Slide 1
A predator changes more than its prey
A sea star feeding on mussels can influence the whole rocky-shore community. Removing the predator can change who occupies limited attachment space.
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Robert Paine tested this relationship experimentally on the Washington coast. Manipulating one participant made it possible to observe consequences that a simple list of species would not reveal.
Slide 2
Space is a resource
Mussels can cover rock that other organisms also need. Predation on mussels can keep some of that space available to competitors.
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A predator can therefore benefit organisms it never feeds or shelters directly. Its effect travels through a chain: fewer dominant competitors can mean more opportunities for other species.
Slide 3
Removal exposes the connection
When sea stars were removed from experimental shore areas, mussels expanded and community diversity declined. The result linked predation to the balance of competitors.
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Comparison with areas retaining sea stars strengthened the interpretation. A control helps separate the effect of removal from changes that might have occurred across the shore anyway.
Slide 4
A keystone effect is disproportionate
A keystone species has an ecological effect large relative to its abundance. The term concerns influence, not simply being common or physically large.
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An abundant habitat-forming organism can be important for another reason. Naming the kind of importance prevents keystone from becoming an empty compliment applied to every species.
Slide 5
Context remains part of the lesson
Paine's experiment established a powerful example, not a rule that every predator always raises diversity. Food webs and habitat conditions differ.
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Repeating experiments and observing other systems reveals when similar effects occur. The general lesson is to trace interactions and test their consequences rather than judge a species only in isolation.
Sources
- Fifty years of ecological insights
University of Washington
- Keystone Species
HHMI BioInteractive