and publishes on this part of the global economy. Natural capital is the stock of materials or information stored in biodiveristy that generates services that can enhance the welfare of communities.^[192] Population losses are the more sensitive indicator of natural capital than are species extinction in the accounting of ecosystem services. The prospect for recovery in the economic crisis of nature is grim. Populations, such as local ponds and patches of forest are being cleared away and lost at rates that exceed species extinctions.^[193]

The WWF 2008 living planet report [13] and other researchers^[195] report that human civilization has exceeded the bio-regenerative capacity of the planet. This means that human consumption is extracting more natural resources than can be replenished by ecosystems around the world. In 1992, professor William Rees developed the concept of our ecological footprint. The ecological footprint is a way of accounting the level of impact that human development is having on the Earth's ecosystems.^[194] All indications are that the human enterprise is unsustainable as the ecological footprint of society is placing too much stress on the ecology of the planet.^[195] The mainstream growth based economic system adopted by governments worldwide does not include a price or markets for natural capital. This type of economic system places further ecological debt onto future generations.^[196]

Human societies are increasingly being placed under stress as the ecological commons is diminished through an accounting system that has incorrectly assumed "...that nature is a fixed, indestructible capital asset." ^[197]:44 While nature is resilient and it does regenerate, there are limits to what can be extracted, but conventional monetary analyses are unable to detect the problem.^[198][199] Evidence of the limits in natural capital are found in the global assessments of biodiversity, which indicate that the current epoch, the Anthropocene^[200], has entered the sixth great extinction period.^[201] The ecology of the planet has been radically transformed by human society and development causing massive loss of ecosystem services that otherwise deliver and freely sustain equitable benefits to human society through the ecological commons. The ecology of the planet is further threatened by global warming, but investments in nature conservation can provide a regulatory feedback to store and regulate carbon and other greenhouse gases.^[202][203] The field of conservation biology involves ecologists that are researching the nature of the biodiversity threat and searching for solutions to sustain the planets ecosystems for future generations.^[204]

"Human activities are associated directly or indirectly with nearly every aspect of the current extinction spasm."^[201]:11472

The current wave of threats, including massive extinction rates and concurrent loss of natural capital to the detriment of human society is happening rapidly. This is called a biodiversity crisis because 50% of the worlds species are predicted to go extinct within the next 50 years.^{[205] [206]} The worlds fisheries are facing dire challenges as the threat of global collapse appears immenent and with serious ramifications for the well-being of humanity^[207] Governments of the G8 met in 2007 and set forth 'The Economics of Ecosystems and Biodiversity' (TEEB) initiative[14]:

In a global study we will initiate the process of analyzing the global economic benefit of biological diversity, the costs of the loss of biodiversity and the failure to take protective measures versus the costs of effective conservation.[15]

Ecologists are teaming up with economists to measure the wealth of ecosystems and to express their value as a way of finding solutions to the biodiversity crisis.^{[208][209][210]} Some researchers have attempted to place a dollar figure on ecosystem services, such as the value of the Canadian Boreal Forest contributing services such as carbon storage is estimated at US$3.7 trillion dollars. These economic values are not currently included in calculations of national income accounts or the GDP.^[211]

[edit] See also

	Environment portal
	Ecology portal
	Biology portal
	Earth sciences portal
	Sustainable development portal

Bachalpsee in the Swiss Alps; generally mountainous areas are less affected by human activity.

[edit] Lists

[edit] Notes

[edit] References

• Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 0132508826. http://www.phschool.com/el_marketing.html. 
• 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.[16]
• 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 [17]
• 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

[edit] External links

Wikimedia Commons has media related to: Ecology

At Wikiversity you can learn more and teach others about Ecology at: The Department of Ecology

Wikibooks has more on the topic of Ecology

Look up ecology in Wiktionary, the free dictionary.

• stanford.edu/entries/ecology/ Ecology (Stanford Encyclopedia of Philosophy)
• Science Aid: Ecology High School (GCSE, Alevel) Ecology.
• Ecology Journals List of scientific journals related to Ecology
• Ecology Dictionary - Explanation of Ecological Terms
• [18] International Society for Behavioral Ecology

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