Disentangling direct and indirect effects of forest structure on biodiversity: Bottom-up and top-down effects between forestry, bats and their insect prey

Timber-oriented forest management profoundly alters forest structure and composition, with complex effects on associated biodiversity. However, while species' responses to forest management and resulting structural characteristics have been the subject of numerous studies, direct and indirect effects that cascade through trophic levels are rarely disentangled. As insectivorous bats are particularly sensitive to changes in forest structure, that shape their available flight space, we investigated how forest structure, composition and management also indirectly modify their habitats, for example, by affecting important insect prey groups. We used structural equation models (SEMs) to test bat responses to forest composition, structure (forest heterogeneity, old-growth attributes) and management intensity, quantifying direct and indirect prey-mediated effects. For that, three bat guilds—short- (SRE), mid- (MRE) and long-range echolocating (LRE) bats—and their prey insects (moths and ground beetles) were analysed from 64 sites in the Black Forest, Germany. We found guild-specific effects on bats: While the structural heterogeneity of forests directly influenced the activity of bat guilds, the main influence of forest management, composition and structure was mediated through their prey-groups. SRE activity responded to moths and LRE activity was associated with ground beetles, with positive effects of the insect groups' abundance, but negative effects of the same group's species richness. In addition, the SEM approach revealed a negative top-down relationship between MRE activities and moths, suggesting predation or avoidance behaviour of moths. While forest management directly or indirectly increased prey insect abundance, it negatively affected the availability of roosting structures for bats. Synthesis and applications. The results highlight the indirect and positive effects of forest management on bats and support the important role of bats in insect regulation within continuous cover forests. Although forest management created small gaps that improved foraging habitats for most bats, it compromised the roosting functionality for bats. The ‘close-to-nature forestry’ currently prevalent in Europe mainly promotes continuous-cover forests in mid-successional stages. Expanding the forest management portfolio towards open and old-growth forests would increase roosting opportunities and provide complementary foraging habitats for different bat species, while promoting high biodiversity in managed forest landscapes.