Assessing landscape connectivity with calibrated cost-distance modelling: predicting common toad distribution in a context of spreading agriculture

P>1. Capturing the relative influence of landscape composition and configuration in real landscapes remains a challenge. Cost-distance modelling provides an interesting approach to the assessment of landscape complexity in a functional way. However, resistances allotted to landscape elements in cost-distance modelling frequently remain defined on the basis of expert advice. To overcome this weakness, we computed resistance coefficients without a priori knowledge through a calibration/validation method enabling us to test the impact of the matrix heterogeneity on the occurrence of the common toad Bufo bufo, the cycles of which imply migrations between complementary habitats. 2. We used cost-distance modelling to elaborate an integrative parameter of landscape composition and configuration in the form of a potential movement zone. We first applied a calibration procedure that systematically tested different resistance values for each landscape element with a large data set. The robustness of the calibrated resistances was then evaluated using two supplementary validation data sets from contrasted landscapes. Finally, in order to isolate the relative influence of landscape configuration, we compared the predictive power of the calibrated potential movement zone with that of landscape composition only. 3. The landscape matrix strongly influences common toad occurrence: selected resistances were low for forests and meadows and intermediate to high for crops. Within the two validation data sets, the potential movement zone was positively and significantly related to toad occurrence and had a better predictive power than landscape composition. 4. Synthesis and applications. This study provides a tool to manage landscape structure in accordance with the ecological requirements of amphibian populations, especially habitat complementation. This method has minimal biological information requirements and therefore could be widely used to investigate the crucial influence of landscape composition and configuration on a broad range of species, and to design functional ecological networks.