Small Mammal Energetics in Grassland Ecosystems

Abstract

Results of quantitative sampling of small mammal populations at different grassland sites for a 3—yr period are compared to evaluate the energy requirements and consumption, according to trophic levels, for total small mammal communities at all seasons in different years. The purpose was to search for patterns of food utilization by these consumers at the different sites. This picture of the bioenergetics of small mammal populations over a wide range of grassland sites was constructed from extensive US/IBP Grassland Biome diet and population data and from physiological information out of the literature. The work represents one step in the process of understanding the role of consumers in the ecosystem. No species occurs at all sites and different trophic strategies predominate among the small mammals at each site: Microtines (herbivores) dominate the tallgrass prairie, sciurids (omnivores) dominate the northern shortgrass prairie, and heteromyids (granivores) dominate the bunchgrass and desert grasslands. Other groups occur at these sites and vary in their importance. At the tallgrass and midgrass sites the small mammal populations are largely dependent on herbage consumption, while at the northern and southern shortgrass prairie sites the rodent fauna is largely dependent on invertebrates. The most uniform distribution of resource utilization by this component of the consumer community occurs at the desert grassland site, where herbage, seeds, and animal matter are all utilized. Relative to the total amount of consumable herbage available (that actually utilized by the small mammal population), this resource is only slightly utilized (from a fraction to a few percent of available). Animal matter, largely invertebrates, is highly utilized at most sites and may in fact be a limiting factor on small mammal populations. The energy consumed by the small mammal population was greatest at the tallgrass prairie site, where the average annual consumption was 172 x 103 kcal/ha (= 720 MJ/ha). However, efficiency of biomass support was greatest at the northern shortgrass prairie site, where consumption of 32 x 103 kcal/ha (= 134 MJ/ha) supported proportionately more biomass (277 g live wt./ha) of animals than at the tallgrass site (935 g/ha). The estimates of population energy requirements presented here compare well with other available estimates. These results emphasize the great between year and site variability, however. Daily population respiration in summer may be as little as 14 and as large as 1,038 kcal°ha—1°day—1. Highest small mammal biomass occurred at the tallgrass (3,075 g live wt./ha) and the desert (2,304 g/ha) sites, and lowest at the midgrass (14 g/ha) site. Within the period of study, small mammal biomass was most stable at the southern shortgrass site and most variable at the tallgrass site. The role of small mammal populations in grassland ecosystem remains incompletely defined. A broader view of the total consumer community in relation to resources is required.