10 BULLETIN 1001, U. S. DEPARTMENT OE 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 moisture 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 
