When More is not Merrier: Shifting Baselines and Invasive Species

Since the early 1980’s my family has been vacationing along the St. Lawrence River in upstate New   York. We’d head up year after year to relax in the beautiful wilderness, and though I was too young to know it at the time, I was witnessing one of the worst ecological invasions of our time. The dreaded zebra mussel, now in Massachusetts, was marching through the great lakes, clearing the water of its precious plankton and forever changing the ecosystem.

But for me, the zebra mussel was just a normal part of the river while I was growing up. I thought the environment seemed healthy enough – there were birds to be seen and fish to be caught. That was because I hadn’t seen the river before the zebra mussel, they’d always been there, that was my baseline. My parents certainly remember a different river, and from their time to mine the baseline had shifted.

It wasn’t until I was much older that I saw the effect of a new invasive species in that same river, the round goby fish. I’d cast a reel and catch this funny looking bottom-feeder on rare occasions. But lately I’ve caught more and more gobies. Now I just expect them as part of the ecosystem, and one day when my kids fish in the St. Lawrence, their baseline will be filled with gobies.

The trouble is that our shifting baselines are dangerous: they cloud our idea of what’s “normal”, and they don’t allow us to see the gradual decline in the environment around us. Be it climate change, ocean acidification, pollution, overfishing, or invasive species, it can be tough to tell what “normal” used to look like.

But invasive species actually provide us with an interesting case. On one hand, if they’ve been around long enough we way think of them as part of the native community (like I did with the zebra mussel). Take, for example, the common periwinkle, Littorina littorea, an invasive species that has been around since the mid 1800’s. They’ve been here so long and they are so ubiquitous that until recently scientists have even debated whether they’re native or invasive. Surely you’ve seen these snails blanketing our rocky shores, and you too may not have thought much of them. But what did our beaches look like before they were here?

On the other hand, some species invasions allow us to witness substantial environmental changes in just a few years, and they can demonstrate to us the fragility of our ecosystems. Like the European shrimp, Palaemon elegans, which was found in Salem Sound just a couple years ago. They eat a lot of other small crustaceans, potentially shifting community interactions, and they are likely to spread rapidly along the coasts. They have already been reported in Maine, Boston Harbor, and in Rhode Island. We’ll actually be able to witness the effects of this invader in just a short while, though we can’t yet predict the severity of the impact they’ll have on native species.

With all of this change it can be easy to lose track of what’s happening in our New England waters, which is why documenting species introductions and the distributional changes of those invaders is so important. A local scientist leading the effort to detect invaders is Dr. Judy Pederson from the MIT Sea Grant College Program. Every few years she and a team of scientists perform surveys to look at the organisms (mostly attached to docks) along New England coastlines. She notes the value of a citizen monitoring program that she says has worked very well for recording invasive species.

“Part of what we do is document the presence and absence of species and their movements up and down the coast,” she told me. “But once an invasive species is found in the marine environment, it’s almost impossible to eliminate.” Since they’re so tough to get rid of, Dr. Pederson also works on helping to control the initial introduction of invaders. The most widely reported vector for spreading invasive species is the discharge of ship ballast water from foreign ports. Microscopic larvae (part of the plankton) taken into ballast water across the ocean often get released into new coastal waters where they may metamorphose and ultimately thrive. But everyday ocean-using citizens spread organisms too, which is why Dr. Pederson has worked to educate divers and boaters about cleaning their equipment after using it, lest they involuntarily transfer species to new places.

It’s a noble effort, tackling such a daunting and complicated problem. Some invasive species are even displacing previous invaders. Remember the European green crab, Carcinus maenas, whose voracious appetite isn’t very easily satiated? Though they are a menace, in some areas even these crabs are being pushed around by a relatively new (1988) invader, the Asian shore crab, Hemigrapsus sanguineus. I can’t turn over a rock at low tide without finding a few of these pugnacious little guys.

In fact, Dr. Pederson had some sobering stats: “About 15% of the species we find during our surveys are non-native, but they can be 40% of the biomass at some locations.” And the increasing effects of climate change are not likely to ease the trouble. As we’ve seen recently, the distributions of native species have been shifting, introducing New Englanders to a host of new organisms. But invaders are on the move too. The devastating lionfish has been found as far north as Narragansett Bay, and is already on the way to becoming an ecological disaster. With no known predators and native communities that aren’t adapted to recognize them as a threat, lionfish can quickly wipe out the biodiversity of entire ecosystems.

Maybe we can’t get rid of the invasive species that have taken up residence here, but without the knowledge of what used to be, we may not notice the slow changes happening all around us. We need to stay vigilant, and you can help keep a watchful eye on our oceans to remind us of how our baselines are always changing.

Casey Diederich is a 5th year PhD candidate in Tuft’s University’s Biology Department, and is conducting his research on slipper-shell snails. We are thrilled to have Casey guest blogging for us about some of the more fascinating plants and animals in our ocean. – Ed.

Image via the National Center for Ecological Analysis and Synthesis