126 ANIMAL ECOLOGY 



do not usually limit the numbers of their hosts except when 

 the latter have increased unduly (as when fish die from tape- 

 worm epidemics or grouse on overcrowded moors from nema- 

 tode disease), or when the parasite has got into the wrong host 

 (as with the trypanosome, which causes human sleeping-sick- 

 ness). Most parasites exist by exploiting the work done by 

 their hosts, without actually destroying them, just as a black- 

 mailer takes care not to ask for too much money at one time. 

 Besides parasites there are also certain animals which are not 

 parasites (in the sense of living on their host), but which at the 

 same time employ similar means of obtaining sustenance from 

 their prey without destroying the latter. For instance, some 

 species of ants keep farms of aphids which they visit for the 

 purpose of getting drops of liquid containing food which has 

 not been completely absorbed by the aphids themselves. 

 These aphid farms may be carefully protected by the ants.^*^ 

 Another example of avoiding killing the goose that lays the 

 golden eggs is described by Beebe.^^^ On one of the Galapagos 

 Islands there lives a scarlet rock-crab {Grapsus grapsus) 

 which inhabits the lava rocks by the seashore. This crab is 

 pursued by a species of blue heron {Butorides sundevalli) 

 which catches the crab by the leg, upon which the crab breaks 

 off the leg by autotomy, leaving it in the possession of the 

 heron. Thus both animals get what they want — the heron 

 its food and the crab its life. 



These cases of exploitation without destruction have been 

 described in order to show that the food- cycle mechanism is 

 not always effective in limiting numbers of animals. In such 

 cases some other means must be employed. Sometimes 

 another animal in the community, which still follows the old 

 method of destruction, acts as a real control of the numbers 

 of the species concerned. 



