Beneath shallow water, benthic communities show seasonal and annual fluctuations (see Section 5.4.4), but over the long term they usually remain fairly constant in numbers and composition, indicating that the major factors moulding the community are stable. Because every community includes a range of species each having slightly different tolerances, there is consequently an inherent capacity to adapt to minor changes of environmental conditions by corresponding adjustments of community structure. Changes in any parameter, for example temperature, are likely to influence recruitment and mortality of one species differently from another, with the result that the proportion of one species increases while another declines but does not necessarily become eliminated. If conditions return to their former state, the balance of species responds accordingly and the overall pattern of the community remains (Buchanan et al., 1978).
Rapid, permanent changes of population are usually associated with major alterations of the environment, often the result of human activity; for example, a change of water circulation or temperature connected with industrial installations, or a change of the sea floor due to dredging or the dumping of waste at sea (e.g. Howell and Shelton, 1970). Changes may also follow the introduction of a new species to an area. Otherwise the continual minor fluctuations are largely self-cancelling. Any tendency for one species to increase is eventually counteracted by increasing competition and predation so that a dynamic equilibrium is maintained and the natural balance of the community is preserved.
Over very long periods, climatic or geological changes may slowly alter the environment, and there are also gradual and permanent modifications brought about by the activities of the organisms themselves; for example, the substrate becomes changed by accumulations of shells or skeletons or through the erosion of rocks and stones by the boring of various plants and animals, and the composition of the water is influenced by biological processes of extraction, precipitation and secretion. The continual interactions between habitat and community lead very gradually to changes in both, and the ecosystem, which comprises both environment and population, is thus a composite evolving unit.
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