Yellowstone

Connectivity

No single unit of the Greater Yellowstone Ecosystem is large enough to hold its wildlife year-round. Its value lies in the connections between two national parks, five national forests, three national wildlife refuges, and the state, tribal and private valleys that link them.

Paradise ValleyJackson HoleUpper GreenWhole ecosystem

Species that migrate

The animals at the centre of the Greater Yellowstone story do not stay in the park, and several are among the longest land migrations in the lower 48 states:

• Elk. The Northern Yellowstone elk winter in two partly separate groups and migrate 10–140 km to at least a dozen summer ranges (White 2010). Clarks Fork elk that migrate into the park saw recruitment fall 70% over 21 years, linked to drought-driven green-up and to predation (Middleton 2013)
• Elk outside the park. Much of the Northern herd winters north of the park boundary, in the Gardiner Basin and Paradise Valley, where Montana manages hunting.
• Bison. Bison migrate across the park's northern and western boundaries in winter (Plumb 2009; Geremia 2011; Geremia 2014), and their grazing reshapes the spring green-up they follow (Geremia 2019). Bison that leave the park are managed under an interagency plan meant to limit the spread of brucellosis to cattle (NPS, USFS & APHIS 2000).
• Pronghorn. Pronghorn travel between Grand Teton National Park and the Upper Green River basin along a corridor that has been used for thousands of years (Berger 2006). Berger estimated that about three-quarters of the region's historic migration routes for elk, bison and pronghorn have been lost (Berger 2004).
• Mule deer. Mule deer migrate about 150 miles one way from the Red Desert to Hoback Junction (Sawyer 2014).
• Songbirds. The two songbirds in the index are long-distance migrants that winter from Mexico southward.

Most of these routes cross from one jurisdiction to another: park, national forest, state, tribal and private land, each managed under different rules (Middleton 2020; Kauffman 2020; Kauffman 2022).

1. White, P. J., Proffitt, K. M., Mech, L. D., Evans, S. B., Cunningham, J. A., Hamlin, K. L. (2010). Migration of northern Yellowstone elk: implications of spatial structuring. Journal of Mammalogy 91:827-837. doi:10.1644/08-MAMM-A-252.1
2. Middleton, A. D., Kauffman, M. J., McWhirter, D. E., Cook, J. G., Cook, R. C., Nelson, A. A. et al. (2013). Animal migration amid shifting patterns of phenology and predation: lessons from a Yellowstone elk herd. Ecology 94:1245-1256. doi:10.1890/11-2298.1
3. Plumb, G. E., White, P. J., Coughenour, M. B., Wallen, R. L. (2009). Carrying capacity, migration, and dispersal in Yellowstone bison. Biological Conservation 142:2377-2387. doi:10.1016/j.biocon.2009.05.019
4. Geremia, C., White, P. J., Wallen, R. L., Watson, F. G. R., Treanor, J. J., Borkowski, J. et al. (2011). Predicting bison migration out of Yellowstone National Park using Bayesian models. PLoS ONE 6:e16848. doi:10.1371/journal.pone.0016848
5. Geremia, C., White, P. J., Hoeting, J. A., Wallen, R. L., Watson, F. G. R., Blanton, D. et al. (2014). Integrating population- and individual-level information in a movement model of Yellowstone bison. Ecological Applications 24:346-362. doi:10.1890/13-0137.1
6. Geremia, C., Merkle, J. A., Eacker, D. R., Wallen, R. L., White, P. J., Hebblewhite, M. et al. (2019). Migrating bison engineer the green wave. Proceedings of the National Academy of Sciences 116:25707-25713. doi:10.1073/pnas.1913783116
7. U.S. Department of the Interior, National Park Service; U.S. Department of Agriculture, Forest Service and Animal and Plant Health Inspection Service (2000). Record of Decision for Final Environmental Impact Statement and Bison Management Plan for the State of Montana and Yellowstone National Park (Interagency Bison Management Plan). Federal Register notice 22 January 2001 (ROD signed 20 December 2000). link
8. Berger, J., Cain, S. L., Berger, K. M. (2006). Connecting the dots: an invariant migration corridor links the Holocene to the present. Biology Letters 2:528-531. doi:10.1098/rsbl.2006.0508
9. Berger, J. (2004). The last mile: how to sustain long-distance migration in mammals. Conservation Biology 18:320-331. doi:10.1111/j.1523-1739.2004.00548.x
10. Sawyer, H., Hayes, M., Rudd, B., Kauffman, M. J. (2014). The Red Desert to Hoback Mule Deer Migration Assessment. Wyoming Migration Initiative, University of Wyoming (report, not peer-reviewed). link
11. Middleton, A. D., Sawyer, H., Merkle, J. A., Kauffman, M. J., Cole, E. K., Dewey, S. R. et al. (2020). Conserving transboundary wildlife migrations: recent insights from the Greater Yellowstone Ecosystem. Frontiers in Ecology and the Environment 18:83-91. doi:10.1002/fee.2145
12. Kauffman, M., Copeland, H., Berg, J., Bergen, S., Cole, E., Cuzzocreo, M. et al. (2020). Ungulate migrations of the western United States, Volume 1. U.S. Geological Survey Scientific Investigations Report 2020-5101. doi:10.3133/sir20205101
13. Kauffman, M., Lowrey, B., Beck, J. L. et al. (2022). Ungulate migrations of the western United States, Volume 2. U.S. Geological Survey Scientific Investigations Report 2022-5008. doi:10.3133/sir20225008

