10 BULLETIN 1001, XJ. S. DEPARTMENT OF AGRICULTURE. 



Plant and animal competition. — In any plant association there are 

 always certain plants which, after a long enough period of adjust- 

 ment, dominate all others and occupy most of the space. These 

 are the characteristic plants of the association and their names 

 are generally used to name the association. Sometimes these 

 dominant species are good forage plants and sometimes they are 

 not. Frequently the dominance of certain plants is due to some 

 other factor than the climatic ones. A good example of this is 

 shown in certain of the plains or prairie regions that formerly were 

 covered with tall grasses but became brush-covered after stock were 

 introduced. Before live stock were brought in, the grass was fre- 

 quently burned over and all shrubs were killed to the ground. 

 Grasses recover quickly from such burning while shrubs grow more 

 slowly, and are only able to get well started by the time another 

 fire occurs. When grazing animals are introduced and they eat the 

 grass, there is nothing left to burn, and the shrubs reach maturity, 

 changing the whole appearance of the region and often modifying its 

 grazing capacity. 



The introduction of grazing animals on any plant association also 

 tends at once to change the relationships existing among the different 

 plants of the association, because the animals do not graze equally 

 all the plants of the association. They always select the plants 

 they like best and eat them first. Thus there is a strong tendency 

 for the plant that is the most palatable feed on the range to be 

 exterminated by the animals. Especially is this the case if the range 

 is too heavily stocked. 



The plants of an association also compete among themselves for 

 place. Hence when certain plants, that ordinarily dominate, are 

 eaten, subordinate and usually less palatable members of the asso- 

 ciation take a higher place in the association than they normally hold. 

 If this overloading of the range is carried to excess for some time, 

 unpalatable weeds occupy the range. It is thus possible for any one 

 who is conversant with the normal. plant associations of a region to 

 tell just about how badly and how long a particular range has been 

 overstocked by observing what plants that should be present are 

 lacking and how subordinate ones in the association have increased 

 in importance. In extreme cases of overstocking the edible plants 

 are practically driven out and the soil is occupied by weeds that the 

 grazing animals never eat. A striking case of this kind is seen where 

 the snakeweed {Gutierrezia spp.) has completely occupied the short- 

 grass land. 



Soil m,oisture and erosion. — When grazing animals reduce the total 

 amount of vegetation on the area that they graze to such a degree 

 as to increase the rate at which the rain water runs off, two things 



