Small mustelid predation slows population growth of Microtus voles: A predator reduction experiment

1. The effects of predator reduction on the breeding performance and sex ratio of Microtus voles were studied in a cyclically fluctuating vole assemblage in western Finland, where the field vole (Microtus agrestis) and the sibling vole (M. rossiaemeridionalis) are the main prey of small mustelids. 2, The densities of small mustelids [the least weasel (Mustela nivalis nivalis) and the stoat (M. erminea)] were reduced experimentally in three large (2-3 km(2)) unfenced areas in 1992, during a crash phase of the vole cycle, and in three different areas during the following crash phase in 1995. The reproductive performance of Microtus voles was compared between control and manipulation areas before and after reducing predators. 3. The reduction of predators increased the productivity of female voles, mainly due to an increased proportion of pregnancies, whereas the body condition of voles was not affected by the manipulation. This suggests that high predation risk in control areas suppressed the breeding of free-living voles. 4. The sex ratio of trapped voles before the manipulation did not differ between reduction and control areas, but after predators were reduced the sex ratio was more male-biased in the control areas. This indicates that small mustelids selectively killed female voles. 5. Our results suggest two possible mechanisms which may have operated concurrently. An adaptive explanation is that voles traded their current reproductive investment against future breeding under high predation risk. A simpler explanation is that small mustelids selectively preyed on pregnant female voles. Both suppressed breeding and selective killing would lead to a lower proportion of reproducing individuals in the vole population. Therefore, our results show that the presence of small mustelid predators slowed the population growth of Microtus voles.