APPLIED ECOLOGY 325 



exclosures of sufficient size, which will keep out rodents and yet 

 will not alter microclimate, presents numerous difficulties. Conse- 

 quently comparative studies on ranges supporting different, but 

 known, animal units are coming into use whenever possible be- 

 cause they do not require exclosures. 



When the results of such studies are evaluated and expressed in 

 general terms, it becomes apparent that several principles have 

 been established that appear to be universally applicable. 59 From 

 an ecological point of view, these principles, determined by ex- 

 periment, would seem to be self-evident since they conform to 

 ecological theory. It must be remembered, however, that these 

 things were originally theory and now can be stated as principles 

 supported by experimental evidence. The testing was necessary to 

 establish them as tried bases for range management. In grasslands, 

 no less than elsewhere, succession is operational, and all trends 

 constantly proceed toward the climax unless they are modified by 

 disturbance or are held in check by an unfavorable swing of cli- 

 mate, as during a series of dry years. Grassland is a climatic life 

 form, which maintains itself in the absence of disturbance and 

 which, if destroyed, reappears when the disturbance is removed. 

 All evidence indicates that perennial grasses become dominant and 

 eliminate annual grasses, forbs, and shrubs in the absence of graz- 

 ing, fire, or similar destructive agencies. The grasses of a particular 

 climax are adapted to its climate and usually have an advantage in 

 terms of competition over introduced ones. 



From the above, it becomes apparent that, as in forestry, prac- 

 tices of management which least disturb the natural balance of 

 grassland and its environment are most desirable. Those that take 

 into consideration the trends of succession and local climax are 

 likely to be most successful at the same time that they require a 

 minimum of expended effort. Although a few exotic species have 

 proved to be easier to propagate than native ones, the introduction 

 of foreign species for range improvement or erosion control is 

 likely to be unsatisfactory unless those species are to be given 

 extra care or special cultural conditions. In fact, there is evidence 

 that seeding of native species should be done only with locally 

 produced seeds since the species may consist of geographic physi- 

 ological races. 



