Forest pollinator richness declines with distance into burned areas

Abstract

The effects of prescribed fire on forest pollinator communities are complex and incompletely understood. One of the least-studied questions concerns how these organisms are affected by the size, or spatial scale, of fire. We sought to address this by sampling bees (Hymenoptera: Apoidea), butterflies (Lepidoptera), and hover flies (Diptera: Syrphidae) at different distances along 500 m transects into forests burned every three years in the southeastern United States. We found combined pollinator richness to decline significantly with distance, being about 23.9 % lower near the centers of burn units than near the edges, and this did not change with time since fire. These patterns held true when small and large pollinators were analyzed separately, as well as for the richness of bees and butterflies (but not hover flies), and regardless of bee nesting guild. We found no relationship between the abundance of all pollinators and distance. However, the abundance of small pollinators decreased with distance whereas that of large pollinators increased. A competing model provided no support for the alternative hypothesis that the observed effects of distance can be explained by the length of road edges nearby as opposed to distance from unburned habitat. The richness and abundance of all pollinators combined and most taxonomic groups and species analyzed separately increased with increasing canopy openness as well as with increasing plant richness. Although pollinator richness declined with distance into large burns and small species appear to be particularly sensitive, we also found the richness and abundance of pollinators to decrease with time since fire. Our results show that fire can improve local conditions for pollinators but that unburned habitats serve as important refugia or sources of flowers following fire, underscoring the importance of pyrodiversity in managed landscapes.