Assessing habitat connectivity of the lesser horseshoe bat using graph theory to explain its distribution

The lesser horseshoe bat, Rhinolophus hipposideros was formerly widespread and quite common in north-western Europe, but has undergone a dramatic decline from the 1960s. The main hypothesis for this decline is a change of habitat quality. Recent works have stressed the importance of good connection between roosts and foraging areas by tree lines and well-structured hedgerows. Thus, landscape connectivity is assumed to be a key-factor for population sustainability. The present study is based on a case study in Franche-Comté (France). Its purpose is to model the distribution of the lesser horseshoe bat and to characterize the functional connectivity of its habitat. Graph theory is used efficiently in landscape ecology as a framework to model landscape connectivity (Galpern et al., 2011). In such approach, habitat patches defined as optimal for the focal species are considered as the nodes of a network. Connections between nodes are set up from ecological assumptions concerning the movements of the species within the landscape. The graph resulting from these connections allows quantifying the connectivity by means of different metrics. In the present case, patches were defined as composite objects requiring both places of potential roosts (i.e. small villages) and wooded environment. Among the different types of graph, we focused on the minimal planar graph, where all pairs of nearby patches are linked by the least-cost distance. This graph allowed computing several patch-based metrics including a parameter of dispersal distance. They were included in a species distribution model as explanatory factors in addition to other potential factors impacting movement and distribution as light pollution.