BACKGROUND

W. Sousa (1979)

In this study, Wayne Sousa tested the intermediate disturbance hypothesis proposed by Connell (1978). In the 70's and 80's ecologists hotly debated factors explaining high diversity in tropical regions and bottom of the deep sea. Popular ideas included: the time hypothesis (older communities are more diverse), the competition hypothesis (in agreeable climates where biological and not physical factors prevail, interspecific competition and niche partitioning results in high diversity), and the environmental stability hypothesis (relatively unchanging physical variables allow more species to exist). Connell questioned all of these and reasoned instead that highest species diversity exists under conditions of intermediate disturbance. He proposed that in recently disturbed communities a few early colonizing species prevail; similarly after a long time diversity is also low, but these few are long-living and competitively dominant organisms. Diversity would therefore be greatest at intermediate points when a variety of species had colonized a habitat but competitive exclusion had not yet taken place.

As Peters (1991) piercingly explains (see Overview) the ecological diversity debates have been plagued by imprecise definitions and inattention to operationalizing hypotheses. One advantage of Connell's Intermediate Disturbance Hypothesis was that it could be experimentally tested as Sousa did in his field study.

Sousa examined effects of wave movement on diversity of organisms living on boulders along beaches in California. In this elegant study Sousa first determined the relationship between boulder size and force required to move them. As expected, small boulders moved often, large ones seldom; when boulders rolled over attached species become damaged or dislodged. Measurement of various sized boulders therefore allowed Sousa to examine a gradient of disturbance frequency and intensity.

Sousa found few species on small boulders; the ones therer were opportunistic early colonizers such as sea lettuce (Ulva) and barnacles (Chthamalus). On the largest boulders species diversity was also low, but in this case a competitively dominant and late colonizing red algae was most abundant (Gigartina canaliculata). Highest diversity was seen on the boulders intermediate in size. These rocks supported early and late colonizers and also bare spots.

To measure the force necessary to move boulders of different sizes Sousa pulled boulders with a chain attached to a spring scale; he pulled in the direction of incoming waves and converted the measured dislodging force from kilograms to Newtons (N).

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J. Lubchenco (1978)

Jane Lubchenco expanded the research of her mentor, Bob Paine, by looking at effects of the herbivorous snail Littorina littorea on diversity of macroalgae on the New England (MA) coast. She studied both tidepools in the upper intertidal zone and rock surfaces lower down.

In the tidepools, Lubchenco first observed that algal diversity could be quite different from pool to pool. Some were dominated by one species (such as the green alga, Enteromorpha intestinalis) while others supported 10 or more algal species. Snail density also differed between pools. Lubchenco conducted 2-way choice laboratory studies on food preferences of the snails. Enteromorpha was clearly a preferred food while others such as the red alga, Condrus crispus, was not.

In the field, Lubchenco manipulated snail density in studies lasting several years. In the tidepools she found that algal species diversity was highest in pools with intermediate snail numbers (about 100-200 m^-2). At low snail densities, Enteromorpha out competed other seaweeds, and the pools looked emerald green. At high densities the snails consumed all the preferred algae and only the inedible Chondrus and encrusting corraline algae remained.

The relationship between snail density and algal diversity was different on the exposed rock faces in the lower intertidal zone. Here diversity decreased with snail density. The snail's food preferences remained the same, but physical disturbance by ice and intense waves limited competition of ephemeral seaweeds like Enteromorpha. At low snail densities more than a dozen algae coexisted; seaweeds grew on bare patches and on each other. As their number increased snails ate preferred algae with declines in diversity as a result.

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Literature Cited

• Connell, J. H. 1978. Diversity in tropical rainforests and coral reefs. Science 199: 1302-1310.


• Lubchenco, J. 1978. Plant species diversity in a marine intertidal community: importance of herbivore food preference and algal competitive ability. American Naturalist 112: 23-39