ENVIRONMENTAL RELATIONSHIPS 



Fig. 19.7. interactions brlween 

 populations of Dif//niiim (pred- 

 ator) and Paramecium (prev) 

 under various experimental con- 

 ditions. In a clean culture (I). 

 the predator dies out after ex- 

 hausting its food supplv. Pro- 

 vided with sediment in which to 

 take refuse, enousjh paramecia 

 survive to flourish ae;ain after 

 Didinium disappears, having; 

 destroved all the prev it can 

 capture (II). When the popu- 

 lation of /-"rtr^wcrn/w is aus;mented 

 by periodic "immigrations," 

 (III), the two species may be 

 maintained in a dynamic equi- 

 librium of numbers. (Adapted 

 from G. F. Gause, The Slruga/c 

 for Existence, copyright 1934 by 

 Williams and Wilkins, Inc. This 

 modification of Gau.se's original 

 figures is from W. C. AUee el al.. 

 Principles of Animal Ecology, 

 copvright 1949 bv W. B. 

 .Saunders, Inc., reprinted by 

 permission.) 



Prey 



Prey 



III 



Prey 



Time 



and drives down the population of the prey species. Principles established by 

 experimentation of this kind can be extended to the interpretation of 

 predator-prey relationships, such as between two mammals, for which it 

 would be impossible to set up exact experiments. An important feature of 

 such experimental work is that the results are capable of mathematical ex- 

 pression, yielding data useful in the construction of mathematical models of 

 conditions in natural communities. From such models it is possible to cal- 

 culate and predict with some confidence the detailed effects of particular 

 environmental changes. 



The ''balance of nature" is a phrase that has long been popularly used to 

 describe the harmonious relationships between organisms and their environ- 

 ments. One important result of ecological studies has been to demonstrate 

 that though there is a balance in nature, it is not a static equilibrium but 

 one that even a slight change may disturb in unexpected ways. It is now 

 possible to understand, for example, that the introduction of a few rabbits, or 

 prickly-pear cactus plants, or Japanese beetles, or gipsy moths, into a 



613 



