PROCESS INDICATORS. 91 



Food and shelter relations are naturally often combined in the same com- 

 munity. When they are found in the same species, the indicator value of the 

 latter is distinctive. This is not infrequent for mammals and birds, as in the 

 case of Neotoma and Yucca or Opuntia in their respective communities, but it 

 is best seen in the case of insects. The classic example is afforded by Yucca and 

 Pronuba, but Xyloscopa, Megachile, and other genera of pollinators furnish 

 similar instances, while the host-plants of gall-producing insects exhibit a 

 like relation. Such examples are naturally rare among birds, but a close rela- 

 tion exists in some cases. Taylor (1912 : 414) has called attention to this in 

 the case of Artemisia tridentata and the sage-thrasher, Oreoscoptes montanus, 

 and it occurs also between the cylindric opuntias and the cactus wren, Heleo- 

 dytes brunneicapillus, as well as between the giant cactus, Cereus giganteus, 

 and the gilded flicker, Colaptes chrysoides (plate 18). 



The indicator relations between plants and animals arising out of the 

 disturbances caused by the latter are numerous, and play a large part in the 

 study of secondary succession. Among the striking examples are ant-hills, 

 rodent burrows, prairie-dog towns, and beaver dams. The indicators of this 

 type are considered further in the section on paleic indicators. 



PROCESS INDICATORS. 



Nature. — Process indicators comprise those plants and communities which 

 indicate definite processes in the habitat. Such processes may be natural, 

 as when they are topographic or climatic, or artificial, when they are the result 

 of disturbances due to man. Such a distinction is convenient rather than 

 essential, since there is no real difference in the overgrazing due to a herd of 

 bison and that caused by a herd of cattle, or in disturbances of the soil pro- 

 duced by primitive or civilized man. The latter, however, does cause dis- 

 turbances in vastly greater number and on a much greater scale, with the 

 result that the majority of process indicators ordinarily encountered are 

 related to his activities. While the two have much in common, the more 

 vital distinction is based upon the nature of the area, and the vegetational 

 development which results (Plant Succession, 33, 60). Primary areas are 

 represented by water-bodies, rock, dune-sand, etc., in which extreme condi- 

 tions prevail, and a long fine of development occurs. Secondary areas are due 

 to disturbance by man or animals, or to superficial erosion or deposition. 

 The conditions are usually much less extreme for the initial invaders, and the 

 development is correspondingly short and simple. Both are alike, however, 

 in that the successional development progresses by more or less well-marked 

 stages, in which there is a definite relation between the dominants and the 

 factors. Each stage or associes thus serves as a community indicator of the 

 conditions of the habitat, each consocies as an indicator of smaller habitat 

 differences, and each socies of still finer differences. 



Kinds. — Process indicators are grouped primarily upon the nature of the 

 process itself. They are all indicators of the successional process in vegeta- 

 tion, and hence this relation is taken for granted. The great majority of them 

 are concerned with unit successions or seres related to physiographic pro- 

 cesses or disturbances, but many of them have to do with climatic processes 

 or cycles, as found in potential succession, and in coseres and cliseres. Hence 

 it is desirable to distinguish the indicators of primary processes, such as 



