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EFFECTIVENESS OF WILDLIFE CROSSING STRUCTURES AND ADAPTED CULVERTS IN A HIGHWAY IN NORTHWEST SPAIN

An intensive monitoring was carried out between June and September 2002 in different passage types across a highway in NW Spain in order to determine their use by terrestrial vertebrates. We used marble dust-beds to get footprints and a complementary photographic system to identify species which cannot be distinguished by tracks. Footprint data (820 passage-days) were collected from 82 passage structures (33 circular culverts, 10 adapted culverts, 14 wide underpasses, 7 wildlife underpasses, 16 overpasses and 2 ecoducts). The number of recorded vertebrates was high (1,424 tracks, 78.8% wildlife, and 21.2% related to human activity; and 490 photographic contacts, 54.3% and 45.7% respectively). Small mammals (mice, voles and shrews) used the passageways most frequently (414 tracks), followed by lagomorphs (Iberian hare, Lepus granatensis, and rabbit, Oryctolagus cuniculus, N= 158), canids (Canis familiaris and C. lupus, N = 142), fox (Vulpes vulpes, N= 137) and lacertids (Lacerta spp. and Podarcis spp., N= 73). Underpasses and non-wildlife-engineered overpasses were the most used structures. Differences were found in the selection of crossing structures by the two lagomorphs, hares selecting wildlife underpasses while rabbits did not show a significative preference. Anurans and ophidians (Fam. Colubridae and Viperidae) showed a clear preference for adapted culverts, avoiding overpasses. Lacertids and small mammals crossed most frequently through circular culverts, but generally used all passage types. Hedgehog (Erinaceus europaeus) and Badger (Meles meles) always selected highway underpasses while small mustelids (Mustela nivalis plus M. erminea) used culverts exclusively. Finally, foxes used all types of crossing structures, showing a preference for wide underpasses. Red deer (Cervus elaphus) were found to use wide passages under or above the road, and more frequently ecoducts, but roe deer (Capreolus capreolus) and wild boar (Sus scrofa) were never detected in crossing structures though very abundant in the area. Four recommendations arise from the study: (1) as a differential use among animal species has been found, it is necessary to keep several crossing structure types; (2) functional structures of the motorway (non-wildlife-engineered) play an important role in the permeability of the road, and their adaptation for wildlife enhances their use by some taxa. Thus, the adaptation of structures related to human activity plays a key role in the achievement of the best solution from a benefit-to-cost point of view. (3) The set of passageways necessary to mitigate the barrier effect suffered by a known mammal community can be established taking into account the animal sizes and the wideness and relative position of crossing structures to the road (over vs. under); however ,(4) it seems that some species may not cross through structures up to 20m wide, and thus some of the passageways should be wider (in the form of tunnels and/or viaducts).