BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



261 



pathogenicity to some kind of innate incom- 

 patibility between the parasite and certain 

 hosts. Ball's main argument is based on the 

 well-authenticated observation that popula- 

 tions of parasites experimentally introduced 

 into new host species do not necessarily per- 

 sist, to say nothing of running riot and kill- 

 ing the new host species. This happens even 

 though the parasite is introduced into fairly 

 close relatives of its usual hosts, and is illus- 

 trated by experimentally introducing various 

 strains of bird malaria into species of birds 

 that they do not normally inhabit. Whatever 

 the evolutionary background may be, in- 

 fected populations often form reservoirs of 

 infection that are a potential danger to other 

 possible hosts in which immunity for the 

 parasite in question has not been developed. 

 Various rodents, including the common rat, 

 ground squirrels in certain regions, Mongo- 

 lian marmots, and others, serve as living res- 

 ervoirs of Pasteurella pestis, the causative 

 organism of bubonic plague. Rat fleas carry 

 the bacteria to man, who notably lacks re- 

 sistance to its ravages save in the disease 

 called pestis minor. 



Rocky Mountain spotted fever, a rickett- 

 sian disease, is carred by ticks of several 

 species. These in turn feed on small wild 

 rodents such as various ground squirrels, 

 without doing them notable injury. If in- 

 fected ticks bite men, the resulting human 

 mortality rate is rather high. Hogs and 

 rats are reservoirs of infection of the nema- 

 tode Trichinella spiralis, the organism that 

 produces trichinosis in man. 



There is a vast literature on this general 

 subject, especially as related to man and his 

 domestic animals. Hull (1930), Riley and 

 Tohansen (1938), Herms (1939),' and 

 Chandler (1944) will serve as introduc- 

 tions. The relations between bacteria and 

 animal parasites and the manv populations 

 of animals in which man is only slightly in- 

 terested have, as yet. received relativelv 

 little attention. Enough has been done to 

 suggest that the key to many present day 

 ecological puzzles may be found as our 

 knowledge of general host-parasite relations 

 is extended. 



Parasites, whether animal, bacterial, or 

 virus, produce diverse kinds of crippling or 

 fatal diseases when thev gain entrance to 

 nonimmune hosts. Study of the various 

 types of natural or acquired immunity is a 

 phase of biology that is essentially an aspect 

 of ecology. It is, however, mainly developed 



in human or veterinary medicine as regards 

 virus, bacterial, and protozoan parasites. 

 Often the immunity reaction is called forth 

 by the presence of a nonlethal population 

 of parasites and hence is a direct reaction of 

 the host organism to its invaders such that 

 the latter are rendered temporarily or per- 

 manently impotent or are completely de- 

 stroyed. Here again the interested reader is 

 referred to the immense medical literature 

 on immunity to which Topley (1933) gives 

 a good introduction. Certain of the direct 

 effects of disease on populations will be dis- 

 cussed in a later section (p. 381), Various 

 students believe that the lack of organiza- 

 tion apparently characteristic of viruses 

 marks them as essentially parasitic organ- 

 isms, the resultants of regressive evolution. 



To return to more general matters: It is 

 well to remember that parasitism is difficult 

 to delimit accurately. There are many and 

 varied conditions in which irreciprocal re- 

 lations occur between members of diflFerent 

 species such that one may benefit and the 

 other may be harmed by the association. 

 Examples will illustrate some types of such 

 irreciprocal associations. Thieving ants feed 

 on termite eggs within the termite nest. 

 Staphylinid and pselaphid beetles some- 

 times prey on the brood of the ant colonies 

 whose nests they inhabit. The organic ma- 

 terial of termite nests may be food of nest 

 inquilines such as tineid caterpillars and 

 fungus-gnat larvae. Many different kinds of 

 nests and burrows are inhabited by diverse 

 animals in addition to the forms that build 

 or dig them. Sometimes the relationship is 

 one of casual occupation, and sometimes the 

 invaders receive more than incidental bene- 

 fits from close association with the original 

 builders. 



A bond may be formed directly between 

 the animals themselves and what may be 

 regarded as neighborly groups in which one 

 species finds protection by the proximitv to 

 another without occupying any part of its 

 nest, if a nest is present. Thus fishes mav be 

 associated with the Portugese man-of-war. 

 with medusae, with sea anemones, or with 

 corals. 



A still more intimate relation exists in the 

 various forms of temporary and permanent 

 social parasitism. A fertilized female ant, of 

 a group in which the power of initiating 

 colony formation has been lost, joins her- 

 self with the fertilized female of some spe- 

 cies that has retained this power. The para- 



