Ecology is the scientific study of the interrelationships of plants, animals, and the environment. In recent years, the word has sometimes been misused as a synonym for environment. The principles of ecology are useful in many aspects of the related fields of conservation, wildlife management, forestry, agriculture, and pollution control. The word ecology (Greek, oikos, "house," and logos, "study of") is generally believed to have been coined by Ernst HAECKEL, who used and defined it in 1869. The historical roots of ecology lie not only in natural history, but in physiology, oceanography, and evolution as well. It has occasionally been called scientific natural history (a phrase originated by Charles ELTON) because of its origin and its heavy reliance on measurement and mathematics.

Ecology is variously divided into terrestrial ecology, fresh-water ecology (limnology), and marine ecology, or into population ecology, community ecology, and ecosystem ecology. ECOLOGICAL CLASSIFICATION OF ORGANISMS Ecologists commonly classify organisms according to their function in the environment. Autotrophs ("self-nourishes," also called producers), which are mainly green plants, manufacture their own food from carbon dioxide, water, minerals, and sunlight, whereas heterotrophs -- a wide assortment of organisms -- lack the metabolic machinery to synthesize their own food and must obtain it from other sources. Some heterotrophs -- the herbivores -- eat plants, and some -- the carnivores, or predators -- eat animals. Some, called omnivores, eat both plants and animals; others eat only dead plants and animals. Some, called scavengers, eat large dead organisms.

Some smaller heterotrophs, such as bacteria and fungi, feed on dead organisms; they are called decomposers. Parasites eat living organisms, but, unlike predators, do not devour them at one time. Parasites include forms such as ticks and fleas, which live on their hosts, and others, such as tapeworms, roundworms, and bacteria, which live within their hosts. COMMUNITIES Organisms live together in assemblages called communities. Some communities are very small, such as those composed of invertebrates and decomposers living within a rotting log. Others may be as large as an entire forest.

The most extensive communities, called BIOMES, occupy wide geographic areas. The major biomes are arctic TUNDRAS, northern coniferous FORESTS, deciduous forests, GRASSLANDS, DESERTS, and tropical JUNGLES AND RAIN FORESTS. CHAPPARALS (shrubby forests) and coniferous rain forests are sometimes also considered biomes. The distinctive appearance of each biome is generally determined by the predominance of characteristic plant species, but the animals that are characteristically associated with it also contribute to its distinctiveness.

Communities are composed of both plants and animals. Each species is distributed according to its own biological requirements, which may be affected by other species. For example, sugar maple seedlings require shade and may therefore mature easily in dense forests, whereas seedlings of eastern white pine require full sunlight for vigorous growth. Therefore, some species are sometimes associated with each other, but the exact degree of dependence is difficult to determine and has led to differences of opinion concerning the extent to which communities are discrete entities. By tabulating all plants found along a line passing through adjacent communities on mountainsides, it has been shown that the distribution pattern of each species is independent of most others, suggesting a continuum rather than a few discrete communities. Communities also exhibit vertical stratification or layering.

In tropical rain forests, for example, the tallest trees, called emergent's, grow above the canopy trees; below the canopy trees are shorter trees; below the shorter trees are shrubs; and covering the forest floor is a layer of herbaceous (non woody) plants growing in soil inhabited by FUNGI and BACTERIA. Some characteristic animals are also found in each of the strata, such as toucans in the canopy, but most animals range through several strata. Another aspect of communities is temporal (time) structure. Some species of animals are diurnal (active in daytime), some are nocturnal (active at night), and still others are crepuscular (active at twilight hours). This structure allows more organisms to occupy the same area without interfering with each other. In addition to these daily activity patterns, there may also be seasonal ones.

In temperate areas, for example, frogs of different species use ponds to reproduce at various times throughout the spring. This prevents excessive competition between species for space and food at any one time. The number of species within a community is called species diversity. Species diversity has two components, richness and evenness. If there are many species in a community, it is said to have a rich diversity. All species, however, are not always equally represented.

If, as commonly happens, only a few species are abundant, the diversity is said to be uneven. If a community is made up of many species and each is relatively abundant, the community is considered relatively stable, because the reduction or removal of any one species would be far less important than the loss of an abundant species in a community where only a few are numerous. If a community that has been disturbed by a disaster such as fire, flood, windstorm, volcanic eruption, plow, or bulldozer, is left undisturbed for a long time, it will eventually restore itself; this process is called succession. A forest completely destroyed by fire may take hundreds or thousands of years to become completely renewed, depending on the climate, the nature of the SOIL, and other environmental factors. A forest destroyed by fire in Minnesota might be restored in a few hundred years, whereas one in Mexico destroyed by a lava flow might not be restored for thousands of years. Succession also occurs very slowly in the desert and in the tundra because of climatic and soil conditions.

The first species to invade a destroyed area are called pioneers. These opportunistic species usually have good means of dispersal and high reproductive capacities. Lichens, grasses, and other herbaceous species are the most common pioneers, but trees such as cottonwood, elm, aspen, and silver maple, which produce abundant windblown seeds, are sometimes found as well. Availability of sources of spores or seeds at the periphery of the disturbed area, as well as the suitability of the disturbed site for each species, determines the species composition of the first community formed. The invading species begin to change the environment by increasing the organic content of the soil with their dead parts and excreted wastes, creating shade, and changing moisture conditions. Some species harbor nitrogen-fixing bacteria that release nitrogenous compounds into the soil and thereby fertilize it.

In the course of succession, conditions are generally made more suitable for new types of organisms that use less energy for reproduction and more energy to maintain themselves. These species gradually win out in competition with the pioneers. Collectively, they produce a new community. The process of replacement of species may continue for a long time, resulting in several visibly different communities, although the changes occur gradually. Eventually a point is reached at which the environmental and species changes are minimal and species diversity is high.

