Testing the intermediate-disturbance hypothesis – managed roe deer populations are not disrupting forest faunal communities

Large herbivores, such as deer, are keystone species in forest ecosystems due to their diverse effects on forest structure, as well as plant and faunal communities. While in production forests deer are often considered pest species due to the browsing impact on woody vegetation, deer are disturbance agents for forest ecosystems as a whole, as their presence and density can influence other trophic levels, while also providing ecosystem services. The effects of deer on ecosystems can be expected to follow the intermediate-disturbance hypothesis (IDH), whereby there is a positive influence on biodiversity up to intermediate deer densities, turning negative at higher densities. To date, most field studies tend to explore either linear effects of deer on the environment, or populations that have extremely high densities, thus ignoring the potential non-linear relationship between deer species and the forest ecosystem. This may lead to the notion that deer are problematic for the conservation of forest biodiversity. To address this gap, we explore the non-linear effect of roe deer densities on the abundance and diversity of multiple faunistic groups including invertebrates, birds, bats and hares, in managed forests of the Black Forest, Germany. Our findings partly support the IDH, as most taxa were either positively related to intermediate roe deer abundances in our system or did not respond to deer abundances. In contrast to the prevalent image of deer as a threat to forest biodiversity, we find few taxa that are negatively affected by roe deer. Conversely, we find that roe deer at low to intermediate population densities may contribute to maintaining forest communities, as with hymenopterans, birds and spiders.