The Original Keystone Species

The Original Keystone Species: The Ochre Sea Star and Its Contribution to Ecology.

Reading time: ~6 minutes

In the 1960s, an ecologist named Robert Paine made a discovery that would reshape our understanding of ecosystems. By removing a single predator, the ochre sea star (Pisaster ochraceus), from a stretch of rocky shoreline in the Pacific Northwest, Paine found that the entire ecosystem fell out of balance (Smithsonian Magazine). That discovery birthed the concept of the keystone species: a species whose role in maintaining ecosystem health is so important that if that species disappears, the entire ecosystem would change drastically.

Nearly sixty years later, scientists have identified hundreds of keystone species worldwide, species found from forests to coral reefs. While some keystone species, like the ochre sea star, are predators, others maintain their ecosystems through very different roles.

Paine’s Experiment

In the rocky intertidal ecosystems along the shores of the Pacific Ocean, Robert Paine studied the community of organisms that lived in the tidepools of the shoreline, which included mussels, barnacles, limpets, algae, and the ochre sea star. At the time in 1966, many ecologists believed that competition for space and resources was the factor that determined which species survived in an ecosystem. Paine wondered if predators might play a more powerful role than anyone realized (Nature Education).

To test his idea, Paine began a bold experiment. In a certain area along the shore, he removed every ochre sea star from a designated plot. In a nearby plot, he left the sea stars alone as a control. Over the months that followed, Paine carefully recorded the changes.

The results were dramatic. Without the ochre sea stars preying on mussels, the mussel population rapidly spread across the rocks and crowded out other species. What had once been a diverse mix of organisms became dominated by mussels, while the plots with sea stars remained more diverse, with many different species able to coexist.

From this experiment, Paine concluded that the ochre sea star was a crucial species in maintaining the balance of the intertidal ecosystem. He published his findings in 1969 and coined the term keystone species to describe the role of the sea star in holding the ecosystem together.

Not All Keystone Species Are Predators

While the predatory ochre sea star was the original keystone species example, scientists now know that keystone species come in many forms. What defines them is not what they eat, but the outsized role they play in supporting ecosystem balance and biodiversity.

Here are a few examples from around the world:

• African Elephants (Loxodonta africana): Elephants shape entire savannas. By knocking down trees, they prevent woodlands from overtaking grasslands, which supports grazing species and the predators that depend on them (WWF).
• Beavers (Castor canadensis): Their dams create wetlands that support fish, birds, amphibians, and insects. Without beavers, many North American watersheds lose critical biodiversity (National Park Service).
• Macaws (e.g., Ara macao): These colorful parrots disperse seeds across the Amazon, ensuring the regeneration of diverse tropical trees (Baños-Villalba et al., 2017).
• Sharks: As apex predators, sharks regulate fish populations and maintain healthy marine food webs. Their decline can lead to collapses in reef systems (WWF).
• Sea Otters (Enhydra lutris): By eating sea urchins, otters prevent overgrazing of kelp forests, which serve as habitat for countless marine species (National Science Foundation).

These examples show that keystone species may be predators, herbivores, seed dispersers, or habitat builders. What they share is an essential role in holding ecosystems together.

The Growth of a Concept

Since the term keystone species was first coined, ecologists have applied the idea across numerous ecosystems worldwide, from coral reefs and rainforests to savannas and the tundra.

Today, nearly 230 animal species have been identified as keystone species, and more are added as research expands (Shukla et al. 2023). The concept continues to shape conservation, helping scientists argue that protecting one crucial species can benefit entire ecosystems.

Why Keystone Species Matter

The ochre sea star experiment revealed that ecosystems are fragile webs of connection. When a keystone species is lost, the effects can disrupt an entire habitat, reducing biodiversity and weakening the natural balance that supports life.

Keystone species matter because they keep ecosystems functioning. They regulate populations, create or maintain habitats, and ensure that many other plants and animals can survive. Without them, ecosystems would shift dramatically, becoming less diverse and less resilient.

Why Protecting Them Protects Us

Conserving keystone species is not just about protecting wildlife. Healthy ecosystems provide essentials that humans rely on every day: clean air, fresh water, fertile soil, and climate stability.  From elephants shaping African savannas to sharks maintaining balance in the oceans, protecting keystone species safeguards the natural systems that sustain us.

How You Can Help

Even small actions can help protect keystone species and ensure that ecosystems remain healthy for future generations.

• Spread awareness about the importance of keystone species.
• Support conservation organizations working to protect these animals and their habitats.
• Make sustainable choices in what you buy and consume to reduce pressure on ecosystems.
  
  

One Last Thing

Almost 60 years after Robert Paine’s shoreline experiment, the ochre sea star still stands as a reminder that one species can hold the fate of an entire ecosystem. By protecting keystone species, we not only preserve biodiversity; we protect the future of our planet and ourselves.