92 ANIMAL ECOLOGY 



the fact that whereas the tropical day and night are always 

 twelve hours long throughout the whole year, the night in, 

 say, the south of England is only eight hours long at mid- 

 summer, and the day, therefore, sixteen hours. A consideration 

 of the time-factor in animal communities opens up a number of 

 interesting lines of inquiry ; we have considered day and night 

 in some detail, as it is a clear-cut phenomenon and a fair 

 amount is known about it in a general way. We may now 

 turn to the subject of tidal variation. 



13. Tides. — In the intertidal zone on the sea-shore there 

 is a marked division of the animals into those which come out 

 or become active at high tide when covered with water and 

 those which appear at low tide. The former group is of course 

 the bigger, and forms the main part of the population, con- 

 sisting of typical marine species. We can only give a few 

 examples of individual cases, owing to lack of fully worked-out 

 data on the subject. Since most of the dominant animals 

 depend for their living upon plankton organisms in the water, 

 they simply close down at low tide and start feeding again 

 when covered by water at high tide. But there are a number 

 of important animals commonly found on the shore at low tide 

 ■ — ^mostly birds such as waders, and these differ as to actual 

 species according to the type of shore habitat. It will often 

 be noticed that shore birds divide themselves up into rough 

 zones when they are feeding, some feeding at the edge of the 

 water, others nearer the shore, while others haunt the upper 

 part of the shore near the drift- line. Again, there are different 

 species found on mud-fiats, sandy shores, and rocky coasts. 

 These differences in habitat of the birds are no doubt correlated 

 with differences in the food, etc., in the various shore habitats. 

 Besides birds, there are a number of insects which live between 

 tide-marks, e.g. Anurida maritima^ which during high water 

 hides in crevices in the rocks, surrounded by a bubble of air, 

 and comes out to feed at low tide.^*^ The number of insects, 

 mites, etc., which behave like this increases as we go towards 

 the higher parts of the shore, since in these places they do 

 not have to withstand such a long immersion in the sea during 

 high tides. 



