Slugs, Bugs & Eelgrass: Unsung Heroes

When we mess with the food web, the food web gets messy.

June 05, 2015

An isopod clings to a blade of eelgrass.
Photo: Eric Heupel/Flickr

All over the world, an army of grazers is hard at work. A nibble here, a nom there. Munch, munch, munch.

These aren’t the herbivores you’re used to hearing about—the deer, buffalo, and elephants of the world. I’m talking about snails, isopods, and amphipod crustaceans. Although you’ll probably never see them, the work of these animals affects the lives of coastal communities the earth over.

Yes, even lowly aquatic bugs and slugs nestled away in the intertidal zone have a role to play in the web of life. And today we sing their song.

The tiny creatures live in eelgrass beds, underwater patches of various species of three-foot-long vegetation found all across the temperate zone. From Norway to Nagasaki, they skitter and slither up and down the blades, dutifully mowing down the algae that grow there. Juvenile fish, crabs, and crustaceans make a living eating these bugs and slugs. Small fish feed big fish. Big fish feed us. 

The problem is, like all food chains, this one is vulnerable to disruptions in the pecking order.

For instance, in a study published in the May issue of Ecology Letters, scientists found that when you remove slugs and bugs from an eelgrass bed—in this case, via slow-release insecticide—algae starts to grow out of control. Algae is a natural part of the seagrass ecosystem, of course, but when let loose, it has a habit of going all Mr. Burns and blocking out the sun. The lack of adequate light strangles eelgrass by preventing photosynthesis.

Eeelgrass provides the architecture for entire ecosystems. Its dense roots anchor shorelines and protect beach communities from storm surges. Ducks, geese, and urchins eat eelgrass. Juvenile salmon and blue crabs hide out in it. Long Island bay scallops can’t live without it.

Photo: NOAAAn eelgrass bed in the Prince William Sound, Alaska

“Seagrass ecosystems are basically like the grasslands and savannahs of the oceans and shallow water,” says lead author Emmett Duffy, director of the Tennenbaum Marine Observatories Network at the Smithsonian Institution. “Unfortunately, they’re also declining all over the world.”

These ribbon-like blades of green, yellow, and blue take the brunt of all kinds of human activities, from sewage to oil spills. Logging, dredging, and unsustainable shellfish harvests all harm eelgrass. According to the National Marine Fisheries Service, rising water temperatures and sea-level change could also cause the vegetation to die off.

Over in the Baltic Sea, there’s evidence that overfishing cod leads to population explosions for the fish the cod would have preyed upon. These smaller fishies then gobble up all the bugs and slugs grazing on seagrass, possibly triggering algal blooms. 

So it seems that just about everything we do mucks up these crucial, coastal ecosystems—but there may be hope yet.

“One of the most exciting things we saw in this study was the fact that high grazer diversity meant better control of algae,” says Duffy. “And also that high eelgrass genetic diversity increased the abundance of [grazers].”

In other words, biodiversity acts as a buffer against shocks to the ecosystem. An algal bloom or oil spill or spate of illegal fishing may clobber an undersea community, but if that community is diverse, it might stand a better chance at eventual recovery. And we can help encourage biodiversity (and thus, resiliency) in a few ways that are relatively easy.

For instance, in some of harbors, the National Oceanic and Atmospheric Association has invested in new “conservation moorings.” These floatable lines allow boats to dock without using chains that rip through eelgrass beds. No-anchor programs, like the one in Washington's Puget Sound, have a similar effect. And targeted restoration efforts—just like you’d see in a terrestrial forest—have allowed eelgrass beds to spread in many areas. In Chesapeake Bay, 250 acres of eelgrass multiplied into 5,000 acres in just a decade

Healthy eelgrass beds beget robust communities of bugs and slugs, which in turn nourish all the successive links in the food chain.

So the next time you eat a sushi roll that contains half a dozen species fish, crab, eel, and roe, give thanks to the many acres of eelgrass—and all the bugs and slugs crawling and sliming within—that helped grow your feast. Mmm. Pass the wasabi. 

onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.

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