Biology
How Keystone Predation Maintains Species Diversity in Rocky Intertidal Zones
Quick fact
In the 1960s, ecologist Robert Paine removed ochre sea stars (Pisaster ochraceus) from a rocky intertidal site in Washington State. Within a year, the number of species plummeted from 15 to just 5, as mussels took over the entire zone.
Why this is interesting
Imagine a rock ledge teeming with life—barnacles, limpets, seaweeds, anemones. But if you remove just one animal from that scene, the entire community can collapse into a single species. What is that animal, and why is it so powerful?
Read the full explanation
Understanding How Keystone Predation Maintains Species Diversity in Rocky Intertidal Zones
Think of a rocky shore as a crowded neighbourhood where space is the most precious resource. Among the residents, mussels are ruthless bullies—they grow fast, attach tightly, and can smother almost everything else. If left unchecked, they would cover the rocks completely, leaving no room for barnacles, algae, or other invertebrates. But a sea star acts like a roaming police officer that constantly removes mussels, keeping their population in check. By eating the dominant competitor, the sea star creates openings for many other species to colonize and survive. This is the essence of keystone predation: a predator that has a disproportionately large effect on its community, not by killing the most individuals, but by controlling a species that would otherwise outcompete all others.
A deeper explanation
The mechanism behind keystone predation in the rocky intertidal is rooted in the interplay between predation and interspecific competition. The ochre sea star prefers mussels, which are the dominant space competitors. When the sea star removes mussels, it prevents mussels from monopolizing the substrate. This 'top-down' control allows subordinate competitors—such as barnacles, limpets, and macroalgae—to persist. The classic experiment by Robert Paine demonstrated this by manually removing starfish from a section of the shore and comparing it to an untouched control. In the removal area, mussels quickly expanded and outcompeted other species, reducing species richness from 15 to 5. This proved that predation, not just resource competition, is a key driver of biodiversity. The term 'keystone species' was coined by Paine to describe species whose impact on the ecosystem is large relative to their abundance. This concept has since become foundational in ecology and has influenced conservation strategies, emphasizing that protecting certain predators can be critical for maintaining overall biodiversity.