Shiels, A.B., Lombard, C.D., Shiels, L., & Hillis-Starr, Z. (2020). Invasive rat establishment and changes in small mammal populations on Caribbean Islands following two hurricanes. Global Ecology and Conservation, 22, e00986. https://doi.org/10.1016/j.gecco.2020.e00986

Summary

Small mammals, such as mice and rats, are some of the most problematic invasive species impacting native island biodiversity. In the Pacific region, rats were found to predate seeds causing several plants to become rare, damage coconut palm fruit production, and reduce seabird, green sea turtle, and crab populations by eating their eggs and juveniles. Severe storms, such as hurricanes and typhoons, can change invasive mammal populations by altering habitat or by facilitating dispersal between islands. Eradicating invasive rodent populations on islands can protect threatened or endangered species, restore native flora and fauna, and build resilience to climate change. On the islet of Irooj in the Marshall Islands, rat eradication restored native seabird populations which increased nutrients through guano deposition, and created healthier, more productive coral reef ecosystems.

In this study, non-native and invasive small mammals, including black rats (Rattus rattus), house mice (Mus musculus), and mongoose (Urva auropunctata), were monitored in 2017 and 2018 to examine the impacts of Hurricane Irma and Hurricane Maria on relative abundance across the tropical islands of St. Croix, United States Virgin Islands (USVI) in the Caribbean. Both hurricanes hit the region in September 2017 just north (Irma) and south (Maria) of the islands and measured as category 3. Comparative data from before the hurricanes in 2017 and after were collected from three study sites: Sandy Point National Wildlife Refuge (Sandy Point), Green Cay National Wildlife Refuge (Green Cay; a smaller island), and Buck Island Reef National Monument (Buck Island). Data were gathered through annual and semi-annual snap-trapping surveys and tracking tunnels that recorded animal presence via inked footprints.

Results

For Sandy Point, there was a significant increase in the relative abundance of mice and on Buck Island, the existing mouse population was found to have doubled after the hurricanes. The relative abundance of rats did not change after the hurricanes on Buck Island, however mongoose relative abundance slightly decreased after the hurricanes. The significant increase in mouse populations at Sandy Point and Buck Island may have been due to the increase in grass cover which increased food availability, and possibly a decrease in predator populations following the hurricanes. No invasive small mammals were present on Green Cay before the hurricanes, but afterwards, rats were found to be present. This rat introduction is likely due to animals rafting across the ocean on debris or being forced to swim between islands after being displaced by storms.

Management Considerations

• Consider increasing monitoring efforts for invasive small mammals both before and after storms to allow for a more rapid response if new incursions are detected. The use of tracking tunnels can enable rapid confirmation of new invasive small mammals in remote locations.
• Consider increased biosecurity efforts related to invasive species prevention on islands after severe storms. The rat incursion on Green Cay may have been an accidental introduction by humans via a rat-infested boat landing on the island.
• Consider increasing both biosecurity and monitoring efforts on offshore islands that are close to main islands because they are likely more vulnerable to invasion than remote offshore islands. Green Cay, which was invaded by rats after the hurricanes is only ~1400 ft. from St. Croix Island, whereas Buck Island, which remained rat-free is ~9000 ft. from St. Croix.
• Some invasive small mammals may decline after intense storms (e.g., mongoose tended to decrease in this study), creating an opportunity for eradication.

Take Home Points

• New introductions of invasive small mammals can occur on islands due to a variety of factors including severe storms.
• Each mammal species had a different population-level response following the hurricanes, with predators (mongoose) showing the smallest response.
• Routine monitoring for the presence, abundance, and species composition of invasive small mammals is essential for establishing baselines and detecting new arrivals.
• Managers should plan for the potential of increased impacts from invasive small mammals after large storm events from new introductions, and re-establishment of species that have been previously eradicated.