Experimental reduction of native vertebrate grazing and addition of logs benefit beetle diversity at multiple scales

Abstract

Summary 1. High densities of vertebrate herbivores can be a significant barrier to ecological restoration in many parts of the world because of their impact on vegetation biomass. A common method for managing vertebrate herbivores is the use of exclosure fences, but very few studies have examined how small-scale structural refugia (e.g. logs) can mitigate grazing impacts. We examined how beetles responded to experimentally manipulated kangaroo Macropus giganteus grazing levels using both exclosure fences and addition of logs over a 16-month period. 2. We analysed beetle responses across (a) one-hectare sites, by focusing on the interaction between grazing level and log volume, and (b) microhabitats, by focusing on the interaction between grazing level and microhabitat structure (in open ground or at experimental logs). 3. At the site scale, we detected significant negative effects of grazing and positive effects of logs on beetle abundance and species richness. Beetle trophic groups responded in the same direction across grazing levels with herbivores, detritivores and predators all having higher abundance and species richness at low grazing levels. Logs applied at 20 t ha−1 in clumped arrangements had the largest positive effect on beetles at low grazing levels. At the microhabitat scale, beetles sampled adjacent to experimental logs showed an increase in abundance and species richness compared with beetles sampled from open ground, indicating logs are acting as microhabitat buffers from grazing. 4. Synthesis and applications. A reduction in grazing level had benefits for the abundance and species richness of beetles at the site scale. Further benefits were achieved at both site and microhabitat scales when logs are used in combination with exclosure fencing. For ecological restoration, exclosure fences and logs can be used to manage the impacts of vertebrate herbivores at different spatial scales. The rapid response of beetles suggests there may be potential for cascading effects on other biota as a consequence of reduced grazing, including increased food availability for insectivorous vertebrates.