Ecological impacts of rat removal on Mediterranean Sea islands

Background/Question/Methods

Travel and trade activities have caused the worldwide spread of invasive species resulting in extensive ecological disruption. Rats (Rattus spp.) are one of the most widespread invasive species as their broad diet allows them to take advantage of numerous food sources. On island ecosystems, rats are thought to increase predation pressure on native flora and fauna, compete with native species for resources, and alter island characteristics such as nutrient availability. Rat eradication projects are becoming a common means to restore island communities; however, management is labor intensive and expensive. This makes it critical that eradication efforts are studied to determine effectiveness and target sites for management.

This project evaluates the short-term impact of rat eradication on 15 islands supporting rats in the Aegean Sea (Greece): seven control and eight treatment sites. On each site, we collected baseline data on all levels of the islands food-web: vegetation biomass, invertebrate biomass and diversity, lizard density, and seabird abundance. Rats were then eradicated from all treatment islands using Brodifacoum baits. The same set of ecological data were collected one year following rat removal and changes were compared using control islands to correct for inter-annual variation.

Results/Conclusions

We found relatively few changes in ecosystem condition following rat removal. We speculate that sites with a long period of rat colonization, like our sites, have lost species susceptible to rats and do not rapidly respond to eradication. However, on one study island, rat colonization occurred early in the study and this rat population was eradicated within a year of establishment. This was the only island to show rapid and strong recovery following rat removal. While more research is needed, it is possible that species vulnerable to rat impacts persist on sites with a short period of rat colonization allowing these sites to quickly rebound following eradication. This also suggests that eradication should be performed of islands as soon as possible following rat invasion.