Forest structure and microclimate predict nesting activity and reproductive success of cavity-nesting Hymenoptera

Landscape-scale habitat alteration causes changes in biotic and abiotic conditions that may drive population declines of insect pollinators and other functional guilds. Such declines may consequentially impair ecosystem function and composition. Our objective was to examine whether habitat structure or microclimate best explained patterns in nesting activity and reproductive output of cavity-nesting Hymenoptera, specifically Megachile (Family: Megachilidae), along a successional gradient following post-mining rehabilitation and restoration in Australia's south-west. To do so, we sampled cavity-nesting Hymenoptera using artificial nesting cavities (trap nests) across twelve survey sites randomly selected across a 22-year restoration chronosequence. We observed that early successional restoration sites had warmer microclimates and supported increased nesting activity and greater fecundity of cavity-nesting bees (principally M. aurifrons). In contrast, cavity-nesting wasps were recorded less frequently but in association with areas of reduced understorey density. Early successional and open habitats appear important in maintaining highly fecund populations of the large cavity-nesting bees that we studied here, suggesting that thermal ecology may underpin the mechanisms driving habitat selection by these important pollinators in heterogenous forested landscapes. This has implications for the management of biodiversity in Australia because cavity-nesting bees represent an important component of the hymenopteran fauna, and often fulfil specialised pollination roles in native ecosystems. Ongoing restoration management could aim to maximise landscape heterogeneity by featuring a mix of successional stages to support the abiotic and biotic requirements of a biodiverse insect community.