Among the problems are bison herds that find natural wintering grounds outside Yellowstone and decimation of the native cutthroat trout population by exotic lake trout.
The three spoke at a convention in West Yellowstone, Mont., celebrating the 30th anniversary of the conservation organization. They said park managers alone may not be able to defend Yellowstone adequately and support from outsiders is critical.
Barbee, who served from 1982 to 1994, said, "things are in better shape today than they ever have been."
Barbee said grizzly bear numbers have increased and wolves are back as part of the landscape in the years since he first served as superintendent.
Finley served from 1994-2001, and Wenk has been in the job since 2011.
Wenk agreed with Barbee about improvements with grizzlies and wolves, but said Yellowstone "still has a number of problems."
Wenk said the National Park Service has not decided how to cope with global warming, Wenk said. He said grizzlies remain a threatened species as key foods have dwindled for a variety of reasons.