Bat communities respond positively to large-scale thinning of forest regrowth.
Over half of the world's forests are secondary regrowth and support considerable biodiversity. Thinning of these forests is a widespread management practice that can affect forest species, including echolocating bats and their prey. We compared total activity of 11 bat taxa, foraging activity of six bat guilds and biomass of 11 insect orders across four forest thinning categories in managed remnant eucalypt forests in south-eastern Australia: unthinned regrowth, forest thinned recently (0-4 years) and in the medium term (5-10 years) and reference (mature open forest). Thinning had been carried out at large (∼350 ha) spatial scales. Total bat activity was 60% less and foraging activity was 80% less in unthinned regrowth, compared to reference sites, but activity levels were similar among thinned and reference sites. Insect biomass was greatest in unthinned sites, and while bat activity was related to prey biomass, this relationship was weak in unthinned sites. Together, this suggests that forest structure was more important than prey availability or time since thinning in influencing bat activity patterns. Synthesizing our findings with the broader literature on bats and thinning, we found support for a clutter threshold of 1100 stems ha-1, above which bat activity was markedly lower, across two continents (the USA and Australia) and four broad vegetation types (eucalypt, conifer, deciduous and mixed forests). While elsewhere bats with adaptive traits for open habitats generally respond positively to thinning, in our study, species with traits consistent with clutter tolerance (high call frequency and low wing aspect ratio) had lowest activity levels (up to 22 times) in unthinned regrowth compared to all other forest types. Synthesis and applications. Widespread dense regrowth forest can restrict movement and foraging of bats, even those adapted to clutter. We recommend thinning dense regrowth or plantations to below 1100 stems ha-1 when targeting bat foraging habitat, but effects of thinning on roost habitat and other forest biota require further investigation.