Red Knots, Horseshoe Crabs and Climate Change
Not far from the casinos of Atlantic City, a different kind of wager takes place each May along the shores of Delaware Bay.
That’s when the red knot, a bird the size of a coffee cup, stakes its future on the ready abundance of eggs laid by tens of thousands of horseshoe crabs. Without enough eggs to fuel them, the long-distance fliers will have a tough time completing the last leg of their 10,000-mile trek from the southern tip of South America to their breeding grounds in the Arctic.
In recent years, the red knots’ bet on the crab eggs has been more of a crapshoot than a sure thing. Scientists blame over-harvesting in the 1990s for the dearth of eggs that’s only recently started to reverse.
Yet they also acknowledge a wild card could be in play: climate change.
Reds knots spend most of their time at the extreme latitudes of the Western Hemisphere, two areas that are expected to show effects from climate change most rapidly. Gregory Breese, the U.S. Fish and Wildlife Service’s project leader for the Delaware Bay Estuary Project, said that models indicate a relationship between snow cover in the Arctic and bird survival. In addition, red knot biologists have observed that the birds are arriving one week later to winter in Tierra del Fuego, an archipelago off South America, than they were a decade ago.
In Delaware Bay, warming waters and increased variation in weather, including more frequent and intense storm events, could throw off the delicate balance between the horseshoe crabs and red knots.
“The peak of horseshoe crabs spawning in Delaware Bay has not always aligned itself with the migration of the red knots. That could be related to climate change,” Breese said.
Since the 1980s, the rufa subspecies of red knot that depends on Delaware Bay has declined dramatically from a population of more than 90,000 to 150,000 birds to between 15,000 and 20,000, according to Kevin Kalasz, the Shorebird Project Coordinator for the Delaware Division of Fish and Wildlife. The population numbers have been steady since 2003, Kalasz said.
The bird, which has a salmon-colored face and breast, is a candidate for federal protection under the Endangered Species Act.
If climate change played any role in the species decline to date, Kalasz said, it would have been overshadowed by the impact from over-harvesting of horseshoe crabs in the Delaware Bay. Now that severe restrictions on crab harvesting are in place, climate change is likely to move to the forefront as a factor in the red knots’ survival.
Once scientists determine how climate change has affected – or will affect – the red knot, it will be possible to develop counteracting management actions, Kalasz said. He called such research “one of our top priorities.”
One outcome could be to replenish critical beach areas to protect the crabs’ spawning grounds from sea level rise.
By Stacy Shelton, public affairs specialist, U.S. Fish and Wildlife Service, Southeast Region
For more information on this topic, please contact...
Gregory Breese, Biologist, Delaware Bay Estuary Project, 302/653-9152