Small Mammals: Pests or Vital Components of the Ecosystem

—Small mammals regarded as "pests" should not be viewed separately from other components in the ecosystem. Small mammals have significant influences on vegetation and soils, exert predatory pressure on other animals, and provide food for predators. Future management efforts should include consideration of these diverse influences. Careful evaluation of the role of small mammals and their relationships with their environment is necessary to fully appreciate the impact of control programs on the ecosystem. Small mammals regarded as "pests" should not be viewed separately from other components in the ecosystem. Rather, small mammals must be viewed in terms of their interrelationships with other components. Alteration of small mammal communities through control programs influence other components and ultimately the whole system. Small mammal influences may be grouped as those effects on (1) vegetation, (2) soils, and (3) other animals. Vegetative influences may include effects on primary productivity, plant species composition, and decomposition rates of plant materials. Small mammals influence both physical and chemical properties of soils. Small mammals prey on insects and occasionally other small mammals, provide a prey base for carnivores, and modify their environments in such a way as to provide habitat for other animals. INFLUENCES ON VEGETATION Researchers have proposed various ways in which small mammals interact with plant communities. The main interactions can be categorized as those relating to primary productivity, plant species composition, plant stature and reproduction, and decomposition rates of plant materials. Paper presented at the 8th Wildlife Damage Control Workshop, April 28-30, 1987, in Rapid City, South-Dakota. Sieg is Wildlife Biologist with the Rocky Mountain Forest and Range Experiment Station, Rapid City, South Dakota, in cooperation with the South Dakota School of Mines and Technology. The Station's headquarters is in Fort Collins, in cooperation with Colorado State University. Primary Production Small mammal herbivores may consume as much as 60% (Migula et al. 1970) to 80% (Taylor and Loftfield 1942) of the total annual primary plant production. They may have localized, large-scale impacts on primary productivity during population explosions. However, the effect of direct consumption of plants by herbivores must be evaluated in terms of what portion of the primary production is actually available to the animal. Estimates of herbage consumption by small mammals ranged from <1% in northern shortgrass and midgrass sites to as much as 20% in desert grasslands (French et al. 1976). Harris (1971) estimated that 0.17-5.01% of the net primary production was transferred to the rodent trophic level. Hayward and Phillipson (1979) concluded that the impact of small mammal consumption on net or available primary production is negligible in most systems. Light grazing by small mammals may stimulate plant production. For example, moderate grazing by voles (Microtus oeconumus and M. middendorffit) increased production of two plant species by stimulating new shoot growth (Smirnov and Tokmakova 1971, 1972). Regrowth of rye grass (Lolium perenne) that had been grazed by hispid cotton rats (Sigmodon hispidus) was faster than regrowth of grass that had been mechanically clipped (Howe et al. 1982). The authors speculated that either a biochemical agent in saliva or the specific manner of tissue removal by the cotton rats stimulated regrowth of the rye