Predation 395 



inroads of the predators on the population will be particularly serious. 

 The degree to which food "preferences" influence the feeding of 

 predators is of considerable importance, both theoretically and prac- 

 tically in relation to pest control, game management, and fishery 

 biology. The forage ratio of a prey species (A) in respect to a 

 predator species (B) is defined as the ratio of the percentage of B's 

 food made up by A to the percentage of the potential food organisms 

 in the environment represented by A: 



„ . %A in B's food 



r'orage ratio = 



%,A in environment 



A forage ratio of 1 indicates that the species is eaten in the same pro- 

 portion as its abundance in the habitat. An investigation of the 

 feeding of the black-nose dace (RJiinic]}tJ}ijs otrattihis atratuhis), for 

 example, revealed a forage ratio of 2.7 for Diptera and of 0.47 for 

 Trichoptera although the latter were about one quarter more abun- 

 dant in the stream. In this instance the Diptera were found to be of 

 size and in positions that rendered them more accessible to the fish 

 than were the Trichoptera (Hess and Swartz, 1940). 



Generally speaking, predators tend to eat the organisms that are 

 most available— either because of their abundance or because of their 

 accessibility. If a predator has a specific food preference or becomes 

 conditioned to continue feeding on the same species, serious conse- 

 quences may ensue both for the predator and the prey; but usually 

 when one type of food becomes scarce, the predator changes its diet. 

 Top minnows (Gambusia) are often stocked in reservoirs because 

 fishes of this type are known to eat mosquito larvae voraciously. Un- 

 fortunately for the mosquito-pestered human residents, when the fish 

 have reduced the larvae to low numbers, they turn to other food and 

 thus allow the mosquito population to recover. 



Studies by N. Tinbergen showed that during one winter, within a 

 certain locality, when voles (Microtus) were abundant, they consti- 

 tuted 86% of the food of long-eared owls (Asio otus) and wood mice 

 ( Apoclemus ) furnished 1%; whereas in the following winter when voles 

 were scarce, they formed only 30% of the owls' food, wood mice fur- 

 nished 15%, and other animals made up the difference. A more elabo- 

 rate investigation of the forage ratios of the sparrow hawk by L. 

 Tinbergen has been reviewed by Hartley (1947). These quantita- 

 tive aspects of prey-predator relations will be considered further in the 

 section on competition and in subsequent chapters dealing with the 

 interdependencies of the community as a whole. 



