The scientific intrigue centers on the degree to which carnivores have an indirect effect on other fauna and flora, a dynamic scientists call a \u201ctrophic cascade.\u201d Beginning in the early 2000s, several studies appeared to show evidence of a transformational relationship between wolves, elk and riparian vegetation in Yellowstone.\u00a0<\/p>\n
But as more time has passed, other studies have suggested that Yellowstone wolves\u2019 connection to the park\u2019s riparian ecosystems may be more subtle than previously believed.\u00a0<\/p>\n
\u201cIt\u2019s not that there\u2019s not evidence consistent with a trophic cascade in Yellowstone,\u201d said Chris Wilmers, a professor of wildlife ecology at the University of California Santa Cruz, and the paper\u2019s lead author. \u201cIt\u2019s that the effects are a lot more complicated and weaker than what was initially thought.\u201d<\/p>\n
The theory of wolves changing the landscapes and vegetation along streams and rivers in Yellowstone was born out of the changes scientists observed in the park after human hunting decimated beaver and wolf populations. More elk and other ungulates dominated the landscape, grazing riparian vegetation on the open banks of streams whose dams were no longer as robustly maintained by beavers. Water began moving through the park more rapidly, carving riverbeds into deep v-shapes and lowering the water table. This, in turn, diminished over-grazed riparian vegetation habitat, leaving less material for any remaining beavers to dam the waterways. All this caused a negative feedback loop that dried out areas around the park\u2019s streams.<\/p>\n
When wolves were reintroduced to Yellowstone, some<\/a> scientists<\/a> reported<\/a> that they altered elk grazing patterns, causing them to become skittish about spending prolonged periods in open riparian habitat. This helped willow and aspen trees along stream banks regenerate, perhaps an early indication of a trophic cascade, one that could give beavers more material to slow flows and reduce waterside erosion.<\/p>\n But other scientists<\/a> have questioned<\/a> these theories, and for the new paper, Wilmers and four other researchers expanded the focus from wolves to include pumas and bears, other large carnivores that are recovering from the brink of human-driven extinction across the West.<\/p>\n In an analysis of about 170 citations published since the 1930s, Wilmers and his team found clear evidence of predator-induced trophic cascades in only limited circumstances. On Isle Royal National Park<\/a>, a nearly 207-square-mile chunk of land in northwestern Lake Superior, for instance, wolves diminished moose populations to the point that trees grew taller. But in Yellowstone, a more than 3,000-square-mile park in the heart of one of North America\u2019s most intact ecosystems, the connection between the predator\u2019s reintroduction and changes in vegetation has generated more debate.\u00a0<\/p>\n In 2024, researchers at Colorado State University published a 20-year study<\/a> finding that willows fenced from grazers near simulated beaver dams in Yellowstone grew faster than both unfenced willows near dams and fenced or unfenced willows without dams, suggesting wolves\u2019 presence alone may not be enough to improve the park\u2019s riparian habitat. Without more beaver activity and healthier riparian vegetation, Yellowstone appeared locked in an \u201calternative stable state,\u201d in which eroded stream banks and lower groundwater levels persist on a dry landscape, \u201ccaused primarily by the extirpation of apex predators during the early 20th century,\u201d the researchers concluded.<\/p>\n Other research has shown that, since wolves returned to the park, human hunting<\/a>, puma recovery<\/a> and grizzly predation on calves<\/a> have also influenced elk populations, and growing bison herds may also account for diminished vegetation heights. Even irrigated fields<\/a> outside the park could be influencing elk behavior.<\/p>\n Without a clear link between wolf predation and the decline of elk populations, the foundation for a scientific determination of a trophic cascade is too shaky to build upon, Wilmers concluded in the paper.<\/p>\n He said he was surprised by how few studies show evidence of wolves, bears and cougars having an effect on elk, moose and deer populations. Instead, the biggest driver of changing elk population numbers across the West is humanity.\u00a0<\/p>\n \u201cIn most mainland systems, it\u2019s only when you combine wolves with grizzly bears and you take away human hunting as a substantial component that you see them suppressing prey numbers,\u201d Wilmers said. \u201cOutside of that, they\u2019re mostly background noise against how humans are managing their prey populations.\u201d\u00a0<\/p>\n In some studies, ungulate populations actually increased slightly in the presence of wolves and grizzlies, Wilmers said, likely because human wildlife managers overestimated the effects of predators as they reduced hunting quotas.\u00a0<\/p>\n This story is funded by readers like you.<\/p>\n Our nonprofit newsroom provides award-winning climate coverage free of charge and advertising. We rely on donations from readers like you to keep going. Please donate now to support our work.<\/p>\n Donate Now<\/a> <\/p>\n![\"A](\"https:\/\/www.newsbeep.com\/ca\/wp-content\/uploads\/2025\/11\/Grizzly_YellowstoneNP.jpg\"\/)
A grizzly bear roams through Yellowstone National Park. Credit: A. Falgoust\/NPS<\/p>\n
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