Long-term persistence of butterfly diversity in a sustainably managed forest ecosystem

In recent decades, intensification in land use has led to severe losses of biodiversity across major parts of the world. Studies from Central Europe revealed severe decline of insect diversity across agro-environments, but to a lower extent in forest ecosystems. Sedentary specialist species are suffering particularly, while mobile generalist species are much less affected. Numerous species are also disappearing from nature reserves. On the long run, biodiversity presumably can only be maintained in ecosystems of sufficient size, but is lost in small and isolated habitats. In order to test this assumption, we conducted butterfly counts over a period of 20 years in a large, heterogeneous and sustainably used forest ecosystem in southern Germany. We found no significant changes in diversity and abundance over the two decades of study. However, our data revealed a significant shift in species´ community structure over time, with the proportion of generalist species increasing and the proportion of specialist species decreasing. These changes are most likely due to changes in habitat structures. At the beginning of our study, wind-blows resulting from the heavy storms in the late 1990s represented open areas, but subsequently reforested in the wake of natural succession. Since these temporary open habitats had attracted a particularly high number of specialised species, the observed changes are probably due to these natural processes in a forest ecosystem. In general, our results show that heterogeneous ecosystems of large size may preserve a species-rich butterfly community in the long run.