ANIMALS AND THEIR ENVIRONMENT 305 



Most soil animals are near the surface, not deeper than 5 or 6 inches 

 during the active season. Many species migrate downward annually 

 to avoid frost and return in the spring. Earthworms have been found as 

 deep as 6 feet, where they went to find moisture in dry seasons. Species 

 which merely make their nests in the ground often go rather deep — 

 gophers 2 feet, termites 5 feet, ants 9 feet, and the prairie dog as deep 

 as 14 feet. 



As a special type of soil environment may be mentioned sand dunes. 

 The chief physical characteristic of dunes is the extreme variation of 

 their temperature and moisture. Even in moderately moist regions, 

 rain water drains out of sand quickly; and in the heat of midday the 

 temperature may rise to 50 or 60°C. The hottest part of a sand dune, 

 when the sun has been shining upon it, is directly at the surface. The 

 air a few inches above it and the sand at a depth of several inches are 

 cooler. Certain wasps which dig burrows are among the most character- 

 istic dune animals; and with them are certain other insects parasitic 

 upon the wasps. Many other animals are occasional visitors but have 

 no particular dependence on dime conditions. 



Associations in Vegetated Areas. — When vegetation has taken hold 

 in the soil in abundance, the physical conditions are modified in several 

 important ways. Sunshine is intercepted, thereby reducing the fluctu- 

 ations of the temperature of both soil and air. The diminution of light 

 by trees is much greater than that by shrubs or herbs, and the reduction 

 by pine trees is much more than by larches or elms. In one forest it 

 was found that the maximum daily temperature was 5 or 6° lower, and 

 the daily minimum an equal amount higher, than in a near-by cutover 

 area. Evaporation of water from the trees is one of the ways in which 

 temperature is lowered. Some of this reduction of temperature is, how- 

 ever, nullified by stoppage of the wind by trees, so that open spots sur- 

 rounded by forest may, when the sun shines long upon them, become 

 warmer than they would if there were no trees. General evaporation is 

 also reduced by this retardation of the wind, beech-maple forest exercising 

 a much greater control than cottonwood, for example. 



Introduction of vegetation modifies the characteristics of an area in 

 very many ways, depending on what plants are present. As a conse- 

 quence the animals become ex(?eedingly varied. The nature of an animal 

 association is determined largely by the plant association. Insects 

 which feed upon the leaves of plants often utilize only one or a few species. 

 Those which produce galls on leaves are commonly limited to one species 

 of plant. Wood-boring and bark insects prefer certain trees. Soil is 

 altered differently by different plants, and root-feeding animals usually 

 specialize in certain roots. Rotten logs in various stages of decay con- 

 tribute to the variety of situations. The general effect of vegetation 



