Category: Invasive species

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.

I don’t like purple deadnettles. It might be my age.

From their blockhouses along the Potomac River bluffs the Union soldiers keep a lookout for any movements of Confederate troops across the river in Virginia. It is 1862, a time of conflict.

I set off down the trail.
My horse remains at home.

Another conflict of sorts continues along these same bluffs, now part of the 639-acre Blockhouse Point Conservation Park on River Road. Today’s combatants are not opposing armies, but plants. Facing off on one side are the trees, shrubs, and wildflowers that are native to this area. On the other side are plant species that came from someplace else: the invasives.

If the invasives gain the upper hand, as they have in many places, they could wipe out the natives. Here in Montgomery County and around the country, people have organized to prevent this from happening.

Alien invaders. Among them is Carole Bergmann, forest ecologist with the Montgomery County Department of Parks. Passionate about her mission, she has founded the Weed Warriors, a group dedicated to uprooting and hacking down these alien invaders.

Bergmann urged me to visit Blockhouse Point as an example of a “really high quality contiguous forest,” still relatively unscathed by change. But keep your eyes open, she advised. For even here, the invasives are at work.

I parked at the little lot on River Road and headed down towards the river on a well-groomed trail. I admired the soaring beeches, oaks, and tulip poplars.

The orange-red sap of the
fragile bloodroot contains
chemicals that might help
fight bacteria and reduce

As Bergmann promised, these are beautiful woods with remarkable plants, each with a story.

My first find was a pair of bloodroot, their leaves still curled around their stems, their delicate flowers poised to open. Native Americans used this plant’s orange-red sap as a dye. Today, the plant is on the U.S. Food and Drug Administration’s list of fake cancer cures.

The trail skirted a swampy area with a stand of skunk cabbage, named for the pungent odor its leaves release when crushed. In the dead of winter, this extraordinary plant produces heat that enables it to push up through cold mud and snow. Well before other flowers open, the skunk cabbage’s hooded blooms are already attracting pollinating insects with their warmth and odor.

On a sun-bathed hillside I found a carpet of wildflowers called spring beauties. You

The skunk cabbage beats
the competition for pollinators
by flowering in the winter.

would never guess from looking at their delicate pink-veined blossoms that their roots form into grape-sized tubers. These are perfectly edible if you don’t object to the taste of dirt.

Honeysuckle taking hold. But here the plot thickens. Among the spring beauties I found snippets of dark-leaved vines poked up among the shy flowers. It was honeysuckle, introduced to the U.S. from eastern Asia. An aggressive tree climber, its dense foliage shades out the native plants living below.

Now I stood on a bluff overlooking the canal and river. Around me a multitude of tiny flowering plants poked up between fallen leaves. Among them the purple deadnettle, a

What looks like the leaves
of this spring beauty
actually belong to an
invasive honeysuckle vine.

Eurasian native that equally thrives along roadsides among the broken glass and cigarette butts.

I slid down the bluff to the floodplain along the canal. Japanese barberry, with its familiar scarlet fruits, pricked my legs. I forced my way through tangles of multiflora rose, also from east Asia, their sharp thorns grasping at my shirt and puncturing my arms and hands and leaving them covered with drips of blood.

As Bergmann explained it, the land along the canal, altered and flooded with sunlight, creates an ideal corridor for invasives. For this reason, she adamantly opposes any proliferation of trails through the park. Her nightmare scenario would be a full-scale assault by Vietnamese stiltgrass. “If we don’t control it,” she said, “stiltgrass will shoot up and down all the trails, even in shade.”

Contrarian on aliens. My walk had come to an end, and now my mind wandered to another setting with invasives—but with subversives as well.

Earlier this year I attended a seminar on non-native species at the annual meeting of the American Association for the Advancement of Science, in Washington, D.C. There I met Mark Davis, a plant ecologist, author of the book Invasion Biology, and professor at Minnesota’s Macalester College.

According to Davis, the invasives issue is less about plants and animals, and more about us—our personal likes and dislikes.

Speaking of aliens, this one
below the Blockhouse Point
bluffs looks straight out
of War of the Worlds.

“When we refer to a healthy ecosystem,” he said, “we’re talking about how we want the ecosystem to be.” Invasive species do affect ecosystems; that’s scientific fact. Some of the changes we like and others we don’t; that has nothing to do with science.

The most vehemently anti-invasive views are typically held by people in their 50s and 60s, he maintains. “Kids growing up today are surrounded by people and cultures from all over,” he continued. In the future, Davis predicts, we will focus less on managing non-native species and more on changing our attitudes towards them.

That said, Davis supports taking firm action when species threaten our health or our economy—alien or native. The emerald ash borer, which is devastating ash trees throughout North America, originally comes from Asia. But the mountain pine beetle, which is wreaking havoc with pine trees across the western states, is 100 percent American.

Back at Blockhouse Point I looked out at the river. The creatures living in its waters tell even more topsy-turvy stories of biology and beliefs. These stories will remain to be told.

