How Reintroducing Wolves Changed Yellowstone National Park

Mark Altaweel


On January 12, 1995, gray wolves were reintroduced to Yellowstone National Park, which is the world’s oldest national park, after decades of likely absence. This reintroduction began one of the most successful wildlife reintroduction programs in the US.

Since 1995, wolves in Yellowstone have been tracked and studied, while the impact on plant and animal life could now be quantified. This also provides valuable lessons for other animal reintroduction efforts that attempt to re-balance fauna and between predator and prey.

How many wolves are in Yellowstone?

Almost all of the gray wolves in Yellowstone are descended from the 41 wolves introduced between 1995-1997.

According the 2022 Yellowstone Wolf Annual Report, there were 108 total wolves living in 10 packet in Yellowstone National Park: 68 adults and 40 pups. This is up from 97 wolves living in 8 packs reported in 2021 but down from 123 wolves in nine packs reported in 2020. Since 2009, the total population of wolves in Yellowstone ecosystem has ranged from 83 to 123.

Free weekly newsletter

Fill out your e-mail address to receive our newsletter!

A brown wolf with yellow eyes stands in the snow in front of evergreen trees.
A wolf in Yellowstone National Park. Photo: Jim Peaco/NPS, public domain.

Studying the Yellowstone wolves

The wolves today in Yellowstone are intensively studied because this population is seen as an example how wider ecosystems are impacted by the reintroduction of a top predator. Wolves have been studied by fitting VHF (very high-frequency) radio collars around them.

In addition, throughout the park bison and elk are observed to see how hunting behavior affects populations of these prey animals.

A group of young wolf pups on a rock in the middle of a grassy area.
A group of wolf pups huddled on a rock in Yellowstone. Photo: Dan Stahler, NPS, public domain.

Overall, it has been determined that the vast majority of prey are elk and wolves have continued a preference for elk even when numbers of this animal significantly diminished below that of bison.[1]

How the hunting behavior of wolves has impacted other species

The hunting behavior of wolves has been very important in impacting not only the number of prey populations, such as elk, but other species. For instance, beavers, which were rare, have at least 9 major colonies around Yellowstone today, when in previous decades there was only 1.

In fact, it is not just beavers but species small and large, from beetles to lynx, wolverine, and others that have been impacted. For the most part, many of these species have rebounded.

Trophic cascade

With elk populations being artificially high due to no or limited wolves before, elk had overrun Yellowstone and displaced many animals due to their feeding behavior. Plants used by various species were eaten by elk, while elk trampling affected what species could grow.

An elk with large antlers stands in front of five wolves on a high with brown grass.
A pack of wolves faces off against a bull elk on a ridge in Yellowstone. Photo: Doug Smith, NPS, public domain.

Researchers argue that with less elk, and elk having to worry about wolves, elk stay away from tree species, such as willow and aspen, while disturbing the habitats of others animals far less.

Less trampling and feeding in certain areas has changed even the physical geography as the ground has stabilized in places, minimizing erosion and enabling vegetation to grow in different places. This has then led to a dramatic change in Yellowstone, making the park far more ecologically diverse.

In fact, the changes caused to many animal and plant species is called a trophic cascade because a variety of plant and animal species have been impacted by the reintroduction. This has made the park generally healthier for animals as there are more diverse food sources throughout the year.[2] 

Wolf tracks in the snow on a sunny day with blue sky, a house, and trees off in the distance.
Wolf tracks in the snow near Old Faithful Geyser, Yellowstone. Photo: Diane Renkin, NPS, public domain.

However, some scientists have disputed the true impact of wolves. For instance, willow trees are likely to be more positively impacted by simply reintroducing beavers.

Wolves have only had a very minor benefit to beavers, and beavers are still rare. Beavers directly benefit willow trees because their dams slows water flow, which benefits willows that grow along banks of streams.[3]

Nevertheless, the generally successful impact of reintroduction of wolves at Yellowstone has inspired other efforts and most scientists agree many species have benefited from wolf reintroduction.[4]

Tracking wolves in Yellowstone

In tracking wolves and even other animals, there have been three general options for researchers. These are VHF radio tracking, satellite tracking, and global positioning systems (GPS).

While satellite and GPS tracking are often used for a variety of animals, and have become more common and could be considered cost-effective, VHF radio-tracking is still a standard technique that has been in use since 1963.

A wolf leaving a crate with a tracking collar on in front of a chain-linked fence.
A Yellowstone wolf with a radio tracking collar in 1996. Photo: Jim Peaco, NPS, public domain.

One critique of VHF is it is considered intrusive since animals need to be live-captured to attach and/or retrieve the collar. The benefit of the VHF system is that it can enable one to study an animal for a long time, often up to the lifetime of an animal.

There is also a longer history of VHF collars used, which means culturally for researchers this is what they are use to and historical data can be easily compared. While there has been a tendency to move towards GPS tracking, historically studied populations, such as the wolves of Yellowstone, have been primarily studied with VHF collars.

This does give us a long running dataset, spanning decades and enabling us to look at historical changes for wolf populations at Yellowstone.[5]

The reintroduction of wolves to Yellowstone has been mostly positive

While the true impact of wolves on Yellowstone is still somewhat debated, scientists generally agree that the reintroduction result has been mostly positive regarding a number of species, small and large, while even plant life has benefited.

Wolves curled up in the snow among the trees.
Wolves bedded down in the snow among the trees in Yellowstone. Phot: Doug Smith, NPS, public domain.

The technology used to track wolves is somewhat old, and some scientists critique the use of VHF collars because it requires sedating or disturbing the animal when collars need servicing; however, some of the older collars have provided us with great long-term data about wolf packs in Yellowstone.

This has now demonstrated how rewilding efforts focused on wolves in Yellowstone have had very positive impacts on wildlife in the park. Similar efforts have also occurred elsewhere and also show great success.

In fact, even in areas where wolves have been gone for hundreds of years, such as in Europe, have begun to reintroduce wolves given the positive benefits to ecosystems.[6]


[1]    For more on bison and elk hunting by wolves and how their hunting behavior changes, see:  Tallian A, Smith DW, Stahler DR, Metz MC, Wallen RL, Geremia C, et al. Predator foraging response to a resurgent dangerous prey. Costa D, editor. Funct Ecol. 2017;31: 1418–1429. doi:10.1111/1365-2435.12866.

[2]    For more on the changing ecology of Yellowstone due to wolves being reintroduced, see:

[3]    For more on willows and the impact of beavers, see:

[4]    More on impacts of the reintroduction of wolves to Yellowstone can be seen here:

[5]    For more on collars and devices used to study animals, see:  Johansson Ö, Simms A, McCarthy T. From VHF to Satellite GPS Collars: Advancements in Snow Leopard Telemetry. Snow Leopards. Elsevier; 2016. pp. 355–365. doi:10.1016/B978-0-12-802213-9.00026-2.

[6]    For more on wolves in Europe, see:


Photo of author
About the author
Mark Altaweel
Mark Altaweel is a Reader in Near Eastern Archaeology at the Institute of Archaeology, University College London, having held previous appointments and joint appointments at the University of Chicago, University of Alaska, and Argonne National Laboratory. Mark has an undergraduate degree in Anthropology and Masters and PhD degrees from the University of Chicago’s Department of Near Eastern Languages and Civilizations.