Shiels, A.B., Ramírez de Arellano, G.E., & Shiels, L. (2022). Invasive rodent responses to experimental and natural hurricanes with implications for global climate change. Ecosphere, 13(12), e4307, 1-14. https://doi.org/10.1002/ecs2.4307

Summary

Tropical storms (hurricanes/typhoons) are expected to increase in severity with climate change, yet how invasive rodent populations respond to tropical storms is not well understood. Dr. Aaron Shiels and colleagues assessed the effects of Hurricane Maria (2017), and a canopy trimming experiment (to simulate a hurricane) on invasive rodent relative abundance and foraging behavior in a wet tropical forest in Puerto Rico. Invasive rodents are responsible for some of the greatest numbers of plant and animal extinctions on islands (see St. Clair, 2011 & Towns et al. 2006). Aside from a small handful of studies, very little is known about invasive rodent responses to tropical cyclones in natural settings, though in urban settings following hurricanes, invasive rodents have been found to become hyperabundant. The severity of tropical cyclones (i.e., hurricanes/typhoons) is expected to increase with climate change (see Kossin et al. 2020 & Emanuel 2020), which could increase the impact of invasive rodents on islands if they increase or change their behavior post-storm.

This study used tracking tunnels to assess rodent presence, and experimental hurricane plots trimmed by an arborist in 2014 as well as reference (closed canopy) plots for comparison. Canopy openness, and percent grass cover was measured in all plots before and after the hurricane. In addition, they assessed rodent foraging behavior using a seed removal trial by comparing the removal of placed seeds from two local tree species in vertebrate-excluded and access plots both 3 months before and 9 months after the hurricane. Trail cameras were used to identify seed predators.

Canopy openness was 3-4 times higher in reference plots in 2018 after Hurricane Maria as compared to the 2016 pre-hurricane state, and grass cover was <0.5% before, but increased to 20% cover in reference plots 2 years after the storm. Rat populations (Rattus rattus) were not affected by either experimental or natural hurricanes. However, seed removal (primarily rats) was significantly less after than before Hurricane Maria, likely due to rats selecting post-hurricane forest patches with greater understory cover for foraging. Mice (Mus musculus), which were not present in the forest interior before the hurricane, were present in forest plots closest to the road after the hurricane, and their forest invasion was associated with increased grass cover resulting from open forest canopy.

Take Home Points

• Cyclones most often negatively impact wildlife, yet invasive rodent populations on islands, such as black rats (Rattus rattus), and mice (Mus musculus) appear largely resistant to the ecosystem changes resulting from large storms.
• Dramatic increases in the opening of forest canopies after major cyclones may be a key factor in changes in the incidence and behavior of invasive rodents.
• Severe cyclones may result in more frequent periods of grassy understories in forests and the increased presence of mice, as well as shifts in the foraging locations of rats away from opened areas and towards areas with a greater understory plant density and cover.
• Invasion corridors, or areas of disturbance post-storm that connect forest edges to interiors may allow the colonization of invasive rodents, such as mice, into new areas.

Management Considerations

• Consider the potential for shifts in the distribution or behavior of invasive rodents on islands after large storm events when planning invasive rodent control activities.
• Consider implementing invasive rodent control after cyclones in areas where there are major shifts in forest canopy structure as those areas may attract different invasive rodent species or affect abundance.
• Consider increased monitoring of the seeds and fruits of rare plants in forests post-storm, as shifts in the foraging behavior of invasive rodents could lead to changes in impacts.
• Be aware of the potential for invasion corridors to be established post-storm, which could allow invasive rodents that prefer more open areas to invade forest interiors.