In biology, the term species refers to all organisms of the same kind that are potentially capable, under natural conditions, of breeding and producing fertile offspring. The members of a species living in a given area at the same time constitute a population. All the populations living and interacting within a particular geographic area make up a biological (or biotic) community. The living organisms in a community together with their nonliving or abiotic environment make up an ecosystem. In theory, an ecosystem (and the biological community that forms its living component) can be as small as a few mosquito larvae living in a rain puddle or as large as prairie stretching across thousands of kilometers.
A very large, general biotic community such as the boreal forest is called a biome.
It often is difficult, however, to define where one community or ecosystem stops and another starts. Organisms may spend part of their lives in one area and part in another. Water, nutrients, sediment, and other abiotic factors are carried from place to place by geologic forces and migrating organisms. While it might seem that a lake and the dry land surrounding it, for instance, are distinctly different in their environmental conditions and biological communities, there can be a great deal of exchange of materials and organisms from one to the other. Insects fall into the lake and are eaten by fish. Amphibians leave the lake to hunt on shore. Soil erodes from the land and fertilizes the water. Water evaporated from the lake surface falls back on the land as rain that nourishes plant life. Every biological community requires a more or less constant influx of energy to maintain living processes.
Several important ecological categories and processes characterize every biological community. Productivity describes the amount of biomass produced by green plants as they capture sunlight and create new organic compounds. A tropical rainforest or a Midwest corn field can have very high rates of productivity, while deserts and arctic tundra tend to be very unproductive. Trophic levels describe the methods used by members of the biological community in obtaining food. Primary producers are green plants that depend on photosynthesis for their nourishment. Primary consumers are the herbivores that eat plants. Secondary consumers are the carnivores who feed on herbivores. Top carnivores are large, fierce animals who occupy the highest level on the food chain or food web. Nobody eats the top carnivores except the scavengers (like vultures and hyenas) and decomposers (like fungi and bacteria) that consume dead organisms and recycle their bodies back into the abiotic component of the ecosystem. Because of the second law of thermodynamics, a majority of the energy in each trophic level is unavailable to organisms in the next higher level. This means that each successive trophic level generally has far fewer members than the prey on which they feed. While there might be thousands of primary producers in a particular community, there might be only a few top predators.
Abundance is an expression of the total number of organisms in a biological community, while diversity is a measure of the number of different species in that community. The arctic tundra of Alaska has vast clouds of insects, enormous flocks of migratory birds, and great herds of a few species of mammals during the brief summer growing season. Thus, it has high abundance but very little diversity. The tropical rainforest, on the other hand, might have several thousand different tree species and an even larger number of insect species in only a few hectares, but there may be only a few individuals representing each of those species in that area. Thus, the forest could have extremely high diversity but low abundance of any particular species. Complexity is a description of the variety of ecological processes or the number of ecological niches (ways of making a living) within a biological community. The tropical rainforest is likely to be highly complex, while the arctic tundra has relatively low complexity.
Biological communities generally undergo a series of developmental changes over time known as succession. The first species to colonize a newly exposed land surface, for example, are known as pioneers. Organisms such as lichens, grasses, and weedy flowering plants with a high tolerance for harsh conditions tend to fall in this category. Over time, the pioneers trap sediments, build soil, and retain moisture. They provide shelter and create conditions that allow other species like shrubs and small trees to take root and flourish. Larger plants accumulate soil faster than do pioneer species. They also provide shade, shelter, higher humidity, protection from sun and wind, and living space for organisms that could not survive on open ground. Eventually these successional processes result in a community very different from the one first established by the original pioneers, most of whom are forced to move on to other newly disturbed land. It was once thought that every area would have a climax community such as an oak forest or prairie grassland determined by climate, topography, and mineral composition. Given enough time and freedom from disturbance, it was believed, every community would inevitably progress to its climax state. It is now recognized, however, that some ecosystems experience continuous disturbance. Certain biological assemblages such as conifer forests that we once thought were stable climax communities, we now recognize as chance associations in an ever changing mosaic of regularly disturbed and constantly changing landscapes.
Many biological communities are relatively stable over long periods of time and are able to withstand many kinds of disturbance and change. An oak forest, for example, tends to remain an oak forest because the species that make it up have self-perpetuating mechanisms. When a tree falls, others grow to replace it. The ability to repair damage and resist change is termed "resilience." For many years there has been an on-going debate between theoretical and field ecologists about whether complexity and diversity in a biological community increase resilience. Theoretical models suggest that a population of a few very hardy, weedy species, such as dandelions and box elder bugs, might be more resistant to change than a more highly specialized and more diverse community such as a tropical forest. Recent empirical evidence suggests that in at least some communities, such as prairies, higher diversity does impart greater resistance to change and a better ability to repair damage after stress or disturbance.
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