Biology 141 Lewis & Clark College

Lecture 20 Predation, disturbance and diversity

K.E. Clifton

Predation and disturbance can both have strong effects on local species diversity.

We've already talked some about Keystone Predation.

The competitive exclusion principle says that two species with the same niche cannot coexist

This principle presumes that resources are in limited supply.

Recall that predation (top-down control) can regulate a population of prey below its K.

Paine's study of predation by seastars in the intertidal zone.

15 different intertidal invertebrates feed by attaching to rocks and filter-feeding tiny organisms and particulates from seawater. They are clearly potential competitors. How can so many spp. coexist?

Paine's hypothesis: predation by seastars reduces competition, allows coexistence.

His experiment: remove seastars from a portion of the intertidal zone, allow them to remain in others (control areas). Both areas had similar species richness at start of study.

result: 8 spp. where seastars were removed, 15 spp. in control areas. Species richness continued to decrease over time where seastars were absent, until these sites were dominated by 1 sp., mussels.

Mussels grew faster than other spp., could overgrow other spp., thus they were competitively dominant.

Why did seastars have this effect on diversity? Feeding trials showed that mussels were seastars' preferred prey.

Conclusion: in the intertidal, seastars are a keystone species, i.e. a species that has an effect on the rest of the community that is stronger than its numbers alone would predict.

Other studies of the effect of predation on prey diversity:

Darwin observed that mown turf (mimicing herbivory) contained more plant spp. than unmown turf.

Sea Otter predation on Sea Urchins influences Kelp Forest community structure (and changes in predation upon sea otters by orca may also be important).

Conclusion: predators/consumers tend to increase the species diversity of their prey. But the effect of grazers/predators on the species diversity of their prey depends on the selectivity of the predators.

Disturbance.

Disturbances create patches of early-successional areas within a larger matrix of later-successional areas, thus increasing species diversity of the overall area.

In addition, disturbance that is moderate in its scope can act like predation.

Study by J. Connell on Heron Island, coral reef in Australia.

highest species diversity of corals in in more exposed areas.

lower diversity in more protected areas (where competitive exclusion had more opportunity to run its course).

generalization: the intermediate disturbance hypothesis.

Implications for management of natural areas.

 

Think more about succession...

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