20 ANIMAL ECOLOGY 



case it would take a good many years for the vegetation to 

 reach its former state. Ahhough bare areas would constantly 

 be formed through various agencies, only a short time would 

 elapse before they were clothed with plants once more. 



There are very few really permanent bare areas to be met 

 with in nature. Rocks which appear bare at a distance are 

 nearly always covered with lichens, and usually support a 

 definite though meagre fauna, ranging from rotifers to eagles. 

 Apparently barren places like lakes contain a huge microscopic 

 flora and fauna, and even temporary pools of rain-water are 

 colonised with almost miraculous rapidity by protozoa and 

 other small animals. 



4. It is the exception rather than the rule for any habitat 

 to remain the same for a long period of years. Slow geological 

 processes, like erosion and deposition by rivers and by the 

 sea, are at work everywhere. Then there are sudden disasters, 

 like fires, floods, droughts, avalanches, the introduction of 

 civilised Europeans and of rabbits, any of which may destroy 

 much of the existing vegetation. There is a third kind of 

 change which is extremely important but not so obvious, and 

 is the more interesting since its movements are orderly and 

 often predictable. This is the process known as the develop- 

 me?it of plant communities. Development is a term used by 

 plant ecologists in a special technical sense, to include changes 

 in plant communities which are solely or largely brought 

 about by the activities of the plants themselves. Plants, like 

 many animals, are constantly moulting, and the dead leaves 

 produced accumulate in the soil below them and help to form 

 humus. This humus changes the character of the soil in such 

 a way that it may actually become no longer suitable for the 

 plants that five there, with the result that other species come 

 in and replace them. Sometimes the seedlings of the dominant 

 plant (e.g. a forest tree) are unable to grow up properly in the 

 shade of their own parents, while those of other trees can. This 

 again leads to the gradual replacement of one community by 

 another. 



When a bare area is formed by any of the agencies we have 

 mentioned, e.g. the changing course of a river, it is first 



