Complementary use by vertebrates of crossing structures along a fenced Spanish motorway

Fenced roads fragment terrestrial vertebrate populations, the individuals of which are forced to cross these infrastructures using transverse structures inherent to the road's construction (culverts, over- and underpasses) or other structures specially adapted or constructed for use by the fauna (enlarged culverts and bridges, ecoducts). The information available on the use of different crossing structure types by vertebrates, and the role played by the structural variables, of the surroundings and of the human use of these passages is still scarce. The use of 82 crossing structures of the A-52 motorway (Zamora, North-western Spain) by terrestrial vertebrates was monitored in summer 2002 using marble dust beds and electronic cameras. A total of 1122 species track-days were recorded, with an average of 1.37 species crossing structure−1 day−1. The results showed that structural aspects were the most determining factors for the species using these passages (MANCOVA test, p < 0.001), and a direct positive relationship between the size of the animal and the size of the pass used existing in general terms. This complementary use of the different passage types by vertebrate species suggest that, mitigation measures in new roads should focus to the establishment of several passages of different characteristics instead of investment in a reduced number of large fauna-specific passages.