766 ANIMALS AND THEIR ENVIRONMENT I 



effects. II this did not occur the parasite would eventually exterminate 1 



the host species and, unless it lound a new species to parasitize, would die 



itself. 



Studies of many examples of parasite-host and predator-prey asso- 

 ciations show that in general, when the associations have been estab- 

 lished for a long time, evolutionarily speaking, the long-term effect on 

 the host or prey species is not very detrimental. Conversely, newly ac- 

 quired predators or parasites are usually quite damaging. The plant 

 parasites and insect pests that are most troublesome to man and his 

 crops are usually those which have recently been introduced into some 

 new area and thus have a new group of organisms to attack. 



The role of the predator-prey relationship in maintaining a balance 

 between the number of predators and of prey is beautifully illustrated 

 by the story of the Kaibab deer. The Kaibab plateau is located on the 

 north side of the Grand Canyon of the Colorado river. In 1907 there 

 were some 4000 deer living on the plateau, together with a considerable 

 population of predators, mountain lions and wolves. When a concerted 

 effort was made to "protect" the deer by killing off the predators, the 

 deer population increased tremendously and by 1925 some 100,000 deer 

 roamed the plateau, far more than the supply of vegetation could sup- 

 port. The deer ate everything in reach— grass, tree seedlings and shrubs— 

 and there was marked damage to the vegetation. Over the next two 

 winters large numbers of the deer died of starvation and the size of the 

 herd fell to about 10,000. In the wild, the size of the predator popula- 

 tion varies with the size of the population of the species which is preyed 

 upon, with the swings in the size of the predator population lagging 

 somewhat behind those of the prey. 



342. Intraspecific Relations 



In addition to the associations between the members of two different 

 species just described, aggregations of animals or plants of a single species 

 frequently occur. Some of these aggregations are temporary, for breed- 

 ing; others are more permanent. Despite the fact that the crowding 

 which accompanies dense aggregations of animals is ecologically un- 

 desirable and deleterious, both laboratory experiments and field obser- 

 vations show that such aggiegations of individuals are able to survive 

 when a single individual of the same species placed in the same environ- 

 ment dies. A herd of deer, with many noses and pairs of eyes, is less 

 likely to be surprised by a predator than is a single one. A pack of 

 wolves hunting together are more likely to make a kill than is a lone 

 wolf. The survival value of aggregations is less obvious, but nonetheless 

 real, in some of the lower animals. It can be shown experimentally that 

 a group of insects is less likely to dry up and die in a dry environment 

 than is a single insect, and a group of planaria is less likely to be killed 

 by a given dose of ultraviolet light than is a single flatworm. When a 

 dozen goldfish are placed in one bowl and a single one in a second 

 bowl, and the same amount of a toxic agent such as colloidal silver is 

 added to each bowl, the single fish will die but the group survives. The 



