Road effects on species abundance and population trend: a case study on tawny owl

Abstract

Urbanization and its inherent road network are one of the major movements that impulse landscape and biodiversity change, and its effects have yet to be fully understood. Few works focus on the effect of this urbanization on abundance and population trend of a certain species, as this study does, using the tawny owl (Strix aluco) as our case study. Although the tawny owl is not threatened at European or global scale, it is often found roadkilled. We studied the effects of different road types on tawny owl abundance in southern Portugal, from 2005 to 2016. In woodlands far from roads, we found high tawny owl abundance, a stable population trend, and low variation in site occupancy. On the contrary, main roads disrupted habitat quality for tawny owls—limiting their abundance and site occupancy and leading to a negative population trend due to disturbance and/or mortality. Secondary roads did not severely disrupt habitat quality, allowing initial occupation and relatively high densities, yet they may act as ecological traps, revealing instability in occupation along the breeding season and a negative population trend. Tawny owl individuals may settle near secondary roads while waiting for a vacant space in woodlands far from roads (the prime high-quality habitats). To avoid the negative effects of roads on tawny owl populations, mitigation efforts should be applied to both main and secondary roads.