Ecological Impacts of Invasive Rat Removal on Mediterranean Sea Islands

Abstract

Travel and trade activities have caused the worldwide spread of invasive species possibly resulting in ecological disruption. Rats (Rattus spp.) are one of the most widespread invasive species as their broad diet allows them to take advantage of various 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, I 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 annual variation. I found minimal changes in ecosystem condition following rat removal. I speculate that sites with a long period of rat colonization have lost species susceptible to rats and do not rapidly respond to eradication. Most study sites likely supported rats for many years. However, one treatment site was colonized by rats for only one year. This site shows a higher degree of positive change when compared to averages for control sites and other eradication islands. While more research is needed, it is possible that species vulnerable to rats persist on sites with a short period of rat colonization allowing for a quick recovery following eradication. It is also possible that, in the Aegean, one year is insufficient time for sites to respond to eradication.