Impact of inundation regime and meadow management on wild bee communities and bee-flower networks in the Donau-Auen National Park
Ulrich Neumüller, University of Vienna (AT)
It is widely known that intensity and predictability of inundation events may have a strong influence on the species composition of plant communities, which in turn shapes the species composition of the associated fauna. Extreme population declines of insect species caused by floods have often been described in the literature. Yet, not much is known about the long term effects of such severe events, especially at the level of entire communities or their biotic interactions. In this study bee communities on meadows which are almost annually flooded by the Danube River were compared with communities on meadows which are rarely subject to inundation events. Flower-visiting bees were sampled on 32 meadows in the National Park Donau-Auen four times between late April and early August 2016. Sixteen of the selected meadows were situated on the flood-prone and another 16 on the protected side of a levee which stretches through the reserve. Altogether, I recorded 92 wild bee species interacting with flowers of 62 plant species. Counter to expectations, wild bee species richness was 16.4% higher on meadows which have a higher risk to be affected by floods. As a potential driver of this pattern, a significantly higher bee differentiation diversity on annually flooded meadows could be identified. Since bees are predicted to be highly vulnerable to floods, it follows that bee assemblages must have recovered following the last unusually severe summer flood in 2013. To obtain further insight into the functional characteristics of the observed bee communities in relation to pollination, three network metrics, which were derived from a bipartite plant-bee interaction matrix, were analysed. None of the network indices was affected by the flooding regime. Hence it is concluded that, although floods may have a devastating effect on wild bee populations in the short term, recolonization happens rather quickly, restoring stable and diverse bee communities only after a few years. These findings reinforce earlier studies in which other insect groups seemed to show a low resistance but high resilience towards flooding.