86 ANIMAL ECOLOGY 



summer, and so on. It would be wrong to get the impression 

 that these time-communities are quite separate from each 

 other. Such is obviously not by any means the case. The 

 point is that the main community changes in personnel to a very 

 large extent at different times ; although the changes are not 

 complete, they are very considerable, and in some cases 

 {e.g. day and night) there may be very few species of animals 

 which live in both communities. The community of animals 

 living in one place still remains a definite and fundamental 

 unit, since its periodic changes are regular and characteristic. 

 We might compare the place-community to an elaborate piece 

 of machinery (e.g. a motor-car), which still remains one unit 

 although there are several more or less independent special 

 mechanisms contained inside it. 



We will now consider some of the time- communities in 

 more detail. 



4. Day and Night.- — In woods, the separation of animals 

 into day and night species may be very easily studied. Take 

 an English oak wood as an example. In the daytime there 

 would be, among a host of other animals, birds like sparrow- 

 hawks, blackbirds, thrushes, woodpeckers, and also bank- 

 voles, weasels, butterflies, bees, and ants. At sunset there is 

 often a short pause when the diurnal animals have gone to 

 rest or begun to think of doing so, and the nocturnal ones 

 have not yet got up full steam. Then there would begin to 

 appear the forerunners of a host of night animals : nightjars, 

 owls, moths, bats, long-tailed field mice, etc. Not only is one 

 kind of animal replaced by another, but one kind of food-chain 

 is replaced by another, and certain niches which are unused 

 by any animal during the day become occupied at night. The 

 weasel — bank-vole industry is changed into a tawny-owl — 

 wood-mouse industry. The woodpecker — ant connection has 

 no equivalent at night, while the moth — nightjar-or-bat chain 

 is almost unrepresented by day. In fact, one food-cycle is 

 switched off and another starts up to take its place. With 

 the dawn the whole thing is switched back again. The two 

 communities of animals are not completely separate, owing to 

 the fact that some animals are not so particular about their 



