Category: Invasive species

Why do weed warriors love to hate the garlic mustard?

The garlic mustard: From potherb
to invasive species of choice.

This is the time of year when weed warriors armed with shovels and pruning shears do battle with that subset of the plant kingdom called Invasive Plants. They march into woods and fields, fearless in confronting any leafed alien that poses a threat to our native plants. They emerge at the end of the day exhausted from their pulling and hacking, often with arms bearing badges of honor in the form of nasty scratches and cuts.

Clearly not all weed warriors fit this heroic mold. There are many others—perhaps the majority—who also believe fervently in the crusade against alien plants, but not so much in hard physical labor. They opt to express their beliefs in symbolic acts, a little like casual church-goers or Earth Day marchers.

A devil’s nursery. I recently came across what seemed like an illustration of this idea at a place near the Potomac River called Hughes Hollow. It’s a kind of “natural” area, favored by birders, where a system of canals and sluice gates control the water level of two artificial ponds covered with lily pads.

A dirt road runs across a dike at Hughes
Hollow, an unnatural area brimming
with life, both native and exotic.

A dirt service road separates the two ponds. Alongside grows a tangle of plants, nearly all of them non-native species. For a weed warrior, I’d imagine it’s like walking down the aisle of a devil’s garden nursery.

As for myself, I admired the exuberance—if not the purity—of this plant profusion. Bumble bees buzzed past my head as a yellowthroat warbler went “witchety-witchety-witchety” in a stand of willows.

A modest experiment. I walked along for a short distance, and then I came upon something curious. It was a neatly laid out stack of withering plants, followed by another, then another.

A withering bunch of garlic mustard
lies on a bed of more fortunate invasives.

The plants were garlic mustard, widely distributed in the Old World. It was brought to our shores in the middle of the 19th century to perk up the salad bowl and vegetable pot with its garlicky flavor.

Out of all of these invasive species it appeared that the weed warrior only pulled out garlic mustard. I found no sign that any of the other invasive plants along the roadway had received similar treatment. Why ?

I had a suspicion, but I needed to confirm it by performing an experiment. My method of analysis would be straightforward, requiring no replication, peer review, or even any actual data. I would simply walk down this service role and pull on every plant I encountered. I set off on my botanical adventure.

Death to dandelions! I first approached a dandelion. I admired the fierce yellow of its blooms, rivaling the sun itself. Italian immigrants once used its bitter leaves in salads. I knelt down and hooked my forefinger around the plant’s stem, right at ground level, and gave it a yank. All I came up with was a rosette of leaves oozing white sap from their base.

My next intended victim was vetch, a plant of slender vines, purple flowers, and delicate compound leaves. The nodules on its roots contain bacteria that convert nitrogen from the air into a form usable to other plants. Again I knelt down. With my fingers I traced the plant’s sinewy stem through the neighboring plants until I found where it entered the ground. This time I pulled more gently. But the results were the same.

Equal-opportunity invasive. I tried uprooting one non-native plant after the other. Some were tiny and delicate. Others were thorny and prickly. In all cases, try as I might, their roots refused to give up their hold on soil from which they sprang.

With one exception. As you have already guessed, it was the garlic mustard. I found a small patch that the weed warrior had evidently missed when he passed through earlier.

Their stems were tall and easy to spot and substantial enough that I could get a firm grip on them. I didn’t even have to kneel down to do so. I pulled, and as I knew would happen, out they came, roots and all. It was almost like these latter day pot herbs welcomed their demise.

I lay my first bunch down on the side of the roadway. I pulled up several more and lay them over the others, noting how naturally they seemed to nestle into a state of repose, like cordwood or neckties in a bargain bin.

This was all the proof I needed. Weed warriors love to pull out garlic mustard because, well, it’s easy to pull out. Science doesn’t always have to be complicated.

The Potomac’s hard-working immigrant

Seeing the bottom of the Potomac River through a facemask is a little like looking through a microscope. It’s a new world down there, and you never know what you might find.

I was looking sponges. The previous year they were right here, at the head of Watkins Island, near Pennyfield Lock. It was apparently the first sighting of sponges in the river’s main stem. What happened to them? Would they come back?

I examined the bottom minutely, admiring the shimmering pebbles and the green patina on the mussel shells. I poked at anything that looked even the slightest bit spongy.

The clams suck water into one
opening and expel it out the other,
removing silt and other pollutants.

