Dynamics of montane vole (Microtus montanus) populations in set-asides, field margins, and orchards: Keystone or pest species?

Abstract

Voles of the genus Microtus are major small mammal species responsible for feeding damage to agricultural crops in Europe and North America. Microtus also occupy non-crop habitats such as field margins, hedgerows, edges, riparian zones, set-asides, and fallow fields where the conundrum of “keystone species” vs. “pest species” arises. We ask if montane vole (M. montanus) populations will occupy small set-asides and potentially move into field margins adjacent to tree fruit orchards during periods of high numbers? In replicated sites in south-central British Columbia, Canada, in 2009–2012, we tested the hypotheses (H) that (H1) small (0.25 ha) perennial grassland sites would be high-quality habitat for voles and generate a multi-annual population fluctuation in abundance; (H2) voles will follow the same population pattern in set-asides and field margins; and (H3) voles occurring in set-aside and field margins will disperse into adjacent tree fruit orchards. In set-asides, voles increased to a mean peak population of 290 voles/ha and then declined to 50/ha thereby providing support for H1. Montane voles generally followed the same pattern of population changes in set-asides and field margins thereby supporting H2. A much reduced abundance of voles in field margins was likely owing primarily to the areal versus linear extent of the two habitats as both had similar mean abundance of herbs, thatch, and structural diversity. Vegetation management in orchards contributed to very low numbers of voles, and hence no relationship in population pattern with the set-asides. Movement of voles from set-asides to field margins was low and from set-asides and field margins to orchards was almost negligible, thereby providing no support for H3. There was no feeding damage to crop production during winter periods. Conservation of even small (0.25 ha) set-asides of perennial grasslands may provide high-quality habitat for fluctuating populations of montane voles without necessarily impacting crop production in adjacent orchards that have appropriate vegetation management. Microtus species may be considered keystone architects of biodiversity in at least some agricultural contexts.