Roadkill islands: Carnivore extinction shifts seasonal use of roadside carrion by generalist avian scavenger

Global road networks facilitate habitat modification and are integral to human expansion. Many animals, particularly scavengers, use roads as they provide a reliable source of food, such as carrion left after vehicle collisions. Tasmania is often cited as the 'roadkill capital of Australia', with the isolated offshore islands in the Bass Strait experiencing similar, if not higher, levels of roadkill. However, native mammalian predators on the islands are extirpated, meaning the remaining scavengers are likely to experience lower interference competition. In this study, we used a naturally occurring experiment to examine how the loss of mammalian carnivores within a community impacts roadside foraging behaviour by avian scavengers. We monitored the locations of roadkill and forest ravens Corvus tasmanicus, an abundant scavenger species, on eight road transects across the Tasmanian mainland (high scavenging competition) and the Bass Strait islands (low scavenging competition). We represented raven observations as one-dimensional point patterns, using hierarchical Bayesian models to investigate the dependence of raven spatial intensity on habitat, season, distance to roadkill and route location. We found that roadkill carcasses were a strong predictor of raven presence along road networks. The effect of roadkill was amplified on roads on the Bass Strait islands, where roadside carrion was a predictor of raven presence across the entire year. In contrast, ravens were more often associated with roadkill on Tasmanian mainland roads in the autumn, when other resources were low. This suggests that in the absence of competing mammalian scavengers, ravens choose to feed on roadside carrion throughout the year, even in seasons when other resources are available. This lack of competition could be disproportionately benefiting forest ravens, leading to augmented raven populations and changes to the vertebrate community structure. Our study provides evidence that scavengers modify their behaviour in response to reduced scavenger species diversity, potentially triggering trophic shifts and highlighting the importance of conserving or reintroducing carnivores within ecosystems.