28 THE ECOLOGY OF ANIMALS 



on several species. Then herbivores can be classified 

 in another way, according to their general method 

 of feeding, or the general type of vegetation that they 

 live upon. There is a very large class of insect plant- 

 suckers, of which the plant-lice or aphids and certain 

 other bugs are the most important. Other groups 

 of insects make mines in the leaves of plants, others 

 again form galls as do also certain species of mites, 

 while certain species live on lichens or on fungi. 

 The list of different types of food-habit among herbi- 

 vores could be multiplied indefinitely. Enough has 

 been said to indicate the importance of ecological 

 niches. The niche means the mode of life, and 

 especially the mode of feeding of an animal. It is 

 used in ecology in the sense that we speak of trades 

 or professions or jobs in a human community. And 

 it is these niches which give any animal community 

 much of its special structure and appearance. It 

 will be noticed that the idea of a niche is a purely 

 ecological one, not taxonomic. Thus pollen feeders 

 which visit flowers may include representatives of 

 many different orders of insects. Grass-eaters in 

 pasture land include cattle, rabbits, voles, grass- 

 hoppers, beetles, moths, snails, etc. Grinnell and 

 Storer (1924) successfully developed the idea of 

 ecological niches during their study of the life zones 

 of the Rocky Mountains. They found that where 

 several species of any one genus of mammal (e.g. 

 ground squirrels) occurred in the same life zone, they 

 usually occupied different ecological niches and did 

 not come into direct competition. On the other 

 hand, the same niche is often occupied by widely 

 differing forms of animal. These forms are in direct 

 competition with one another for the same food. 

 It seems probable that, since the number of niches 

 that exist at all for vegetarian animals in a 

 community is limited, we have a glimpse of one of 

 the reasons why the number of species is rather 

 limited. 



