Like wolves, non-native lake trout have radically altered Yellowstone ecosystems (commentary)

Yellowstone National Park is often told as a story of recovery. Wolves returned and then elk changed their behavior. Willows and aspens then rebounded and rivers, it is said, changed course. The reintroduction of wolves in 1995 is one of the most familiar ecological narratives in the world.

But the Yellowstone ecosystem was not waiting in a stable state for wolves to return, it had already been reorganizing for centuries. Beavers declined following widespread trapping, altering vegetation and hydrology. Grizzly bears were heavily persecuted across the region, reducing another major connector between aquatic and terrestrial systems. Bison were reduced to near extinction in the late 19th century and later rebuilt under protection. Large predators were removed in the early 20th century, and elk populations expanded in their absence. Each of these changes altered how nutrients moved and how water, plants, and animals interacted.

Wolf reintroduction did not occur in isolation; it entered a system already in motion. At almost the same time, something else was happening. It did not involve a visible predator and did not occur in valleys or along rivers. It did not lend itself to photography or documentary.

It happened beneath the surface of Yellowstone Lake.

For many people, the idea of a non-native predator reshaping an ecosystem is not abstract. The Burmese python in the Florida Everglades provides a clear example, with an introduced predator reducing prey populations and altering how energy moves through the system.

The same process can occur in water: Lake trout function as a non-native predator in Yellowstone Lake, but their effects are less immediately visible. Rather than reshaping only the visible landscape, they alter how energy and nutrients move through the ecosystem, beginning deep beneath the lake’s surface.

Lake trout are widely believed to have been illegally introduced into Yellowstone Lake in the 1980s, though some research has suggested that connected waterways during high-water events may also have provided pathways for movement within the broader watershed.

Unlike native Yellowstone cutthroat trout, which occupy shallow waters and migrate into tributary streams to spawn, lake trout remain deep. They are efficient predators and eat cutthroats, but their ecological significance is not just that they eat these fish. It is also that they concentrate the ecosystem’s nutrients in deep waters instead of moving them onto land.

Historically, cutthroat trout moved that energy outward, in spawning runs that pushed their biomass from the lake into shallow waters and up tributary streams in late spring and early summer. The fish concentrated in narrow corridors of the landscape, and in early summer, spawning trout created a short but important food pulse for grizzlies, particularly near Yellowstone Lake.

Otters and mink also hunted trout in the shallow water, while eagles and ospreys gathered: Aquatic insects cycled with these seasonal pulses, too. So, nutrients that originated deep in the lake spread outward into streams and surrounding landscapes. And as cutthroats moved into small streams to reproduce, their making of nests (“redds”) to lay eggs in stirred up gravel and released nutrients that fed aquatic life.

But when the lake trout population expanded, that movement collapsed as cutthroat trout declined by roughly 90%. Nutrients that once moved annually from Yellowstone Lake’s nearshore waters into tributaries instead remained trapped in deeper offshore waters.

Energy that had spread across shorelines, streams, and riparian systems became less available to the species that depended on those seasonal pulses. Osprey pairs nesting around Yellowstone Lake fell by approximately 85% over the same period, reflecting the loss of surface feeding cutthroats that once concentrated along shorelines and tributaries and which often fell prey to the raptors.

While the lake did not stop functioning, it began to function differently.

Energy was held deeper and connections to the surrounding landscape thinned. The lake-to-land transfer that once redistributed nutrients across tributaries and riparian corridors diminished. Bird populations declined. Aquatic insect communities reorganized. Disturbance in spawning streams decreased. What had been episodic, pulsed systems became flatter and more continuous. The streams did not disappear, but they did not function the same way.

Grizzly bears are often used as indicators of ecological change in Yellowstone, but even here, the drivers overlap. Alongside the decline of cutthroat, the loss of whitebark pine has reduced another key high-energy food source, while climate variability is also affecting the availability of army cutworm moths in alpine environments. Grizzlies historically depended on this suite of dense, seasonal foods — trout, moths, and whitebark pine — each concentrating energy and becoming available at different times of year.

At the same time, wolves were changing elk behavior. When wolves were absent, elk often calved in concentrated areas, allowing bears to search efficiently and obtain substantial energy in a short period. Following wolf reintroduction, predation risk increased and calving became more dispersed across the landscape. The calves did not disappear, but their energy became harder to find, harder to capture, and in many cases, it was no longer metabolically profitable.

These pressures overlap. In different ways, wolves and lake trout have both reduced concentrated energy available to bears.

