Population dynamics and bioenergetics of a fossorial herbivore, Thomomys talpoides (Rodentia: Geomyidae), in a spruce-fir sere

Abstract

Studies of the bioenergetics of the northern pocket gopher, Thomomys talpoides , are coupled with data on demography, activity budgets, and microclimates to model the energy requirements of individuals and populations in the Wasatch Mountains of Utah during 1976-1979. Metabolic rates during rest increased linearly with decreasing ambient temperature, but burrowing metabolic rates (16.3 mL O 2 • h -1 • g -9.75 ) were independent of both temperature and physical properties of the soil. Radio-telemetry studies indicated that free-ranging gophers are active =50% of each day. Conservative estimates of true energy consumption were calculated using estimates of habitat-specific minimum daily burrowing requirements. Rates of burrowing measured in the laboratory were either ∞ 0.0 or ∞ 2.0 cm/min. The low burrowing rate was observed when the soil was frozen or saturated with water, as would occur in the field in early winter and in spring, respectively. Gophers burrowed through soil at the study site at an average rate of ∞ 1.5 cm/min. Belowground food energy densities at gopher foraging depth declined from 24.6 to 3.2 J/cm 3 along a successional gradient (subalpine forb meadow to Engelmann spruce dominated forest). We conclude that individual gophers are food limited within the climax spruce seral stage. Further, daily energy costs associated with reproduction in females may exceed the belowground energy supply available in intermediate seral stages (aspen and subalpine fir). Reduction of burrowing rates for any reason will affect gophers in the late seral stages proportionately more than those resident in the meadow. The peak gopher densities recorded (from 62 individuals/ha in the meadow to 2 individuals/ha in spruce forest) support these inferences. Detailed demographic information was obtained only in the meadow seral stage. Adult survivorship was lower in winter than in summer and varied greatly between years (0.18-0.70 yr -1 ). Juvenile survivorship from weaning through the first year was comparable to adult annual rates. The fertility rate was 3.75 young • female -1 • yr -1 . The energy supply and demand analyses indicate that the growth of Thomomys talpoides populations in the early seral stages is seldom directly limited by the amount of food present. From our demographic, environmental, and autecological studies we conclude that stochastic events associated with weather affect energy acquisition (burrowing) rates, and thus survivorship. In montane environments, such events may prevent populations from attaining sizes at which territorial behavior would hypothetically limit further increases. The energy flow through the meadow population at moderate to high )1976-1977) densities (at least 1100 MJ • ha -1 • yr -1 ) indicates that pocket gophers are proficient energy movers relative to non-fossorial small mammals. Subalpine T. talpoides populations appear commonly to attain such densities. More than 30% of the annual primary productivity allocated to belowground parts of meadow forbs may be consumed by gophers.