GENERAL ZOOLOGY 



This also offers an explanation for the fact that there are seldom more than 

 two or three successive carnivorous links in a food chain. 



The relationships just discussed in terms of the pond community exist, in 

 a generally comparable way, in any natural community. In all, there are 

 plants, herbivores, and varying numbers of carnivorous animals forming inter- 

 locking food chains. Among ecologists, each of these levels of activity is 

 spoken of as a niche,- and the niche occupied by a particular species in a com- 

 munity conveys an idea of its relationships with other members of the com- 

 munity. The types of animals occupying the various niches differ, of course, 

 in different environments, but the parts they play are always comparable. 

 For example, the very important niche of the chief herbivore in the com- 

 munity is filled in aquatic habitats by small crustaceans; in woodland and 

 grassland communities, various small rodents, such as mice and rabbits, 

 serve this function; in the arctic tundra, the principal herbivore is the 

 lemming. In any community the maintenance of the whole superstructure of 

 the food chains depends directly on the activities of the herbivores, and the 

 activities of these animals are often referred to as the "key industries" of 

 the various communities. 



Exploring these relationships a bit farther, we find that in any environ- 

 ment there are usually a certain number of ecologically important niches to 

 be filled. If through some catastrophe all the occupants of a particular 

 niche in a community become extinct, the niche will eventually be occupied 

 by some other type of animal. The new occupant may be an immigrant from 

 another community, whose spread has been favored by the availability of the 

 unoccupied niche. Alternatively, over a longer period of time, the vacant 

 niche may be filled by the evolutionary appearance of a new form of life, 

 adapted to the functions of the particular niche through natural selection 

 from some ancestral type. In the fauna of oceanic islands, and of larger but 

 similarly isolated land masses such as Australia, there are many examples of 

 the formation of entire communities of animals through "adaptive radiation" 

 from a generalized common ancestor. For instance, until the introduction of 

 placental mammals into Australia and New Zealand, relatively late in historic 

 time, all the mammalian occupants of various niches there were marsupials, 

 with the minor exception of a few prototheres such as the platypus. The 

 marsupials had evidently descended from a primitive, generalized ancestor 

 of the opossum type, adapted in a bewildering variety of ways as herbivores 

 and carnivores, with habits and requirements suited to the functional 

 demands of the environment and the community (Fig. 19.5). 



The complex interrelationships of the community are not necessarily in- 

 variable, however. There are many instances, as with ruminant mammals, 

 in which herbivores are relatively large animals, and above these the food 

 chain may consist of only a single carnivorous member. Also, many animals, 

 by the development of special feeding adaptations or techniques, have found 

 it possible to by-pass food chains. The whalebone whales, among the largest 

 animals of the earth, feed almost exclusively upon microscopic crustaceans. 



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