The impact of vehicular traffic on two frog populations of differing vagility, Rana pipiens and Rana clamitans

Vehicular traffic can be a major source of dispersal related mortality for some species. Highly vagile organisms should be at a disadvantage in landscapes with roads because they are more likely to encounter roads and incur traffic mortality. Population abundance of two sympatric anuran species of differing vagility, Rana pipiens (vagile) and Rana clamitans (less vagile) was assessed at 30 breeding ponds. Traffic density, an index of the amount of potential traffic mortality, was measured in concentric circles radiating from the pond, out to 5 km. Multiple linear regressions relating population abundance to traffic density, pond and landscape variables concluded that Rana pipiens populations were negatively affected by traffic within a radius of 1.5 km, while Rana clamitans populations were not correlated with traffic density. These results imply that vagile species are vulnerable to traffic mortality which can cause population decline.