GENERAL ZOOLOGY 



"balanced" community is by no means such a minor event as it might seem. 

 Relieved of the checks which restrain them in their normal environments, 

 presented with abundant food supplies for which there is no strong competi- 

 tion, out of reach of their natural predators, they flourish unhindered, with 

 far-reaching effects on the equilibria of established communities. Where 

 man's livelihood and food supplies are affected, the economic cost of such 

 importations is staggering. Under normal conditions the establishment of a 

 new equilibrium, adjusted to the presence and activities of the immigrant 

 species, is a very gradual process. However, through careful studies of the 

 environments from which the immigrants came, it is often possible to discover 

 parasites which operate there to keep them in check. Very often these para- 

 sites can be introduced into the invaded area, and through their activities 

 the undesirable alien species may be brought under control. Rabbits have 

 become a severe economic pest in Australia since they were introduced there; 

 their numbers have increased in spite of hunting, trapping, fencing, and 

 poisoning. Recently, their numbers have been much reduced by the intro- 

 duction of a virus disease to which most of them are susceptible. It has been 

 reported, however, that strains resistant to the disease have appeared, and it 

 will not be surprising if breeding from these stocks repopulates the range with 

 rabbits. The prickly-pear cactus was introduced into parts of Australia as an 

 ornamental plant, but it spread so widely that millions of acres of grazing 

 land in Queensland and New South Wales became covered with impene- 

 trable thickets of the cactus. In 1925, several thousand eggs of the cactus 

 moth, Cactoblastis, were collected in Argentina and sent to Australia to 

 establish a colony of this species. The results of this introduction have been 

 remarkable; the larvae of the moth feed upon the cactus plants, and, with the 

 aid of a fungus which grows in their burrows, have destroyed the plants in a 

 large part of their former range. Many other examples of the utility of such 

 methods of "biological control" could be cited. These importations of con- 

 trolling organisms must, of course, be conducted with the utmost care that the 

 parasite does not, itself, have unexpected effects in a new environment on 

 species other than its normal host. 



Ecological Succession. Even the most stable environment, though 

 equilibrium conditions prevail, is not constant or unchanging but undergoes 

 continual ffuctuations in both physicochemical and biotic characteristics. 

 One aspect of successful adaptation to a habitat involves the ability to sur- 

 vive the customary periodic changes in any of its features. Environments 

 may, however, undergo progressive alterations in a particular direction, as a 

 result of which they become at length completely different environments. If 

 these changes are sufficiently gradual, they may permit progressive adaptation 

 on the part of the indigenous species, which thus survive with new adjust- 

 ments. With more rapid environmental shifts, the limits of tolerance of the 

 indigenous species are eventually exceeded in one or more respects, and these 

 species, dying out or emigrating, cease to exist in the changed environment. 

 Ordinarily, such consequences do not leave the habitat sterile, for other 



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