490 



THE COMMUNITY 



described in this report has been recognized 

 for a long time by naturalists working in 

 the tropics, but is not recognized by all 

 the other animals living there." Allee found, 

 for example, that leaf -cutting ants (Atta) 

 ranged through the three lower strata of the 

 forest, and Azteca ants were found in all 

 levels above the shrub stratum. 



Bates (1944, p. 169), in a study of 

 Haemogogus capricornii, an important mos- 

 quito vector of jungle yellow fever in 

 Colombia, states: "Each species of diurnal 

 mosquito found in the forest seems to have 

 a particular type of flight distribution, some 

 species showing a preference for ground 

 levels, some for higher (canopy) levels, 

 while others show a random distribution." 

 Even within a given forest stratum, the 

 stratal occupants tend to vary in their popu- 

 lation density and taxonomic composition as 

 a consequence of the regular march of 

 seasonal events (Chap. 28). During the 

 summer months the vertical distribution of 

 stratal occupants is relatively stabilized, but 

 allowance must be made here for the move- 

 ments of nocturnal and diurnal species and 

 for local movements as a consequence of 

 shifts in physical factors. Even so, the 

 composition of a given stratum of a given 

 community type is usually predictable, 

 after completion of sufficient research 

 to allow for several years' normal variation. 

 Consequently, the appearance of certain 

 typical species serves not only to identify 

 the stratum, but may have value as an in- 

 dicator of immediate stratal microclimate 

 (Fichter, 1939). 



Biotic factors are important in the vertical 

 distribution of animals. Vegetation is di- 

 rectly important to herbivores and, there- 

 fore, indirectly of importance to pred- 

 ators. The vertical distribution of spiders, 

 during the summer months, in a river- 

 terrace forest community in western Tennes- 

 see was directly associated with stratal 

 vegetation as well as stratal microclimate 

 (Gibson, 1947). The horizontal distribution 

 of characteristic spiders was associated with 

 the moisture content of the soil. 



The principle of vertical distribution of 

 animals in a community is based on the 

 several responses of the primary residents 

 in the gradient. Herbivorous animals re- 

 spond to the plant gradient in so far as 

 their limits of species toleration to the 

 operating physical gradients permit. Sapro- 



phytes, saprozooics, omnivores, predators, 

 and parasites respond to the herbivore 

 gradients within their limits of toleration. 



The result is a complex vertical and 

 horizontal distribution that is peculiarly 

 reactive to the factors involved. It may be 

 thought of as a vast community response 

 to the many specific reactions to the prin- 

 ciple of the minimum (pp. 198, 205). We 

 may apply this rule to a higher level of 

 integration and speak of the principle of the 

 community minimum. At the level of com- 

 munity integration, as well as at individual 

 or population levels, the harsh operation of 

 the principle of the minimum is ameliorated 

 by relations summarized under the principle 

 of partial equivalence (see p. 223). For 

 example, a certain amount of shelter is 

 necessary. Some of this is furnished by leaf 

 mold. Where leaf accumulation is at a 

 minimum, animals may find a partial 

 equivalent in the shelter furnished by moss, 

 by the crevices under stones, or they may 

 even penetrate the upper layers of the soil. 

 Usually large animals (deer) forage over 

 more area than small animals (mites); 

 animals with great climbing agility (tree 

 squirrels), or those that fly (birds), cover 

 more area than animals that are sedentary 

 (sloths) or that move slowly (majority of 

 scale-insects for most of their life cycle). 

 Other things being equal, species have a 

 definite distributional pattern in a com- 

 munity that suffices to meet their feeding, 

 sheltering, and breeding requirements. 



Animals tend to be more abundant where 

 their food is plentiful. As the requirements 

 become more restrictive, we pass from the 

 third to the fourth type of vertical distribu- 

 tion, that of specific stratal localization. 



The leaf miners are species of insects 

 whose larvae live and feed, for all or a part 

 of their larval period, between the upper 

 and lower epidermal layers of leaves. The 

 leaf-mining habit converges with the 

 petiole-twig-branch-trunk borer group, and 

 with the leaf-eating group. There is little 

 difference between a leaf miner and a 

 borer, save that the borer feeds deep in 

 plant tissue, while a miner burrows just be- 

 neath the surface of the plant (Frost, 

 1942). 



The leaf miners are found chiefly in four 

 orders of insects: certain Chrysomelidae, 

 Buprestidae and Curculionidae in Coleop- 

 tera; Agromyzidae and Anthomyiidae in 



