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Wolves Have Stabilised Yellowstone’s Ecosystem 25 Years After They Were Reintroduced


A study spanning more than 20 years has found the reintroduction of wolves into Yellowstone National Park helped stabilise the ecosystem. 

Wolves, along with other predators such as grizzly bears, black bears and mountain lions, thrived in the park before the 1900s, but a coordinated campaign by the federal government led to the extermination of nearly all of those that lived in the area.

The US National Park Service and the state of Montana controlled elk populations by killing them between 1932 and 1968, but due to the lack of predators in the area populations rose dramatically when the killings stopped. In the following decades elk populations saw both booms and collapses, their numbers fluctuating along with the climate.

Wildlife officials reintroduced 41 wolves to Yellowstone between 1995 and 1997, and grizzly bears and mountain lions increased due to more protections from states and the federal government.

Over the last 20 years, Chris Wilmers, a wildlife ecologist at the University of California, and Doug Smith, Yellowstone’s senior wildlife biologist, analysed the wolves and the elk deaths in Yellowstone, which is located mostly in Wyoming but also spreads into parts of Montana and Idaho.

The scientists tracked wolf packs for a month at the beginning and end of each winter and located every elk the wolves killed, noting the animal’s age, sex and physical condition before death. The team also kept track of how much plant life was available for the elk each year, dependent on climate.

Their findings, published in the Journal of Animal Ecology, found that during years with normal amounts of rain and snow wolves primarily killed older cow elk, as they’re easiest to hunt.

In more dry years however, the wolves switched to hunting bulls, in turn giving the cows a chance to reproduce and allowing the population to stay afloat. For the past 12 years, elk numbers in the park’s largest herd have levelled off between about 6,000 and 8,000, National Geographic reports.

Wolves hunt bulls in dry years because the males are particularly diminished due to energy exerted in autumn, when they focus on fighting other males instead of eating.WolfPixabay

The wolves, which now have a population between 300 and 350, could also help elk herds withstand a more volatile climate because elk herds that maintain consistent numbers can better withstand more frequent droughts.

In the winter of 2010 to 2011, for example, the elk had to deal with abnormally deep snow and cold temperatures, and they managed to fare well compared with the mass deaths seen during similar winters in the 1980s and 1990s.

Commenting on the findings, Smith said:

These systems are better evolved or better adapted to that way of life than elk starving to death. What elk starving to death means is they’re eating themselves out of house and home.

In a future that will be very unpredictable, we want a buffer… humans help stabilise elk populations, but they don’t do the same thing as wolves.


The study will be relevant to future initiatives such as an upcoming vote that will see Colorado residents decide whether to reintroduce wolves to their state, home to about 287,000 elk – the largest number in the US.