ECOLOGY. 363 



more. Complete tabulations have been made of the flora and fauna of 

 each formation and epoch, and these are being organized into climax 

 and successional communities on the basis of the system set forth in 

 "Plant Succession." The results already secured seem to assure the 

 discovery of a number of new correlations between the climate, vegeta- 

 tion, and fauna of the past. 



Indicator Plants, by F. E. Clements. 



While some attention has been given to practically all types of in- 

 dicators, it has been centered chiefly upon those that have to do with 

 climate, grazing, serai sequence, and competition. A striking instance 

 of the value of subdominants as climatic indicators was found in the 

 Red Beds valley inside the Black Hills rim. The societies of the mixed 

 prauie were developed here to a marked degree, equaled only in the 

 true prau'ies far to the east, and they furnished a decisive indication 

 of greater rainfall and reduced evaporation. It was also clearly rec- 

 ognized for the first time tliat the presence of Amorpha, Rosa, Ceano- 

 thus, and other bushes indicated a marked tendency toward a woodland 

 climate. The indicator value of these plants has been obscured by 

 grazing and mowing, and was first clearly revealed in protected places, 

 where Amorpha in particular grew 5 or 6 feet high. 



The most detailed indicator studies have dealt with the response 

 of the various grassland dominants and subdominants to grazing. 

 Each species has its particular response, which is often largely charac- 

 teristic of the genus as weU. This is best shown in the case of Stipa, 

 the climax species of which bear essentially the same relation to 

 overgrazing. They are the first dominants to disappear, whether 

 it be Stipa setigera and eminens in the bunch-grass prairie, Stipa 

 comata and pennata in the mixed praiiie, or Stipa spartea in the true 

 prauie. This is chiefly due to their early start in the spring, as a result 

 of which they must stand the brunt of early grazing, and partly also 

 to the absence of vigorous rootstocks or stolons. The same causes 

 explain the fact that Koeleria cristata usually disappears next, and 

 hence indicates a somewhat greater degree of overgrazing. Agro- 

 pyrum glaucum is much more persistent by vntue of its remarkable 

 rootstocks, and it is often abundant where both Stipa and Koeleria 

 have vanished. It is a tall-grass, however, and overgrazing handicaps 

 it in comparison with the short-grasses because of the much greater 

 consumption of its leaves. Its disappearance marks the destruction of 

 the most valuable of all forage types, the mixed prairie, and the final 

 dominance of short-grass. Still more serious overgrazing is indicated 

 by the gradual destruction of the open mats of Bouteloua gracilis, 

 which is much m.ore easily trampled out than Carex filifolia or Bulhilis 

 dactyloides. When both are present, Carex next succumbs and 

 Bulhilis alone persists by virtue of its dense mat and vigorous stolons. 



