TABLE 9.7 Size of ar 

 May to September, 



populations in forest and forest-edges, 

 isive of nnesofauna and microfauna of 



Taxonomic 

 group 



Deciduous Coniferous 



forest, forest, Chaparral, 



central Ill.» Utatf Utatf 



'Including and extendi 

 ^Hayward 1945 

 'Hayward 1948 



Iford 1951 (a, b) 



THE FOREST COMMUNITY 



Since the censusing of each group of ani- 

 mals furnishes special problems, there have been no 

 studies of total animal populations in single forest 

 communities. By Table 9-7, however, it appears that 

 the ratio in numbers of individuals per hectare be- 

 tween different animal groups is of the order : 1 bird, 

 3 mammals; 13,000 snails and slugs, 20,000 centi- 

 pedes, millipedes and sowbugs ; 35,000 arachnids ; 

 and 225,000 large insects. The mesofauna would 

 number in the tens of millions (Table9-8), and the 

 microfauna in numbers so large as to be scarcely con- 

 ceivable. In general, the number of individuals rep- 

 resenting a species varies inversely with the body 



size characteristic of the species. There is, however, 

 considerable variation in population levels both geo- 

 graphically and temporally. We must give special 

 consideration to each of these various groups of ani- 



Soil animals 



Some animals, geobionts (Table 9-8), spend 

 all their lives in the ground ; certain protozoans, flat- 

 worms, nematodes, annelids, tardigrades, snails, milli- 

 pedes, centipedes, some spiders, mites, pseudoscor- 

 pions, true scorpions, many small wingless insects, 

 some beetles and other winged insects, and a few 

 mammals are examples. Other animals, geophils, live 

 in the ground only as eggs, larvae, or pupae, such as 

 do many flies and beetles ; in cocoons, as do some 

 moths : or for hibernation, as do many beetles and 

 bugs. 



Soil animals are most abundant in undisturbed 

 virgin areas. In a longleaf pine forest suffering fre- 

 quent burning, the number of small animals in the 

 humus layer was reduced to one-fifth and the num- 

 ber in the top 5 cm of the mineral soil was reduced 

 to one-eleventh of the number in unburned areas 

 (Hey ward and Tissot 1936). 



Some 250 species of flagellate, amoeboid, and 

 ciliate protozoans have been recorded in the soil 

 (Sandon 1927), but only a few species are limited in 

 distribution exclusively to the soil since they also 

 occur in freshwater habitats. Many species occur in 

 practically worldwide distribution. Flagellates may 

 range from 100,000 to 1,000,000 or more individuals 

 per gram of soil ; amoebae, from 50,000 to 500,000 ; 

 and ciliates, from 50 to 1 ,000 (Waksman 1952) . Over 

 150 species of rotifers are known as ground inhabi- 

 tants, and about one-third of these species have been 

 found only in the soil. They feed on organic material 

 and, to a lesser extent, on nematodes and proto- 

 zoans. Nematodes may occur to the extent of 1,000 

 to 10,000 individuals per cubic centimeter. Most of 

 these forms belong to the Anguilluliformes and are 

 more or less worldwide in distribution. They com- 

 monly possess mucous glands in the skin, the secre- 

 tions of which aid locomotion. These nematodes are 

 very resistant to desiccation and will quickly become 

 active when moisture is added to soil that has been 

 dried out for years. Tardigrades occur regularly, 

 sometimes abundantly ; they too are very tolerant of 

 desiccation (Kiihnelt 1950). Land planarians are not 

 common except in moist tropical regions. Some of 

 these soil animals are detritus-eaters, some bacterial 

 and algal feeders, some partly carnivorous, and some 

 partly parasitic on plant roots. 



The majority of these small organisms are active 

 only in soil water, present as a thin film lining the 



130 Habitats, communities, succession 



