Hurricane Irma caused extensive damage to the mangrove forests of Everglades National Park when it made landfall in 2017. The storm also deposited a large amount of sea kelp along the coastline.
Photo courtesy of Stephen Davis, Everglades Foundation
by David Malmquist, VIMS
March 14, 2022
In a new study on the ecological impacts of hurricanes, an international research team addresses a question that has been asked for centuries: in the face of a storm, is it better to be resistant like an oak or resilient like a willow?
The team’s findings published in the March 2 issue of Scientists progress, can help managers plan for climate change and a growing coastal population threatened by tropical storms that are more intense and extend farther into temperate latitudes. The results also provide a framework to guide management decisions related to other disturbances, such as nutrient pollution or wildfires.
The study’s lead author, Christopher Patrick of William & Mary’s Virginia Institute of Marine Science, says, “We found that coastal ecosystems exhibit consistent trade-offs between resistance and resilience to tropical cyclones. Our results highlight that managing increased resistance can lead to decreased resilience, and vice versa. This knowledge is essential for coastal decision-making, especially as climate change alters the risk profile with stronger and more frequent mid-latitude storms.
Patrick illustrates these management trade-offs with an example from his role as director of the seagrass monitoring and restoration program at VIMS. “In the Chesapeake Bay,” he said, “ eelgrass tends to be more stable over time than whistling grass, but takes longer to recover from disturbances such as hurricanes. This trade-off, which would also apply to dieback due to water quality or thermal stress, is an important consideration for coastal managers when choosing which seagrass species to restore.
The research team includes 23 scientists from 11 states, Puerto Rico and Taiwan. Their study is linked to a research coordination network funded by the National Science Foundation for synthesizing knowledge about ecosystem responses to hurricanes. Joining Patrick as co-authors and members of the network’s leadership team are John Kominoski from Florida International University, Bill McDowell from the University of New Hampshire, and Beth Stauffer from the University of Louisiana at Lafayette.
A repeated pattern of resistance/resilience trade-offs
In total, the researchers used pre- and post-storm monitoring surveys to analyze patterns of ecosystem resistance and resilience from 26 Northern Hemisphere storms. These made landfall between 1985 and 2018 in states from Texas to North Carolina, as well as Puerto Rico and Taiwan.
Researchers measured storm characteristics and impacts via total precipitation, maximum precipitation rate, and wind speed; then grouped their study areas into four ecosystems (freshwater, saltwater, wetland, and terrestrial) and five “response categories,” for a grand total of 4,138 time series. The response categories documented post-storm changes not only in the distribution and abundance of living things – populations of mobile animals like fish, sedentary animals like oysters, and vascular plants like mangroves – but also in ecosystem biogeochemistry (eg, salinity, nitrogen) and hydrography (eg, shoreline depth and position).
“Our study revealed a repeated pattern of trade-offs between strength and resilience across all categories,” says Patrick. The authors note that these patterns are likely the result of evolutionary adaptation and are consistent with ecological perturbation theories, suggesting that consistent rules govern ecosystem susceptibility to tropical cyclones.
For example, researchers cite the fate of Jamaican forests after Hurricane Gilbert. When this intense Category 5 storm crossed the island in 1988, it devastated hardy and normally resilient species such as the Jamaican tree fern, allowing myrtles and other shrubby and weedy species to colonize the now open spaces of the canopy.
In another example, When Hurricane Harvey hit Texas in 2017, erosion from this Category 4 storm carved deep channels into local coastal wetlands, favoring recovery by the greater salt marsh cordgrass over marsh species. shorter, while wetlands dominated by mangroves suffered less erosion than marshes.
Hurricane Harvey also reduced coastal phytoplankton biomass in waters off the Texas coast, and changed which groups were dominant. Such changes in the community structure of microscopic organisms – the basis of aquatic food webs – can affect the amount of energy available to larger organisms that are ecologically and economically important in the region.
Knowledge to guide effective strategies
The team’s findings suggest that managers seeking to improve both the resistance and resilience of coastal ecosystems may face an impossible task. On the other hand, their findings provide valuable insights for choosing the most effective management strategy for a particular location.
“If you can’t handle both resistance and resilience,” Patrick asks, “what should you focus on? The answer depends on both the specific project objectives and the expected intensity and frequency of disturbances.
Under a relatively static and predictable climate, resistance would generally be the best restoration strategy in undisturbed areas. “In this scenario,” says Patrick, “managers would ideally choose resilience as the primary feature of their restoration strategy, so that the function you care about — slowing coastal erosion, preserving water quality — doesn’t waver. not when hit by a large disturbance Managers could, for example, choose to plant mangroves rather than marsh grasses to protect against coastal erosion, as mangroves are more resilient to large storm surges .
However, when and where conditions change, resilience may appear to be a better option. According to Patrick, “If disturbances are expected to be more severe, more frequent, or both, exceeding the potential resistance of a particular species, managers can focus on resilience so that recovery time from disturbances is quick. »
“If it takes 25 years for a tree species to grow large enough to withstand an average hurricane, but hurricanes are now starting to hit an area every 20 years, that’s probably a waste of effort. ‘try to cultivate it,’ he adds. “The best restoration strategy depends on the frequency and intensity of current and future disturbances.”
As one of the first comprehensive studies of the ecological impacts of tropical cyclones, the team’s analysis raises as many questions as it answers and points to several important areas for the group’s future research, officially known as the name of Hurricane Ecosystem Response Synthesis Network, or HERS.
Future areas of research, guided by a HERS Steering Committee – include studies on how species traits such as reproductive potential, mode and distance of dispersal, and physiological tolerance might explain patterns of resistance and resilience at the population level; or how the long-term or recent environmental history of an ecosystem can influence its response to subsequent disturbances. For example, scientists believe that Hurricane Agnes in 1972 was particularly disruptive to seagrass beds in the Chesapeake Bay because it arrived in June, before most species had gone to seed. With a better knowledge of past conditions, managers could better assess the likely sensitivity of an ecosystem to a predicted disturbance.
Another key area for future HERS research is determining the stability of highly developed coastal ecosystems in the face of tropical cyclones. According to Patrick, “Future studies will improve our ability to understand how local human stressors like nutrient pollution might interact with global stressors like climate change to influence a specific ecosystem or locality, and thus help to target efforts to improve resilience or coastal resilience.”