Road encroachment mediates species occupancy, trait filtering and dissimilarity of songbird communities

Assessing the road effects on biodiversity is challenging because impacts may depend on both wildlife responses to roads and on the spatial arrangement of roads. We questioned whether an increase in road encroachment i.e., the advancement of roads into non-urban areas, leads to significant changes (positive and negative) in species occurrence, and if so, whether those changes are linked to specific traits related to perturbation sensitiveness, therefore acting as filters that increase the community compositional dissimilarity. Using a large citizen-science dataset of point-counts performed throughout Iberian Peninsula (4,459 unique survey sites), we modelled the effect of road density on the occurrence of common songbirds (n=58), while accounting for potential confounding effects of environment and survey effort. We then tested if species occurrence patterns would be linked to specific traits related to the ability to cope with human presence. Finally, we assessed how road density affect the community compositional dissimilarity. We estimated 24 (41%) and 12 (21%) species to be negatively and positively affected by roads, respectively. Increased road encroachment was positively related with a higher prevalence of urban dwellers and negatively related with the occurrence of species nesting and foraging on the ground. Furthermore, increasing road density translated into an increasing community compositional dissimilarity, mostly due to species turnover. Our study support previous research showing that roads have different effects on the occurrence of different species, but we revealed that at least three species' traits have a clear relation with such road responses. Such trait filtering is probably causing a high species turnover between songbird communities occurring in roaded and nearby roadless areas. Overall, we found that different species-specific responses to roads translates into changes at the community level. Landscape and road-network management should be conceived acknowledging that roads are contributing to biodiversity changes. As so, building upon the concepts of Land Sharing / Land Sparing, conservation actions should be tailored according to the different species responses e.g., road verge management targeting species having a positive relation with road density; and compensation actions targeting species showing a negative response toward roads.