12 ANIMAL ECOLOGY 



associations, and we may also note that they are all living a 

 similar life as regards food and general habits. Each associa- 

 tion has some kind of large vegetarian bird, although the 

 actual species is different in each case. Another well-known 

 example is the common grass-mouse {Microtus agrestis) which, 

 except when it is extremely abundant and " boils over " into 

 neighbouring habitats, is chiefly found living underground in 

 grassland, where it feeds on the roots of the grass. A great 

 number of vegetarian insects are attached to one species of 

 plant, and if that plant only occurs in one association, the 

 animal is also limited in the same way. The oak {Quercus 

 robur) supports hundreds of insects peculiar to itself, and if 

 we include parasites the number will be far greater. 



10. Continuing our survey of one zone, we should find 

 that there are certain species which occur in particularly large 

 numbers there, although they are not exclusively confined to 

 it. These we call '* characteristic " species. A good example 

 of this type is the long-tailed mouse (Apodemus sylvaticus) 

 which occurs in woods, but is not confined to them. Trapping 

 data for one area near Oxford showed that 82 per cent, of 

 specimens were caught in woods, while 18 per cent, occurred 

 outside in young plantations and even occasionally in the open. 

 Here the animal is not so strictly limited to one habitat as, 

 for instance, Microtus^ but we are quite justified in calling it a 

 wood-mouse in this district. Thorpe 1^2 has described some 

 of the exclusive and characteristic British birds with reference 

 to plant associations. 



In most cases in which we have any complete knowledge 

 (and they are few) it is found that these two classes of animals — 

 the exclusives and the characteristics — may often form only 

 a comparatively small section of the whole community, that 

 there are many species of animals which range freely over 

 several zones of vegetation, either because they are not limited 

 by the direct or indirect effects of the vegetation, or because 

 they can withstand a greater range of environment than the 

 others. As an example of this type of distribution we may 

 take the common bank vole {Evotomys glareolus) which, in 

 contrast to the Apodemus mentioned above, comes both in 



