494 THE COMMUNITY 



could be prepared to show such stratifica- selection of stratum, substratum, or habi- 



tion in mirid bugs (Knight, 1941), aphids tat- niche by warblers in their breeding 



(Patch, 1938), scale insects (Britton, season. 



1923), sawflies ( MacGilHvray, 1916) or It must not be supposed that food is the 



borers (Felt, 1905, 1906). only limiting influence restricting a species 



In all these instances the genetic back- to a group of communities, to a single 



ground is reenforced by the proximity of community type, a particular stratum of a 



the food plant, often in abundance, from community, or to an especial habitat- 



which an ovipositing female has fed during niche within a stratum, 

 her earlier life (cf. Hopkins' Host Principle, Over-all stratification is the result of 



in Index). many influences, of which food is an im- 



Feeding upon these countless herbivores, portant component. Elsewhere in this book 



including many larger animals (cf. Table numerous other influences have been dis- 



40), are predators and parasites. For ex- cussed that have to do with the spatial 



ample, Mantidae in Orthoptera, Reduviidae organization of species populations and in- 



in Hemiptera, the aphid-eating ladybird dividuals, with respect to the stratified 



beetles (Cocoinellidae), Syrphidae and structure of the community, and the selec- 



Table 41. Warbler Stratification in a Forest Community 

 (Reorganized from Kendeigh, 1945) 

 Habitat-Niche Warbler 



Top level of evergreen trees Blackburnian 



Middle level of evergreen trees 



usually Black-throated Green 



Low level of evergreen trees Magnolia 



Secondary deciduous trees Redstart 



Tree trunks Black and White 



Shaded shrubs Black-throated Blue 



Sunlit shrubs Chestnut-sided 



Wet shaded floor Canada 



Wet sunlit floor Yellow-throat 



Dry shaded floor Oven-bird 



Dry sunlit floor Nashville 



Asilidae in Diptera, spiders as a group, tion of the community by a species or the 



ants, numerous parasitoids in the Hymen- selection by the community of a species, 



optera, many forest mammals and birds This problem has been studied by orni- 



(cf. Balduf, 1935; Clausen, 1940; Hamil- thologists. They find, in general, that com- 



ton, 1943). These predators and parasites munity selection, stratum selection, and 



are less definitely stratified than are the habitat selection by birds are complex, 



herbivores on which they feed. Even so. For example, on a broad zoogeographic 



numerous parasites prey upon a given basis, species of birds are limited by physi- 



herbivore and hence may be limited to a cal barriers and climatic conditions they 



particular stratum, can not tolerate (Kendeigh, 1934). When 



Even such highly motile animals as birds such limits of tolerance coincide with a 



are often characteristic residents of a par- range of communities, selection of a given 



ticular forest stratum, or even of a partic- community type may be the consequence 



ular level within a given stratum, although of a variety of influences, 



food may or may not play a leading role In such case the selection may be the 



in such organization. For example. Ken- result of some obligatory relation of the 



deigh (1945) found breeding warblers bird species to a particular type of com- 



generally stratified, and substratified, in a munity (Beecher, 1942; Pitelka, 1941). 



sugar maple-beech-hemlock forest of New Within this community type, local segrega- 



York. He finds that such (p. 433) "diversi- tion of species into diflFerent communities, 



fication in niche requirements reduces in- strata, and habitats may be obvious, but 



terspecific competition and permits a the causes may or may not be obscure, 



greater and more varied population to in- Lack and Venables (1939) discuss the 



habit an era." Table 41 demonstrates this limitation of British woodland birds to a 



