Keystone Species & Rewilding | Rewilding Report

Q: How do you know if a reintroduction has worked?

Success is measured at multiple levels: has the population established, is the species resuming its ecological role, and are ecosystem-level effects measurable? Research published in Restoration Ecology found that 9 out of 11 studies assessing ecosystem-level effects following keystone species reintroduction found evidence of resumed ecological functioning.

Q: Can keystone species reintroduction fail?

Yes. Reintroductions can fail if the habitat has been too severely degraded, if key prey or food sources are absent, if there is insufficient connectivity, or if human conflict leads to illegal killing. Reintroductions are most likely to succeed when preceded by habitat assessment, stakeholder engagement, legal protection, and ongoing monitoring.

Key Takeaways

→The keystone species concept was introduced by ecologist Robert T. Paine in 1969, based on experiments showing that removing a single starfish species from Pacific intertidal rock pools caused the entire ecosystem to collapse into a monoculture of mussels.
→Keystone species exert effects far larger than their abundance would suggest. Their removal triggers trophic cascades that alter the abundance, behavior, and distribution of species across multiple levels of the food web.
→Nine out of eleven peer-reviewed studies assessing ecosystem-level effects following keystone species reintroduction found evidence of resumption of keystone ecological roles (Hale & Koprowski, 2018, Restoration Ecology).
→Trophic rewilding, the strategic reintroduction of keystone species to restore top-down ecological processes, is now recognized as a major tool for biodiversity recovery and natural climate solutions.
→Central keystone species in rewilding programs include wolves (trophic cascades in terrestrial ecosystems), beavers (wetland engineers), sea otters (kelp forest protectors), European bison (grassland grazers), and large whales (nutrient cycling in marine systems).

Keystone species in rewilding

🐺

Grey Wolf (Canis lupus)

The apex predator that rebuilds rivers

Wolf reintroduction triggers trophic cascades that restructure vegetation, stabilize rivers, and support dozens of dependent species.

Active reintroductions in USA, Europe

🦫

Eurasian & North American Beaver (Castor fiber / canadensis)

Nature's wetland engineers

Beavers create wetlands that reduce floods, filter water, store carbon, and generate habitat for hundreds of other species.

Recovering across UK and Europe

Coming soon 🦦

Sea Otter (Enhydra lutris)

Guardian of the kelp forest

Sea otters control sea urchin populations, protecting kelp forests that shelter thousands of marine species and sequester vast amounts of carbon.

Recovering along Pacific coast after near-extinction

American bison grazing among mesquite trees in Texas

Coming soon 🦬

American Bison (Bison bison)

The grassland shaper

Bison grazing creates habitat heterogeneity in grasslands, maintaining the diverse mix of plant communities that support hundreds of other species.

Reintroduction programs ongoing across North America

Coming soon 🐋

Large Whales (multiple species)

The ocean's nutrient pumps

Whales fertilize ocean surface waters with nutrient-rich excretions, driving phytoplankton growth that underpins marine food webs and sequesters carbon.

Recovering in some regions following commercial whaling bans

Coming soon 🐆

Eurasian Lynx (Lynx lynx)

The forest's silent regulator

Lynx reintroduction controls deer and roe deer populations in temperate forests, reducing overgrazing and allowing forest regeneration.

Present in parts of Europe; reintroduction debated in UK

What is a keystone species?

The term keystone species was coined by ecologist Robert T. Paine in 1969, based on a simple but startling experiment on the rocky intertidal shores of Washington state. Paine removed all individuals of a single species, the ochre starfish (Pisaster ochraceus), from a section of coastline and observed what happened. The result was dramatic: within months, the diverse intertidal community collapsed into a near-monoculture of mussels, which without the starfish's predation rapidly dominated and excluded almost every other species from the zone.

Paine called the starfish a keystone species, after the architectural keystone, the wedge-shaped stone at the top of an arch that holds all the others in place. Remove it, and the arch collapses. The concept has since expanded beyond predators to include ecosystem engineers (like beavers), mutualists (like fig trees, which support hundreds of dependent species), and large herbivores (like bison and elephants) whose grazing shapes entire landscapes.

Paine published his findings as a three-page letter in The American Naturalist, arguing that a single species, disproportionate to its numbers, could hold an entire community's structure in place. The paper has since been cited more than 1,600 times.

Paine, R.T. (1969). A Note on Trophic Complexity and Community Stability. The American Naturalist, 103(929), 91–93. doi.org/10.1086/282586

What is a trophic cascade?

A trophic cascade is the chain of ecological effects that follows when a keystone species is added or removed. The classic example is the reintroduction of wolves to Yellowstone National Park in 1995. Without wolves, elk populations had grown large and bold, overgrazing riverbanks and preventing the regeneration of willow, aspen, and cottonwood. When wolves returned, they not only reduced elk numbers, they changed elk behavior. Elk avoided lingering in open river valleys where they were vulnerable to predation.

Riverbank vegetation recovered. Songbirds returned. Rivers narrowed and deepened, stabilizing banks. Beaver populations recovered, ironically aided by the return of their rival, the wolf, which kept elk away from the willow the beavers depended on.

This is the power of the trophic cascade: a single species reintroduction changed the behavior of prey, the structure of vegetation, the hydrology of rivers, and the presence of dozens of other species. The ecosystem was transformed from the top down.

