Scavenging dynamics on Guam and implications for invasive species management

Deployment of mouse carcasses laced with acetaminophen has become a common management tool to control invasive brown tree snakes (Boiga irregularis; BTS) on Guam. Additionally, anticoagulant rodenticides may be used to control invasive rats (Rattus spp.) if their populations increase due to predator release in the wake of BTS eradication. However, there has been little research examining how scavengers on Guam could be incidentally exposed to toxicants by scavenging carcasses of animals that die from these population control strategies. Furthermore, there is a limited understanding of how the proliferation of invasive species on Guam has influenced the composition of the scavenger community. We investigated these topics by examining scavenger consumption of mouse, rat, and BTS carcasses on Guam in both a coastal and upland site during the wet (May-Aug 2016) and dry season (Jan-Apr 2017). We documented carcass consumption by 9 species, which scavenged 48% of carcasses. Interactions between season, habitat, and carcass type influenced probability of scavenging, and appeared to be driven by consumption by the two main scavenger species, BTS and cane toads (Rhinella marina), both of which are invasive on Guam. Baiting programs should consider the potential for toxin exposure to land crabs (Coenobita spp., Birgus latro), native species that scavenged at every combination of carcass type, habitat, and season. Overall, 60% of scavenging events were attributed to species considered pests that are recent introductions to Guam. Invasive species on Guam are the primary scavengers of small vertebrate carrion, suggesting a substantial role in trophic dynamics that extends beyond predation.

Automated summary

The deployment of mouse carcasses laced with acetaminophen has been effective in controlling invasive brown tree snakes on Guam, while anticoagulant rodenticides are used to control invasive rats. However, there is limited research on how scavengers on Guam could be exposed to toxicants by consuming carcasses from these control strategies, and the effect of invasive species on the scavenger community is not well understood. Our study found that the consumption of carcasses by scavengers was influenced by season, habitat, and carcass type, with the two main scavenger species being invasive. Baiting programs should consider the potential exposure of land crabs, native species that scavenge on a wide range of carcasses. Overall, invasive species play a significant role in scavenging small vertebrate carrion on Guam, impacting trophic dynamics beyond predation.