niche means that they must in some way recognize 

 the merits of that niche by definite characteristics of 

 it that may be in the nature of sign stimuli (Table 

 18.1). Such characteristics are usually prominent, 

 though they need not necessarily be the most essen- 

 tial features of the niche (Lack 1937). 



Probably most animals exercise a deliberate, al- 

 though not necessarily conscious, evaluation process 

 in choosing one niche from those available. This has 

 been tested experimentally by exposing the animals to 

 gradients of environmental factors, either in the labo- 

 ratory or field (Harris 1952) ; a variety of apparatus 

 and procedures is available for such tests (Shelford 

 1929). Usually there is a coincidence of the species' 

 experimentally ascertained preferendum and its nat- 

 ural preferendum. For instance EUpsocits melachlani 

 and E. iveshvoodi, both psocid insects, occur abun- 

 dantly on larch trees, but E. melachlani frequents 

 those dead branches heavily encrusted with lichens, 

 and E. westivoodi frequents living branches covered 

 with the alga Pleurococciis. Laboratory experiments 

 showed clearly that when each species was given a 

 choice, each selected its customary habitat. Further- 

 more, the feeding of E. zvcstwoodi was restricted al- 

 most entirely to the alga, although E. melachlani 

 would feed on both the alga and on lichens (Broad- 

 head and Thornton 1955). 



For certain species, niche preference can be at- 

 tributed to appropriate behavioral patterns alone. 

 Isopod species occur in water and on land but only 

 in places where the humidity is high. What success 

 the group has achieved on land appears to be the re- 

 sult of their avoidance of the rigors of ordinary ter- 

 restrial conditions by means of behavior mechanisms 

 that retain them in these moist cryptozoic niches, 

 rather than to the development of any special morpho- 

 logical or physiological adaptations (Edney 1954). 



A stereotyped behavior pattern appears to make 

 the magnolia warbler build a nest supported in the 

 interlocking leaves or twigs of a conifer rather than 

 in the vertical fork of a tree or shrub, as the redstart 

 regularly does. The black-throated green warbler 



originally had a nest-building behavior similar to that 

 of the magnolia warbler, but in some regions it has 

 taken to building in forks, a behavior which has ex- 

 panded its range into both deciduous and coniferous 

 forests. Why is the American robin restricted to lo- 

 calities where it can get mud to put into its nest? 

 Others members of the family Turdidae do not use 

 mud in their nests. There is, on the other hand, an 

 advantage for barn and cliff swallows to use mud in 

 constructing their nests because it enables them to use 

 locations on the vertical sides of cliffs or buildings 

 free of competition from other species. There ap- 

 pears to be no physical reason why a barn or cliff 

 swallow could not build its nest in crevices or holes 

 like other swallows, why a bank or rough-winged 

 swallow could not build a mud nest like the barn or 

 cliff swallow, or why a robin could not build like other 

 thrushes. Such niche segregations are apparently 

 consequences of restrictions imposed by behavior pat- 

 terns alone, although one can never be sure but that 

 each species has some hidden adaptation that keeps 

 its characteristic kind of nest the best nest for it, and 

 its preferred niche the best niche for it. 



It is possible that certain species of birds are con- 

 fined to coniferous forests because they are of north- 

 ern origin and coniferous forest was the original 

 community available to them ; similarly, the broad- 

 leaved deciduous forest is conjectured to have been 

 the original community inhabited by species of south- 

 ern ancestry. Presumably each group evolved herit- 

 able, instinctive behavior patterns which continue to 

 drive it back to the ancestral community in which it 

 belongs, so to speak, even though other types of 

 communities have become available (Lack and Vena- 

 bles 1939). 



Where two species with similar niche require- 

 ments come into competition, one species must possess 

 better adaptation to it if it gains full possession of 

 the niche to the exclusion of the other species (Lack 

 1944). Preadaptation or possession of suitable ad- 

 aptations also appears a necessary prerequisite for 

 a species to invade a new niche or habitat and suc- 



250 Ecological processes and dynamics 



