Seasonal Maintenance and Net Production of Two Sympatric Species of Subarctic Microtine Rodents

Estimates were made of production and productivity (rate of production) of net biomass and metabolic rates for two species of mocrotine rodents in interior Alaska. Net production was estimated at two—wk intervals for natal, nestling, juvenile, subadult and adult components of each population. Maintenance energy requirements of nestling and adults were estimated on a seasonal basis using metabolic rates measured in the laboratory at concurrent field temperatures and photo—period. Maximum densities of Clethrionomys rutilus were nearly equal during each year of the study (° 60/hectare), yet yearly net production (1969 = 11565 kJ/hectare, 1970 = 6363 kJ/ha, 1971 = 1168 kJ/ha) varied nearly twofold. Yearly variability was due to a delay in production or a shorter production season. Net productivity during the low production year (1970) was significantly greater than the following high production year. As a result production intensity (maximum bimonthly production) was similar for each year studied. The density of Microtus oeconomus was not similar during each year of the study. A peak density during 1969 (° 80/ha) was followed by 2 yr of very low density and an increase during 1972. Net production during 1969 was 31359 kJ/ha. The average net production for high and low density years was approximately the same as the average yearly production for Clethrionomys rutilus. Maintenance production values are quite different from those cited for other microtine populations in two respects: First, seasonal maintenance estimates for the Clethrionomys rutilus population indicate that those of summer periods were lowest in spite of the cost of reproduction. Second, winter individual maintenance was nearly 2x summer maintenance for both species. Production estimates for the two microtine populations suggest that low species density typical of northern habitats should not always imply low production over a 3— 4—yr cycle, from year to year or even on a short term (bimonthly) basis. Dispersal and weight loss are discussed as possible mechanisms for regulating production variability.