FIG. 12-3 Development of a parasitoid black digger wasp, (a) 

 eggs in position on the host larva, (b) the developing larva, 

 (c) the fully grown larva devouring the remainder of the host 

 (courtesy Illinois Natural History Survey). 



munitively to the alien parasite, and may sicken, even 

 die, of the effects of the parasite's presence, the same 

 effects in kind and intensity which the normal host 

 easily takes in stride. For instance, a trypanosome 

 that is a natural parasite in many of the larger wild 

 mammals of Africa evokes no spectacular effects in 

 its usual hosts. But when the parasite is vectored by 

 the tsetse fly to man, it causes sleeping sickness ; to 

 cattle, nagana. A good bibliography of references to 

 diseases in wild mammals and birds is given by Hal- 

 loran (1955) : Davis (1946) discusses diseases of 

 fish, especially trout. 



PARASITOIDISM 



Some Diptera and Hymenoptera deposit 

 their eggs in the immature stages of other insects ; 

 the larvae on hatching feed on the host until they are 

 full grown. The relation of the larva to its host is fre- 

 quently described as one of parasitism. But it is 

 fundamentally different from parasitism in that the 

 host invariably dies of the larval depredations before 

 the larva emerges, but the larva invariably lives in 

 spite of the host's death. The relationship resembles 

 that of predator to prey, except that, unlike the true 

 predator, the larva lives within the body of its prey 

 and kills it slowly as it feeds, not suddenly before it 

 feeds. Such larvae are, for these reasons, best thought 

 of as parasitoid s. 



Parasitoids may in turn be infested with hypcr- 

 parasitoids. In the Chicago, Illinois, region Samia 



cecropia, a saturniid moth, suffers the destruction of 

 nearly 23 per cent of its cocoons by an ichneumonid 

 parasitoid, Spilocryptus extrematis, which deposits 

 an average of ii eggs on the inside of each cocoon or 

 on the surface of the larva. The host larva dies in a 

 few hours after the parasitoid hatches, and the ich- 

 neumonid larva moves about freely, feeding on the 

 cuticle or burrowing into the tissues to drink the 

 body fluids. Another ichneumonid, Aenoplex smithii, 

 was found as a secondary parasitoid, feeding on the 

 larvae of -S". extrematis in about 13 per cent of the 

 cecropia cocoons infested by the latter species. A 

 chalcidid, Dibrachys boucheaniis, fed both upon S. ex- 

 trematis and, as a tertiary parasitoid, upon A. smithii. 

 Another chalcidid, Pleiirotropis tarsalis. infected co- 

 coons containing D. boucheaniis and eventually killed 

 the larva as a quarternary parasitoid (Marsh 1937). 

 To have five links in an inverted parasitoid food chain 

 is perhaps unusual, but hyperparasitoidism is com- 

 mon and of importance in controlling the size and 

 interrelations of animal populations. 



Predation is a form of disoperation, at least in 

 point of immediate effects, since one animal kills an- 

 other for food. Predation is important in community 

 dynamics in so many ways that we will postpone dis- 

 cussion of it until we consider food coactions (Chap- 

 ter 13), productivity (Chapter 14), and regulation of 

 population size (Chapters 16, 17). 



COMPETITION 



Competition is the more or less active de- 

 mand in excess of the immediate supply of material 

 or condition exerted by two or more organisms 

 (Clements and Shelford 1939: 159). The materials 

 and conditions sought by animals include food, space, 

 cover, and mates. When these materials are in more 

 than adequate supply for the demands of those organ- 

 isms seeking them, competition does not occur ; when 

 they are inadequate to satisfy the needs of all the or- 

 ganisms seeking them, the weakest, least adapted, or 

 least aggressive individuals are forced to do without, 

 or go elsewhere. Competition may result in death for 

 some competitors, but this is from fighting or being 

 deprived of food or space rather than being killed for 

 food as in predation, or by disease as in extreme 

 parasitism. 



Competition may be either direct or indirect. It 

 is direct where there is active antagonism, struggle, 

 or combat between individuals : indirect, when one 

 individual or species monopolizes a resource or ren- 

 ders a habitat unfavorable to the establishment of 

 other organisms having similar requirements. Direct 

 competition, or interference, is evident in the fighting 

 of bull seals for larger harems and of grouse for a 

 better position in the social hierarchy ; in chasing and 



182 Ecological processes and dynamics 



