86 



THE HABITAT 



the exception of snow, cover is, however, a question of vegetation, living 

 and dead, and consequently is to be referred to the discussion of biotic 

 factors. The surface of the soil itself often shows irregularities which 

 must be taken into account. Such are the rocks of boulder and rock fields, 

 the hummocks of meadows and bogs, the mounds of prairie dog towns, 

 the innumerable minute gullies and ridges of bad lands, the raised tufts 

 of sand-hills, etc. The influence of these is not profound, but they do 

 have an appreciable effect upon the run-off, temperature, and wind. In 

 many cases, this is distinctly measurable, but as a rule little more can be 

 done than to indicate that the surface is even or uneven, and to describe 

 the degree and kind of unevenness. 



125. Record of physiographic factors. Altitude, slope, exposure, and 

 surface are essentially constant factors, and are determined once for all, 

 after a few check readings have been made, except in those relatively rare 

 habitats in which dynamic forces are very active. The form of record 

 used is the following: 



127. Topography. As heretofore indicated, questions pertaining -to the 

 form and development of the land concern groups of habitats within which 

 each habitat is the unit of investigation after the manner already laid down. 

 A knowledge of topography is essential to the accurate mapping of a 

 region, for which the simple methods of plane table and contour work are 

 employed, while the geology of the surface is of primary importance in 

 the study of successions. 



BIOTIC FACTORS 



128. Influence and importance. Biotic factors are animals and plants. 

 With respect to influence they are usually remote, rarely direct. Neverthe- 

 less, they often play a decisive part in the vegetation. Their effect is, as 

 a rule, felt directly by the formation rather than the habitat, but in either 

 case the one reacts upon the other. Such factors are not themselves sus- 

 ceptible of exact measurement, but their influence upon the habitat is 

 usually measurable in terms of the physical factors affected. In the case 



