COMBINATIONS OF ENVIRONMENTAL FACTORS 



215 



them if they are confined thereon (Chap- 

 man, 1931). 



In addition to the partial freedom from 

 the environment allowed by homiothermy 

 or by the possession of a dry, impervious 

 body covering and other similar devices, 

 man and some other animals have partial 

 control over their immediate surroundings 

 primarily as a result of group behavior. The 

 closed nests of termites control the imme- 

 diate moisture relations of the colony, ex- 

 clude air currents, and retard temperature 

 changes. The winter clusters of honeybees 

 allow the inner bees to escape the full im- 

 pact of low temperature, and their own 

 activity within the insulating shell of their 

 fellows exerts a temperature control unless 

 the outside cold becomes too great. Bees 

 and wasps cool their nests in summer by 

 self-fanned ventilation and by evaporation 

 of transported water. Beavers secure a par- 

 tial freedom from several limitations by 

 building dams and lodges. Social animals 

 tend towards securing greater control over 

 their environment than that possessed by 

 more solitary forms. A forest, a coral reef, 

 and similar ecological biocoenoses are en- 

 vironment-controlHng mechanisms in which 

 the dominant forms meet the full impact of 

 the habitat and so modify it that sensitive 

 elements of the community can live in re- 

 gions that they could not otherwise occupy. 

 Freedom from physical surroundings is 

 never complete. Even the impressive free- 

 dom achieved by man often leaves him at 

 the mercy of common phenomena such as 

 fogs, winds, and rainstorms, except as they 

 can be walled out of restricted spaces. 



The range of an ecological unit is a prac- 

 tical expression of the distribution of the 

 habitat niches it can tolerate and of its 

 abihtv to reach them. Range provides a 

 concrete test of ecological valence and 

 vagility. A stenokous community, like that 

 of the hot springs, may be cosmopolitan 

 in distribution, although the habitable 

 niches are restricted in size. The physi- 

 cal factor of time plays an important 



role at this point, as well as the biotic 

 factor of aggressive vigor. New habitats 

 may open faster than they can be en- 

 tered. This is strikingly illustrated by the 

 lag between the appearance of new man- 

 modified areas and their invasion by forms 

 well suited to the changed conditions. In 

 North America, the gray ground squirrel 

 (Citelhis franklini) is still extending its 

 range from the western plains into the nev/ 

 grasslands of the recently cleared forest 

 areas to the east of the climatic prairies. 

 Similarly, in Western Europe, the hamster 

 (Cricetus cricetus), a postglacial relic, is 

 also actively expanding its range into the 

 grasslands created by man. Neither species 

 has as yet had time to reach equilibrium 

 with its environment. Similar instances that 

 do not involve the human biotic factor form 

 the factual basis for the much-disputed 

 age-and-area hypothesis of Willis (1922, 

 1940); the limited apphcation of this con- 

 cept should not prevent due appreciation 

 of its validity under some conditions. Time 

 is a factor in ecology. 



Extent of a tolerable habitat, its geo- 

 graphic position, together with present and 

 past relations to surrounding physiographic 

 and biotic features, are often effective in 

 determining occupancy. An island, whether 

 of land surrounded by water, of forest sur- 

 rounded by grassland, or of mountain 

 meadow surrounded by peaks, or of some 

 other sort, normally supports a different 

 biota than that found in a similar habitat 

 with a more extensive range. The southern 

 part of a grassland that extends far to the 

 north supports different animal communi- 

 ties as compared with the northern portion 

 of a south-extending grassland, even though 

 both are in the same latitude and are sub- 

 ject to generally similar conditions, provided 

 only that the two are fairly well separated 

 from each other and from other grassland 

 communities. These are as truly physical 

 aspects of the environment as are direction 

 and degree of slope, type of substrate, or 

 temperature and rainfall. 



