INTRODUCTION 



439 



physical environment (burrows of Citellus, 

 Cijnomijs, Gopheriis, Cambarus) or of the 

 biological environment (bracket fungi, 

 Tachigalia, epiphytes), or may be a product 

 of the biological environment (hermit crabs 

 occupying snail shells). In all these exam- 

 ples the original or primary occupant is 

 joined by other, secondary occupants. 

 These fill spaces not otherwise in use, and 

 by their multifold secondary adjustments 

 pyramid the complexity of the habitat.' Oc- 

 cupants of these habitats modifv their 

 homes; consequently the community at 

 large, by aerating the substratum, altering 

 its temperature and the rate and amount 

 of gas exchange of the medium; by their 

 catabolic wastes and feces, their deciduous 

 integumentary products, and eventually 

 their decomposing protoplasms, share in 

 this equilibration. Organisms die and are 

 replaced by their descendants or ecologi- 

 cally equivalent organisms; habitats are de- 

 stroyed or modified while other habitats 

 are created. During this continual activity 

 the community remains relatively stable, 

 and its characteristic aspect and taxonomic 

 composition are substantiallv unchanged. 



This is a relative stability, since com- 

 munities tend to evolve, imder normal con- 

 ditions, to a highly stable end point, the 

 climax community. 



In many cases the functional boundaries 

 of a communitv are not clearly discernible. 

 \ given area mav be subjected to more or 

 less periodic flooding, and if such a condi- 

 tion occurs in regions supporting deciduous 

 forest communities, the flooded depression 

 forms a temporary pond during the spring 

 while the same area supports a woodland 

 elade by summer; or temporary ponds re- 

 place marshy meadow in prairie areas; or 

 such ponds replace semidesert in arid re- 

 gions. In all such cases these temporary 

 ponds form more or less rapidly and have 

 a characteristic biota whose ephemeral 

 active phase places a premium upon repro- 



• An unhackneyed example showing plasticity 

 of adjustment is furnished by the observations 

 of Mr. Henry Dybas, of the Chicaeo Natural 

 History Museum. Early in the 1940's, hermit 

 crabs were found on the island of Saipan 

 (Marianas) in numbers as much as three miles 

 inland from the nearest salt water, and up to 

 about 1200 feet altitude. These crustaceans 

 were inhabiting shells of an African land snail 

 (Achatina fulica) that was introduced by the 

 Japanese in the early 1920's. (Personal com- 

 munication. ) 



duction, and whose prolonged dormancy 

 places a premium upon capacity for aesti- 

 vation, hibernation, encystment, and dis- 

 persal. Such a vernal pond usually sup- 

 ports an abundant and varied fauna 

 and flora, including amphibious animals 

 such as ducks and frogs, burrowing 

 crayfish whose subterranean burrows supply 

 dormancy niches for the rich plankton 

 (Greaser, 1931), sunfishes from ephemeral 

 stream connections, colonial flagellates 

 (Volvox), Cladocera, Copepoda, Ostracoda 

 and notably phyllopod crustaceans (Eu- 

 branchipus, Eiilimnadia, Apus) . These 

 ephemeral communities hold many ecologi- 

 cal equivalents, to be discussed later, and 

 as they gradually disappear, there is a 

 space of time in which pond-glade, pond- 

 meadow, or pond-desert is not clearly de- 

 fined. Their relation to the community as a 

 whole may be seen in the gathering of 

 predators to their borders as they dry up. 



Another example of this lack of defini- 

 tive boundary is seen in the food supply 

 of caves. The cave community is clearly 

 defined and is composed of a distinctive 

 fauna (Bailey, 1933; Banta, 1907; Eigen- 

 mann, 1909; Hyman, 1937; Jeannel, 1926; 

 Maheu, 1926; Packard, 1888; Valentine, 

 1932). The absence of chlorophyll-bear- 

 ing plants is accompanied by the nearly 

 complete absence of herbivores, so that 

 cave animals tend to be saprophagous 

 or carnivorous. This gives no normal base 

 to the community food supply; such as- 

 semblages, although typically communities 

 in other respects, often rely on periodic 

 floods for the base of the food chain 

 (Hawes, 1939; Park, Roberts, and Harris, 

 1941), or upon bat dung in special cases. 



Finally, societies of man, ants, and ter- 

 mites, although they have an increased 

 control over the environment, still normally 

 form a part of a general ecological com- 

 munity.* This control over otherwise peri- 

 odic influences is much less developed in 

 nonsocial communities, and forms a distin- 



• The large city is a peculiar case in that it 

 has evolved a notable degree of dependence 

 on adiacent communities, since, unlike the 

 societies of ants, termites, and smaller human 

 settlements, its food supply is transported bv 

 various types of carriers at various times of 

 the twenty-four hour period: the waste products 

 of its metabolism are incinerated or processed 

 in sewage disposal plants instead of being re- 

 turned directly to the community (O. Park, 

 1941a). 