Reintroduction of Wolves

Wolves were gone from Yellowstone by the mid-1920s (Ripple 2004). Seventy years later they were brought back. Since then the Northern Range has been the most closely watched test anywhere of what an apex predator does to an ecosystem, and of how hard that is to measure.

Northern RangeParadise ValleyBoth

The reintroduction

A federal environmental impact statement came first (USFWS 1994). The U.S. Fish and Wildlife Service then designated the released wolves a nonessential experimental population under section 10(j) of the Endangered Species Act (USFWS 1994). Fourteen wolves captured in Alberta were held in acclimation pens and released in the park in 1995, and 17 from British Columbia followed in 1996. Ten more came from north-west Montana in 1997, making 41 released in Yellowstone in all. Another 35 were released in central Idaho (Fritts 1997; Bangs 1996; Smith 2016).

The Yellowstone Wolf Project has counted the population every year since. At the end of December 2024 it counted 108 wolves in nine packs, seven of them breeding pairs, living mainly in the park (Cassidy 2025). A count of 84 wolves in eight packs at the end of 2025 was announced in March 2026 and reported in the press (International Wolf Center 2026). The project's 2025 annual report had not been published when this page was written. The park count has risen and fallen since the late 1990s, and it is not a trend for the whole ecosystem, because many packs live mainly outside the park.

Subsequent ecological effects

Before the release, researchers expected wolves to reduce elk and coyotes, with knock-on effects on plants and smaller carnivores (Smith 2003). Within a few years, studies led by William Ripple and Robert Beschta reported less browsing by elk and new growth of aspen, willow and cottonwood on the Northern Range. They read this as a trophic cascade: wolves lowered elk numbers and changed where and how elk fed (Ripple 2001; Beschta 2003; Ripple 2012; Beschta 2016; Painter 2015; Painter 2018).

Other effects were documented alongside the plants:

• Wolf kills spread carrion through the winter, feeding scavengers (Wilmers 2003).
• Coyote densities fell where wolves were common (Berger 2007).
• Active beaver colonies in the park rose from 44 in 1996 to 127 in 2007. The authors attributed this, probably, to willow recovery (Smith 2012).
• Willow flycatcher and Wilson's warbler, two of the eight species in this landscape's Priority Species Index, were found only in taller willow (Baril 2011).