Kill this fish by cutting, bleeding, or freezing

I recently came across a pair of posters on a tree by the Potomac’s C&O Canal at Riley’s Lock. The upper poster shows the familiar largemouth bass and warns any fisherman catching one to release it immediately.

The lower poster, solidly fastened with anti-rust screws, depicts a creature with a small head, boldly patterned body, and fins that run along its top and bottom. It looks like a fish that could have swam with the plesiosaurs, certainly unlike anything ever seen in this part of the Potomac. It’s called the snakehead.

Two non-native fish. Kill the first by cutting/bleeding or freezing, and report catches to the authorities. And the second? Release it immediately.

“Have you seen this fish?” the poster wants to know. It goes on to demand that if you do catch one, you must act immediately. “PLEASE DO NOT RELEASE,” the poster says. “Please KILL this fish by cutting/bleeding or freezing.”

Why does the Maryland Department of Natural resources want us to protect one fish and execute the second, no questions asked? The poster tell us. The snakehead comes from China. As a non-native, it will out-compete the Potomac’s natives and upset the river’s ecosystem. And the bass? This fish is the icon of the Potomac with a lineage that surely stretches back into the river’s misty past.

Except for one thing. Bass are not native to the Potomac either. Largemouth bass come from the southern US, and smallmouths from the Ohio Valley. What’s more, nearly all of the Potomac’s other fish that people know and love also came from somewhere else.

I took another look at the two posters. They’re not really about two fish, I concluded, one good, the other bad. The posters actually are about us.

Environmental heresy. Downstream from Riley’s Lock, not far from where the tidal Potomac runs past Washington, D.C., the American Association for the Advancement of Science was holding its annual meeting. Speaking at one session was Mark Davis, a biologist and expert on invasive species. Maybe he would clear up the paradox of the two posters.

Davis told the audience that in his view, much of the scientific-sounding debate over non-native, so-called invasive species, has little to do with science. Instead, it has a lot to do with personal belief and a kind of biological jingoism.

“Groups in society have different values,” he said, “and may come to different conclusions with respect to different species and the harm, change, or even benefits they cause.”

Yes, but what about snakeheads? Shouldn’t we be worried that such a fish could destroy our healthy Potomac River ecosystem?

This is how Davis put it:  “There’s no such thing as healthy ecosystem. When we refer to a healthy ecosystem, we’re talking about how we want the ecosystem to be.” Following this line of reasoning, invasives such as the snakehead admittedly do affect ecosystems. But the key point, according to Davis, is that some of the changes we like and others we don’t. It’s all about us.

You would think that evidence-based science would solve the matter of what’s good and what’s bad. But according to Davis, many scientists themselves are responsible for distorting the issue. “Scientists influence the decision-making environment by influencing the ‘valuescape,’” he said.  Take the statement, “genes from invasive species contaminate native gene pools.” This is what Davis could call hybrid language―seemingly scientific, but containing a hidden element of value judgment, in this case the loaded word “contaminate.”

“When we declare something as invasive and harmful,” he continued, “we oblige society to take action. ‘Get out chemicals, trample other plants, spray, spray, spray!’”

Borers and beetles. That said, Davis perfectly accepts the need to combat pest species—non-native or otherwise—when they threaten our health and safety or negatively impact ecosystem services on which we depend. “It comes down to change versus harm,” he said.

For example, the emerald ash borer is killing ash trees throughout North America. It originally comes from Asia. The mountain pine beetle is devastating pine trees across Western North America. But this second bug is a native. Which is worse?

In some cases the “valuescape” contains “facts” that seemingly come out of nowhere. For example, even respected authorities perpetuate the notion that invasive species are the world’s second greatest extinction threat (after habitat destruction). “This is cited thousands of times,” said Davis, adding that it is “sometimes not even cited, just repeated as fact.” He noted that this “fact” originated in a scientific article by a respected ecologist at Princeton. The article itself didn’t rank extinction threats, but instead merely reported on what people thought were threats. In fact, the article stated that its conclusions were based on little or no empirical data.

Attitude management. The reason the snakehead ended up on the wanted poster may be because our attitudes about invasive species were formed in previous generations, said Davis. The proponents of these attitudes today are typically people in their later years.

“Kids are now growing up in a completely globalizing world, surrounded by people and cultures from all over, he continued.” Davis advised that rather than trying to manage new species, we should be learning to manage our attitudes towards them.

In addition there are the high costs of insisting on our beliefs. “We simply don’t have the money to eradicate every species we don’t like,” Davis said, “even if we could.”

“And we certainly don’t have the luxury to use public money to fund personal preference,” he added. “We should reserve our resources to try to eliminate species that threaten human health or cause economic harm,” he said.

Given all of this, it seems ironic that public money is even now being spent on helping  non-native species get a finhold in the Potomac. Among them are the muskellunge, walleye, and rainbow trout. In the Potomac, many of our most beloved fish got here because people (and government agencies) believed that they provide better sport or better eating than the natives.

Maybe so. Certainly an introduced bass is more fun to catch than a native white sucker. But how about rolled in breadcrumbs and baked? From the culinary reports I’ve read, this honor must surely go to the snakehead.