THE DOUBLE-CRESTED CORMORANT. 19 



Food supply is largely dependent upon conditions outside 

 the species and without its control; inasmuch as food habits 

 can be changed, within certain broad lines, they are flexible. 

 Should the usual source of supply fail, the species can usually 

 adapt itself to others. However, food supply must be obtained 

 and is a most important agent in limiting the numbers of any 

 and all species. It is also a necessity that occurs throughout 

 the life of an individual. It avails nothing that food should be 

 abundant for the adult if it is scarce or absent during develop- 

 ment. The effect of starvation is immediate. A subnormal 

 birth-rate reduces population slowly, acting over generations. 

 Reduction of numbers through enemies may be much quicker 

 and completed within a few seasons or less, but starvation acts 

 almost instantaneously and in most cases is an operation of 

 days instead of weeks, years, or generations. 



The effect of enemies upon a species is complicated in 

 results; the species preying and preyed upon re-acting on each 

 other in various ways. In general, a species is seldom if ever 

 absolutely exterminated by these means. As soon as a food 

 animal becomes too scarce to be profitably hunted, its pursuit 

 is neglected and thereafter only occasional or accidental in- 

 dividuals are taken. On the other hand, any marked increase 

 of food animals is followed by an increased attention from their 

 present enemies and an influx of new ones from adjoining ter- 

 ritories. If reduction of food supply stopped when normality 

 was reached the result would be comparatively simple, but, 

 while the number of enemies in the locality is supernormal their 

 food supply has at this stage been reduced to normal. Attention 

 is eventually turned towards other food sources, but, as the 

 enemy population is temporarily greater than the supporting 

 powers of the habitat, it is unusually active and keen, hunting 

 more carefully, with greater persistence than usual, and con- 

 sequently with greater effect. For a while, at least, the re- 

 duction process is thus continued and the food supply, or species 

 preyed upon, falls below normal. Finally, however, the enemies 

 also are reduced either through starvation or movement to 

 other localities until their population is also brought below normal 

 numbers in harmony with the reduced resources of the habitat. 



