Voles on small islands: effects of food limitation and alien predation

Abstract

Ecosystems of three trophic levels may be bottom-up (by food-plant availability) and/or top-down (by predators) limited. Top-down control might be of greater consequence when the predation impact comes from an alien predator. We conducted a replicated two-factor experiment with field voles (Microtus agrestis) during 2004-2005 on small islands of the outer archipelago of the Baltic Sea, south-west Finland, manipulating both predation impact by introduced American mink (Mustela vison) and winter food supply. In autumn 2004, we live-trapped voles on five islands from which mink had been consistently removed, and on four islands where mink were present, and provided half of these islands with 1.8 kg oats per vole. Body mass of female voles increased as a response to supplementary food, whereas both food supplementation and mink removal increased the body mass of male voles in subsequent spring. During winter, there was a positive effect of supplementary food, but in the subsequent summer, possible positive long-term impacts of food supplementation on field voles were not detected. Mink removal appeared not to affect density estimates of field voles during the winter and summer immediately after food addition. Trapping data from 2004 to 2005 and 2007 suggested, however, that in two out of three summers densities of voles were significantly higher in the absence than in the presence of mink. We conclude that vole populations on small islands in the archipelago of the Baltic Sea are mainly bottom-up limited during winter (outside the growing season of food plants), when food availability is low, and limited by mink predation during summer which slows population growth during the reproductive season of voles.

Food resources and intestinal parasites as limiting factors for boreal vole populations during winter

“…Thus far my studies have dealt with parasites and food as independent entities. In the final two studies I sought to investigate the relationship between food and parasite infection on vole populations over the resource limited boreal winter (Huitu et al 2003, Fey et al 2008. In the first winter experiment, I used the same enclosures and applied a two-factor experiment design with ad libitum food supplementation and antihelminthic treatment of intestinal parasites of voles.…”

“…The concept of food limitation of high-density small mammal populations during the boreal winter is well established (Huitu et al 2003, Korslund and Steen 2006, Fey et al 2008. However, this effect has been found to be attributable primarily to variation in survival (Huitu et al 2003, Korslund and Steen 2006.…”

Does removal of an alien predator from small islands in the Baltic Sea induce a trophic cascade?

“…We therefore used live-trapping data from three additional years to confirm that mink removal increases vole densities. In a previous experiment on the small islands of the outer archipelago of the Baltic Sea (Fey et al 2008), we have found that the number of visited traps the next day was positively correlated with the number of individuals trapped (Spearman rank correlation, r s 00.672, p00.047). In 2005, on some islands we recorded the number of visited traps after one day of pre-baiting with standard laboratory mouse pellets (estimated from the number of pellets disappearing from the traps) without actually trapping voles, because poor weather conditions and logistic constraints forced us to survey vole populations as quickly as possible.…”

“…Mink have continuously escaped from Fennoscandian fur farms for 60 yr and are now resident along the coastlines of the Baltic Sea and in the archipelago (Kauhala 1996). Because none of the native terrestrial predators of the region have had a corresponding ability to exploit both marine and terrestrial resources, the invasion of this alien predator has created a qualitatively new situation: an area which was previously practically free from all mammalian predators has now a fairly dense predator population, suppressing the densities of voles (Banks et al 2004, Fey et al 2008. Because none of the native terrestrial predators of the region have had a corresponding ability to exploit both marine and terrestrial resources, the invasion of this alien predator has created a qualitatively new situation: an area which was previously practically free from all mammalian predators has now a fairly dense predator population, suppressing the densities of voles (Banks et al 2004, Fey et al 2008.…”

“…Mink have a diverse diet comprising mainly fish and migratory birds, but they also feed on small rodents as a supplemental component of their diet (Niemimaa and Pokki 1990). Because none of the native terrestrial predators of the region have had a corresponding ability to exploit both marine and terrestrial resources, the invasion of this alien predator has created a qualitatively new situation: an area which was previously practically free from all mammalian predators has now a fairly dense predator population, suppressing the densities of voles (Banks et al 2004, Fey et al 2008.…”