THE PHENOMENON OF PREDATION — ERRINGTON 517 



ing, and part of tlie annual production of a pasture depends upon the 

 grazing pressure that it receives. 



PREDATION ON INVERTEBRATES 



The literature on biological control has among its bewildering fig- 

 ures and variables and mathematical models and claims and counter- 

 claims some examples of causes and effects that look quite clear. Some 

 of the evidence as to controlling or regulating influence of predation 

 upon invertebrate prey populations can be duplicated by experimenta- 

 tion practically at will, or verified by repeated observations of natural 

 events that fall into patterns. 



Granted that we must know what preys upon what, it is not dis- 

 advantageous to know about relative severities of predation drawn 

 by the prey, provided that we do not thereby conclude overmuch. 

 I have nothing against the idea of exploring what can be explored with 

 the aid of theoretical means, but I would hesitate to endorse anything 

 following the line of thought that a given theory must be correct 

 because it has no alternatives its proponents would rate as logical. 

 I confess also to a distrust of conclusions derived from mathematical 

 models that assume more randomness of contacts between predator 

 and prey than I am accustomed to see under natural conditions — 

 though, by this, I do not contend that randomness cannot or does 

 not occur in true-to-life equations. 



In general, the more patently the evidence comes from the land — 

 or the water — itself, the more reassured I feel as to its validity as any 

 sort of proof, one way or another. And, while even long-term experi- 

 mentation on the land with predator-prey (or parasite-host) relation- 

 ships very frequently gives rise to negative or inconclusive results, 

 there are enough convincing cases of populations of especially insect 

 prey responding either to increased or decreased predator (or parasite) 

 pressure to demonstrate causes and effects. Some of the examples 

 coming out of biological control experiments are by now classics in 

 the literature on predation. I suppose that almost everyone who has 

 done much reading in biology knows about lady-bird larvae preying 

 upon plant lice. Similar examples that are scarcely less celebrated 

 have been reported from many regions of the world. Indeed, the 

 books and review papers on biological control attest to a tremendous 

 amount of collective experience with this sort of thing and to the 

 frequency with which, among the invertebrates, a predator can in- 

 fluence the population levels of its prey; and the idea of managing 

 entomophagous insects through environmental manipulation, estab- 

 lishment of "refuge stations" in intensively cultivated areas, etc., is 

 not new in applied entomology. I am uncertain, however, as to how 

 effectively this type of management may mcrease an economically 

 desirable type of predation. 



