Regio7ial Differences 487 



has already been pointed out. Furthermore it should be noted that 

 the biotic potential of prey species of animals is usually much higher 

 than that of the predatory species. This relationship helps to retard 

 the growth of the more remote links of the food chain. As prey be- 

 comes scarcer, predators have more and more difficulty in finding 

 them so that their rate of feeding is automatically reduced. Often 

 predators turn to other species at this time, relieving the pressure on 

 the species originally used as a source of food. 



This type of check also applies to man as a predator to some extent. 

 When deer become scarce, sportsmen may voluntarily desist from 

 hunting sufficiently to allow the population to recover. In some man- 

 aged fish ponds unrestricted fishing by hook and line can be permitted. 

 After the number of fish in the pond has been greatly reduced by the 

 anglers, so much food is available for the remaining fish that they 

 are too well fed to take bait. When the depletion of an oceanic 

 fishery reaches the point where fishing for this species is no longer 

 profitable, fishermen turn to other species. In such instances, if the 

 population has not been reduced below the point of recovery, as dis- 

 cussed in Chapter 9, and if the habitat has not deteriorated in the 

 meantime, the population may be able to return to its former 

 abundance. 



Regional DiflFerences. The total quantity of life that can be sup- 

 ported in an area at its full carrying capacity may be made up of a 

 few individuals of a large number of species or many individuals of a 

 few species. Generally speaking tropical communities tend to ex- 

 hibit the former condition and non-tropical communities the latter, but 

 there are many exceptions. The fauna and flora of tropical biotopes 

 are typically very rich in species but an observer often has difficulty 

 in finding more than one or two specimens of each kind of animal or 

 plant. The diversified nature of tropical vegetation is illustrated by 

 the results of a transect 2 m wide and 150 m long run from the edge 

 to the center of Castellow Hammock near Miami, Florida, in which 

 111 species of flowering plants were found belonging to 98 genera and 

 48 families (Phillips, 1940). 



In contrast, the vegetation in high latitudes may be very monotonous 

 from the point of view of species composition. The great spruce 

 forest extending across the North American continent and Eurasia con- 

 sists of but a few species of trees with a relatively small number of 

 subordinate plants. The same general contrast in relation to numbers 

 of species and individuals occurs among the land animals and also in 

 the biota of aquatic areas ( Table 21 ) . Tropical marine areas, for 

 example, support hundreds of species of fish with relatively small 



