SMALL MUSTELID PREDATION SLOWS POPULATION-GROWTH OF MICROTUS VOLES - A PREDATOR REDUCTION EXPERIMENT

1. The effects of predator reduction on the breeding performance and s ex ratio of Microtus voles were studied in a cyclically fluctuating vo le assemblage in western Finland, where the field vole (Microtus agres tis) and the sibling vole (M. rossiaemeridionalis) are the main prey o f small mustelids. 2, The densities of small mustelids [the least weas el (Mustela nivalis nivalis) and the stoat (M. erminea)] were reduced experimentally in three large (2-3 km(2)) unfenced areas in 1992, duri ng a crash phase of the vole cycle, and in three different areas durin g the following crash phase in 1995. The reproductive performance of M icrotus voles was compared between control and manipulation areas befo re and after reducing predators. 3. The reduction of predators increas ed the productivity of female voles, mainly due to an increased propor tion of pregnancies, whereas the body condition of voles was not affec ted by the manipulation. This suggests that high predation risk in con trol areas suppressed the breeding of free-living voles. 4. The sex ra tio of trapped voles before the manipulation did not differ between re duction and control areas, but after predators were reduced the sex ra tio was more male-biased in the control areas. This indicates that sma ll mustelids selectively killed female voles. 5. Our results suggest t wo possible mechanisms which may have operated concurrently. An adapti ve explanation is that voles traded their current reproductive investm ent against future breeding under high predation risk. A simpler expla nation is that small mustelids selectively preyed on pregnant female v oles. Both suppressed breeding and selective killing would lead to a l ower proportion of reproducing individuals in the vole population. The refore, our results show that the presence of small mustelid predators slowed the population growth of Microtus voles.