In 2025 Ripple and colleagues reported that mean willow crown volume on the Northern Range rose about 1,500% between 2001 and 2020, and called this one of the strongest cascades on record (Ripple 2025).

The landscape of fear

The "ecology of fear" is the idea that predators affect prey through fear as well as by killing them. Prey avoid risky places and times, and eat less where danger is high (Brown 1999). The idea was applied to Yellowstone within years of the release:

• Female elk and bison with young were more vigilant where wolves had returned (Laundré 2001).
• Elk movements, habitat choice and grouping shifted when wolves were present (Fortin 2005; Creel 2005; Gude 2006).
• Ripple and Beschta argued that fear alone could let streamside plants recover, even where elk numbers had not fallen (Ripple 2004).

Later tests with GPS collars and physiology gave weaker and mixed results:

• Creel and colleagues reported lower pregnancy rates as elk-to-wolf ratios fell (Creel 2007), and diet shifts when wolves were present (Christianson 2010).
• Yet elk ate more willow, not less, when wolves were near (Creel 2009).
• Northern Yellowstone elk had similar body fat and pregnancy rates before and after wolf restoration (White 2011).
• Elk met wolves only about once every nine days, even in high-risk areas (Middleton 2013).
• Elk used risky places while wolves were inactive (Kohl 2018; Kohl 2019), and showed little spatial avoidance of wolf risk at all (Cusack 2020).

Conflicting views: do wolves explain it?

Few dispute that the Northern Range changed after 1995. The disagreement is over how much it changed, how to measure it, and why. This section gives each position with its authors; it does not settle the question.

Syntheses describe a system with many causes that varies across space and time, where no single study isolates the wolf (Peterson 2014; Boyce 2018). Others have cautioned against turning the wolf into a symbol of ecosystem repair (Mech 2012; Middleton 2014). An earlier landscape-wide test found no sign of aspen recovery even in high-risk sites (Kauffman 2010). That finding was disputed, and its authors replied (Beschta 2013; Kauffman 2013).

Many of the animals in this story leave the park each year. Where they go, and what they cross, is on the Connectivity tab.

The most complete synthesis of the first 25 years is the Wolf Project's edited volume (Smith 2020).

Every reference below was checked on 2026-09-27, against Crossref or against the agency that published it. Several are under non-commercial licences, so this page cites them but reproduces no figure or text from them. The 2025 wolf count and the 2014 opinion piece in The New York Times come from secondary sources.

