TABLE 9 2 Re 

 strata. 



Mixed species 



snts conducted in the field to establish light orientation of arthropods taken 



Light inten- 



sity gradient Control 



Number of Inter- (no gradient) 



experiments Strong mediate Weak Left Middle Right 



Grassland animals from herb stratum 5 



Forest animals from herb and shrub strata 7 



Grassland animals from ground stratum 5 



Forest animals from ground stratum 8 



47% 34% 19% 40% 31% 29% 



46 21 33 33 30 37 



24 27 49 34 23 42 



18 23 59 34 29 37 



change in microhabitat in the forest gradient requires 

 a shift of several meters in vertical position. Experi- 

 ments show, however, that each forest animal species 

 occupies a stratum approximating its preferendum for 

 a particular microhabitat, in response especially to 

 the relative humidity factor (Todd 1949). 



between these microclimates and the prevailing 

 macroclimates of the region must be demonstrated. 



THE GRASSLAND COMMUNITY 



Slope exposure 



Microclimatic diiferences between North- and 

 South-facing slopes are great. South-facing slopes 

 receive a greater amount of solar radiation and are 

 commonly exposed to the prevailing winds. As a 

 consequence, both air and soil temperatures are 

 higher on South-facing slopes than on North-facing 

 slopes : relative humidity is lower, soil moisture lower, 

 and the rate of evaporation is higher. The differences 

 between the two slopes are most marked close to the 

 ground, increasingly less so at higher levels (Cantlon 

 1953). 



The vegetation on the protected North-facing 

 slopes is usually more mesic in type and more luxuri- 

 ant than on the exposed South-facing slopes, and 

 there is a deeper organic leaf litter on the ground. 

 Types of vegetation characteristic of arid habitats 

 penetrate humid climates on South-facing slopes ; 

 mesic vegetation penetrates the relatively arid cli- 

 mates obtaining on North-facing slopes. Southern 

 types of vegetation invade boreal climates on the 

 warm South-facing slopes, and boreal vegetation in- 

 vades southward on North-facing slopes. In moun- 

 tain areas, vegetation characteristic of lower altitudes 

 penetrates higher on South-facing slopes, and vege- 

 tation of the upper altitudes penetrates farthest down- 

 ward on North-facing slopes. Animals are locally 

 distributed in a similar manner, partly as a direct 

 response to the climate, partly to the differences in 

 vegetation. Many other differences in microclimate 

 occur in various parts of the forest and forest-edge 

 (Wolfe et al. 1949), and in grassland. 



In studying the distribution of animals in relation 

 to climate, it is obviously not sufficient to consider 

 only the macroclimate. Animals respond to the micro- 

 climate of their particular niches, and the relation 



Invertebrates 



Snails, earthworms, and myriapods are not nu- 

 merous in grassland because of the dry habitat. In- 

 sects, however, are abundant: some 1175 species and 

 varieties have been listed for different grassland plant 

 communities in Iowa (Hendrickson 1930). These 

 belong principally to the orders Orthoptera, Hemip- 

 tera, Homoptera, Coleoptera, and Diptera. Ants, 

 bees, and wasps (Hymenoptera) are also numerous. 

 Spiders make up about 7 per cent of the total 

 arthropod population in grassland. In one study 

 made in Nebraska (Muma 1949), 111 species were 

 collected from 128 hectares (320 acres) of mixed 

 high and low prairie containing some shrubs. Less 

 than a dozen species were web-builders ; there is a 

 lack of suitable web-building sites in grasslands. The 

 vast majority were wandering cursorial forms. In re- 

 gard to strata in this prairie, 45 species were re- 

 stricted to the soil and litter, 30 to the herbs, 1 to the 

 shrubs. Thirty-five species occurred in two or more 

 strata. The total population for the area was least in 

 the spring and greatest in the autumn. Peak popula- 

 tions in the ground stratum were reached during the 

 winter, however, because of the presence of many 

 hibernating immature forms. Similar seasoiial fluctu- 

 ations occur with other invertebrates, although the 

 peak populations of insects are usually attained dur- 

 ing the summer (Shackleford 1939, Fichter 1954). 



In grazed pastures and in grassy meadows in 

 New York State, invertebrates average 777 individ- 

 uals per square meter (Wolcott 1937). Of this pop- 

 ulation, ants make up 26 per cent, leafhoppers 15 

 per cent, other insects 34 per cent, spiders 9 per cent, 

 millipedes 9 per cent, sowbugs 2 per cent, snails and 

 slugs 2 per cent, earthworms 2 per cent, and large 

 nematodes 1 per cent. 



124 Habitats, communities, succession 



