120 COACTION: THE INTERRELATIONS OF ORGANISMS 



of North America are the bison, pronghorn antelope, musk ox, and 

 tundra caribou. The first three prefer grasses and sedges; the tundra 

 caribou seem to favor lichens, grass, and forbs. Elk, deer, sheep, and 

 goats may shift according to the conditions from the grazing habit 

 through various combinations to a browsing coaction, or the reverse. 



Quantitative studies of the food coactions of feral grazing animals 

 have been rare, and the scattered observations are for the most part 

 incomplete and undependable. Even the few examinations of stomach 

 contents leave much to be desired, owing to inherent difficulties and 

 the scattered nature of the observations. There is an increasing body 

 of knowledge as to the food habits of domestic cattle, sheep, and goats, 

 all of which are unfortunately of European or Asiatic origin, and some 

 of the observations are both quantitative and specific as to plants 

 used. This knowledge will be steadily augmented by the experimental 

 installations now in existence at the several range reserves and grazing 

 experiment stations. However, these need to be supplemented in 

 detailed manner by extensive observations on the actual process of 

 grazing in terms of life forms and species, closeness of cropping, sea- 

 sonal preference, etc., with reference to the most important wild 

 ungulates. 



The observed effect of grazing coactions upon the plant matrix 

 is primarily an outcome of the selection and utilization of the con- 

 stituent plant species by domestic animals. Every association of the 

 grassland climax in North America has been thus modified, some of 

 them in most striking and puzzling fashion as an outcome of the 

 choices of domestic stock, but the results of grazing by native animals 

 cannot be expected to be the same. Similar modifications have oc- 

 curred in grassland the world over. 



Probably the best-known example of such a modified community, 

 viz., the "short-grass plains," was long supposed to be climax in char- 

 acter (Pound and Clements, 1898; Clements, 1920, 1922). Similarly, 

 the bunch-grass prairie of California has been converted almost en- 

 tirely into an associes of annual grasses, chiefly Avena and Bromus, 

 while the true prairie of the Middle West has been largely transformed 

 into a tall-grass postclimax of Andropogons. Under more intense 

 grazing pressure, the grasses have yielded to dominants of adjacent 

 scrub climaxes, such as Artemisia or Larrea, to consocies of such un- 

 dershrubs as Gutierrezia and Haplopappus, or have been replaced by 

 introduced annuals like Salsola. Thus, each kind and degree of 

 overgrazing produces its proper indicators, and in consequence it is 

 possible to reconstruct the history of a range by the indicators that 

 characterize it. 



