Complementary use by vertebrates of crossing structures along a fenced Spanish motorway C. Mata * , I. Herva ´s, J. Herranz, F. Sua ´rez, J.E. Malo Departamento Interuniversitario de Ecologı ´ a, Facultad de Ciencias, Universidad Auto ´ noma de Madrid, E-28049 Madrid, Spain Received 18 October 2004; received in revised form 11 January 2005; accepted 21 January 2005 Abstract Fenced roads fragment terrestrial vertebrate populations, the individuals of which are forced to cross these infrastructures using transverse structures inherent to the roadÕs 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 ver- tebrates 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 À1 day À1 . The results showed that structural aspects were the most deter- mining 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 ver- tebrate species suggest that, mitigation measures in new roads should focus to the establishment of several passages of different char- acteristics instead of investment in a reduced number of large fauna-specific passages. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Barrier effect; Corridor; Fauna passage; Fragmentation; Road ecology 1. Introduction Roads and railways are common infrastructures in developed countries and generate a wide range of envi- ronmental effects. Among them, there is growing atten- tion to the fragmentation of terrestrial vertebrate populations. Linear infrastructures section up the land- scape and divide terrestrial animal populations into more-or-less isolated sub-populations, compromising their conservation through the so-called barrier effect (Bennett, 1991; Mader, 1984; Trombulak and Frisell, 2000; Forman et al., 2003). Roads with high levels of traffic are usually fenced to avoid the entry of animals given that vehicle collisions with animals of medium to large size represent a serious hazard (Conover et al., 1995; Groot and Hazebroek, 1996; Romin and Bisson- ette, 1996). Road fencing is relatively successful in avoiding animal–vehicle collisions (Ludwig and Brem- icker, 1983; Putman, 1997), but reinforces the barrier ef- fect of the infrastructure on the surrounding animal populations. Medium and large vertebrates along fenced roads are forced to use the transverse structures (over- passes, bridges, culverts) in order to cross. For this rea- son, new roads frequently include passages specifically designed for use by the fauna or show modifications to the integral transverse structures to achieve the same objective (Keller and Pfister, 1997; Rosell and Velasco, 1999; McGuire and Morrall, 2000). After almost three decades of developing this type of mitigation measures, the knowledge about their effec- tiveness is still rather fragmentary, despite the interest 0006-3207/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2005.01.044 * Corresponding author. Tel.: +34 914978011; fax: +34 914978001. E-mail address: cristina.mata@uam.es (C. Mata). www.elsevier.com/locate/biocon Biological Conservation 124 (2005) 397–405 BIOLOGICAL CONSERVATION