Invasive Species and the Edge Case

Could invasive species actually be benign?

I recently visited a friend’s fishing camp along a legendary trout stream in the Catskills. I didn’t wield a rod, but I had a grand time communing with nature.

Catching up with emails when I returned, I found in my inbox an issue of Paul Hormick’s “The Green Dispatch,” whereby he waded into a debate about whether invasive species are so terrible after all.

What got Paul’s hackles up were a pair of essays by a science writer named Sam Matey entitled “Even the ‘World's Worst’ Invasive Species Often Aren’t Actually Bad” and “The Dawn of ‘Immigrant Species’ Biology.”

I hadn’t read Matey’s work before. It seems his thing is finding positive aspects among our rapidly changing natural world to counterbalance the collective freakout over climate change and other environmental degradations. Kudos to him on that. Everyone needs a little hope.

Matey’s efforts reminded me that Michael Mann, who created the famous “hockey stick” graph that showed the accelerating impact of human activity on climate change (and was relentlessly vilified by oil company shills for half his career as a result), has often resisted joining climate doomsayers.

Pollyannaism? Maybe Sam Matey really believes the glass is always half full, and Michael Mann believes a mere course correction can save us from what we’ve wrought—if only he could convince the right people. But these approaches seem to me more like coping mechanisms than objective observations, even coming from scientists citing certain studies. Matey in particular seems to be using edge cases to make broader generalizations about invasives that I don’t think are justified.

Indeed, a couple of well-informed folks pushed back hard against Matey’s initial essay (if you have time to read his piece, I’d urge you to scroll down to the comments), and Hormick was one of them, with a brief response entitled “Invasive species: there isn’t an upside.”

You will note, just comparing titles, that we humans have a tendency toward the binary no matter which direction we’re coming from. Like Hormick, I hate hate hate invasive species. Matey, especially in the title of his follow-up piece, seems to suggest that such haters are bigots. Isn’t it equally possible that we just don’t go in for moral relativism when it comes to kudzu and emerald ash borers?

Interestingly, when he staked out his initial position for this debate he conflated invasive species with naturalized ones in order to set up another binary (native vs invasive), when in fact those of us worried about invasive species take care to draw distinctions between non-native populations that do little harm and those that appear to be crowding out natives to the detriment of the local ecology.

During my brief Catskills visit, we ventured several times into the woods, which were nearly pristine in the densest parts, full of native species in balance with one another, with the exception of some huge swaths of ferns in places that to me indicated heavy white-tailed deer browse (deer don’t eat ferns). We saw mature hemlocks untouched by woolly adelgid, mountain azalea, low-bush blueberry, club moss (Lycopodiopsida spp.), tall white-aster (Doellingeria umbellata), white meadowsweet (Spiraea alba), Indian pipe—which I thought were fungus but turn out not to be—(Monotropa uniflora) and meadow rue (Thalictrum spp.), among many other natives.

The property is occasionally logged, however, and in the temporary logging roads I spotted common invasives such as dock, thistle and bindweed, which I found disappointing.

Worse, invasive mugwort and Japanese knotweed dominated both sides of a path we used to get from the house through a meadow to the river. Each had established its own monoculture, crowding out everything else in its way, no doubt more so next year than last.

Matey used Japanese knotweed as an example of an invasive that we haters have maligned. I will quote him in full:

Japanese knotweed, Reynoutria japonica, is a plant I’ve often seen serve as a food source for native bumblebees in Maine. It’s also been widely deemed “destructive” to homes and other structures (to the point where its presence can sometimes lower a property’s value!), and expensively attacked with herbicides. But an exhaustive research project by the University of Leeds and global infrastructure company AECOM found no evidence at all that the species caused significant damage to buildings!

Seeing a native bumblebee visiting a plant is not prima facie evidence that the plant is good for the local ecosystem or even good for the bumblebee. Many native butterflies visit invasive butterfly bushes, for example, but not a single one of their caterpillars can survive on it. The West Virginia white butterfly (Pieris virginiensis) confuses invasive garlic mustard with the host plant it evolved with (toothwort), wasting its eggs on a decoy upon which its larvae cannot feed. When tempted to eat the berries of invasive plants, migratory birds that require certain nutrients to sustain their seasonal flight, can arrive at their destination malnourished and vulnerable.

Notwithstanding the anecdotal evidence of bumblebee visits, here’s how the New York Invasive Species website explains the potential harm of Japanese knotweed:

Japanese knotweed spreads rapidly, forming dense thickets that crowd and shade out native vegetation. This reduces species diversity, alters natural ecosystems, and negatively impacts wildlife habitat. The ground under knotweed thickets tends to have very little other growth. This bare soil is very susceptible to erosion, posing a particular threat to riparian areas. Once established, populations of Japanese knotweed are extremely persistent and hard to eradicate.

I was so concerned at having walked near this fiend at the fishing camp that I washed our boots off before placing them in the car. Is that plant racism or the simple acknowledgment that noxious plants often spread by human movement?

A clematis with a white flower also grew along the path, and I immediately identified it as invasive Japanese clematis (Clematis terniflora)—having been influenced by the mugwort and knotweed nearby. But, though abundant, it was not overtopping the solidago to which it clung, as many invasives do. Based upon that behavior and its earlier flowering time, I decided (as my host had insisted all along) that it was more likely native virgin’s bower (Clematis virginiana), a lookalike.

Incidentally, a recent paper in the Annual Review of Ecology, Evolution, and Systematics entitled “Observed and Potential Range Shifts of Native and Nonnative Species with Climate Change” makes a further case for wariness with regard to introduced species, thus:

There is broad concern that the range shifts of global flora and fauna will not keep up with climate change, increasing the likelihood of population declines and extinctions. Many populations of nonnative species already have advantages over native species, including widespread human-aided dispersal and release from natural enemies. But do nonnative species also have an advantage with climate change? Here, we review observed and potential range shifts for native and nonnative species globally. We show that nonnative species are expanding their ranges 100 times faster than native species, reflecting both traits that enable rapid spread and ongoing human-mediated introduction. We further show that nonnative species have large potential ranges and range expansions with climate change, likely due to a combination of widespread introduction and broader climatic tolerances. With faster spread rates and larger potential to persist or expand, nonnative populations have a decided advantage in a changing climate.

As climate change accelerates (three worldwide heat records fell just this week), many native species will not keep up. Most arthropods and animals higher on the food chain cannot evolve in time to find adequate nourishment from non-native plants, especially dominating invasives. It doesn’t help to pretend otherwise.

Having spent ten days away from the string trimmer, I arrived home with work to do. Invasive mugwort, porcelainberry, and Japanese stilt grass are coming on strong, and I don’t intend to show them any mercy. How about you?

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This native brown-eyed Susan (Rudbeckia triloba), growing at the edge of the wet woods, looks so specimen-like that you’d think I planted and nurtured it, but it’s totally wild:

Native grass-leaved goldenrod (Euthamia graminifolia) has begun to bloom in the barn meadow:

Native Allegheny monkeyflower (Mimulus ringens) blooms near the bottom of the big pond embankment:

Brown knapweed (Centaurea jacea), a native of Europe, looks pretty by the big pond:

Native phlox sparkles in front of the pawpaw woods by the small pond (yup, invasive Japanese stilt grass in there, too):