256 



ANALYSIS OF THE ENVIRONMENT 



Whether a parasite finds all the suitable 

 habitats that exist depends on four main 

 relations: (1) The plant or animal host 

 must live in the same geographical region 

 inhabitated by the parasite. (2) The gen- 

 eral habits and ecological relations of the 

 two must be such that parasite and host 

 come together when the parasite is infec- 

 tive and the host is open to invasion. Among 

 other considerations this means that (3) 

 the life cycles of the two must be suffi- 

 ciently synchronized so that the parasite can 

 gain the necessary foothold or entry; (4) 

 population density and mode of dispersal of 

 both parasite and host may be governing 

 factors, as in the relations between many 

 plant-eating insects and their insect para- 

 sites (see p. 380), These and other aspects 

 of parasitology are discussed by Hegner, 

 Root, Augustine, and Huff (1938) with 

 some general ecological emphasis. 



The organisms that have become estab- 

 lished in living habitats belong to two in- 

 formal series that are more or less distinct. 

 There are those that undergo regressive evo- 

 lution (p. 676) and become finally, as in the 

 adult stages of the crustacean Sacculina, 

 primarily bags enclosing reproductive or- 

 gans with suitable devices for attachment 

 and for absorbing host tissues as food. At 

 the other extreme are the active, unde- 

 generate trypanosomes such as cause Afri- 

 can sleeping siclcness in man and rinder- 

 pest in many ungulates. Size relations ex- 

 cepted, there is much similarity between 

 the feeding of these internal carnivores and 

 external ones, such as the coyotes or pumas 

 (with which we are more familiar), pro- 

 vided we focus on generalities rather than 

 on details, however important the details 

 mav be in other connections. 



Elton (1927) stressed the different size 

 relations between predator and prey and 

 parasite and host as one of the outstanding 

 characteristics of parasitic rektionships. 

 "The parasite," he states correctly, "cannot 

 exceed a certain size without harming its 

 host too much." Another sentence of Elton's 

 that is much ouoted seems to us to be less 

 apt. He says (p. 72) : "The difference be- 

 tween the methods of a carnivore and a 

 parasite is simoly the difference between 

 living on capital and tipon income; between 

 the habits of the beaver, which cuts down 

 a whole tree a hundred years old, and the 

 bark beetle, which takes a daily toll from 



the tissues of the tree; between the burglar 

 and the blackmailer." 



This whole matter falls into truer per- 

 spective when we remember that the beaver 

 is preying on some few individuals annually 

 from a population of trees, just as the bark 

 beetle is taking a toll from the population 

 of cells that together make up an individual 

 tree. Similarly, the marsh hawk is not hving 

 on capital when it kills off vulnerable bob- 

 white quail that have been produced be- 

 yond the year-round carrying capacity of 

 the area. Both external predator and inter- 

 nal parasite are normally adjusted to their 

 food supply so that in an ecosystem that 

 approaches balance, the welfare of the pop- 

 ulations preyed upon, like that of the spe- 

 cies serving as host to parasites, is not dis- 

 turbed too much. Some of the similarities 

 between herbivore and animal parasite 

 within a plant are even more readily appar- 

 ent. Grasshoppers are important grass con- 

 sumers; they also obtain shelter from the 

 dense tufts, especially when near the 

 ground. Nematode worms, parasitic in the 

 grass blade or stem, likewise obtain both 

 food and shelter. Both grasshoppers and 

 nematodes depend on present or former 

 growth processes of the grass; both normally 

 live on income rather than on capital. 



The similarity between predatism and 

 parasitism is emphasized when the blood- 

 sucking habit is considered. Are blood-suck- 

 ing bats, bugs, flies, or leeches predators or 

 parasites? They may or may not remain on 

 the host; lice do; bedbugs do not. They may 

 fly actively and still remain near their food 

 supply, as do even the winged species 

 among the hippoboscid flies, or they may 

 live somewhat apart from their sources of 

 food and hunt it actively, as tabanids fly 

 towards horses and other suitable animals 

 that come near their habitat. Still another 

 variation in the comparison between preda- 

 tism and parasitism is furnished by the 

 parasitoid insects. The larva of an ichneu- 

 mon fly slowly devouring a caterpillar from 

 within is scarcely less predatory than is a 

 wolf rapidly devouring a deer from without. 

 In the former case the hunting was done by 

 the preceding generation, but even this may 

 be compared to a wolf hunting prey for its 

 recently weaned cubs. The basic difference 

 lies in the relation to dispersal. 



A type of social parasite, the robber is 

 exemplified by various insects and birds. 

 The bald eagle (Haliaetus leucocephalus) 



