A damped precipitation-driven, bottom-up model for deer mouse population abundance in the northwestern United States

Small-mammal population densities can be regulated by bottom-up (food availability) and top-down (predation) forces. In 1993, an El Niño Southern Oscillation event was followed by a cluster of human hantavirus with pulmonary syndrome in the southwestern United States. An upward trophic cascade hypothesis was proposed as an explanation for the outbreak: Increased plant productivity as a consequence of El Niño precipitations led to an unusual increase in distribution and abundance of deer mice (Peromyscus maniculatus; reservoir host of Sin Nombre virus). Could such drastic events occur in mesic habitats, where plant productivity in response to climate conditions is likely to be much less dramatic? In this work, we investigate to what extent deer mouse populations follow a precipitation-driven, bottom-up model in central and western Montana and discuss important conditions for such a model to be possible. We found positive correlations between deer mouse abundance and on-the-ground measured plant productivity with a several-month lag in three of six study sites. This effect was weaker when deer mouse populations were more abundant, indicating density-dependent effects. Dispersal resulting from territoriality may be important in attenuating local density increments in spite of high food availability. In addition, there is evidence that population abundance in the study area could respond to other abiotic factors. In particular, precipitation in the form of snow may reduce deer mice survival, thus compensating the benefits of improved plant productivity. Deer mouse populations in Montana study sites follow complex dynamics determined by multiple limiting factors, leading to a damped precipitation-driven bottom-up regulation. This prevents dramatic changes in rodent abundances after sudden increments of food availability, such as those observed in other regions.