Altered species interactions at forest edges: contrasting edge effects on bumble bees and their phoretic mite loads in temperate forest remnants

1. The way in which species interactions are altered at habitat edges is an emerging concern in conservation biology, with some theories predicting edge-affected habitats to be less stable and exhibit intensified species interactions relative to interior habitats. One little-studied interaction is phoresy, where an individual of one species attaches itself to an individual of another species for the purpose of dispersal. Bumble bees (Bombus spp.) are important crop pollinators around the world and at least 10 species of phoretic mites have been associated with various bumble bee species. 2. Here, we investigated changes in the abundance of the introduced European B. terrestris across forest-grassland edges in the Southern Alps of New Zealand, and concomitant changes in the phoretic mite loads on these mobile pollinators. 3. Bumble bees penetrated up to 250 m inside forest fragments, with abundance declining with distance from the anthropogenic matrix habitat. By contrast, phoretic mite loads increased towards the forest interior. Overall, phoretic mite loads on B. terrestris were greatest in the forest canopy, suggesting that this species interaction is intensified in natural habitat compared to that in the anthropogenic land use surrounding forest fragments. 4. Our data indicate that phoretic loads on bumble bees are elevated in forest canopies. The functional significance of altered bumble bee - mite interactions for pollination services at forest edges remains to be tested.