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MCAT Biology Question — Pisaster Ochraceus

  • by John
  • Dec 01, 2014
  • MCAT Biology, MCAT Blog, MCAT Question of the Day

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

 

Explanation

 

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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