Combined wildlife passages and ecological corridors best mitigate highway impacts on a threatened species

Habitat loss and fragmentation are two of the main threats to biodiversity. These processes can generate isolation of animal populations and cause local extinctions. Biodiversity management requires an evaluation of the effects of landscape connectivity loss and of potential mitigation measures on the long-term viability of terrestrial vertebrate populations. Our objectives are to: 1) assess the impact of habitat fragmentation caused by linear transport infrastructures, such as highways, on the viability and functional connectivity of terrestrial vertebrate populations; 2) analyse whether different mitigation measures, individually or combined, such as wildlife passages and ecological corridors, improve population viability. We used Testudo graeca, a threatened reptile that inhabits a fragmented area of southeastern Spain, as a real study context. We conducted simulation experiments using STEPLAND (an individual-based model for T. graeca movement and demography) in a Before-After-Control-Impact design. We modelled population dynamics in a real landscape scenario impacted by a highway, and we simulated scenarios involving different mitigation measures. Our results showed that the highway significantly decreased the area with viable population densities of T. graeca. In isolation, the scenario with wildlife passages and the scenario with ecological corridor could not fully mitigate the effects of the highway, whereas their combination produced a complementary effect that was more than twice the sum of their individual effects, thereby fully mitigating the effect of the highway. Our approach can support informed decision-making for the management and conservation of T. graeca. Moreover, our general insights can be extrapolated to other species with similar demographic characteristics.