342 Relations within the Species 



find conditions suitable for rapid growth and reproduction. Unim- 

 peded, geometric increase in the population ensues until all suitable 

 food or other necessity on the island is exhausted. Thereupon starva- 

 tion, and often disease, decimate the population, and the death of all, 

 or nearly all, of the animals quickly follows. 



More usual than either of the foregoing is the third type of situa- 

 tion in which the population overshoots its equilibrium level, but, 

 after a reduction in numbers, conditions are ameliorated sufficiently 

 for the population to increase again. The repetition of this process 

 causes fluctuations. If these are small, the population curve is desig- 

 nated as flat; if the amplitude is large and regular, the curve is termed 

 cyclic, and, if irregular, especially with sudden periods of great in- 

 crease, it is called irruptive (Fig. 9.13). Some ecologists refer to 



Time 

 Fig. 9.13. Diagram of types of fluctuations in populations. 



regular changes in abundance as oscillations and to irregular changes 

 as fluctuations, but the more specific terminology given above will be 

 employed here. The reader should understand that all gradations ex- 

 ist between these types of population change. 



Fluctuations after a population has approached its equilibrium level 

 may be caused in whole or in part by changes in physical features of 

 the environment or in biotic influences such as the abundance of 

 predators, diseases, or food organisms. These population changes 

 due to the interaction of different species will be considered in detail 

 in later chapters. At this time we wish to emphasize the fact that 

 conditions within the population of a species can themselves be re- 



