CHAPTER VII 



TIME AND ANIMAL COMMUNITIES 



Many of the animals in a community (i) never meet owing to the fact that 

 they become active at different times. This is because (2) the environ- 

 ment is subject to a number of rhythmical changes which (3) result in 

 corresponding variations in the nature of the animal communities at 

 different times. (4) There is the day and night rhythm which affects 

 both free-living animals and (5) some parasites. (6) This rhythm miay 

 be of practical importance, e.g. from its influence upon blood-sucking 

 insects and (7) is most strongly marked in deserts, but (8) there is really 

 very little detailed information about day and night communities. 

 (9) Some of the changes in the fauna are caused by migration, as in the 

 vertical strata of a wood or in the plankton. (10) There is not always a 

 very sharp limit between day and night communities. (11) In polar 

 regions there is no night fauna, while (12) in the tropics the latter is 

 very rich. (13) Other rhythms are those of the tides and (14) of weather 

 (caused by the passage of depressions) which (15) produce variations 

 in the composition of the active animal communities, e.g. those of dry 

 and (16) wet conditions, which (17) may override the day and night 

 rhythm. (18) Weather changes also have important effects on blood- 

 sucking insects. (19) Then there is the annual cycle of the seasons 

 which (20) has a particular interest from its relation to bird migration 

 and (2t) to changes both in the food-habits of animals and (22) in the 

 particular species occupying any one niche. (23) These rhythmical 

 changes in communities enormously increase the difficulty of studying 

 the latter completely, so that (24) it is advisable to choose extremely 

 simple ones in order to work out the principles governing animal com- 

 munities in general. (25) Larger pulsations in climate chiefly affect 

 the numbers of animals, and so we are led on to the next chapter. 



I. One of the commonest rodents of the South African veld 

 is the gerbille, springhaasrot, or rooiwitpens (Taterona loben- 

 gula)y which lives sociably in warrens in sandy country where 

 there is plenty of sweet grass and bulbs to eat. Quite often it 

 makes its network of burrows in places already occupied by 

 two species of carnivorous animals, the yellow mongoose 

 {Cynictis pencillata) and the suricat {Suricator suricator). But 

 although the rodents and the carnivores live in close contact 

 with one another, actually using to some extent the same system 



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