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, tropical jungles and rain forests.

Chapparals (shubby forests) and coniferous rain forests are sometimes also considered biomes. The distinctive appearance of each biomes 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 required 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 th 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 emergents, grow above the canopy trees; below the canopy trees are shorter trees; below the shorter trees are shrubs; and covering the forests floor is a layer of herbaceous (nonwoody) plants growing in soil inhabited by fungi and bacteria. Some charcteristic animals are also found in each of the strata, such as toucans in the canopy, but most animlas range trhough 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 (acitve 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 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 condition.

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 of seeds at the periphery of the disturbed area, as well 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, 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.