ENVIRONMENTAL NICHES 



FIG. 18-1 Species I finds optimal conditions in niche C, and 

 reaches greatest abundance in that niche. It is also able to 

 utilize niches B and D, but with less efficiency, and niches A and 

 E only very poorly. Species 2 cannot utilize niches A, B, and C 

 at all, finds D only partially suitable, but E and especially F very 

 favorable. Species I and 2 overlap in niche D, but species 2 

 prevents any occupancy of niche E by species I. The absence 

 of competition in niche A makes it open to a species evolving 

 adaptations to it (from Mayr 1949). 



4. When competition occurs, it is severest be- 

 tween organisms with the most similar require- 

 ments. 



5. In general, the closer the taxonomic relation- 

 ship between them, the more similarity there 

 is in needs and habits of species. 



6. When new forms appear in a given locality, 

 either by evolvement there or by invasion after 

 evolutionary divergence elsewhere, they will 

 either eliminate or be eliminated by their near- 

 est relatives if they compete with them, unless 



7. Each form becomes adapted to a different 

 niche, in which case competition between them 

 will cease, and they may occur in proximity. 



Evidence that interspecific competition is the most 

 critical factor confining a species to this or that niche 

 is available with the expansion of the species beyond 

 the usual limits of its niche when this competition is 

 removed. This expansion is often evident in geo- 

 graphic differences in the niche characteristics of a 

 species. In Scotland, the mountain hare occurs at 

 high elevations, the common hare at lower ones. In 

 Ireland, which was isolated as an island before the 

 common hare could reach it, the mountain hare oc- 

 curs at both high and low elevations and is differ- 

 entiated into distinct subspecies (Huxley 1943). In 

 the Canary Islands, the chaffinch Fringilla teydea 

 breeds only in pine forests. The closely related F. 

 coelcbs usually breeds in broad-leaved forests above 

 and below the pine, but not in the pine forests. On 



certain islands. F. tcydca is missing, and on those 

 islands F. coelebs occurs in the pine as well as in the 

 broad-leaved forests (Lack 1944). Other examples 

 of a similar sort are given, for birds, by Moreau 

 (-1948). 



Success in competition between species of turbel- 

 larian flatworms depends on temperature and water 

 current, but when the competing species is absent, 

 the remaining species disperses far into the micro- 

 habitat usually occupied by its competitor (Beau- 

 champ and Ullyott 1932). 



Under natural conditions in habitats favorable to 

 it. the male Anna's hummingbird is usually success- 

 ful in maintaining a high population and forcing the 

 male Allen's hummingbird into less favorable periph- 

 eral territories. The success of the Anna's humming- 

 bird in competition with the Allen's hummingbird is 

 attributable to its establishment of defended terri- 

 tories earlier in the season, by reason of which it is 

 more familiar with the terrain and alert to intrusions. 

 It sings persistently to warn off competing males, 

 and the larger size and flashier coloration of the 

 Anna's hummingbird give it authority (Pitelka 

 1951). In other habitats, however, the male Allen's 

 hummingbird may consistently displace the Anna's 

 hummingbird (Legg and Pitelka 1956). 



The general effect of interspecific competition is 

 restriction of a population more closely within its op- 

 timum niche. Intraspecific competition exerts pres- 

 sure impelling individuals to disperse into less favor- 

 able situations. The relative pressure exerted by 

 these two forces determines whether at any particular 

 time the species is contracting or expanding its range 

 (Svardson 1949). 



Interspecific competition is reduced or eliminated 

 altogether when the combined requirements of all 

 species are less than the supply of materials available. 

 Land snails, insects, aquatic clams, and copepods 

 sometimes occur together in a considerable profusion 

 of species but with little evidence of competition be- 

 tween them (Boycott 1934, Fryer 1957, Ross 1957). 

 Voles, during upswings in population, may become 

 superabundant for periods of two or three years. 

 During such periods several species of hawks and 

 owls may feed in the same field without competing 

 because there are more than enough voles to supply 

 all. During downswings in the rodent population, 

 however, competition does occur, and some or all the 

 predator species are forced to turn to other prey 

 (Lack 1946). 



Parasites or predators may keep the populations 

 of competing species below the level which available 

 food resources of the habitat can sustain so that com- 

 petition is reduced or disappears. For instance, when 

 two species of weevil are placed together in a limited 

 amount of food, one species is eliminated by the other 

 in about five generations. However, when a wasp 



252 Ecological processes and dynamics: 



