That elk migrate in and out of Yellowstone every year in vast herds is more than a triumph of movement.

Every spring, tens of thousands of elk divided among nine different herds traverse perilously rocky terrain, swim across fast-moving rivers, and scale precipitous mountain slopes. They do what researchers refer to as "surfing the green wave" in search of freshly sprouted grass. Their healthy, active movement acts as the life-blood for the greater Yellowstone ecosystem.

As they become more abundant, so do their predators. And with a healthy balance of predators, Yellowstone thrives.

In fact, elks' existence within Yellowstone's ecosystem is so critical that the Crow Nation native to the region originally named the river flowing through its boundaries Elk