YELLOWSTONE NATIONAL PARK, Wyo.  The grassy sweep of the Lamar Valley in the northeastern corner of this park is famous for its wildlife, especially its vast herds of elk and bison and the wolves that hunt them.

But while walking across the Lamar last fall, Robert L. Crabtree, chief scientist with the Yellowstone Ecological Research Center in Bozeman, Mont., pointed out a cascade of ecological changes under way. The number of grizzly bears and gophers in the valley has increased, Dr. Crabtree said, an increase supported by the spread of an invasive plant from the Mediterranean that a warming climate benefits.

“It’s the early stages of a new ecosystem,” he said, “one that hasn’t been seen here before.”

The plant, Canada thistle, provides food for grizzlies in more than one way but may also be squeezing out native plants that cannot compete.

Canada thistle first appeared in North America several hundred years ago and has been present in Yellowstone at least since the 19th century, Dr. Crabtree said. Because of its extensive root system, the plant defies spraying, and park officials have largely abandoned efforts to control it. Warming temperatures have helped its fortunes.