♪ MUSIC ♪ MILES O’BRIEN: Yellowstone National Park–where the bison roam, the elk graze, and hot springs bubble to the surface. [WOLF HOWLING] Loathed as a threat and nuisance, the wolf population in Yellowstone was essentially wiped out by the mid 1920s. That changed in 1995, when the National Park Service reintroduced them here. So, 20 years on, how has the Yellowstone ecosystem responded to the return of the wolves? That’s what brings Utah State University wildlife ecologist Dan MacNulty to this dirt road, near Yellowstone’s fabled Roosevelt Arch. [DAN MACNULTY TALKING] MILES O’BRIEN: With support from the National Science Foundation and working in partnership with the National Park Service, MacNulty and his team are hot on the trail of the wolves’ primary prey: elk. DAN MACNULTY: We’re interested in understanding how wolves are affecting the numbers of elk and how that changes over time. And then we’re also interested how wolves are affecting the behavior and the movement of elk. MILES O’BRIEN: Today, more than 4,500 elk graze Yellowstone’s Northern range. That’s down from a peak of about 18,000 before the wolf reintroduction. Just under 100 elk are outfitted with radio collars, all of them females. DAN MACNULTY: So, they are the reproductively key part of that population and that’s why we’re focusing on them. [TEAM MEMBER COMMENTING] MILES O’BRIEN: MacNulty and his team track them closely, especially those that have calves. It turns out that for much of her life, a female elk is actually highly unlikely to be killed by a wolf. Wolves tend to go for the easy pickings. DAN MACNULTY: Soon after an elk is born, roughly 1 year of age, they’re highly vulnerable to wolf predation because they’re small. They’re easy for wolves to capture. Elk that are between around 2 and 10 years of age, female elk are largely invulnerable, so they can survive attacks by wolves because they’re aggressive. MILES O’BRIEN: When a collar indicates an elk hasn’t moved for a while, that’s an indication she might be dead. MacNulty or one of his Park Service colleagues always check it out. If a wolf killed her, they want to know. DAN STAHLER: We can hike in to that area, and in a very timely fashion, usually the next day, we try to follow up on these signals of mortality and we can investigate the area. If we find a dead elk and we find enough evidence, we can determine cause of death. MILES O’BRIEN: Biologists with the Wolf Project track the wolves using collars as well. They say that, for now anyway, the wolf population in Yellowstone seems to be holding steady at roughly 100 animals or so. DOUG SMITH: Since about 2008, our wolf numbers have been fairly flat, stable and, so, how long is that going to last? I think where we’re at now is pretty much what we expected 20 years ago, and unless there’s some kind of a major change, of course, climate change is a huge wrench in everything–we think this could be some kind of long-term equilibrium. MILES O’BRIEN: And MacNulty’s research so far suggests the elk population may have stabilized for the time being, too. DAN MACNULTY: So, there’s all kinds of natural brakes on the predation process that prevents what we would refer to as runaway predation. Killing is a very dangerous difficult activity for wolves, and that’s a fairly underappreciated fact. MILES O’BRIEN: Fuller understanding of what’s happening here could translate to better predator management decisions all over the globe. Good thing scientists are keeping an eye on things. For Science Nation, I’m Miles O’Brien.