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MCAT Biology Question — Pisaster Ochraceus

Sea stars of the species Pisaster ochraceus prey on mussels and other shellfish that have no other natural predators. In one bay, a toxic chemical is introduced that creates reproductive anomalies, wiping out the sea star population in a handful of years. As a result, the mussel population explodes, driving out two dozen other species in the local ecosystem. Within less than two decades, the mussels over-consume available resources and their population crashes, leaving the bay with a total population of all species that is vastly reduced.


The Pisaster ochraceus plays what role in the ecosystem?


A. Tertiary consumer

B. Apex predator

C. Detritivore

D. Keystone species




A keystone species is one that exerts a very large influence on the overall balance of an ecosystem, often out of proportion with the total biomass that species represents. Here, the sea stars provide a crucial role in keeping down populations of several other species, and without the sea stars much of the ecosystem collapses. Thus the sea stars are a keystone species and (D) is the right answer.


A: Tertiary consumers are those species with prey on secondary consumers (e.g a human eats a lion which eats gazelles). Here, the sea star is eating a mollusc, which are typically herbivores. Thus the sea star is a secondary consumer, not tertiary.


B: Apex predators have no predators themselves. Nothing in the question indicates that the sea stars have no predators. In fact, sea stars are often eaten by gulls and otters.


C: The question does not indicate that the sea stars are eating dead or decaying matter.


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