Keystone species and the Yellowstone wolves

When Robert Paine pried the sea stars off a strip of rocky shore in Washington State, the number of species living there fell from 15 to 8.

The sea star experiment

In the early 1960s the ecologist Robert Paine studied the shore at Mukkaw Bay, near Neah Bay in Washington State. The rocks there are covered by barnacles, mussels, limpets, chitons and other animals, and the main predator is the ochre sea star, Pisaster ochraceus. Paine picked a strip of rock about 8 m wide and 2 m high, kept a neighbouring strip as a control, and began removing every sea star he found from his strip.

The result went against the usual expectation that taking away a predator helps its prey. Mussels, the sea star's favourite food, spread across the bare rock and crowded out the other species that need the same space. In his 1966 paper Paine reported that the strip went from a 15-species system to an 8-species one. The sea star had been keeping the space open by eating the strongest competitor. Three years later he used the word keystone for such a species, after the stone at the top of an arch that holds the others in place.

OpenStax defines a keystone species as one whose presence is key to keeping an ecosystem's biodiversity and structure. Its examples include Pisaster and the banded tetra, a fish of tropical streams that supplies nearly all of the phosphorus the rest of the community uses. A keystone species is different from a foundation species, which builds or strongly shapes the habitat. Kelp is a producer; reef-building corals are animals with photosynthetic partners in their tissues.

Wolves return to Yellowstone

According to the US National Park Service, the last wolf pack in Yellowstone National Park was killed in 1926. After decades of planning, wolves captured in Canada were brought in: 14 in January 1995 and 17 in January 1996, followed by 10 from northwestern Montana in 1997, for 41 in all. They were held in acclimation pens for about ten weeks before release. The park count grew by about 10% a year on average, reached a high of 174 wolves in up to 16 packs, and later settled at around 100 wolves in about 10 packs.

Did the wolves save the willows?

The northern Yellowstone elk herd counted about 17,000 in winter when wolves arrived in 1995 and fell to 3,915 by early 2013, before rising again to 6,673 in 2022. Over the same years some willow and aspen stands grew taller. A popular story says the wolves caused a trophic cascade: fewer or more fearful elk, so more young trees survive, so more beavers and songbirds. The Park Service calls this a big scientific debate. Cougars and bears also recovered, elk were hunted outside the park, and water supply matters too; in dry places willows did not respond, and the area of willow and aspen has not grown, only the height of existing stands.