Summary

Extreme Climatic Events (ECEs) are rare, high-impact events such as hurricanes, heat waves, and extended drought, and they are increasing in frequency and intensity across the Pacific. These extreme events are one of the most immediate threats caused by climate change, and can provide increased opportunities for invasive species to colonize and spread. Despite potentially severe consequences, however, ECEs are rarely considered in planning efforts for ecological restoration. The authors examined the impacts of ECEs on restoration projects and the degree to which they were resilient to ECEs, and they found overwhelmingly negative impacts on restoration efforts. Impacts varied across geographies, species, and within sites, highlighting the need for restoration practitioners to adopt a “portfolio approach” to increase resilience of projects to ECEs. By diversifying the sites, species, and genotypes used as well as the methods employed, managers can reduce the risk of an entire restoration project failing when an extreme event occurs.

Results

Hurricanes and severe storms were the most reported ECEs, impacting 76% of the projects examined via wind, floods, and/or waves. The severity of impacts varied substantially by project and ECEs were not uniform across the restoration site, across all restoration methods, or across species, life stages, or genotypes.  Types of impacts included mortality, community shifts, impacts to reproduction of target species, change in vegetation structure, and changes in species cover. ECEs had overwhelmingly negative impacts on restoration projects, however a few reported both positive and negative impacts or neutral/no-damage impacts. One study reported a positive impact.

Management Considerations

• Plan for ECEs by considering the potential impacts of extreme events most likely to impact your area, possibly exceeding historical events.
• To spread risk through the “portfolio approach”, consider having multiple restoration sites instead of one; place restoration sites across scales (e.g., across elevational or rainfall gradients, or across multiple years or seasons to increase the likelihood of favorable conditions), make use of topographic complexity, and use multiple species.  If possible, identify likely spatial and temporal refugia and incorporate these into project design to reduce the impacts of ECEs.
• Consider selecting a diversity of propagule sources to enhance genetic diversity and adaptive potential to climate change. For example, incorporate species with traits that are more tolerant of extreme conditions such as droughts, heatwaves, or high winds.
• Consider having a post-ECE response plan in place that includes a budget for monitoring and invasive species control. Data on impacts from ECEs can help inform future restoration designs and guide adaptive management.

Take Home Points

• ECEs can create major setbacks for restoration projects by destroying or damaging structures or sites, and by threatening restored species.
• To increase the resilience of restoration projects to ECEs, spread risk across time and space by using the portfolio approach.
• Adapting restoration projects to ECEs or post-ECE recovery may require the use of propagules whose genotypes are more tolerant or resistant to ECEs.
• To adapt to climate change, including to an increasing frequency and severity of ECEs, restoration projects will need to plan for greater uncertainty, secure increased funding for monitoring and adaptive management in response to ECEs, and anticipate setbacks and longer timeframes for success.

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.

Summary

Extreme climate events (ECEs) can influence different stages of invasion and create an ‘invasion window’, or an opportunity for invasive species to take advantage of resources made available after an ECE. ECEs often alter ecosystem structure or function and may cause abrupt mortality of resident species. This disturbance provides introduced species the opportunity to establish and take advantage of resources available post-disturbance. Other ECEs may cause stress to resident species and limit their ability to recover and compete for resources with invasive species. Invasive species often are more likely to succeed post ECE because they tend to have broader environmental tolerances than co-occurring native species, however responses will depend on each species’ tolerance to ECEs. ECEs, however, do not uniformly favor non-native species and many non-natives that benefit may have no substantial ecological impact, or impacts may be context dependent. Some ECEs may negatively affect established invaders, providing opportunities for restoration of native species.

Take Home Points

• ECEs can allow invasive species to transform ecosystems to new and persistent states.
• ECEs can increase the transportation of invasive species and reduce the current ecosystem’s ability to resist or be resilient to new invasions.
• ‘Resource pulses’ (e.g., a disturbance-induced sudden increase in light, space, or water) created by ECEs can allow invaders to establish and spread.

Management Considerations

Pre-ECE (Planning):

• Identify what types of ECEs will likely affect your region and evaluate your current management goals and objectives to see how they may be affected. Successful mitigation of the future impacts of ECEs will likely require planning and mobilization of resources.
• Include the potential responses of invasive species to expected ECEs in regional risk assessments and species watch lists.
• Rank the value and importance of cultural and natural resources that may be impacted by ECEs, so that rapid protection efforts can be targeted after an ECE.
• Identify management areas that are vulnerable to disturbance and work to increase the resilience of those areas prior to ECEs. Develop greater capacity for post-ECE restoration such as saving a supply of seeds for future restoration efforts.
• Consider how barriers or buffers between invaded and uninvaded areas could be bolstered or increased prior to ECEs.
• Develop and coordinate efforts across agencies, individuals, and landscapes for addressing ECEs. Cooperative agreements may assist with cross-boundary impacts.

Post ECE (Response):

• Increase early detection and rapid response (EDRR) efforts after an ECE to eradicate new invasions. ECEs that greatly reduce populations of invasive species may also increase the likelihood of eradication of those same species if managers can respond quickly.
• Prioritize management of the most detrimental invasive species first, such as those that are known to transform ecosystems or are highly ranked on risk assessments.
• Enlist volunteer and community groups to help monitor for invasive species after an ECE and respond to those that are found.

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.