UMR 241 EIO, University of French Polynesia, France
Abstract: On intact coral atolls, the resident terrestrial fauna is dominated by omnivorous land crabs, which play key roles as predators and by scavenging and influencing litter and vegetation dynamics. Unfortunately, many atolls have been invaded by rodents, with potentially severe impacts on land crab populations. Most of the studies that have considered potential impacts report differences in the number of crabs counted along transects on rat invaded vs. rat free islets. Such apparent differences in abundance may, however, also reflect changes in crab behavior in response to rat presence. In depth investigations of the impact of rats on crab survival, densities and behavior are needed to acquire a more mechanistic understanding of the impact of rats on crabs. We took advantage of a rat eradication program on a Polynesian atoll (Tetiaroa) to investigate the impact of rats and rat removal on horned ghost crabs (Ocypode ceratophthalmus). Tetiaroa offers a unique experimental setting with 12 islets including three never colonized by rats and others where rats were eradicated in 2018 (one islet), 2020 (two islets), or June 2022 (six islets). We quantified predation risk (using tethered ghost crabs), population density (using capture mark recapture [CMR] approaches), crab morphology, and activity periods on rat-infested and rat-free islets. We also measured changes in predation risk following the 2022 eradication. We found that predation risk was about three times higher on rat-infested as compared to rat-free islets, and significantly decreased post-eradication on islets that had been infested with rats. By quantifying ghost crab abundance at nine sites distributed across the atoll using CMR approaches (three nights of CMR per site), we also found that crabs were significantly larger and more abundant on rat-free as compared to rat-invaded islands. To characterize crab activity, we set up trail cameras to count the number of crabs active within a 10m² surface every 15min and related this to rat activity. Our results clarify the impacts of rats as important invasive predators of ghost crabs. The strength of the effects we detected also suggests that the eradication of rats will have long term effects on the functioning of the intertidal ecosystems, where ghost crabs play a pivotal role in scavenging, predation, and soil aeration.
