How Hurricanes Michael, Florence May Have Spread Nonnative Species

Hurricane Florence’s floodwaters and Hurricane Michael’s storm surge caused obvious devastation to natural areas, but a subtler set of harms is harder to see. Potentially destructive nonnative aquatic species, such as fast-growing plants that can choke waterways and hungry snails that can attack crops, can fan out across the landscape in the storms’ waters, spreading unseen and becoming hard to eradicate.

To help land managers find and manage these flood-borne newcomers before they get established, scientists at the U.S. Geological Survey have created two preliminary online maps, one for each hurricane. These early “storm tracker” map sets show that more than 160 nonnative aquatic plant and animal species had the potential to spread during the 2018 hurricane season. The maps are available at https://nas.er.usgs.gov/viewer/Flooding/.

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Nationwide, more than 1,280 freshwater aquatic species have been reported as found beyond their home ranges. Some have caused no obvious ill effects on their new habitats. Others, like the zebra mussels introduced into the Great Lakes, have caused damage to fisheries, shipping, water utilities and other industries. 

Storm surges and floodwaters can quickly spread nonnative aquatic species into waterways where they weren’t found before. The USGS’ innovative storm tracker maps were first produced in 2017 after Hurricane Harvey made landfall on Texas’ Gulf Coast, and became a useful tool for land managers. Scientists with the USGS Nonindigenous Aquatic Species Program have made six storm trackers so far: four for 2017 hurricanes Harvey, Irma, Maria and Nate, and the preliminary trackers for 2018 hurricanes Florence and Michael.

“These powerful hurricanes affected wide expanses of open spaces, including farmlands, forests, wetlands and wildlife refuges,” said USGS fishery biologist Pam Fuller, who leads the program. “It’s very difficult for land managers to search all the places where flooding or storm surge occurred. Our results can help them concentrate on areas where nonnative aquatic species are most likely to appear.”

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For example, in the Florida Panhandle Michael’s storm surge had the opportunity to spread curly pondweed, a South American aquatic plant that out-competes native species and can form dense mats that can foul boat propellers, said USGS research biologist Matthew Neilson, one of the storm tracker developers. Nonnative giant apple snails, which feed heavily on native plants and can become a pest in rice fields, also could have been spread by the hurricane’s waters.

In North Carolina, floodwaters from Hurricane Florence potentially spread two Mississippi Basin catfish species, blue catfish and flathead catfish, which feed on native pan fish like sunfish. The flathead is the subject of an eradication campaign in nearby Georgia, Neilson said.

“It’s important to note that the storm tracker shows where a nonnative species has been found, but it doesn’t show how abundant it was before the hurricane,” said USGS research biologist Wesley Daniel, a storm tracker developer. “And the more abundant a species is, the more likely it is to be picked up and carried along on floodwaters and start a new population somewhere else. So the tracker can show which species had the potential to spread, but it can’t show which ones were the most likely to spread.”

The Gainesville, Florida-based research group maintains the Nonindigenous Aquatic Species database, the nation’s most complete record of freshwater plant and animal species found outside their native range, and used it to develop these preliminary maps within a few days of each hurricane’s passage. From the database, the researchers identified all the nonnative plant and animal species known to occur along the hurricane’s path. Then they used storm surge and rainfall forecasts to identify places where lakes, rivers, streams and other waterways merged, giving aquatic species the opportunity to spread.

In the coming weeks the team will incorporate more information collected by USGS hydrologists, including measurements from USGS streamgages and thousands of high water marks left on the landscape. That data will be used to develop revised maps, available in three to six months, showing precisely where the natural or human-made barriers between watersheds were breached.