Effects of invasive rat removal on Mediterranean island ecosystems
Rats (Rattus sp.) are among the most widespread invasive species and have, by virtue of their behavioral flexibility and broad diet, achieved a very wide distribution across the world’s island ecosystems. Whereas rats have been shown to have potentially severe impacts on island species communities, most of this information comes from recently invaded islands. For this study, we evaluated the effects of rats on multiple trophic levels of Mediterranean island ecosystems that have been occupied by rats for several hundreds of years. This was accomplished by conducting whole-island rat removal experiments on eight islands in the Aegean Sea (Greece) and comparing before and after responses across multiple trophic levels (plants, invertebrates, and vertebrate populations including nesting seabirds). These results were then compared to seven ecologically matched nearby control islands.
We assessed ecological effects one year after rat removal and detected only minimal changes in the study island species communities (p>0.05 for all of vegetation biomass, invertebrate biomass, lizard populations and seabird abundance). The exception to this pattern was a single, unusual island (Panagia) that had been invaded early in the study by rats. Rat eradication on this island was accomplished within approx. 1 year of invasion and resulted in a strong and rapid recovery of the native vegetation and seabird populations, all of which had declined following the initial rat invasion.
We speculate that sites with a long period of rat colonization have already lost species susceptible to rats and instead are inhabited by heavily defended, slow reproducing taxa that do not respond rapidly to rat eradication.
While more long-term island species community shifts may be detected as the study continues, our results suggest that prompt eradication of rats following invasion is likely to produce the most immediate conservation benefits.