Within the biosphere—the total expanse of water, land, and atmosphere able to sustain life—the basic ecological unit is the ecosystem. An ecosystem may be as small as a tidal pool or a rotting log or as large as an ocean or a continent-spanning forest. Each ecosystem consists of a community of plants and animals in an environment that supplies them with raw materials for life, i.e., chemical elements and water. The ecosystem is delimited by the climate, altitude, water and soil characteristics, and other physical conditions of the environment.
The energy necessary for all life processes reaches the earth in the form of sunlight. By photosynthesis green plants convert the light energy into chemical energy, and carbon dioxide and water are transformed into sugar and stored in the plant. Herbivorous animals acquire some of the stored energy by eating the plants; those animals in turn serve as food for, and so pass the energy to, predatory animals. Such sequences, called food chains, overlap at many points, forming so-called food webs. For example, insects are food for reptiles, which are food for hawks. But hawks also feed directly on insects and on other birds that feed on insects, while some reptiles prey on birds. Since a severe loss of the original energy occurs with each transfer from species to species, the ecologist views the food (energy) structure as a pyramid: Each level supports a smaller number and mass of organisms. Thus in a year's time it would take millions of plants weighing tons to feed the several steer weighing a few tons that could support one or two people. The ecological conclusion is that if human beings would eat more plants and fewer animals, food resources would stretch much further. Once the energy for life is spent, it cannot be replenished except by the further exposure of green plants to sunlight.
The chemical materials extracted from the environment and elaborated into living tissue by plants and animals are continually recycled within the ecosystem by such processes as photosynthesis, respiration, nitrogen fixation, and nitrification. These natural processes of withdrawing and returning materials are variously called the carbon cycle, the oxygen cycle, and the nitrogen cycle. Water is also cycled. Evaporation from lakes and oceans forms clouds; the clouds release rain that is taken up by the soil, absorbed by plants, and passed on to feeding animals—which also drink directly from pools and lakes that catch the rain. The water in plant and animal wastes and dead tissue then evaporates and can be recycled. Interference with these vital cycles by disturbance of the environment—for example, by pollution of the air and water—may disrupt the workings of the entire ecosystem. The cycles are facilitated when an ecosystem has a sufficient biological diversity of species to fill its so-called ecological niches, the different functional sites in the environment where organisms can act as producers of energy, consumers of energy, or decomposers of wastes. Such diversity tends to make a community stable and self-perpetuating.
A climax community is one that has reached the stable stage. When extensive and well defined, the climax community is called a biome. Examples are tundra, grassland, desert, and the deciduous, coniferous, and tropical rain forests. Stability is attained through a process known as succession, whereby relatively simple communities are replaced by those more complex. Thus, on a lakefront, grass may invade a build-up of sand. Humus formed by the grass then gives root to oaks and pines and lesser vegetation, which displaces the grass and forms a further altered humus. That soil eventually nourishes maple and beech trees, which gradually crowd out the pines and oaks and form a climax community. In addition to trees, each successive community harbors many other life forms, with the greatest diversity populating the climax community.
Similar ecological zonings occur among marine flora and fauna, dependent on such environmental factors as bottom composition, availability of light, and degree of salinity. In other respects, the capture by aquatic plants of solar energy and inorganic materials, as well as their transfer through food chains and cycling by means of microorganisms, parallels those processes on land.
The early 20th-century belief that the climax community could endure indefinitely is now rejected because climatic stability cannot be assumed over long periods of time. In addition nonclimatic factors, such as soil limitation, can influence the rate of development. It is clear that stable climax communities in most areas can coexist with human pressures on the ecosystem, such as deforestation, grazing, and urbanization. Polyclimax theories stress that plant development does not follow predictable outlines and that the evolution of ecosystems is subject to many variables.
See E. P. Odum, Fundamentals of Ecology (3d ed. 1971); R. L. Smith, ed., The Ecology of Man: An Ecosystem Approach (1971); P. A. Colinvaux, Introduction to Ecology (1973); R. M. Darnell, Ecology and Man (1973); T. C. Emmel, An Introduction to Ecology and Population Biology (1973); D. B. Sutton and N. P. Harman, Ecology: Selected Concepts (1973); K. E. F. Watt, Principles of Environmental Science (1973); D. Worster, Nature's Economy (1977); R. Brewer, The Science of Ecology (1988).
Ecology (from Greek οἶκος, oikos, "house(hold)"; and -λογία, -logia) is the scientific study of the distribution and abundance of life and the interactions between organisms and their natural environment. The environment of an organism includes physical properties, which can be described as the sum of local abiotic factors such as insolation (sunlight), climate, and geology, and biotic ecosystem, which includes other organisms that share its habitat.
The word "ecology" is often used more loosely in such terms as social ecology and deep ecology and in common parlance as a synonym for the natural environment or environmentalism. Likewise "ecologic" or "ecological" is often taken in the sense of environmentally friendly.
