Predatory Hymenopteran assemblages in boreal Alaska: associations with forest composition and post-fire succession

Abstract

Predatory Hymenoptera play key roles in terrestrial foodwebs and affect ecosystem processes, but their assemblage composition and distribution among forest habitats are poorly understood. Historically, the boreal forest of interior Alaska has been characterized by a fire disturbance regime that maintains vegetation composition dominated by black spruce forest. Climate-driven changes in the boreal fire regime have begun to increase the occurrence of hardwood species in the boreal forest, including trembling aspen and Alaska paper birch. Replacement of black spruce forests with aspen forests may influence predatory hymenopteran assemblages due to differences in prey availability and extrafloral nectar provisioning. Furthermore, changes in the frequency and extent of boreal forest fires increase the proportion of forests in earlier successional stages, altering habitat structure. The primary goal of this study was to characterize predatory hymenopteran assemblages in post-fire boreal forests of interior Alaska. To investigate this, the abundance, species richness, and composition of predatory hymenopteran assemblages were compared among forests at different stages of succession that were dominated by black spruce pre-fire, but that vary in their tree species composition post-fire. Predatory hymenopterans were separated into three groups: ants, macropterous wasps, and micropterous wasps. Ant species richness and abundance were not related to forest composition, but both were significantly higher in early-successional forests than in mid-late successional forests. In contrast, macropterous wasp morphospecies richness and abundance, as well as micropterous wasp abundance, were positively related to the basal area of aspen, suggesting that aspen forests benefit macropterous and micropterous wasps, perhaps due to extrafloral nectar provisioning and the availability of greater quality prey than is provided by black spruce. Wasp assemblages did not differ between successional stages. This study is the first to characterize the influence of post-fire succession on predatory hymenopteran assemblages of the boreal forest at a large spatial scale. The results suggest that continued warming of the boreal forest will have cascading influences on the insect assemblages of boreal Alaska.