40 ANIMAL ECOLOGY 



signals which indicate to it that if it goes much further into 

 this unsuitable habitat, or remains there too long, the results 

 will be dangerous. For instance, the fresh-water amphipod 

 Gamfnarus pulex is a nocturnal animal, which lives under stones 

 and plants during the day, its habitat being determined by a 

 simple negative reaction to light. But light is not in itself 

 harmful to Gammarus (except possibly in a very early stage 

 of development), and the reason for living in the dark is 

 almost certainly because of the protection which it gets from 

 enemies, since the animal has no effective means of defending 

 itself against enemies except by swimming away or hiding. 

 Often, however, the signs by which animals choose their 

 habitats are not warnings of danger to the animal itself, but 

 have the effect of keeping it out of places in which it could 

 not breed or bring up its young successfully. 



9. The point which we are trying to make is that most 

 animals are, in practice, limited in their distribution by their 

 habits and reactions, the latter being so adjusted that they 

 choose places to live in, which are suitable to their particular 

 physiological requirements or to their breeding habits. The 

 latter may often be much more important than the former : 

 the willow wren, Phylloscopus trochilus, and the chiff chaff, 

 P. colly bita, both range through similar kinds of vegetation, 

 and do not appear to be affected directly by the physical or 

 biotic conditions at different levels of a wood or in different 

 plant associations. At the same time, the habitats for 

 breeding are very markedly different, the willow wren 

 choosing places where the ground vegetation is low, while 

 the chiffchaff occurs in woods which have grown up and in 

 which the undergrowth has correspondingly changed. ^^^ We 

 might put the matter this way : every animal has a certain 

 range of external conditions in which it can live successfully. 

 The ultimate Hmits of environment are set by its physiological 

 make-up ; if these limits are reached the animal will die. 

 It is therefore undesirable that the animal should run the risk 

 of meeting such dangerous conditions, and it has various 

 psychological reactions which enable it to choose, to a large 

 extent, the optimum conditions for life. The animal is not 



