Ecology and impacts of invasive rodents on tropical islands, in relation to eradication operations: Science-based restoration

Abstract

Central to the growing field of ecological restoration is the removal of invasive rodents, especially from insular ecosystems. However, the success of eradication attempts in the tropics is limited, compared to temperate regions. This research was undertaken to investigate and produce information needed for successful eradication of rodents on tropical islands. It covered a range of topics, including rodent ecology, improvement of declaring eradication success, and measurement of the effects of rodent removal on invertebrates. Field work was conducted for over three years on eight islands in the Mexican tropics, six of them with rodents (Rattus rattus and Mus musculus), using eradication projects as large-scale experiments. These tropical islands range from dry to wet ecosystems. I found that: • Overall rodent densities were higher and individuals were larger on wet islands, but the pattern changed with predator presence. Rodent reproduction occurred throughout the year. • Both rodent species were omnivorous but highly opportunistic, so there were major differences between nearby island populations, even for the same species, positively related to local food availability. • Statistical modelling could be used to improve evaluation of success after rodent eradications. A spatial-survey model was developed for assessing the probability of eradication within weeks, rather than years of an operation, and predicting the required survey effort to achieve an acceptable probability of success. • Rodent impacts on terrestrial invertebrates were apparently low for some taxa. Detection of immediate changes after rodent removal was positive, but limited, possibly because large invertebrates were rare even on rodent-free islands. • Rodent impacts on land crabs (native ecosystem engineers) can be severe, as evidenced by the dramatic changes recorded after rodent removal. Also, land crab activity can drop significantly during the dry season. • Rodent eradications on tropical islands have inherent challenges which vary between dry and wet islands. However, more and larger tropical islands can be cleared of rodents if directed research informs planning and implementation. A series of management recommendations drawn from this research are listed.