The term ecology or oekologie was coined by the German biologist Ernst Haeckel in 1866, when he defined it as "the comprehensive science of the relationship of the organism to the environment. Haeckel did not elaborate on the concept, and the first significant textbook on the subject (together with the first university course) was written by the Danish botanist, Eugenius Warming. For this early work, Warming is often identified as the founder of ecology.
Ecology is usually considered a branch of biology, the general science that studies living organisms. Organisms can be studied at many different levels, from proteins and nucleic acids (in biochemistry and molecular biology), to cells (in cellular biology), to individuals (in botany, zoology, and other similar disciplines), and finally at the level of populations, communities, and ecosystems, to the biosphere as a whole; these latter strata are the primary subjects of ecological inquiry. Ecology is a multidisciplinary science. Because of its focus on the higher levels of the organization of life on earth and on the interrelations between organisms and their environment, ecology draws on many other branches of science, especially geology and geography, meteorology, pedology, genetics, chemistry, and physics. Thus, 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. In support of viewing ecology as a subject in its own right as opposed to a sub-discipline of biology, Robert Ulanowicz stated that "The emerging picture of ecosystem behavior does not resemble the worldview imparted by an extrapolation of conceptual trends established in other sciences.
Agriculture, fisheries, forestry, medicine, and urban development are among human activities that would fall within Krebs' (1972: 4) explanation of his definition of ecology: where organisms are found, how many occur there, and why.
Ecological knowledge such as the quantification of biodiversity and population dynamics has provided a scientific basis for expressing the aims of environmentalism and evaluating its goals and policies. Additionally, a holistic view of nature is stressed in both ecology and environmentalism.
Consider the ways an ecologist might approach studying the life of honeybees:
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:
Ecology can also be sub-divided according to the species of interest into fields such as animal ecology, plant ecology, insect ecology, Marine Ecology, and so on. Another frequent method of subdivision is by biome studied, e.g., Arctic ecology (or polar ecology), tropical ecology, desert ecology, etc. The primary technique used for investigation is often used to subdivide the discipline into groups such as chemical ecology, genetic ecology, field ecology, statistical ecology, theoretical ecology, and so forth. These fields are not mutually exclusive.
Ecology can be studied at a wide range of levels, from large to small scale. These levels of ecological organization, as well as an example of a question ecologists would ask at each level, include:
For modern ecologists, ecology can be studied at several levels: 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, after the ozone layer protecting living beings from UV rays formed. 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. Recently technology has allowed the discovery of the deep ocean vent communities. This remarkable ecological system is not dependent on sunlight but bacteria, utilizing the chemistry of the hot volcanic vents, are at the base of its food chain.
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 simulated the biosphere in a small-scale model, called Biosphere II.
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.
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:
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 (inbreeding) 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.
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 recyclings 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).
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 poles, 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 approaches 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.
These relations form sequences, in which each individual consumes the preceding one and is consumed by the one following, in what are called food chains or food networks. In a food network, there will be fewer organisms at each level as one follows the links of the network up the chain.
These concepts lead to the idea of biomass (the total living matter in a given place), of primary productivity (the increase in the mass of plants during a given time), and of secondary productivity (the living matter produced by consumers and the decomposers in a given time).
These last two ideas are key, since they make it possible to evaluate the load capacity -- the number of organisms that can be supported by a given ecosystem. In any food network, the energy contained in the level of the producers is not completely transferred to the consumers. And the higher one goes up the chain, the more energy and resources are lost and consumed. Thus, from an energy and an environmental point of view, it is more efficient for humans to be primary consumers (to subsist from vegetables, grains, legumes, fruit, etc.) than to be secondary consumers (from eating herbivores, omnivores, or their products, such as milk, chicken, cattle, sheep, etc.) and still more so than as a tertiary consumer (from consuming carnivores, omnivores, or their products, such as fur, pigs, snakes, alligators, etc.). An ecosystem(s) is unstable when the load capacity is overrun and is especially unstable when a population doesn't have an ecological niche and overconsumers.
The productivity of ecosystems is sometimes estimated by comparing three types of land-based ecosystems and the total of aquatic ecosystems:
Ecosystems differ in biomass (grams carbon per meter squared) and productivity (grams carbon per meter squared per day), and direct comparisons of biomass and productivity may not be valid. Ecosystems are often compared on the basis of their turnover (production ratio) or turnover time which is the reciprocal of turnover.
Humanity's actions over the last few centuries have seriously reduced the amount of the Earth covered by forests (deforestation), and have increased agro-ecosystems (agriculture). In recent decades, an increase in the areas occupied by extreme ecosystems has occurred (desertification).
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 perturbations that interfere with that ecosystem, landscape or species survival. 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 favored 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.
Some common examples of ecological crises are: