Here is a good article about the benefits of cougar populations in Zion National Park. The deer have nearly eliminated cottonwood reproduction in Zion Canyon, the main tourist attraction. It is full of people and deer. The large number of people in this congested canyon have scared away the cougar. There are a huge number of tame deer, which anyone who has been there has seen. There are no cottonwood seedlings, few flowers, and not many other species either except wild turkeys.
In nearby, unvisited canyons the biodiversity is much greater due to the indirect effects of the presence of cougar.
The hypothesis, called the “Ripple effect” is still controversial. It is named after OSU Professor William Ripple’s hypothesis that the presence of large predators creates a “landscape of fear” among ungulates, serving to keep them from eating so much in riparian zones. The improved conditions in riparian zones ripples throughout the ecoystem leading to many important secordary and third level changes.
Wolves have been reported to be having a similar effect in Yellowstone on aspen, willows, and cottonwood.
Large carnivores promote healthy ecosystems by keeping browsers on edge From Terra, the world of research at Oregon State University.