STUDENT INSTRUCTIONS

To prepare the class for a discussion of disturbance and the intermediate disturbance hypothesis you have data from either of 2 papers. One is Wayne Sousa's 1979 study called "Disturbance in marine intertidal boulder fields: the non-equilibrium maintenance of species diversity" (Ecology 60: 1225-1239). The other is Jane Lubchenco's. 1978 study. "Plant species diversity in a marine intertidal community: importance of herbivore food preference and algal competitive ability." (American Naturalist 112: 23-39) Both focus on the intertidal zone between high and low water on coasts.

In this "pairs-share" you first work on your figure/table by yourself. Use the step one-step two approach you have practiced in class (see footnote below). After this you will pair up with a student who has focused on the other study and the two of you will explain the figures/table to each other in class. When you finish this compare and contrast the studies and make a list of similarities and differences.

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Sousa 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 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.

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.

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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Lubchenco Study

Jane Lubchenco looked at effects of the herbivorous snail Littorina littorea on diversity of macroalgae (seaweed) on the New England (MA) coast. She studied both tidepools in the upper intertidal zone and on surfaces of very large (immovable) rocks lower down in the low intertidal zone. (The intertidal zone is the zone between highest and lowest tides).

The research began from visual observations. In the tidepools, Lubchenco noticed that algal diversity was different from pool to pool. Some were dominated by one species (such as the green alga, Enteromorpha intestinalis) while others supported 10 or more seaweed species. Snail density also differed between pools. Lubchenco then 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. She measured diversity of seaweed in the pools and on the rock surface and compared the relationship between snail density and algal diversity in the two habitats.

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* In step one you first figure out how the figure or table is set up (e.g. what the labels on the axes mean). You also need to have a pretty good idea of the experimental design - how Sousa or Lubchenco set up the experiments - and the hypotheses the scientists address. In step two you can go on to interpreting the data. For both steps write down any questions you have.