Then I saw something that made me come to a full stop. Wedged among the pebbles was a pair of tiny white tubes, then another, and another. Hundreds of them. I had found a colony of freshwater clams.

This was interesting, but not because clams are unusual in the Potomac. Far from it. The shells of this little bivalves literally pave parts of the river in Montgomery County. Along some stretches of shoreline, their shells crunch under every footstep.

But I had never seen these little creatures actually at work. And here they were, drawing water through one tentacle-lined tube, retaining nutrients other matter, and then expelling the same water through the other tube.

It struck me that they might be a little like the Higgs boson of the Potomac. Even though most people don’t know anything about them, their vast numbers could give them a decisive role in the river’s ecology. Are they another of Charles Darwin’s humble creatures that rule the earth, such as the  barnacles that were the subject of his first book and earthworms which he lovingly saved for his last?

Oysters of the Upper Potomac. It seems so, at least according to biologist Harriette Phelps, a professor emeritus at the University of the District of Colombia. Phelps is widely known for her work with this clam species in the tidal Potomac.

Biologist Phelps sorts through
a haul of Asian clams destined
for research on pollution.

She told me the clam’s scientific name is Cobicula fluminea. Its common name will have to wait, so as not to give away the second part of the story.

“Hard working Corbicula,” she calls them. Put a layer of them on the bottom of a pail of murky water, and an hour and a half later the water will be clear. Multiply this effect by the countless millions of clams in the river, and you have what Phelps calls a “key species.”

Forget the feisty bass, the graceful heron, the majestic eagle. One of the real drivers of life in the river is this humble mollusk, most of them no bigger than a quarter. Darwin would have understood.

Corbicula almost seems designed with the express purpose of purifying water. The particles it extracts from the river are not simply cast back again. Instead, they are combined with mucus to build up the sediment in which they live.

Does this sound familiar? Elsewhere on this site I wrote how oysters were the cheapest and most efficient water purifier system in the Chesapeake Bay, until they were nearly wiped out.

Stands of stargrass grow so thickly
that canoes barely penetrate.

Clearing up the water kicks off a chain of happy events. More sunlight penetrating the water fuels the growth of aquatic plants. With oxygen-producing plants come fish and birds, and pretty soon, the river environment becomes whole again.

Phelps saw this happen in the tidal Potomac.  There’s “absolutely every reason” to assume that Corbicula is providing the same service in our upstream portion as well, she says.

Certainly something good is going on out there. In some places in the river, plants are so thick that you can barely plow your kayak through them. In the channels through the vegetation you see darting shoals of minnows and bass slipping back into the cover. Big catfish chug along the bottom like monster tadpoles.

A dark secret. But these sunlit waters contain a dark secret.  Corbicula is not a native of the Potomac region or anywhere else in North America. It originally came from eastern Asia, and is know by the popular name Asian clam. It arrived first on the West Coast around 1930, then showed up in the Potomac in 1977.

Once in the Potomac the clams rapidly increased their numbers, carpeting many parts of the river bottom. This would seem to qualify Corbicula for membership in that increasingly familiar “genus” of the world of advocacy ecology called “invasive.” Here we have another foreign creature that has invaded our own natural communities, like a shiploads of Vikings burning monasteries, slaughtering townspeople, and carrying off treasure.

Clam gathering isn’t much
sport, but it’s a sure thing.

But not according to Phelps. “Corbicula were not invaders,” she said emphatically. “They occupied areas that were previously unoccupied.” And like many immigrants, they got their start by doing jobs that natives wouldn’t or couldn’t do themselves.

In the tidal Potomac, Corbicula inhabits the upper layer of the river bottom, she explained. The estuary’s native clam lives deeper down. “Even with Corbicula, the native clams are all over,” said Phelps.

Paradoxically, Phelps worries that this “invasive” species might disappear. It already has in her former study areas in the tidal Potomac. The same could happen in the upper river as well.

If Corbicula disappears, the river’s ecological clock could start to go in reverse. With no more clams filtering out so much stuff that gets washed into the river, the water could again lose its clarity, and with it, many of its plants, fish, birds, and other life forms.

Tiny, but considered a
perfect addition to soup.

The clams’ disappearance would also be felt by one group of local people whose ties to Corbicula extend to long ago and far away. You can sometimes see them, the men bent over the water, raking out clams with fan covers, the women sitting in the river, plastic baskets resting on their outstretched legs, picking the clams out from the stones and empty shells.

