As described by Paine (1966), some sea stars (e.g., Pisaster ochraceus) may prey on sea urchins, mussels, and other shellfish that have no other natural predators. If the sea star is removed from the ecosystem, the mussel population explodes uncontrollably, driving out most other species. This means that sea stars are the _____ species in these ecosystems.

Question

BlackTom AI · Accepted Answer

This question asks you to identify the role of a sea star predator in its ecosystem based on the Paine (1966) classic example. 
Option a: foundation. Foundation species are those that physically structure the environment and create habitats for other organisms (like corals or trees). Sea stars do not primarily build habitat; their key role is not to physically form the environment, so this option mischaracterizes their function. 
Option b: keystone. A keystone species exerts a strong control on community structure disproportionate to its abundance—for Pisaster ochraceus, predation on mussels and other organisms prevents any one species from dominating, allowing greater species diversity. Removal of the sea star leads to mussel domination and collapse of other species, illustrating a classic keystone effect. This option aligns with Paine’s findings and standard ecology definitions. 
Option c: secondary. The term secondary typically relates to succession (secondary succession) or to organisms that occupy a site after disturbance, not to a species that stabilizes community structure through top-down effects. It does not capture the interaction strength and the community-wide influence described in the scenario. 
Option d: dominant. A dominant species has high abundance and substantial influence by sheer numbers, but Pisaster ochraceus is not described by high abundance; its impact is due to its ecological role as a predator, not sheer numbers. The example emphasizes functional importance rather than sheer biomass. 
Option e: iteroparous. This is a reproductive strategy meaning producing multiple offspring across years. While reproductive strategy informs life history, it does not describe the species’ ecological role in controlling community composition as in this scenario. It’s unrelated to the observed ecosystem effect. 
In summary, options a, c, d, and e each misstate or misinterpret the sea star’s ecological function in this context, whereas option b correctly identifies the sea star as a keystone species whose predation maintains community diversity and structure. The reasoning hinges on understanding how a single predator can regulate competitive interactions among prey species and thereby shape the entire ecosystem.

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