THE ANIMAL COMMUNITY 57 



parasites are smaller than their hosts. For instance, in a 

 pine wood there are various species of aphids or plant-lice, 

 which suck the juices of the tree, and which are preyed on by 

 spiders. Small birds such as tits and warblers eat all these 

 small animals, and are in turn destroyed by hawks. In an 

 oak wood there are worms in the soil, feeding upon fallen leaves 

 of plants, and themselves eaten by thrushes and blackbirds, 

 which are in turn hunted and eaten by sparrow-hawks. In the 

 same wood there are mice, one of whose staple foods is acorns, 

 and these form the chief food of the tawny owl. In the sea, 

 diatoms form the basic plant food, and there are a number of 

 Crustacea (chiefly copepods) which turn these algae into food 

 which can be eaten by larger animals. Copepods are living 

 winnowing fans, and they form what may be called a *' key- 

 industry " in the sea. The term " key-industry " is a useful 

 one, and is used to denote animals which feed upon plants 

 and which are so numerous as to have a very large number of 

 animals dependent upon them. This point is considered 

 again in the section on " Niches." 



9. Extremely little work has been done so far on food- 

 cycles, and the number of examples which have been worked 

 out in even the roughest way can be counted on the fingers of 

 one hand. The diagram shown in Fig. 3 shows part of a 

 marine plankton community, which has been studied by 

 Hardy ,102 ^nd which is arranged to show the food-chains leading 

 up to the herring at different times of the latter's life. To 

 complete the picture we should include the dogfish, which 

 attacks the herring itself. Fig. 4 shows the food- cycle on a 

 high arctic island, and is chosen because it is possible in such a 

 place to work out the interrelations of its impoverished fauna 

 fairly completely. 



At whatever animal community we look, we find that it is 

 organised in a similar way. Sometimes plants are not the 

 immediate basis of the food-cycle. This is the case with 

 scavengers, and with such associations as the fauna of tem- 

 porary fresh-water pools and of the abyssal parts of the sea 

 where the immediate basic food is mud and detritus ; and the 

 same is true of many parasitic faunas. In all these cases, which 



