16 THE ECOLOGY OF ANIMALS 



characteristic of a community though not confined to 

 it, play a very important part in determining the 

 structure of the whole, since their numbers will 

 influence the numbers of other species that depend 

 on them for food, or are preyed upon by them, or 

 compete with them. We may say, then, that where 

 species have only a narrow range of environmental 

 conditions that they can live in, they tend to become 

 confined to one particular major habitat or minor 

 habitat (e.g. a plant association or one species of 

 plant). But this does not always occur : studies on 

 Rocky Mountain life zones have shown that many 

 species end their altitudinal range irrespective of the 

 limits of the main vegetation zones. Species with a 

 wider range of physiological reactions range over 

 more than one major habitat, and their speciahza- 

 tion is shown more by the relative numbers of indi- 

 viduals that they are able to maintain in each. The 

 combination of specialization, varying numbers, and 

 inter-relations of species, gives each animal com- 

 munity its characteristic facies and structure. 



A further conclusion concerns the difi"erence between 

 animal and plant associations. The lists of animals 

 collected during a survey are generally swollen by 

 the presence of many accidental visitors from other 

 places. Animals move about a great deal, both 

 passively (as when spiders are blown about on 

 gossamer) or else actively (as with flying beetles and 

 flies or with migrating birds). The result of this is 

 that animal communities can hardly ever be studied 

 as isolated units, owing to the constant action of 

 these outside influences. On the fjaeldmark tundra 

 of high Arctic regions, the food supply of small 

 spiders is greatly increased by Chironomid flies whose 

 larvae are aquatic and whose adults emerge and fly 

 about over land. In summer in England you may 

 see mayflies flying over fields by a river and being 

 snapped up by sand martins which make their nests 

 in the sides of sand and gravel pits higher up on the 



