Population Dynamics and Bioenergetics of a Fossorial Rodent, the Northern Pocket Gopher (Themomys talpoides), in a Spruce-Fir Ecosystem
Laboratory studies of the burrowing and resting energetics of the northern pocket gopher, Thomomys talpoides, are coupled with data on reproduction, activity budgets, and microclimates gathered during a 3 year field study to model the daily energy of free-ranging individuals in the Wasatch Mountains (2550 m), Utah. Radio telemetry data indicated that gophers are active about 50% of each day. No estimate of the time actually spent burrowing could be made. Estimates of average total winter (1 October to 1 June) consumption were, e.g., 40 to 52 megajoules for a 104 gram male, assuming burrowing periods of 2 to 10 hours day. Estimates of minimum consumption, based on minimum burrowing requirements commensurate with maintenance energy demands, are derived by comparing costs of various activity budgets to the belowground energy (plant material) the budgets would make available. Rates of burrowing through soils found in the study area were determined in laboratory experiments to be either '1.0.0 or '12.0 centimeters minute. The low burrowing rate was observed when soils were frozen or saturated, as would occur in the field in early winter and spring, respectively. The belowground standing crop of preferred plant biomass was estimated in 4 seral habitats (subalpine meadow to Picea-dominated forest) by applying root/shoot ratios determined for 42 broad-leaf species to peak aboveground standing crop estimates. Belowground energy densities at the depth of gopher foraging tunnels (0-15 centimeters) declined from 25 to 3 joules centimeter along the sere. I conclude that individual gophers are food-limited within the climax spruce seral stage. Further, costs associated with reproduction in females may exceed the available belowground energy supply in the later seral stages. Reduction of burrowing rates will affect gophers resident in the later seral stages more than it will meadow residents because of differences in energy acquisition rates. The peak gopher densities found during the study (summer 1976; 62 to 2 individuals hectare along the sere) support these inferences. Detailed demographic parameters were estimated only for meadow residents. Adult survivorship was lower in winter than summer (0.971 versus 0.989 week ), and varied greatly between years (0.16, 0.27 and 0.70 year). Juvenile survivorship from weaning through the first winter was comparable to adult annual rates. About 75% of females produced litters; fertility was 3.75 young per adult female per year. Comparison of demographic patterns predicted from the energetics analyses to those actually found leads me to conclude that growth of T. tolpoides populations is seldom food-limited. Rather, stochastic events associated with weather affect burrowing rates and thus survivorship. In montane environments, such events may occur with sufficient frequency to prevent populations from attaining sizes at which territorial behavior would limit further increases. Estimates of minimum energy flow through populations of T. talpoides at high densities (1100 megajoules hectare year) indicate they are proficient energy movers relative to non-fossorial small mammals. Indirect calculations suggest more than 30% of primary production allocated to belowground plant parts may be removed by gophers each year. More attention should be directed towards these and other fossorial rodents in studies of ecosystem structure and functioning.
Andersen, Douglas C. (1980). Population dynamics and bioenergetics of a fossorial rodent, the northern pocket gopher (Themomys talpoides), in a spruce-fir ecosystem. PhD. 134p.