Rodent Threat Defeated As Delmarva Battles Rising Seas

A rodent resembling a rat was found dead in a trap on a warm day last spring in marshland popular with hunters and fishers. It was the last time anybody saw a nutria on the Delmarva Peninsula.

A $1 million-a-year federal program that has killed 13,000 of the wetland-destroying pests on the peninsula since 2003 has failed to capture or detect signs of any more since three were trapped and killed at Maryland's Ellis Bay over three consecutive days last May.

Nutria breed fast and devour marsh plants.
Credit: Simone A. Bertinotti/flickr

That’s raising cautious hopes that the sinking coastlines of the 170-mile long peninsula, encompassing all of Delaware and parts of Maryland and Virginia, may have been been purged of an urgent coastal threat.

“At this juncture, it’s a good sign,” said Marnie Pepper, a wildlife biologist with the U.S. Department of Agriculture, which has overseen the eradication effort. “But it doesn’t mean that they’re not there.”

Removal of the invasive nutrias from the peninsula and its parks, refuges, farms and cities is expected to help marshes adapt to rising seas by bulking up vertically, instead of succumbing and being inundated.

The mid-Atlantic is experiencing some of the world’s fastest rates of regional sea level rise, with the soggy effects of climate change being exacerbated by shifting ocean currents and by the sinking of land, which is caused by groundwater pumping and natural geological processes.

Nutrias were brought to the U.S. more than a century ago, valued for their pelts. In their native South American habitat, the semi-aquatic rodents, which can weigh more than 15 pounds, are called coypus. They’re sometimes called river rats.

The animals were first released in Chesapeake Bay in 1943 to support the local fur industry. Lacking natural predators, the population flourished in the kinds of dense and mucky marshlands that are widespread along the mid-Atlantic coast.

By devouring marsh plants and their roots, nutrias have destroyed marshes from the Delmarva Peninsula to the Gulf Coast and across the Atlantic in the United Kingdom, leaving mudflats and open water in their wake. Marshes are key breeding sites for fish and birds, and they protect coasts from floods and storm waves.

Nutrias breed quickly, and officials have pivoted from focusing on trapping them on Delmarva to seeking survivors. They’re using sniffer dogs, fixed cameras, visual inspections from boats and hair traps on floating platforms to detect any surviving pockets of the population.

“If you leave a handful of animals, well, my goodness — this is how this problem all started,” Pepper said. “It started with a handful.”

Marshes are powerful features of the Delmarva landscape that can grow vertically to keep up with rising seas.
Credit: Jim Brickett/flickr

The attempted eradication came too late for many of the peninsula’s marshes. In Blackwater National Wildlife Refuge, on Maryland’s eastern shore, 5,000 acres of marsh disappeared from 1938 to 2006 — about half of the original marshland. (Meanwhile, 3,000 acres of new marsh appeared during the same time, taking over former forestland as the peninsula shrinks.)

“Blackwater is really known as a refuge that has seen a mass conversion of tidal marsh to open water,” said Matt Whitbeck, a federal wildlife biologist who works at the refuge and at others nearby.

“A lot of that can be attributed to sea level rise and subsidence. But also a lot of it can be attributed to nutria,” Whitbeck said. “Removing that artificial stressor to the tidal wetlands on Blackwater is a critical and huge step forward in making these marshes more resilient.”

Unlike the eradication of rats from islands in the Galapagos and elsewhere, officials didn’t use poisons to remove nutrias from Delmarva. “It just wouldn’t be very feasible — nor the best approach,” Pepper said. Nor did they offer bounties, which could have created incentives for hunters to reintroduce the pests to the wild.

The Delmarva eradication program relied on trapping and some hunting. It was guided by a successful 1980s program in the U.K., which also did not use poisons.

Newcastle University emeritus professor Morris Gosling, a population biologist who worked on the British eradication project, and who advised the U.S. government on its early work on Delmarva, warned his American counterparts they will have a tougher time preventing potential new populations from emerging.

“The benefit of being in the U.K. is that once you have gotten rid of them, you have some confidence — because the U.K. is an island — that you’re not going to get immigrants,” Gosling said.

Intensive monitoring programs in place on Delmarva now and in the future will be cheaper, he said, than allowing new populations to become established — which would require laborious work to remove.

“Since you’re in a continental situation, you’re potentially always going to get odd numbers of immigrants coming into the area,” Gosling said. “You’re going to have to maintain continual vigilance.”

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