TABLE 9-9 Number, biomass, and metabolism of ground Invertebrates per m' 



mer populations commonly vary from 25 to 100 per 

 hectare (10 to 40 per acre). In rich, moist, undis- 

 turbed forests, populations may sometimes attain 

 temporary levels of up to 500 per hectare (200 per 

 acre). Considerable data on population sizes and 

 biomasses of individual species have been compiled 

 by Mohr (1940, 1947). 



In forests of eastern North America (Hamilton 

 and Cook 1940:469), small mammals fall into several 

 categories. The deer mice and the flying squirrels are 

 adept climbers and often have their homes thirty feet 

 or more [10 meters] from the ground in some hollow 

 snag, deserted nest, or abandoned woodpecker hole. 

 Flying squirrels feed among the trees and descend to 

 the ground to forage about old logs and brush piles. 

 They also dig down into the litter from the surface. 

 Chipmunks forage in much the same manner, al- 

 though they climb less frequently. Deer mice occupy 

 several levels, from the trees to the burrows of moles 

 and shrews. The red-backed mice, the lemming mice. 

 and probably the jumping mice dig fairly permanent 

 tunnels and runways through the soil and the litter. 

 These they use as bases for food-gathering in both the 

 litter and the upper layers of the mineral soil. These 

 runways are often used by the shrews and the deer 

 mice. The short-tailed shrews dig substantial tunnels. 

 The diminutive long-tailed shrews thread their zvay 

 through the easily parted litter and top-soil, and make 

 intricate temporary labyrinths daily in search of food. 

 The moles remain in their . . . tunnels during the 

 daylight hours, but often come to the surface at night, 

 no doubt attracted by the countless invertebrates that 

 swarm over the ground with darkness. In winter they 

 remain safe under the snotv. 



In mixed deciduous and coniferous forests in the 

 Sierra Nevadas the total number of rodents varied 

 from 150 per hectare (60/acre) in July- August to 

 52 per hectare (21 /acre) in December; their bio- 

 mass varied from 27 to 4 kg per hectare (24 to 3.3 

 lb/acre) (Storer et al. 1944). The home ranges of 

 individuals of these small rodents and insectivores are 

 commonly only a fraction of a hectare (Blair 1953). 



Of forest species, the larger the mammal, the 

 fewer its numbers ; and, usually, the wider an indi- 

 vidual's range. The home ranges of the weasel, rac- 

 coon, and bear, are more extensive than those of ro- 



dents and insectivores. In the aggregate, their biomass 

 does not exceed that of the more numerous smaller 

 species. A population density of one deer per 20 

 iiectares, for instance, translates into a deer biomass 

 of about 2.8 kg per hectare (2.5 lb/acre) . 



Stratification 



The community is a structure of the subter- 

 ranean, ground, herb, shrub, and tree strata, and spe- 

 cies are separated into different niches in relation to 

 these strata (Elliot 1930). Food, shelter, and micro- 

 climatic differences are the chief limiting factors. Be- 

 cause microclimate in each stratum varies from hour 

 to hour, day to day, and season to season, classification 

 of a species by stratum must be in terms of the stra- 

 tum it is observed to frequent for the major portion 

 of a relatively long period of time. The inhabitants of 

 the five strata divide into two major groupings, or 

 societies. The soil invertebrates and some mammals 

 move freely back and forth between subterranean and 

 ground strata, and may be considered a society dis- 

 tinct from that which occupies the combined herb, 

 shrub, and tree strata. Insects and birds depending on 

 the foliage for food and reproduction sites move more 

 freely among the latter three strata than between these 

 strata and the ground. In terms of the strata which 

 each occupies, however, the societies are not mutually 

 exclusive, for ground animals such as millipedes and 

 snails climb up onto herbs and tree trunks during 

 humid weather, and foliage animals often rest on the 

 ground, and search for food, hibernate, and lay eggs, 

 there. 



The majority of arthropod species carry on their 

 main activities within a single stratum for their major 

 activities (Table 9-10). The tree stratum spans a 

 greater vertical distance than any other. Within this 

 broad stratum, arthropods often show segregation to 

 particular heights above the ground (Davidson 

 1930). Ants, spiders, and beetles appear to move 

 more freely between different strata than do other 

 species. 



In point of macrofauna population densities of in- 

 vertebrates, the ground stratum ranks highest, fol- 

 lowed by subterranean and herb strata, while shrub 



136 Habitats, communities, succession 