Types of keystone species

Apex predators

Wolves, lynx, lions, sharks, orcas. These regulate prey populations and behavior through direct predation and the fear effect, preventing any single herbivore species from overgrazing and allowing plant communities to diversify.

Ecosystem engineers

Beavers, elephants, prairie dogs, woodpeckers. These physically modify the landscape, creating wetlands, digging burrows, opening tree cavities, in ways that generate habitat for hundreds of other species. Their modifications persist long after they have moved on.

Large herbivores

Bison, wisent (European bison), wild horses, rhinoceroses. These graze and browse in ways that maintain the structural diversity of grasslands, savannas, and scrublands: habitats that without large herbivores would often transition to closed-canopy forest or monoculture grassland, reducing overall biodiversity.

Mutualists

Fig trees, sea otters, large frugivores. These species maintain the productivity or diversity of ecosystems through positive relationships. Fig trees support hundreds of dependent species year-round in tropical forests. Sea otters protect kelp forests by controlling sea urchin populations.

Why keystone species reintroduction is central to rewilding

Traditional conservation typically protects what remains. Rewilding goes further, it actively restores what has been lost. Keystone species reintroduction is the most direct way to restore the ecological processes that make an ecosystem function: predation, grazing, seed dispersal, wetland creation, nutrient cycling. Without these processes, even large, protected areas tend to shift toward simpler, less biodiverse states over time.

The evidence is increasingly clear: restoring keystone species accelerates ecosystem recovery far more quickly than passive protection alone. And because keystone species drive processes that benefit many other species, a single successful reintroduction can substitute for dozens of individual species recovery programs.

References & Further Reading

1.Hale S.L., Koprowski J.L. (2018). Ecosystem-level effects of keystone species reintroduction: a literature review. Restoration Ecology, 26(3), 439–445. doi.org/10.1111/rec.12684

2.Svenning J.C., et al. (2016). Science for a wilder Anthropocene: Synthesis and future directions for trophic rewilding research. PNAS, 113(4), 898–906. doi.org/10.1073/pnas.1502556112

3.Perino A., Pereira H.M., et al. (2019). Rewilding complex ecosystems. Science, 364, eaav5570. doi.org/10.1126/science.aav5570

4.Schmitz O.J., et al. (2023). Trophic rewilding can expand natural climate solutions. Nature Climate Change. doi.org/10.1038/s41558-023-01631-6

5.Ripple W.J., et al. (2022). Rewilding the American West. BioScience. doi.org/10.1093/biosci/biac069

6.Jepson P.R., Lemoine R. (2025). De-extinction beyond species: Restoring ecosystem functionality through large herbivore rewilding. Cambridge Prisms: Extinction. doi.org/10.1017/ext.2024.27

Keystone species: frequently asked questions

What makes a species a keystone species?

A keystone species is defined by the disproportionate size of its ecological impact relative to its abundance. A species that is rare or modestly abundant but whose removal or addition causes dramatic changes across the ecosystem, altering the abundance, behavior, or distribution of many other species, qualifies as a keystone species. The key distinction is impact per individual, not total biomass. A forest may contain millions of insects but only a handful of wolves; yet the wolves may have more impact on the overall structure of the ecosystem than all the insects combined.

How do you know if a reintroduction has worked?

Success is measured at multiple levels. First, has the reintroduced population established and started reproducing? Second, is the species resuming its ecological role? For example, are wolves hunting elk, are beavers building dams, are sea otters eating sea urchins? Third, are ecosystem-level effects measurable? Are changes in vegetation structure, hydrology, species diversity, or prey behavior seen? The most rigorous studies track all three levels over many years. Research published in Restoration Ecology found that 9 out of 11 studies assessing ecosystem-level effects following keystone species reintroduction found evidence of resumed ecological functioning.

Are all apex predators keystone species?

Not necessarily. A species is only a keystone species if its removal causes disproportionate ecosystem-level change. Some apex predators have strong top-down effects; others exist at such low densities or in such complex food webs that their removal is absorbed without dramatic consequences. The keystone designation must be demonstrated empirically, not assumed from trophic position alone. That said, large predators are disproportionately represented among confirmed keystone species because their effects on prey behavior, not just prey numbers, can cascade through entire ecosystems.

What is the most important keystone species for rewilding?

It depends entirely on the ecosystem. In temperate forests and grasslands of the Northern Hemisphere, wolves and beavers are the most impactful. In kelp forest ecosystems, sea otters. In African savannas, elephants and lions. In marine systems, large whales, whose nutrient-rich excretions fertilize surface waters, are increasingly recognized as keystone species. The key principle is always the same: identify which species, if restored, would most rapidly and broadly restore ecological processes in that specific landscape.

Can keystone species reintroduction fail?

Yes, and understanding why is important. Reintroductions can fail if the habitat has been too severely degraded for the species to establish, if key prey or food sources are absent, if there is insufficient connectivity for the population to grow, or if human conflict leads to illegal killing before the population can self-sustain. Reintroductions are most likely to succeed when they are preceded by habitat assessment, stakeholder engagement, legal protection, and ongoing monitoring. The science is clear that reintroduction works when the conditions are right. The challenge is getting those conditions right.