90 KINDS OF INDICATORS. 



quently throughout the summer. The light changes little in quality or inten- 

 sity with the altitude in the Rocky Mountain region generally, though this may 

 be due to low humidity. The relative humidity increases, but evaporation 

 and transpiration are greater at higher elevations, owing to reduced pressure, 

 wind, etc. The annual precipitation rises steadily with altitude, and an increas- 

 ing amount of it occurs as snow. The excessive snowfall of subalpine and 

 alpine regions accounts for many of their characteristic features and explains 

 the generally high water-content. Winds are usually prevailing and force- 

 ful, and have both a direct and indirect effect in the dwarfing of trees at 

 timber-line. The indicator values associated with high altitudes are primarily 

 due to temperature or water, or usually to both acting together. With the 

 exception of the wind and snow forms of trees and shrubs, all alpine and sub- 

 alpine indicators are related to these factors in the region concerned. 



The sharp changes of climate with altitude produces a corresponding 

 sequence of climaxes, which serve as the most outstanding of indicators. 

 These are considered in more or less detail in the following chapter. In addi- 

 tion, the majority of montane and alpine species have rather definite lower 

 and upper limits, and may be used as indicators of altitude, though a cor- 

 rection is necessary for those of wide range in latitude. Cockerell (1906 : 861) 

 has made an analysis of the alpine species of Colorado, based upon Rydberg's 

 Flora of Colorado (1906), which brings out their altitudinal relations clearly, 

 and makes it possible to use many of them as altitude indicators for the 

 central Rocky Mountains (plate 17). 



Organism indicators. — The many basic relations between plants and 

 animals make it clear why the plants often serve as definite indicators of 

 animals. Animals may also act as indicators of plants, but to a less degree 

 and in a less definite manner. In addition, plants regularly serve as indicators 

 of such other plants as bear a distinct nutritional relation to them. This is 

 particularly true of the fungi and bacteria, of which one of the most striking 

 indicator relations, the fairy ring, has already been discussed (p. 12). The 

 use of plants as indicators of animals is based upon the relation of food, 

 shelter, disturbance, or pollination. In all of these the indications may be very 

 definite, a certain plant or community denoting a particular animal, but as a 

 rule the relation is necessarily more general. In some cases, moreover, the 

 relation may be concomitant rather than causal, as in the case of the alpine 

 conies and marmots, where the control seems to be rather one of climate than 

 of the alpine plants upon which they feed. Furthermore, the indicator rela- 

 tion varies from region to region with the range of local occurrence of the 

 species concerned. A striking example of this occurs in the relation of the 

 kangaroo rat (Dipodomys deserti) to the shrubs about which it makes its 

 mounds. In the savannahs of the desert plains it occupies every clump of 

 Celtis pallida as its first choice. In the usual desert mixtures of Larrea and 

 Prosopis where Celtis is absent, the preference is almost exclusively for 

 Prosopis, but when the latter is lacking or has been destroyed by the rats, 

 the mounds are made about Larrea. In portions of the Colorado Desert, 

 mounds of remarkable size are built about Dalea spinosa, and both Prosopis 

 and Larrea are practically ignored. Throughout the desert scrub, one or the 

 other of these four genera will be the indicator, depending upon their grouping. 



