Effects of the availability of herbaceous food on vole attacks on birch seedlings

According to the optimal diet theory, predators should consume low-quality prey only when the availability of better-quality prey is low. The model of short-term apparent competition predicts, somewhat contradictorily, that low-quality prey co-occurring in the same patches with high-quality prey should be attacked more than low-quality prey in patches containing no high-quality prey. The prediction should apply, in particular, to poor environments. We compared vole attacks on experimental birch seedlings (low-quality prey) in patches with and without herbs (high-quality prey) in experimental enclosures with non-depleted or depleted herbaceous vegetation. Voles attacked fewer birch seedlings in the non-depleted enclosures compared to the depleted ones. They attacked more birch seedlings in patches with herbs than in patches without herbs independently of food depletion in the enclosure. Tree-seedling predation increased with vole density, but the increase was more pronounced in the depleted enclosures. These results may be useful for explaining and controlling natural predation of tree seedlings by voles.