This relatively stable community is called a CLIMAX COMMUNITY. ECOSYSTEMS Many ecologists study communities in the context of an ecosystem, which includes interactions involving mineral cycling, energy flow, and population control. The study of ecosystems facilitates a functional approach to ecology. One of the major aims of ecological research is to determine how organisms retain and recycle the minerals within an ecosystem. All minerals used by organisms are important, but some, such as those rich in nitrogen and phosphorous, are used in larger amounts, or may be less available than others. Herbivores speed the recycling of minerals by eating plant parts and then excreting some of these materials; minerals in the plants are thus returned to the soil faster than if the plants had not been eaten.

Scavengers help by breaking down large dead organisms faster than bacteria or fungi alone, and bacteria and fungi eventually break down fecal material and other smaller organic matter into its mineral components. Carnivores feeding on herbivores also help to recycle the minerals found in the bodies of herbivores. Roots of plants may absorb these materials for reuse or may absorb minerals from soil made available by weathering processes. Some minerals, however, will escape from the ecosystem and end up eventually in the oceans. A second functional aspect of ecosystems is energy flow. In the 1920 s, Charles Elton pointed out that there was a pyramid of trophic (food) levels: the base of the pyramid was composed of producers; above them were a smaller number of herbivores and a still smaller number of carnivores.

Ecosystem ecologists prefer to express these relationships by the amount of energy passing through each of the trophic levels in a given period of time. Because of the properties of energy, as expressed in the laws of CONSERVATION and of ENTROPY, there is always more energy passing through the producers in a given unit of time than there is in the herbivores that eat them, and still less in their predators, and so on. In ecosystems, all energy comes from the Sun. A small part of this energy (1 to 2%) is captured by plants and stored as energy-rich chemical bonds in the compounds of their PROTOPLASM. Only 1 to 20% of this energy is passed on to herbivores, and a similar energy loss occurs at each higher trophic level. This phenomenon has three important effects: it limits the possible number of trophic levels (there are usually no more than five); it affects the size of the organisms in each succeeding trophic level, so that carnivores are usually larger than other species; and it absolutely limits the amount of energy available to organisms in each level.

Energy is lost at each trophic level for several reasons: heat is created as chemical bonds are changed into mechanical energy or other forms; energy is used for metabolic processes (especially respiration); the organisms at one trophic level are not completely utilized by those at the next higher level; and food passing through animals' digestive tracts is not completely digested. For example, only about 10% of the leaves of trees, and even less of the roots and stems, are eaten by insects. Much of the energy that does not pass through herbivores or carnivores eventually passes through scavengers and decomposers. Some of this energy may then flow into predators that feed on these forms and may thereby be passed back into the main energy pyramid. Aphids excrete much of the sugary sap that they suck from veins in leaves, which then appears as honeydew beneath the trees. When glucose is respired in cells, no more than 50% of its energy is converted into other chemical bonds; the rest is given off as heat.

The amount of energy stored in a given unit of time by green plants as a result of photosynthesis is called primary productivity. The amount of energy stored by animals in a given unit of time is called secondary productivity. The percentage of energy that passes from one trophic level to the next is called ecological efficiency. A third major ecological function is population regulation. One of the functions of herbivores is to control plant populations. The moth Cactoblastis cactorum, for example, was introduced into Australia to control a runaway population of prickly pear cactus (Opuntia) that had been introduced earlier.

Predators and parasites may also serve as controls over organisms they eat. Insects that prey on other invertebrates may indeed control their prey, which often cannot escape or hide. Such is the case with the ladybird beetle, which can control such pests as the mealy bug. With larger animals, such as foxes and rabbits, the situation is much more complex; the control of rabbits, for instance, may involve density- independent factors such as severe winters and drought, as well as density-dependent controls such as predators and parasites. (The control effectiveness of density-independent factors does not change as the population size changes, whereas that of density-dependent factors does. ) EVOLUTION AND ADAPTATION Darwin's theory of evolution was essentially ecological; he postulated that the organisms which will survive to reproduce are those that are best adapted to their environment.

Ecologists are concerned with how organisms adapt to their environment in order to survive. The ecological functions of an organism are said to constitute its niche. For example, an insect may be a predator, but it may not prey on organisms much larger or much smaller than itself. Also, diurnal predators normally do not capture organisms that are nocturnal or that live in HABITATS unsuited to the predator. Many ecologists are proponents of a principle called competitive exclusion, which states that each niche can be occupied by only one species, because when a required resource is limited and two or more species compete for it, one of the species will be eliminated unless it can evolve to occupy a slightly different niche. MODERN TRENDS AND FUTURE DEVELOPMENTS One of the major trends in ecology is the increased use of mathematical modeling, which often requires the use of computers.

Mathematical formulas are used to simulate population fluctuations, mineral cycling, and energy flow. Models can be used to discover where human knowledge is inadequate, to aid in making generalizations and formulating ecological principles, and to help predict the fate of ecosystems under given sets of circumstances. The growing field of systems ecology uses theoretical analysis and experimental methods to study the disruption of ecosystems and the dynamics of their reconstruction. Systems ecology requires people trained in a wide variety of disciplines: mathematics, computer technology, physiology, microbiology, biochemistry, climatology, and taxonomy. Ecologists are more and more involved in solving problems caused by increased human population, increased pollution, increased need for energy, and increased attempts to destroy ecosystems for human use. Ecology will be needed to help solve the problems of feeding the world's people, as well as to solve conservation problems ranging from the management of all kinds of native plants and wildlife to the preservation of ENDANGERED SPECIES..