[explosion]

(Describer) Streams of light collide to create a globe filled with water. Title: Science Nation. Small dark crabs have light claws.

(male narrator) These purple marsh crabs are being tethered to fishing line. Oop. Get back here, bud.

(Describer) A man catches one on a table.

(narrator) Who would think these crustaceans could wreak so much havoc in the marshes of Cape Cod? Blue crabs are their main predator, but they're being over-fished and their numbers are dwindling. The result is an abundance of hungry purple marsh crabs, gorging unchecked on healthy fields of cordgrass, leaving behind lumpy areas of mud.

(man) The cordgrass that's being destroyed here is the foundation species that builds salt marshes.

(Describer) Mark Bertness:

(narrator) With support from the National Science Foundation,

(Describer) He walks onto a beach.

Brown University marine ecologist Mark Bertness studies this threatened ecosystem. Marshes protect our coastal environment by nurturing a complex web of plants and animals, filtering nutrients, and serving as a critical storm barrier.

(Bertness) When I see decimated marshes, I feel sad.

(narrator) Bertness has set up experiment stations all over Cape Cod to monitor the impact of purple marsh crabs on the local habitat. He's ready to go.

(narrator) Bertness leaves them tethered to sticks, offering an easy meal for hungry blue crabs. He also sets up predator traps.

(Describer) He drops a cage in the water.

(Bertness) And that's what's left.

(Describer) He finds an empty line.

(narrator) Here, all the purple crabs get eaten, and the trap is full of predator blues.

(Describer) They're shaken out.

It's no coincidence that this is a healthy-looking marsh, but where the grass is eroded, the tethered crabs remain untouched...

(man) Still alive.

(narrator) and the predator trap is empty. These are waters that have been depleted by recreational fishing from that marina right over there.

(Bertness) People like fishing, and they like salt marshes, and they don't understand there's a tight linkage between the health of both of them. Salt marshes are such an important nursery ground habitat for both recreational and commercial fisheries that it's in their best interest to understand these linkages.

(narrator) Bertness says as long as purple marsh crab populations are high, this ecosystem will remain... Aaah! This one pinched me.

(narrator) in a real pinch.

(Describer) The globe turns.

For Science Nation, I'm Miles O'Brien.