ZOOGEOGRAPHY 301 



the same nature as those wliich have been observed within the range of a 

 species. A good example is the mouse plague of 1907-1908 in the Hum- 

 boldt Valley,. Nevada. This irruption has been described^ as follows: 



"Always present in Humboldt Valley, these mice (Microtus montanus) attract 

 little attention when in small numbers. Usually they are not uniformly abun- 

 dant in the district; in fact in many of the fields thej' may not be present at all. 

 They live in scattered colonies in swampy places; along the borders of sloughs 

 and irrigation ditches; in salt grass patches and in similar damp areas. WTien in 

 normal numbers a little damage may be noticeable about the borders of fields or 

 along ditches where plants have been killed during winter and spring. Ordinarily 

 the mice are very prolific, each pair producing 4 to 6 litters of about 6 young each 

 during the long breeding season, which extends from March to November; and 

 probably the young born early in the season breed before fall. Occasionally 

 conditions favor excessive multiplication, and under such circumstances damage 

 soon becomes evident, and in a single season may increase locally to the serious 

 injury of fields. Extending from such centers during the next breeding season, 

 and increasing not only by reproduction but by joining with other colonies, a vast 

 army of mice is formed. Because of overcrowtling and the limitation of food, 

 such armies invade adjoining districts, and this progress becomes more rapid with 

 the disappearance of green food in the fall. Through the combination of several 

 such armies, entire districts are overrun. 



"Damage by field mice attracted the attention of the ranchmen in the lower 

 part of the Humboldt Valley early in the spring of 1906 and became severe during 

 the following summer. In the fall and winter of 1906-1907, damage had increased 

 until fields here and there in the valley were seriously injured. Extensive ravages 

 first occurred above and about Lovelocks. In May, 1907, fields on the Rodgers 

 ranch, 5 miles below Lovelocks, were invaded from the lands farther up the valley, 

 the progress of the mice being plainly marked, as the fields above the Rodgers 

 ranch suffered first. The movement of this great bodj^ of mice, it should be noted, 

 was a gradual, scattering progression, first by a few and later by increasing num- 

 bers, until the greater part had moved to fresh fields. Numbers, however, 

 finding conditions improved, remained as stragglers in the fields deserted by the 

 main body. 



"By October, 1907, a large part of the cultivated lands in this district had 

 been overrun by vast numbers of mice. The yield of hay had been reduced by 

 one-third ; potatoes ond root crops were largely destroyed; many alfalfa fields were 

 ruined by the mice eating the roots of the plants; and the complete destruction of 

 this, the chief crop of the valley, was threatened. 



"The height of abundance was reached in November, when it was estimated 

 that on many large ranches there were from 8000 to 12,000 mice to each acre. 

 The fields were riddled by their holes, which were scarcely a step apart, and over 

 large areas averaged 150 to 175 to the square rod. Ditch embankments were 

 honeycombed, and the scene was one of devastation. Serious losses in hay and 

 root crops during the summer proved but a slight forerunner of the damage 

 which began in the fall with the disappearance of green food. Burro wmg down 



*U. S. Dept. of Agriculture, Farmers' Bulletin 352. 



