In Massachusetts' Cape Cod, the cordgrass salt marshes have been mysteriously dying off for decades. Now, some of the marsh is just as mysteriously beginning to grow back. This die-off and regrowth has finally been traced back to two little crabs and, of course, to human error.
Researchers have figured out that the exploding population of native purple crabs was gobbling up the grass faster than it could reproduce. Purple crabs, in turn, were booming because recreational fishermen have severely depleted their main predators, blue crabs and striped bass. Drainage ditches dug to reduce mosquito populations now became highways for the purple crabs, allowing them to easily invade cordgrass habitat and mow it down.
The strange convergence of coincidences doesn't end there. According to a new paper, the invasive European green crab has been accidentally restoring the Cape Cod salt marshes to some of their former glory. The aggressive green crab is feasting on the abundance of purple crabs, and scaring away the survivors. In the purple crab's absence, cordgrass is bouncing back. This may be one of the few cases where an invasive species actually helped an ecosystem – though, of course, plenty of blue crabs, bass and no drainage ditches are preferable to having to trust an non-native aggressor to do the work.