Griz 101: What’s a Keystone Species?

In the 1960s, an experimental ecologist named Dr. Robert Paine traveled to the tip of the Olympic Peninsula to pry starfish loose from from their habitat with a crowbar before flinging them out to sea like frisbees. Although the process sounds slightly absurd, Paine had done less exciting work while earning his Ph.D—sifting through sand looking for quarter-inch-long mollusks, for example—and had already earned a reputation for his unorthodox methods.

Paine took the four-hour journey to Makah Bay from Seattle twice a month for five years to systematically remove Pisaster ochraceus from the rock faces and relocate the purple and ochre sea stars as far out into the bay as he could hurl them. His goal was simple: to measure the impact of that single species on a thriving tidal plain community.

Paine chose Makah Bay, a rocky outcrop off the edge of the northwestern tip of the lower 48 states, not only because it had tide pools filled with abundant sea life, but also because it was remote and had been largely undisturbed by humans. Pisaster ochraceus feeds on mussels and barnacles, and in between Paine’s trips to Makah Bay—the mussels took over, advancing slowly but surely toward the low-tide mark. Without their natural predator regulating the environment, the mussels crowded out algae, bivalves, and limpets. Within a year, the tidal plain’s biodiversity was cut in half, and Paine’s idea of a keystone species was confirmed. So what does this have to do with grizzly bears? As it turns out, a lot!

At first glance, it seems like a stretch to connect the study of sea stars to grizzly bears, but by Paine’s definition, both are keystone species making them critical components of both local and global biodiversity. Keystone species are creatures that impact their ecosystems to such a degree that without them the ecosystem would not only be dramatically different, but could also disappear completely. Paine determined that, “certain species exert a disproportionate impact on their ecosystems and that their elimination—as a result of climate change, pollution or some other natural or man-made factors—can produce unexpected and far-reaching consequences for the local environment.”

The name “keystone species” comes from the wedge-shaped stone that is used at the top of a bridge or arch to support all of the other stones and lock them into position. Without the keystone at the apex, the arch collapses. Keystone species are not always predators, but they define their ecosystems by keeping the inhabitants in check, and affect their community’s composition both by the plants and animals they eat as well as those they do not. Grizzlies —like Paine’s starfish—are an excellent example.

As an apex predator, the grizzly bear is at the top of the food chain and has no natural predators, yet despite being relatively few in number, grizzlies control both the population and distribution of a significant number of other species. Grizzly bears are omnivorous, opportunistic eaters and their diet ranges from young elk and roadkill to tubers, berries, insects, and even pinecone seeds. Grizzlies are a multi-faceted keystone species in that their role in an ecosystem extends beyond their success as hunters.

According to National Geographic, “Keystone species have low functional redundancy. This means that if the species were to disappear from the ecosystem, no other species would be able to fill its ecological niche. The ecosystem would be forced to radically change, allowing new and possibly invasive species to populate the habitat.”

In addition to maintaining populations through predation, grizzly bears impact their local flora and fauna in other ways. The grizzly bear’s home range is extensive, and a single bear can cover up to 600 miles. Grizzlies spend their days (when they’re not hibernating) walking and looking for food, and in the process they aerate the soil through foraging and digging. This enhances local growth, but also facilitates the spreading of seeds both on grizzly bear’s fur and through elimination.

Along the Pacific coastline in British Columbia and Alaska, grizzly bears feast on salmon and then “act as nutrient conveyor belts, dispersing the nitrogen- and phosphorus-rich salmon carcasses across the forest floor like fertilizer.”

In addition to predator and gardener, grizzly bears solidify their importance as keystone species by also acting as somewhat of a hybrid between garbage man and vacuum cleaner. Grizzlies eat decaying carcasses off the forest floor that other wildlife pass by, and they also eat mass amounts of insects such as army cutworm moths—up to 40,000 in a single day—which are a pest to both oat and wheat farmers, but which provide grizzlies with necessary fats and energy-dense nutrition as they prepare for hibernation.

Despite knowing how to hunt, forage, and flourish on the Rocky Mountain landscape, grizzly bears became threatened only after humans jockeyed into position as both top predator and top consumer, not only in grizzly bear habitats, but across the board. Despite our self-instated, top-dog status we’d be remiss not to heed Paine’s words when he said that humans, “will be the ultimate losers if the rules are not understood and global ecosystems continue to deteriorate.”

Dr. Robert Paine’s original discovery of keystone species was done through hands-on ecology and the systematic removal of a species. Simon Levin, an ecologist and professor at Princeton University, wrote that Dr. Paine’s “influence cannot be overestimated,” particularly his “notion that to understand systems one had to perturb them.” We wouldn’t attempt this with grizzly bears—their populations have already been reduced to species-threatening numbers.

We have already perturbed the grizzlies, and not in a “don’t poke the sleeping bear” kind of way, but rather because the numbers don’t lie; the 98% reduction in grizzly bears over the course of the 20th century makes it clear that grizzlies have been sufficiently perturbed and that it was humans, the “overdominant keystones,” as Paine described us, who did the poking.