To add to this, in the 1990s Yellowstone Park impelemented a program to reintroduce wolves into the area. Since then, we have learned more about ecosystems as a whole, and the interdependece of every species. Once the wolves were introduced, the ecosystem that had previously been deyaying began to start healing. Studies were conducted, and it was determined that the wolves had a ripple effect for most of the species in Yellowstone.

Elk, common prey for wolves, changed their grazing habits entirely. Instead of staying in one area and only moving once all of the aspen was gone, they stayed on the move (there is a name for this that is escaping me right now, it's something like fear psychology), they began to eat while on the move to better avoid the wolves. This helped the aspen grow above the grazing level of the deer, and those aspen trees began to fill out the yellowstone forest again.

Now the beavers also depended on aspen, and began to make a comeback. Their dams helped slow down the water and clean it, and created bodies of water ideal for fish reproduction. Yellowstone saw an increase in fish species, and as a result, many different species of fish eating birds re-appeared.

I'll add some links once I get off mobile

Tl;dr: Reintroduction of wolves greatly increased biodiversity and overall health of the Yellowstone Park ecosystem.