ENVIRONMENTAL RELATIONSHIPS 



forms of life capable of survival under the changed conditions immigrate and 

 flourish. During long-continued progressive changes the environment may 

 harbor a series of many dififerent communities, each composed of plants and 

 animals adjusted to conditions of the moment, and each replaced by another 

 as conditions continue to change. The orderly sequence of communities thus 

 occurring is termed ecological succession. 



Illustrations of ecological succession are common. For example, there is an 

 annually recurring, seasonal succession in every temporary vernal pond. In 

 the early spring such a pond is inhabited by communities of aquatic organisms 

 requiring low temperatures; these are successively replaced by others as the 

 water becomes warmer, and when it eventually evaporates in summer, there 

 remains a damp or dry terrestrial habitat for exploitation by still other com- 

 munities. Over shorter periods, succession may be studied in laboratory 

 cultures of pond water. In these, we may observe the successive appearance, 

 dominance, and decline of various kinds of protozoans, and of gastrotrichs, 

 rotifers, copepods, and small annelids, as conditions change in the culture 

 from day to day. 



More striking examples of succession in nature occur on a larger scale and 

 over longer periods of time. These may be set in motion by such events as 

 the drying of rivers and lakes in prolonged droughts, the inexorable en- 

 croachment of sand dunes upon adjacent woodlands, and the filling of a pond 

 with silt to produce a bog and, eventually, dry land. Studies of such events 

 enable ecologists to chart the successive appearance and disappearance of 

 specific plant and animal types, and to correlate each with prevailing 

 conditions. 



The most impressive changes, and those that should provoke thought in 

 every citizen, are commonly the result of man's activities which alter the 

 environment, usually as a negligent side result of some worthwhile project. 

 Indiscriminately cutting or burning timberlands, cultivating or overgrazing 

 dry prairie grasslands, polluting harbors and streams with oil, industrial 

 waste, and sewage, man has destroyed ecological equilibrium conditions 

 established through centuries of slow successions of communities. There is 

 a growing public awareness of these problems; by analyzing and calling atten- 

 tion to the consequences of such practices, the ecologist makes a significant 

 contribution to the conservation and proper utilization of natural resources. 



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