What dramatically changes when starfish are removed from the stimulated system?
A. Acorn and Gooseneck Barnacle Populations increase in size
B. The Mussel population increases in size
C. The coral weed population increase in size.
D. The system remains largely unchanged.
The Correct Answer and Explanation is:
Correct Answer: B. The Mussel population increases in size
The removal of starfish, specifically the Pisaster ochraceus, from a marine ecosystem leads to a dramatic increase in the population of mussels (particularly Mytilus californianus), which can profoundly alter the structure and diversity of the community. This phenomenon was first studied in a landmark experiment by ecologist Robert Paine in the 1960s, who examined rocky intertidal zones along the Pacific Northwest coast of the United States.
Starfish, particularly Pisaster, are keystone predators. This means that although they may not be the most abundant species in the ecosystem, their presence has a disproportionately large effect on the community structure. Pisaster primarily preys on mussels, controlling their population and preventing them from monopolizing space on the rocks.
When starfish are removed from the system, their predatory control over mussels disappears. This allows the mussel population to grow rapidly and dominate the available space. Since mussels are strong competitors for space, their unchecked growth leads to the displacement or exclusion of many other species, such as barnacles, algae, limpets, and other invertebrates.
As a result, species diversity significantly declines, and the once-rich and varied community becomes overwhelmingly dominated by mussels. This illustrates the critical role that keystone predators like starfish play in maintaining biodiversity and ecological balance.
In contrast:
- Option A is incorrect because mussels often outcompete barnacles for space.
- Option C is incorrect because coral weeds (or encrusting algae) are typically overgrown by mussels.
- Option D is incorrect because the system changes dramatically with the loss of biodiversity and mussel overgrowth.
This case is a classic example used in ecology to demonstrate how a single species can regulate an entire community and how its removal can lead to ecological collapse or simplification.