Population Fluctuations of Peromyscus maniculatus and Other Small Mammals as Revealed by the North American Census of Small Mammals

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Abstract
Data collected in the "North American Census of Small Mammals" were analyzed to reveal fluctuations in local populations of several species of small mammals through successive years of trapping. The failure of a species to fluctuate widely is suggested as indicative of effective biological or sociobiological controlling mechanisms. Peromyscus maniculatus and Blarina brevicauda populations exhibited a markedly smaller average range of population fluctuations than did Microtus montanus, M. californicus, M. pennsylvanicus and Reithrodontomys megalotis. These ranges in fluctuations between maximum and minimum population levels were evaluated according to the average number of years required for the change to take place. This evaluation was referred to as the Fluctuation Index (F. L) and reflected the potential for a species to fluctuate in a short period of time. Blarina brevicauda (9.6) and Peromyscus maniculatus (10.6) exhibited the two lowest F. I. Values, while Microtus montanus and M. calif ornicus exhbited the two highest (52.3 and 80, respectively). The data support the hypothesis that Peromyscus maniculatus and Blarina brevicauda may be more sensitive to factors controlling population growth and thus their growth is limited within a narrower numerical range of variability than is true for more widely fluctuating forms.

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