Mass mortality events and declining obligate scavengers in the Anthropocene: Social feeders may be critical

Abstract

A pervasive but understudied global change is occurring in the Anthropocene. Wildlife mass mortality events (MMEs) are increasing in frequency causing the abrupt entry of unusually large amounts of carrion into ecosystems, while most vertebrate obligate scavenger species are declining. We hypothesized that behavioral plasticity could still allow obligate-vertebrate scavengers to maintain carrion recycling with increasing carrion biomass as a result of relaxed competition, and that consumption is likely to be driven by those species less behaviorally constrained. We designed an experiment by establishing plots with increasing carrion biomass. The lowest carrion biomass represented a normal level of carrion with a single carcass, with each subsequent level of carrion biomass roughly doubling until reaching a carrion density similar to a recent MME. We monitored behavior of two obligate scavenger species: a social feeder (i.e., black vulture, Coragyps atratus) and a relatively solitary feeder (i.e., turkey vulture, Cathartes aura). In support of our hypothesis, group size and number of individuals feeding increased at the highest carrion biomass level for the more social feeder, while the less social increased solely the number of individuals feeding. Likewise, activity pattern overlap between the species increased with carrion biomass likely because both spent more of the day consuming larger carrion inputs. The effects of these behavioral changes resulted in an increase in estimated consumption with carrion biomass, which was primarily driven by the more social species. Our results indicate that vulture behavioral plasticity may be critical to maintain carrion recycling services in the Anthropocene.