GENERAL ZOOLOGY 



falling off of their main article of food. We may thus reasonably assume that the 

 black bass would recover its numbers because of the decrease of pickerel and bull- 

 heads, the enemies of its young. A further study of the diagram shows that a 

 balance between the numbers of the various groups of the community would soon 

 result. Under other circumstances, such as the extinction of the black bass, the 

 resulting condition would be entirely different from the original one, but a balance 

 between supply and demand would nevertheless finally be established. ' 



Thus, it is clear that any change in the environment, at any point in the 

 food web, disturbs the existing dynamic equilibrium. The disturbance then, 

 so to speak, echoes and re-echoes through the community, until a new, equally 

 unstable equilibrium is established. 



Shelford's statement also reveals something of the highly competitive nature 

 of the relationship between different predators. Members of the same species, 

 as well as different species of the same size and food habits, compete for a 

 limited amount of food. Competition is not limited to food relations, how- 

 ever; individuals of the same species compete with each other for mates, and 

 many diverse members of a community are in competition for space and for 

 other requirements supplied by the environment. These competitive relation- 

 ships should not be thought of as necessarily involving actual striving, or 

 physical conflict; it is quite possible for two competitors never to encounter 

 one another, if their habits are sufficiently different. The presence of one 

 species may affect the environment in such a way as to make it unsuitable for 

 another species, and very slight and indirect effects may be sufficient to tip 

 the scales and lead to the extinction of a competitor. 



With some knowledge of the approximate competitive relationships within a 

 community, it is possible to predict the probable results of certain environ- 

 mental changes, as Shelford has done in his analysis of the pond community. 

 It is much more difficult to isolate two interacting species from a community 

 for controlled experimentation, and to obtain quantitative data describing 

 their interaction. This has been done, however, with two species of ciliate 

 protozoans, with the results shown in Figure. 19.7. When Didimum nasutum, 

 a predatory ciliate, is introduced into a sedimentlcss culture of Paramecium 

 caudatum, the predator multiples until it has exhausted the supply of its food, 

 Paramecium, after which the population of Didimum declines and dies out. 

 When sediment is added to a similar culture, Didimum thrives for a short 

 time and then disappears, whereupon Paramecium, some of which have escaped 

 destruction by hiding in the sediment, is found to have survived the predator 

 and to flourish with its extinction. To imitate another possibility in nature, 

 periodic "immigrations" of Didimum can be arranged, with the result that 

 the two populations show successive reciprocal peaks and declines. It is 

 interesting, and has been substantiated by observations in natural communi- 

 ties, that the population peaks of predator and prey do not coincide; rather, 

 as the numbers of predators increase, the intensity of predation also increases 



From V. E. Shelford, Animal Communities in Temperate America, copyright 1913 by Geo- 

 graphic Society of Chicago. Reprinted by permission. 



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