Rewilding, Explained: The Data from Yellowstone’s Wolves and England’s Knepp Estate
Willow volume rose 1,500% after Yellowstone’s wolves returned. Nightingales at Knepp rose 511% in 25 years. Here is what rewilding actually does.
Rewilding is a conservation approach that restores natural processes and, where feasible, key species — particularly large herbivores and predators — rather than managing a landscape toward a fixed, human-defined target state. Two long-running, well-documented projects illustrate what the approach has produced over multiple decades: the reintroduction of gray wolves to Yellowstone National Park in 1995-96, and the conversion of the 3,500-acre Knepp Estate in West Sussex, England, from conventional farmland to a rewilding project beginning in the early 2000s.
The Yellowstone trophic cascade, updated
A 2025 study by William J. Ripple, Robert L. Beschta, Christopher Wolf, Luke E. Painter, and Aaron J. Wirsing, “The strength of the Yellowstone trophic cascade after wolf reintroduction,” found that the cascade effect of reintroducing wolves — a “trophic cascade,” in which a top predator’s return alters herbivore behavior and, in turn, vegetation — strengthened considerably over time rather than leveling off. Tracking willow recovery in riparian areas from 2001 to 2020, the researchers documented willow crown volume increasing by approximately 1,500%, from about 0.3 cubic meters to 4.8 cubic meters, and average willow height more than doubling, from 92 centimeters in 2001 to 192 centimeters in 2020. The study’s response-ratio measurement of cascade strength exceeded 82% of cases reported in a global meta-analysis of trophic cascades, and the authors note their figures likely understate the true recovery, since willow regrowth had already begun before their 2001 baseline measurements started. The researchers describe the effects as having “unfolded more gradually and non-linearly” than early post-reintroduction studies suggested, underscoring the value of multi-decade monitoring over short-term assessment.
Twenty years of data from Knepp
Knepp’s own 20-year review, published in January 2026, documents concrete recovery figures rather than general claims of increased wildlife. Breeding birds in the estate’s southern block increased 916% since 2007. Nightingale numbers rose from 9 singing males in 1999 to 62 in 2025, a 511% increase. Common whitethroat numbers surged 2,200% between 2007 and 2025. Turtle doves, a species in steep national decline across the UK, climbed 600% to reach 23 singing males by 2020. Purple emperor butterflies, recorded at up to 283 individuals counted in a single day in 2025, now represent what Knepp describes as the largest population of the species in the country. Butterfly species richness grew 107% between 2005 and 2025, dragonfly and damselfly numbers rose 871% over the same period, and invertebrate species of conservation concern increased approximately 31% between 2015 and 2020.
How rewilding is organized as a movement
Rewilding Europe, whose stated mission is “to demonstrate the benefits of wilder nature through the rewilding of diverse European landscapes, and to inspire and enable others to engage in rewilding by providing tools and practical expertise,” now coordinates work across 11 landscapes spanning from Portugal’s Greater Côa Valley and Spain’s Iberian Highlands to Finland’s Nordic Taiga and Scotland’s Affric Highlands. The organization is a member of the International Union for Conservation of Nature (IUCN).
What remains unresolved
Rewilding does not always proceed as planned: the Dutch Oostvaardersplassen reserve, an earlier and more hands-off rewilding attempt involving large herbivores without predator control, faced significant public and scientific criticism over winter starvation among its grazing animals, illustrating that outcomes depend heavily on site-specific design rather than a single universal rewilding method. Both the Yellowstone and Knepp results documented above are specific to their ecosystems — a boreal-adjacent US national park in one case, lowland English farmland in the other — and neither research team claims the same magnitude of recovery would occur in a different climate, soil, or species context without site-specific study.
Sources: The strength of the Yellowstone trophic cascade after wolf reintroduction
Featured image: photo by David Solce on Pexels (free Pexels license).
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I specialize in sustainability education, curriculum co-creation, and early-stage project strategy. At WINSS, I craft articles on sustainability, transformative AI, and related topics. When I’m not writing, you’ll find me chasing the perfect sushi roll, exploring cities around the globe, or unwinding with my dog Puffy — the world’s most loyal sidekick.
