Scientists with the U.S. Geological Survey have implicated climate change in a decline in horseshoe crab populations in the Delaware Bay and other spots along the East Coast.
In recent years, fishermen have been blamed for the decline, leading to a complete horseshoe crab harvest moratorium in New Jersey beginning in 2008 and cutbacks in other states.
Commercial harvesting may have had short-term effects, but USGS conservation geneticist Tim King said his study shows the species has been suffering for thousands of years, beginning with the end of the last ice age more than 12,000 years ago.
"The overall trend is a decline as temperatures have warmed. All the things associated with that, such as warmer waters and sea level rise, have had an impact," said King, lead author of the study just published in the trade publication Molecular Ecology.
King said the trend has implications for other species, including threatened shorebirds such as the red knot, which times its migration each spring from South America to the Arctic to stop on Delaware Bay beaches and feast on horseshoe crab eggs. Other species, including endangered Atlantic loggerhead sea turtles, feed on adult horseshoe crabs and have already been forced to seek other food sources.
"If horseshoe crabs do better, then turtles and red knots should do better," King said.
While the overall horseshoe crab population has declined since the ice age ended, King said a bigger concern is the "effective population" decrease. The effective population has to do with the number of horseshoe crabs it takes to create the necessary genetic diversity. King said genetic diversity is "the tool box" a species needs to successfully adapt to changing conditions.
"The fact that the effective population decreased gives one pause. Over the long haul, there is less and less genetic diversity for the species to make a living with. We would hope we don't allow the effective population size to get below the point where they're not able to adapt to their environment," King said.
As the earth has warmed, King said, horseshoe crabs have increased their range to the north, as far as Maine, but numbers and diversity have dropped. The rocky terrain to the north may be affecting breeding since the species uses sandy beaches to deposit and fertilize eggs.
"As the range increased, the population did not increase. Something is limiting population growth," King said.
If the species is spread too thin, it could be harder to survive a cataclysmic event, which King called an "extinction vortex."
USGS studies of horseshoe crabs are designed give managers the information they need to protect them. A 2005 study showed females return to the bays where they are born, while males move from bay to bay. Such habits could lead to practical applications, such as moving females to different bays.
"Males are the only ones exchanging genes from one place to the next. Females and males should be managed separately," King said.
The study looked at genetic variation through time and found more variation before the Earth's most recent warming trend.
The Atlantic States Marine Fisheries Commission, a compact of East Coast states that regulates horseshoe crabs, has recently reported an increase in horseshoe crabs, likely from harvest cutbacks due to concerns about a declining red knot population.
King said that is a good thing but noted that, even though the population of a species can rise quickly, a boost in genetic diversity, which occurs with mutations during reproduction, takes much longer.
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