These studies have shown that some species of mammals have 
habits that cannot be made.to. fit in harmoniously with such economic 
interests as stock raising and crop farming. These are chiefly the 
rodents--ground squirrels, prairie dogs, rats, mice, pocket gophers, 
and rabbits--and the carnivorous animals—-coyotes, bobcats, and moun- 
tain lions. If they remained at a nominal population, where their 
depredations would be small, stockmen and ranchers would have little 
objection to them. But in the case of these animals, naturé does not 
regulate the population to a level satisfactory to man. 
Conditions Favoring Pest Increases 
Observations show thet animal populations respond to increase 
in food. A low population may be rapidly increased when a liberal 
food supply causes an increase in the size of their litters. The 
‘introduction of field crops into an area often results in an increase 
of rodent pests to a point where they would take the greater part of 
the products if effort was not made to reduce their numbers. Bringing 
domestic stock (especially sheep) on a Western range soon causes an 
increase in the coyote population. These conditions have made it 
necessary to carry on artificial control measures for carnivorous 
animals and rodents. 
The Bureau of Biological Survey has research units whose inves- 
_ tigations proceed along two lines: (1) To study the habits of the 
animals that have become pests to learn if there are some natural 
methods of control that will stop their depredations; (2) to devise 
artificial methods when natural control is not effective. To aid the 
effectiveness of natural control may require years of research, so 
the two lines of study are carried on simultaneously. Artificial 
methods, which generally can be devised more quickly, are used to 
stop the damage to crops, livestock, and game without endangering the 
existence of any species. 
Meadow Mouse Control 
To develop a metnod of reducing the numbers of a species of 
. animal and the damage inflicted by it, a careful and detailed study 
must be made of its habitat, its food, and its relations to other 
animals, in order to determine why it selects a particular kind of 
place in which to live. For instance, the meadow mouse (Microtus), 
which often inhabits fruit orchards, chooses a place where there is 
vegetative cover under which its extensive feeding runways on the 
surface of the ground connect with its shallow burrows. The mice 
do not often come out into. open, clear spots to feed, although such 
places may be only a few feet wide. They will at times make shallow 
tunnels under ground to the trunk and roots of a tree, if it is not 
too far away. 
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