EVOLUTION OF INTERSPECIES INTEGRATION AND ECOSYSTEM 



705 



or a parasite that causes a great decrease 

 in the numbers of its prey or hosts is also 

 eliminating its own food supply, so that 

 exploitation has elements of disoperation. 

 Disoperation is more severe when a 

 parasite or predator attacks a single species 

 of host or prey, and is probably less acute 

 when many species are exploited. More 

 efficient adaptation, however, is attained 

 through speciahzation, and these two tend- 

 encies lead toward a balanced compro- 

 mise, depending upon the quantitative 

 pressures involved in each case. Natural 

 selection must favor adaptations that tend 

 to bring opposing systems into equilibrium, 

 with a resulting evolution, at least in num- 

 erous instances, approaching toleration be- 

 tween species, one of which exploits the 

 other (Baylis, 1938). 



Animals depend upon plants directly 

 or indirectly for carbohydrates, certain es- 

 sential amino acids, and certain vitamins. 

 Plants in turn have become dependent to 

 a degree upon the activities of animals 

 through the nitrogen, carbon, and phos- 

 phorus cycles (pp. 497-499), the geologi- 

 cal influence of animals as soil modifiers 

 (Chap. 16), and through their role in 

 controlling plant enemies, dispersing seeds, 

 and as pollinating agents. A balanced 

 equilibrium between plants and plant-eat- 

 ing animals may be favorable to both. 



It is an interesting hypothesis that plants 

 have slowly become adjusted to the evolu- 

 tion of herbivores through various regenera- 

 tive and protective devices. Grasses, 

 through growth from the base of the leaf, 

 are adjusted to the grazing herbivores and 

 thus dominate the prairies where other 

 types of plants that grow at the end of the 

 stem are largely eliminated (Gunderson 

 and Hastings, 1944). Cacti, through the 

 development of spines, survive in desert 

 areas where the plant population is reduced 

 by the lack of water; without the spines 

 these same plants would probably be elim- 

 inated by the larger herbivores. Cattle 

 readily eat the prickly pear (Opiintia) 

 when the spines are burned off by man. 

 Camels have become adjusted to feeding 

 on desert plants and will even eat the spiny 

 prickly pear that has been introduced into 

 North Africa. Other desert plants may be 

 equally spiny or, hke the creosote bush 

 (Larrea), may evolve a repellent taste that 

 gives as effective protection as spines. The 



herbivores, near the bottom of the food 

 chain, are in turn exploited by the preda- 

 tors and parasites that tend to keep their 

 numbers down and thus prevent the over- 

 exploitation of the plants. 



Animals that are likely to be attacked by 

 predators have evolved defensive adapta- 

 tions such as the spines of porcupines and 

 hedgehogs; the poisonous spines of various 

 caterpillars (e.g., lo); the stinging appara- 

 tus of scorpion fishes and of bees; the se- 

 cretion of repellent fluids by millipedes, ter- 

 mite soldiers, ants, and toads; the protec- 

 tive coloration of walking sticks, sargassum 

 fishes, and mice; the swiftness of escape of 

 antelopes, squirrels, and crayfish; all match- 

 ing the evolution of the predaceous adapta- 

 tions of their enemies. The whole com- 

 munity tends, through the process of 

 natural selection operating on complex 

 coactions, to attain a relative equilibrium 

 sufficient to carry the quantitative pattern 

 of interspecies relations over long periods 

 of time. 



Equilibrium is here regarded as the con- 

 dition in which the rate of change of popu- 

 lation density on the average is approxi- 

 mately zero, and is, of course, independent 

 of absolute density (Smith, 1939). An over- 

 all equilibrium may be established for 

 average densities even for populations that 

 may fluctuate periodically in relation to 

 both regular and irregular environmental 

 variations. Elton (1930) says: "The num- 

 bers of wild animals are constantly varying 

 to a greater or less extent, and the varia- 

 tions are usually irregular in period and 

 always irregular in amphtude." This state- 

 ment is essentially true, but does not con- 

 tra-indicate the attainment of a compara- 

 tive balance in nature based upon long- 

 term population relations (pp. 305, 391, 

 507-522). 



The gross equihbrium of communities 

 can sometimes be detected through the in- 

 troduction of a species that has not evolved 

 with the system (p. 723). A good example 

 is the effect of the gypsy moth (Porthetria 

 dispar), introduced into Massachusetts in 

 1869. By 1890 so much destruction of 

 forest and shade trees had occurred that 

 the State Board of Agriculture instituted 

 control measures and the pest was reduced 

 considerably. In 1900 the state appropria- 

 tions were discontinued with consequent 

 rapid increase and spread that still con- 