1. Ripple, W. J., Beschta, R. L. (2004). Wolves and the ecology of fear: can predation risk structure ecosystems?. BioScience 54:755-766. doi:10.1641/0006-3568(2004)054[0755:WATEOF]2.0.CO;2
2. U.S. Fish and Wildlife Service (1994). The Reintroduction of Gray Wolves to Yellowstone National Park and Central Idaho: Final Environmental Impact Statement. U.S. Fish and Wildlife Service, Helena, Montana. link
3. U.S. Fish and Wildlife Service (1994). Endangered and Threatened Wildlife and Plants; Establishment of a Nonessential Experimental Population of Gray Wolves in Yellowstone National Park in Wyoming, Idaho, and Montana (final rule). Federal Register 59:60252. link
4. Fritts, S. H., Bangs, E. E., Fontaine, J. A., Johnson, M. R., Phillips, M. K., Koch, E. D. et al. (1997). Planning and implementing a reintroduction of wolves to Yellowstone National Park and central Idaho. Restoration Ecology 5:7-27. doi:10.1046/j.1526-100x.1997.09702.x
5. Bangs, E. E., Fritts, S. H. (1996). Reintroducing the gray wolf to central Idaho and Yellowstone National Park. Wildlife Society Bulletin 24:402-413. link
6. Smith, D. W., Stahler, D. R., Metz, M. C., Cassidy, K. A., Stahler, E. E., Almberg, E. S. et al. (2016). Wolf restoration in Yellowstone: reintroduction to recovery. Yellowstone Science 24(1). link
7. Cassidy, K. A., Stahler, D. R., Stahler, E. A., Metz, M., SunderRaj, J., Rabe, T. et al. (2025). Yellowstone National Park Wolf, Cougar, and Elk Project Annual Report 2024 (YCR-2025-01). National Park Service, Yellowstone Center for Resources. link
8. International Wolf Center (reprinting Livingston Enterprise reporting) (2026). Yellowstone National Park wolf population declined in 2025. news report (secondary source). link
9. Smith, D. W., Peterson, R. O., Houston, D. B. (2003). Yellowstone after wolves. BioScience 53:330-340. doi:10.1641/0006-3568(2003)053[0330:YAW]2.0.CO;2
10. Ripple, W. J., Larsen, E. J., Renkin, R. A., Smith, D. W. (2001). Trophic cascades among wolves, elk and aspen on Yellowstone National Park's northern range. Biological Conservation 102:227-234. doi:10.1016/S0006-3207(01)00107-0
11. Beschta, R. L. (2003). Cottonwoods, elk, and wolves in the Lamar Valley of Yellowstone National Park. Ecological Applications 13:1295-1309. doi:10.1890/02-5175
12. Ripple, W. J., Beschta, R. L. (2012). Trophic cascades in Yellowstone: the first 15 years after wolf reintroduction. Biological Conservation 145:205-213. doi:10.1016/j.biocon.2011.11.005
13. Beschta, R. L., Ripple, W. J. (2016). Riparian vegetation recovery in Yellowstone: the first two decades after wolf reintroduction. Biological Conservation 198:93-103. doi:10.1016/j.biocon.2016.03.031
14. Painter, L. E., Beschta, R. L., Larsen, E. J., Ripple, W. J. (2015). Recovering aspen follow changing elk dynamics in Yellowstone: evidence of a trophic cascade?. Ecology 96:252-263. doi:10.1890/14-0712.1
15. Painter, L. E., Beschta, R. L., Larsen, E. J., Ripple, W. J. (2018). Aspen recruitment in the Yellowstone region linked to reduced herbivory after large carnivore restoration. Ecosphere 9:e02376. doi:10.1002/ecs2.2376
16. Wilmers, C. C., Crabtree, R. L., Smith, D. W., Murphy, K. M., Getz, W. M. (2003). Trophic facilitation by introduced top predators: grey wolf subsidies to scavengers in Yellowstone National Park. Journal of Animal Ecology 72:909-916. doi:10.1046/j.1365-2656.2003.00766.x
17. Berger, K. M., Gese, E. M. (2007). Does interference competition with wolves limit the distribution and abundance of coyotes?. Journal of Animal Ecology 76:1075-1085. doi:10.1111/j.1365-2656.2007.01287.x
18. Smith, D. W., Tyers, D. B. (2012). The history and current status and distribution of beavers in Yellowstone National Park. Northwest Science 86:276-288. doi:10.3955/046.086.0404
19. Baril, L. M., Hansen, A. J., Renkin, R., Lawrence, R. (2011). Songbird response to increased willow (Salix spp.) growth in Yellowstone's northern range. Ecological Applications 21:2283-2296. doi:10.1890/10-0169.1
