The consequences of tree disease and pre-emptive felling on functional and genetic connectivity for woodland invertebrates

Abstract

Trees outside woodlands facilitate dispersal of woodland invertebrates and may buffer against fragmentation impacts. European ash (Fraxinus excelsior) is common outside woodlands but is threatened by the fungal disease ash dieback (Hymenoscyphus fraxineus). Loss of ash trees to disease or pre-emptive felling could represent a substantial loss in connectivity. We assess the impact of tree disease and the pre-emptive felling of non-woodland ash trees on dispersal and gene flow of woodland invertebrates. We use a stochastic individual-based modelling platform, RangeShifter, to explore impacts of tree loss on the spatial dynamics of ‘virtual’ ash-reliant insects, species which depend on ash to complete their life cycle, with varying dispersal abilities and population densities. We simulate the loss of individual trees in and out of woodlands using current tree cover data from 24 real-world landscapes and estimate functional and genetic connectivity in relation to species-specific habitat-dependent movement costs and the likelihood to move in a straight line. Removal of 10% of ash trees resulted in an increase in dispersal mortality of up to 14.6%, and an increase in isolated woodlands (receiving no immigrants) of up to 2.9%. In some landscapes this resulted in increased isolation by distance (IBD - correlation between genetic and geographic distance). Carrying capacity impacted the proportion of isolated patches and IBD. Species experiencing high dispersal cost were less successful at dispersing under high tree loss, and this decreased geneflow. The consequences of tree loss for woodland connectivity are influenced by the species dispersal traits, but the consequences for gene flow depends on the arrangement of trees within the landscape. Therefore, the focal landscape must be represented explicitly when predicting the impacts of tree diseases on connectivity for a given species.