These people are also immigrants, harvesting the same clams in the same way they they and their families have done for many generations in their southeast Asian homelands. Following age-old traditions they will cook the clams in a wok with ginger, chile peppers, basil, and soy sauce.

Evidently it’s an acquired taste. “They’re good,” says Phelps, “but not really clam-like to me.  I like clams that are saltier.”

How invasive species came in second

What is the most serious cause of species extinction?

I think the answer is pretty clear. Just looking at my own Potomac River watershed I can see that it’s changed a lot over the past few centuries. For many creatures, the changes have not been for the better. The streams feeding into the river are much warmer, forcing the native trout to retreat up into the highest mountain brooks. The dam at Little Falls bars shad, herring, and sturgeon from their upstream spawning areas. Silt caused by erosion smothers insect nymphs and other creatures that live on the stream bottom.

So without hesitation I reply, “Habitat loss.”

Now, how about the second most serious cause? I know that most of the familiar fish in the river originally came from somewhere else. I tick them off: Smallmouth bass, channel catfish, sunfish, walleye pike, the toothy muskellunge, and now the equally toothy snakehead from China. These must have had a tremendous impact on native creatures when they moved into the neighborhood.

My answer for number two is, “Invasive species.”

An apparent consensus. It turns out I’m right, at least according to a quick Google search. Here’s a sampling of what I found:

“Invasive species represent the second leading cause of species extinction and loss of biodiversity in aquatic environments worldwide,” says the U.S. Environmental Protection Agency.

“Exotic invasive species are a major threat to many ecosystems worldwide and the second leading cause of extinction behind habitat destruction.” This is from the University of Michigan’s Botanical Garden.

“Invasive species are second only to habitat destruction as the major cause of extinction,” says the U.S. Park Service in a publication on the Point Reyes National Seashore.

“Invasive species are the second leading cause of animal decline and extinction, worldwide!!!” Red is the color of choice for Wyoming’s Natrona County Weed and Pest district.

“The introduction of exotic species that replace local and native species is cited as the second largest cause of biodiversity loss.” The United Nations Environment Programme said this as part of its observance of the “International Year of Biodiversity.”

“After habitat loss, nonnative species are the second leading cause of endangerment.” This appears in a “Solutions Paper” on forests and biodiversity loss in Latin America commissioned by the Inter-American Development Bank.

Fact or something else? Or at least I thought I was right about the number two cause. Now, after attending an American Association for the Advancement of Science meeting in Washington, D.C., I’m not so sure.

One of the speakers was Mark Davis, plant ecologist at Minnesota’s Macalester College and author of the book Invasion Biology. According to Davis, the claim that invasive species are the second greatest cause of species extinction has been cited more than 700 times. It’s anybody’s guess how many more times the same claim has appeared in technical papers, reports, journalistic articles, power point presentations, and essays written by earnest Environmental Studies majors.

According to Davis, few of these claims cite the source for this information, which was a 1998 paper published in BioScience magazine. Its lead author was David Wilcove, a respected professor of ecology and evolutionary biology at Princeton University.

It so happens that Wilcove’s paper said something very different than the “findings” that are so widely being cited. “We emphasize at the outset that there are some important limitations to the data we used,” stated the paper. “The attribution of a specific threat to a species is usually based on the judgment of an expert source.”

The Wilcove paper continues: “[The] evaluation of the threats facing that species may not be based on experimental evidence or even on quantitative data. Indeed, such data often do not exist.”

Again, the paper emphasizes: “. . . assessments of the threats to individual species are often based on the subjective opinions of knowledgeable individuals, rather than experimental evidence or quantitative data.”

In other words, the researchers asked people—albeit experts—what they thought. In most cases the judgements were not based on actual facts. They were opinions.

A snowballing ‘fact.’ I can imagine how this whole chain of misunderstanding may have happened. Hard-line defenders of native species may have seen the Wilcove paper and used its findings—minus Wilcove’s caveats—to add scientific oomph to their anti-non-native positions.

A block of misleading display text on the paper’s first page might also deserve some of the blame. Likely written by an editor with little interest in scientific nuance, it asserted, “Habitat loss is the single greatest threat to biodiversity, followed by the spread of alien species.”

As time went on, a mounting number of Google searches found the same “fact,” and it snowballed from there, along the way gaining authority and legitimacy.

Can anything now be done to clear this up? Not likely, unless someone can find a way to get a snowball to roll back up a hill all on its own. What’s done is done.