There is no doubt that the Northern Yellowstone elk herd has declined in numbers, BUT, and that is a big BUT, that herd size is down from a record size that the northern range could not support long-term. Other herds, such as the Dome Mountain Herd, are growing and are at near record size. The Yellowstone Bison population is well over the preferred carrying population for the ecosystem.
Today there is a study out that has concluded that the Grizzly population may be overstated due to some changes in how the bears are counted. Not surprisingly, this study was commissioned by groups who are interested in keeping the Grizzly on the endangered species list.
In my opinion, the absolute most important consideration when counting animals is who is paying for it. I can pretty much guarantee that any count or study will always support the positions of the people who commissioned and paid for that study or count. These studies will take all possible precautions to prevent the facts from getting in the way of their desired results.