While wolves are visible and their effects can be photographed, tracked, and narrated — and the return of this predator fits a familiar arc: Loss then restoration and response, and feels complete — the lake does not offer that kind of story. Its changes unfold below the surface, distributed across species and processes that do not announce themselves.

The ecological consequences emerge as absences — fewer otters, fewer eagles, reduced nutrient movement — and absence is harder to see. There is no clear before and after, no dramatic visual marker. No single moment when the system visibly shifts.

Wolves produced measurable responses across multiple species. In some riparian areas where elk browsing pressure declined, willow growth increased, supporting greater abundance of riparian songbirds. Carrion from wolf kills subsidized scavengers, and some localized increases in beaver activity followed. These changes are often presented as evidence of ecological restoration, but they occurred in a system already reorganized.

Aquatic insect communities likely simplified as cutthroat spawning and nutrient pulses weakened. The songbirds that have since increased — species such as common yellowthroat, warbling vireo, and song sparrow — are among the more flexible and generalist riparian birds, whereas earlier aquatic conditions may have supported a broader suite of species and interactions than the simplified system visible today.

So, what appears as recovery may instead be a constrained response, an ecosystem reorganizing within narrower limits. Yellowstone has been reshaped repeatedly, and these changes do not stop after a decade or even two. Some unfold over centuries. Others slow and then accelerate again when new pressures arrive. Predator removal, ungulate expansion, beaver decline, grizzly persecution, wolf reintroduction, lake trout introduction, climate-driven food changes — each is layered onto a landscape already in transition.

What emerged was not a single transformation, but overlapping ones. The wolf literature reached consensus that wolves mattered, but not that they alone restored Yellowstone, while the lake trout literature concluded that the lake itself had been fundamentally reorganized — a change that rarely entered the broader Yellowstone narrative. Both reshaped the system, but change is often attributed to only one.

This is not a critique of wolves but a recognition of how we see. Humans are drawn to presence, but often struggle to recognize absence. We see wolves returning to the landscape, but do not see native cutthroat trout being replaced by a very different predator beneath the surface.

We see animals moving through valleys. We do not see connections breaking between lakes, streams, and the surrounding landscape. We tend to build our understanding of ecosystems around what is most visible, most intuitive, and most easily explained. What happens out of sight is harder to hold.

Ecosystems are not organized around presence alone. They are organized around connection. When those connections are altered — whether by the removal of a predator or the collapse of a fish population — the effects do not remain isolated. They move. They accumulate. They reshape ecosystems in ways that are not always immediately apparent.

Yellowstone cutthroat trout have increased in recent years, but only under continued intervention, while lake trout suppression requires millions of dollars annually and has been described by managers as a long-term, possibly perpetual commitment. And if effort declines, lake trout will rebound. Suppression efforts target deep spawning areas, but a small fraction of lake trout may persist in alternative habitats, making long-term eradication unlikely. The system is, therefore, held in limbo — somewhere between collapse and an echo of what once existed — depending on the intensity of intervention.

Yellowstone Lake is a microcosm of a much larger pattern. Salmon and steelhead once moved enormous quantities of nutrients from the ocean deep into the continent as they migrated inland to spawn. Yellowstone cutthroat trout performed a similar function, just at a smaller scale.

Dams and agriculture have done at a continental scale to salmon what lake trout did within the Yellowstone ecosystem. Yellowstone changed not only in the presence of wolves, but in the absence of fish.

We did not miss what happened. We just told a different story. And beneath that story, the lake remains — still there, still functioning, but no longer what it was.

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.

See related coverage:

How one woman’s wolf ‘moon shot’ changed Yellowstone forever: Interview with director Tom Winston

Yellowstone’s wolves defied extinction, but face new threats beyond park’s borders

Citations:

Koel, T. M., Bigelow, P. E., Doepke, P. D., Ertel, B. D., & Mahony, D. L. (2005). Nonnative lake trout result in Yellowstone cutthroat trout decline and impacts to bears and anglers. Fisheries, 30(11), 10-19. doi:10.1577/1548-8446(2005)30[10:nltriy]2.0.co;2

Baril, L. M., Smith, D. W., Drummer, T., & Koel, T. M. (2013). Implications of cutthroat trout declines for breeding ospreys and bald eagles at Yellowstone lake. Journal of Raptor Research, 47(3), 234-245. doi:10.3356/jrr-11-93.1

Cite this article

Lyle Lewis (2026). Like wolves, non-native lake trout have radically altered Yellowstone ecosystems (commentary). Mongabay Conservation news. DOI: https://doi.org/10.66709/news-322890