Revilla, E. Wildlife crossing structures in the mitigation for transportation projects: an evaluation of ex post monitoring in Spain. Proceedings of the 2019 International Conference on Ecology and Transportation. Raleigh: Center for Transportation and the Environment, North Carolina State University. 2019.
Wildlife crossing structures are used to mitigate barrier effects and to reduce mortality. Many factors affect their effectivity, including typology, location and maintenance. The monitoring of the use of these structures is frequently legally binding, often by the environmental impact assessments (EIAs) in the form of environmental monitoring or within mitigation plans that are carried out during the first years after the construction of the infrastructure. This ex post monitoring (EPM) is intended to detect potential problems that were not foreseen during the EIA. Here we review the information provided in 48 studies conducted between 2001 and 2018 on the use of 1078 different crossing structures located at 57 roads and 11 high-speed railways across Spain, and evaluate the usefulness of this type of studies within the framework of adaptive management. Studies were mostly made by the companies in charge of the construction or the exploitation of the infrastructure and normally by subcontracted consulting. Most field work was done by means of camera trapping and marble dust monitoring. When provided, sampling effort is normally given in an aggregated way (e.g. number of crossing events per sampling period). After removing the studies that did not provide the total number of sampling days, we extracted data on 13371 sampling periods (232963 structure-days of sampling) in 922 crossing structures, including culverts (46.5%), underpasses (34.7%), overpasses (10.9%) and bridges, tunnels, viaducts and ecoducts (5.7% grouped). A minimum of 91134 animal crossing events (some studies did not record the number of individuals crossing) plus 27319 vehicles or people crossing events were recorded. Many different species used the structures, including rare and endangered species such as Iberian lynx or grey wolves. Nearly half of the crossing events corresponded with small mammals and lagomorphs (49.8%), followed by carnivores (18.2), birds (9.1), wild ungulates (3.2%), reptiles (2.2%) and amphibians (1.8%); while domestic animals were also frequent (15.5%). In spite of its heterogeneity, the data shows evidence of higher usage rates for structures specifically designed for wildlife crossing (ecoducts). EPM types of studies are critical in the mitigation of the impact of infrastructures. However, we found several problems in using the data: lack of clear objectives and sampling designs; raw data is not provided; data offered in aggregated, non-standardized formats, and no clear record of sampling efforts; very rarely there was monitoring on the occurrence or abundance of the focal species and normally it was only in the form of ancillary information associated with casual observations or obtained using different methods; finally, data on the mortality associated with the same infrastructures and periods was not recorded. With some notable exceptions, it was difficult to use data from EPM studies to make generalizations on the design of crossing structures. It is therefore necessary to improve the standards on how to design, implement and report EPM studies in the mitigation of the impact of transportation projects.
España
