230 The Evolution of Communities 



proportion of insects (Scott and Klimstra, 1955). 



Conversely, the larger the number of predator species of all 

 categories, the more the prey populations are kept below the level 

 of competition and the larger the number of prey species which 

 can coexist. 



These two reactions lead theoretically to the conclusion that an 

 increase in number of species and in complexity of food chains 

 automatically tends to perpetuate the host-prey complexity. 



AGE OF HOST-PREDATOR RELATIONSHIPS 



Many host-predator relationships are between extremely distantly 

 related groups. In the marine Mollusca which feed on microorgan- 

 isms, the predatory relationship may be extremely old and may 

 perhaps trace back to a pre-Cambrian time when the ancestors of 

 the Mollusca were closely related to the ancestors of the micro- 

 organisms. In other cases of distant relationship between host and 

 prey the predatory relationship is not this old. In the case of insects 

 feeding on plants, for example, the insects and the plants represent 

 two specialized phylogenetic lines whose common ancestor was 

 back in the far pre-Cambrian time, probably a billion years ago. 

 The host-predator relationship between them represents a much 

 more recent change in food habits on the part of the insects long 

 after insects as such came into being. The cynipid gall wasps of 

 the genus Cynips and allied genera make galls in plant tissues, but 

 the ancestors of these wasps were parasites of other insects. After 

 cynipids had evolved from other parasitic Hymenoptera, therefore, 

 one line made the transfer of food habits from insect larvae to 

 plant tissue, probably from a form parasitizing a leaf-mining or 

 twig-boring insect. The plant-inhabiting Cynipidae comprise so 

 many species, however, that the host transfer to plants must have 

 occurred at least as long ago as earliest Cenozoic time or more 

 probably in Mesozoic time. 



The complex food chains in a modern biotic community there- 

 fore represent food relationships of various ages, ranging from 

 very ancient to relatively recent changes in community relation- 

 ships. Being a dynamic situation, similar changes are undoubtedly 

 taking place now. 



It is generally conceded that the predator species in any one 

 feeding level (called a trophic level) can consume on the average 

 only 15 to 20 per cent of the mass of the food species. This per- 

 centage, however, refers to total weight of predators in relation 

 to total weight of prey. The number of species involved bears no 

 relation to these weight proportions. A single species of mammal 

 may be preyed upon by dozens of species of internal and external 



