More on Roads as Barriers to Wildlife

Psammodromus algirus a forest species that does not appear to cross roads. 
Photo Credit: Mario M.

Roads, railways, fences and other linear structures may act as barriers to dispersal of a variety of species. The long term effects may be to interrupted gene flow between populations, but the impact of linear structures may differ for species. In a recent study Telleria et al (2011) examined the effect of a 25 year old motorway on the distribution of several forest species in a fragmented landscape in northern Spain. The patches of forest in the study area are becoming scarcer and increasingly fragmented to the west. The highway runs perpendicular to the westernmost tips of the fragmented area, isolating forest patches from eastern habitat patches. These eastern habitat patches are better connected to large forests located further east in the ‘Sierra de la Demanda’ mountain range, and might act as source habitats. When the motorway was constructed populations of forest vertebrates were expanding westwards from these mountains as a consequence of human abandonment of the low productivity highlands. A barrier effect might therefore have caused asymmetric population dynamics on either side of the motorway, obstructing the recovery of local extinctions in western forest patches by individuals moving from mountain forests. The researchers aim was to look for evidence of a barrier effect . They found clear evidence of a barrier effects on the distribution of the forest lizard Psammodromus algirus.The roe deer (Capreolus capreolus) was also unequally distributed on both sides of the motorway, but this may also be due, at least in part, to fragmentation. The eyed lizard (Timon lepidus),a species that can move through open fields, showed no evidence of barrier effects. The distribution of two small birds (Erithacus rubecula and Phylloscopus bonelli) was unaffected by the road. Their results demonstrate roads may severely restrict the distribution of species which can withstand high levels of forest fragmentation but have limited dispersal ability.

Tellería JL., Díaz JA, Pérez–Tris J, De Juana E, De la Hera I, Iraeta P, Salvador A, Santos T. 2011. Barrier effects on vertebrate distribution caused by a motorway crossing through fragmented forest landscape Animal Biodiversity and Conservation 34: 331-340.