20. Ripple, W. J., Beschta, R. L., Wolf, C., Painter, L. E., Wirsing, A. J. (2025). The strength of the Yellowstone trophic cascade after wolf reintroduction. Global Ecology and Conservation 58:e03428. doi:10.1016/j.gecco.2025.e03428
21. Brown, J. S., Laundré, J. W., Gurung, M. (1999). The ecology of fear: optimal foraging, game theory, and trophic interactions. Journal of Mammalogy 80:385-399. doi:10.2307/1383287
22. Laundré, J. W., Hernández, L., Altendorf, K. B. (2001). Wolves, elk, and bison: reestablishing the "landscape of fear" in Yellowstone National Park, U.S.A.. Canadian Journal of Zoology 79:1401-1409. doi:10.1139/z01-094
23. Fortin, D., Beyer, H. L., Boyce, M. S., Smith, D. W., Duchesne, T., Mao, J. S. (2005). Wolves influence elk movements: behavior shapes a trophic cascade in Yellowstone National Park. Ecology 86:1320-1330. doi:10.1890/04-0953
24. Creel, S., Winnie, J., Maxwell, B., Hamlin, K., Creel, M. (2005). Elk alter habitat selection as an antipredator response to wolves. Ecology 86:3387-3397. doi:10.1890/05-0032
25. Gude, J. A., Garrott, R. A., Borkowski, J. J., King, F. (2006). Prey risk allocation in a grazing ecosystem. Ecological Applications 16:285-298. doi:10.1890/04-0623
26. Creel, S., Christianson, D., Liley, S., Winnie, J. A. (2007). Predation risk affects reproductive physiology and demography of elk. Science 315:960. doi:10.1126/science.1135918
27. Christianson, D., Creel, S. (2010). A nutritionally mediated risk effect of wolves on elk. Ecology 91:1184-1191. doi:10.1890/09-0221.1
28. Creel, S., Christianson, D. (2009). Wolf presence and increased willow consumption by Yellowstone elk: implications for trophic cascades. Ecology 90:2454-2466. doi:10.1890/08-2017.1
29. White, P. J., Garrott, R. A., Hamlin, K. L., Cook, R. C., Cook, J. G., Cunningham, J. A. (2011). Body condition and pregnancy in northern Yellowstone elk: evidence for predation risk effects?. Ecological Applications 21:3-8. doi:10.1890/09-2123.1
30. Middleton, A. D., Kauffman, M. J., McWhirter, D. E., Jimenez, M. D., Cook, R. C., Cook, J. G. et al. (2013). Linking anti-predator behaviour to prey demography reveals limited risk effects of an actively hunting large carnivore. Ecology Letters 16:1023-1030. doi:10.1111/ele.12133
31. Kohl, M. T., Stahler, D. R., Metz, M. C., Forester, J. D., Kauffman, M. J., Varley, N. et al. (2018). Diel predator activity drives a dynamic landscape of fear. Ecological Monographs 88:638-652. doi:10.1002/ecm.1313
32. Kohl, M. T., Ruth, T. K., Metz, M. C., Stahler, D. R., Smith, D. W., White, P. J. et al. (2019). Do prey select for vacant hunting domains to minimize a multi-predator threat?. Ecology Letters 22:1724-1733. doi:10.1111/ele.13319
33. Cusack, J. J., Kohl, M. T., Metz, M. C., Coulson, T., Stahler, D. R., Smith, D. W. et al. (2020). Weak spatiotemporal response of prey to predation risk in a freely interacting system. Journal of Animal Ecology 89:120-131. doi:10.1111/1365-2656.12968
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38. MacNulty, D. R., Cooper, D., Procko, M., Clark-Wolf, T. J. (2025). Flawed analysis invalidates claim of a strong Yellowstone trophic cascade after wolf reintroduction: a comment on Ripple et al. (2025). Global Ecology and Conservation 63:e03899. doi:10.1016/j.gecco.2025.e03899
39. MacNulty, D. R., Brice, E. M., Bergeron, N. J., Larsen, E. J. (2026). Overstating trophic cascade strength following large carnivore restoration in Yellowstone: a comment on Painter et al. (2025). Forest Ecology and Management 605:123448. doi:10.1016/j.foreco.2025.123448
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55. Smith, D. W., Stahler, D. R., MacNulty, D. R. (2020). Yellowstone Wolves: Science and Discovery in the World's First National Park. University of Chicago Press (book). doi:10.7208/chicago/9780226728483.001.0001