Stratification of the Community 423 



The subdivisions of the forest biocenose above the ground are 

 variable and are determined by the arrangement of the vegetation, 

 but in most instances herbaceous, shrub, and tree strata are distin- 

 guishable. The herbaceous stratum varies in height up to a meter or 

 so and frequently overlaps with the shrubs that extend to heights of 

 perhaps 1 to 5 m. The tree stratum occurs between heights of 5 and 

 15 m in the typical oak forest, but it extends to 25 or 30 m in the 

 coniferous forest and to about 40 m in the rain forests, with individual 

 large-crowned trees towering to 50 m or more. The upper limit of 

 the forest canopy in groves of giant redwoods may surpass 100 m. 

 The herbaceous stratum is poorly represented or absent in some in- 

 stances as, for example, under a thick stand of pines or spruce, and 

 the shrub stratum may vary greatly in prominence. On the other 

 hand, the tree stratum may be elaborately developed and divisible 

 into sublayers. The trees of the rain forest on the Gold Coast of 

 Africa display a stratification that extends from a height of 2 m or so 

 to about 40 m ( Foggie, 1947 ) . 



The herbaceous, shrub, and tree strata are each inhabited by a 

 characteristic assemblage of epiphytes and epizoans. Herbivorous 

 insects and web spiders are particularly abundant in the herbaceous 

 stratum. In the higher strata numerical superiority is held by the 

 insect group in enormous variety but snails, lizards, snakes, frogs, 

 arboreal mammals, and many kinds of birds are also present in greater 

 or lesser abundance. Most of the species are primarily associated 

 with one of the principal strata— or perhaps with one of the subdivi- 

 sions of the tree stratum— but individuals of many of the species range 

 above or below their usual abode. 



In certain aquatic habitats vertical gradients in environmental fac- 

 tors cause a recognizable stratification among the members of the 

 community. Such layering may have dimensions of less than a meter 

 in shallow ponds, or it may involve strata many meters thick in the 

 open ocean. Mention has been made in earlier chapters of the sharp 

 limits commonly found for the vertical ranges of attached plants and 

 animals in the littoral zone, and particularly in the tidal zone, as a 

 result of the rapid change in water and light conditions. Where these 

 sessile organisms exert a controlling influence on a variety of de- 

 pendent species, they cause subdivisions of the community to be 

 formed that may be directly observed (Stephenson and Stephenson, 

 1949). 



The stratification of benthic communities at greater depths is not so 

 easily recognized, but it is being investigated by means of dredges 

 and underwater cameras, and by divers with aqualungs. The situa- 



