Invasive Plant Drives Ecological Change 

By Lyle Lewis / Mongabay

The Selway–Bitterroot Wilderness was part of the original class of lands designated under the United States’ 1964 Wilderness Act: 1.3 million acres, or about 526,000 hectares, of steep river canyons, cold subalpine ridges, dense forest, and weather so unforgiving it shapes everything that survives there. It remains one of the most remote places in the continental U.S.

The Selway offers a window into a broader global pattern: ecosystems that appear intact from afar are already being structurally reshaped by invasive species, climate change and hidden biodiversity loss. The idea of “untouched wilderness” persists in our imagination, but ecologically, it is no longer true.

Its remoteness is literal. Trails disappear under the brush that regrows faster than crews can cut it. Fire and blowdowns reshape entire drainages in a year. Maps may show routes, but the land often says otherwise. Without intervention, trails vanish, reclaimed by vegetation and gravity.

I know this because I’ve spent days clearing them. By 2016, roughly half the trails shown on topographic maps were already impassable, in a failure for recreation but a quiet triumph for wildlife. There are square miles of habitat here that likely haven’t seen a human in decades.

The Selway River is one of the most technically demanding whitewater runs in North America. Rafts flip, boats wrap, and accidents turn fatal almost every year. People call it wilderness because it feels like one. But feeling wild and intact functioning are not the same thing.

Wilderness held together by weather

The Selway’s remoteness is sustained not just by distance but by precipitation. North of the River of No Return Wilderness, the Selway receives far more moisture. Vegetation grows thick and fast, trails vanish in months, weather creates inaccessibility, and inaccessibility creates the illusion of intactness.

But that illusion is thinning.

I first came here in the mid-1990s to conduct wildlife surveys, and returned between 2010 and 2015 for multiday stretches each year. In 2016, I spent three months as caretaker on one of three private inholdings inside the wilderness — the same site where the U.S. Fish and Wildlife Service conducted “soft” releases of gray wolves in 1995. The woven-wire pens still stand, more artifact than infrastructure, fading into brush.

Before wolves returned, elk densities were high. After reintroduction, wolf numbers rose for roughly 15 years while elk and deer declined. About a decade ago, wolf numbers fell alongside their prey. Today, the system has settled into a dynamic equilibrium that benefits many birds, insects, small mammals and native plants.

But unlike Yellowstone, it isn’t wolves or ungulates driving the most dramatic changes here.

It is something far smaller and easier to overlook: a lavender-flowered invader spreading through meadows, ridgetops, and the dim understory of the forest.

Rewriting the food web

Thirty years ago, spotted knapweed (Centaurea stoebe) crept in from private inholdings and from hay carried by packhorse hunters. Few noticed until it was impossible to ignore. A biologist I knew was spraying it from horseback in the early 2000s, trying to hold the line. It didn’t matter.

When I worked here 15 years ago, it spread mainly along lower-elevation trails. Now it dominates large areas.

This past November, a month when snow should bury the high country, there was no snow at all. Not in the canyons. Not on the ridges. In warm light, spotted knapweed was blooming everywhere.

Pulling it out is an education in futility. Anchored by a deep taproot, it rarely comes out clean, snapping below the crown and resprouting. I spent hours removing a patch no larger than 10 square feet (less than 1 square meter). Each plant felt engineered for persistence. What struck me most was where it thrived.

Spotted knapweed repeatedly colonized small fern microhabitats, pockets of moisture where native ferns trap humidity and buffer temperature swings. These microclimates evolved to help native seedlings survive, but now they provide ideal germination beds for an invader.

In a few inches of soil, the story becomes visible: native plants building stability, knapweed exploiting it, and climate change amplifying the latter’s advantage.

And knapweed is not merely a competitor; it is a slow-motion trophic cascade.

It suppresses native forbs, reducing nectar for pollinators. Fewer insects mean fewer birds and bats, and fewer prey for predators. Its taproot alters soil moisture and nutrient cycling. Small mammals lose plant diversity. Ungulates lose forage diversity. Mycorrhizal networks shift. The deeper impact is simplification.

Soils have finite biological productivity and limits to how many interactions they can support. When one highly adaptive plant converts a living mosaic into pseudo-monoculture, effects reverberate through the food web. What appears wild begins functioning more like a simplified system dominated by a single species.

In many places, knapweed is already competing with brush and likely precluding tree seedling establishment, altering forest regeneration for decades.

Extinctions in a place few are watching

Even here, knapweed is almost certainly driving extinctions we will never detect. Soil ecosystems hold thousands of species — nematodes, springtails, micro-annelids, root-associated beetles, mycorrhizal fungi, moss and lichen specialists — most undescribed and unmonitored.

These organisms live where biodiversity truly resides: the top inch of soil, the underside of a rotting log, the moist duff beneath a fern. When knapweed converts diverse understories into simplified stands, many micro-specialists disappear.

There are no surveys and no headlines.

From a distance, the Selway still looks intact. But at the level of its living fabric — the layer supporting insects, birds, amphibians, mammals and forest regeneration — losses are underway.

We often speak of wilderness as if it stands apart from the world we are changing. But remoteness no longer insulates ecosystems from global pressures.

Spotted knapweed does not care that a trail lies 25 miles (40 kilometers) from the nearest road. Warming winters do not stop at wilderness boundaries.

The Selway–Bitterroot remains beautiful and ferocious. But it is also a microcosm of a connected planet — a system under stress, shifting faster than many species can adapt.

Standing here in November — a month that no longer behaves like itself — it became clear that one of the wildest landscapes remaining in North America is changing rapidly, even as cultural narratives continue to frame wilderness as untouched.

If we continue to treat wilderness as pristine, we will miss the most consequential ecological changes unfolding within it. Recognition is not pessimism. It is prerequisite to protection.

Lyle Lewis is a former U.S. Fish & Wildlife endangered species branch chief with more than three decades of experience in wildlife conservation and ecological policy.