Red knots, horseshoe crabs and climate change
Here's how they're connected: Red knots feast on horseshoe crab eggs in Delaware Bay before flying 10,000 miles. Now, spawning and migration are not always synching up correctly. Is climate change to blame?
Mon, Dec 21 2009 at 6:13 PM
LONG-DISTANCE FLIGHT: The red knots will have a tough time completing a 10,000-mile trek without proper fuel — that's where the crabs come in. (Photos: U.S. Fish and Wildlife Service )
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 overharvesting in the 1990s for the dearth 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 ends of the Western Hemisphere, two areas 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 modeling indicates 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 than a decade ago in Tierra del Fuego, an archipelago off South America.
In Delaware Bay, warming waters and increased variation in weather — including more frequent and intense storm events —could throw off the delicate dance 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 knots that depend on Delaware Bay has declined dramatically from a population of more than 90,000 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 has 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 was overshadowed by the impact from overharvesting the 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.
If scientists could determine how much of the red knots’ decline is due to climate change versus other factors, Kalasz said it would be possible to develop counteracting management actions. One outcome could be to replenish critical beach areas to protect the crabs spawning grounds from sea level rise.
Stacy Shelton is a public affairs specialist for the Southeastern region of the U.S. Fish & Wildlife Service.