12/13/2025
Importance of predators
Yellowstone is seeing something it hasn’t witnessed in nearly a century: young aspen trees finally growing into real forest again. For decades, the ecosystem was stuck. Aspens sprouted every spring but never made it past a few inches because elk and bison ate them down year after year. By the late 20th century, you could walk through entire valleys filled with tiny shoots but almost no young trees gaining height. The forest was aging without replacement, and scientists feared the collapse was permanent.
The turning point came in 1995 and 1996, when wolves were reintroduced. What followed wasn’t instant, but it was sweeping. Elk numbers shifted, their movement patterns changed, and the relentless browsing pressure that had suppressed aspens for more than 70 years finally eased. With fewer elk staying in one place for long periods, more aspen shoots survived long enough to rise above browsing height — the critical six-to-seven-foot threshold that determines whether a sprout becomes a tree.
Now, almost 30 years later, ecologists returning to long-monitored sites are finding young aspens where none existed for generations. In a new survey of 87 aspen stands, roughly a third now contain tall, healthy saplings pushing toward the canopy — a dramatic contrast to the 1990s, when researchers documented zero. It’s one of the clearest examples of a trophic cascade ever recorded: restoring a top predator triggered changes that rippled through elk behavior, vegetation, and entire habitat systems.
The story isn’t perfect. Some areas are still struggling, especially where growing bison herds continue to browse young trees, and where snow, drought, and other predators shape elk movement in complex ways. But the pattern is unmistakable: where browsing pressure drops, aspens rise. Where wolves influence the landscape, forests begin to rebuild.
After nearly a century of stalled regeneration, Yellowstone’s aspens are growing again — not because humans planted them, but because an ecosystem regained a missing piece and began to correct itself.