Gray-tailed vole population responses to inbreeding and environmental stress

Habitat fragmentation and other types of disturbance are creating an increasing number of small, isolated populations, conditions that lead to inbreeding. The effects of inbreeding and environmental stressors may interact to hasten extinction of small populations. Also, some species may be adversely affected by edge conditions in fragmented landscapes. In a series of experiments conducted in the laboratory and in terrestrial mesocosms, the gray-tailed vole (Microtus canicaudus) was used as a model organism to study the effects of inbreeding and environmental stressor interactions on population dynamics. An organophosphorous insecticide was used as a model environmental stressor. Also, animal responses to habitat edges were evaluated.; In the laboratory, several correlates of fitness such as mean litter size at birth and weaning, and the proportion of young that survive to weaning, decreased upon inbreeding. The magnitude of inbreeding depression in the M. canicaudus studied was similar to that of other undomesticated mammals. Inbreeding also caused reduced population growth rates and lower population densities in the field. Similarly, insecticide applications temporarily reduced densities and population growth rates of both inbred and noninbred populations. The effects of inbreeding and insecticide applications on various population measures were approximately additive. The primary effect of inbreeding was an apparent reduction in reproductive success, and the chief effect of exposure to insecticides was reduced survival probabilities. However, inbred and noninbred populations had similar extinction rates, suggesting that stochastic environmental factors may be important determinants of persistence times of small populations.; Fewer captures of voles occurred in edge traps than expected based on the availability of traps set near the edge. Neither male nor female body weights, or female reproductive rates, were associated with capture locations within enclosures. As females aged, they tended to occupy more interior portions of habitat patches. Experimental model systems using small-mammals as study organisms may be effective tools for studying both important determinants of extinction processes in small populations and the effects of habitat fragmentation on population dynamics and individual behavior.