Addressing the impact of land crabs on rodent eradications on islands

Abstract

Consumption of rodent bait by land crabs, leaving some rodents unexposed, has been described as one potential reason why several rodent eradications undertaken on mesic-tropical islands have failed. Strategies to overcome the issue have been proposed but all increase the risk, cost or logistics of running an eradication operation. To quantify the impact of land crabs and assess the feasibility of achieving rodent eradication using a standard bait application rate used in temperate climates, we measured crab density, rates of bait take and exposure of rats to bait on Vahanga Atoll in French Polynesia. The two methods used to measure crab density were closely correlated and agreed with anecdotal observations, suggesting they were a reliable index of crab numbers. Rates of bait take were closely correlated with crab density providing a potential means of predicting bait take in a crab dominated ecosystem such as Vahanga, an advantage when planning a rodent eradication attempt. At some sites on Vahanga, crabs were in sufficient numbers (up to 5 900/ha) to rapidly reduce bait availability but even at these sites rats were able to access bait. The result suggests that achieving rat eradication on some mesic-tropical islands may be possible using a typical temperate climate bait application rate. However, our results should be applied with caution because we did not determine the amount of bait consumed by rats.