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Probing Diversity's Complexity

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Science  20 Jul 2012:
Vol. 337, Issue 6092, pp. 286
DOI: 10.1126/science.337.6092.286

By identifying all trees growing along an elevation gradient from above 6000 meters in the mountains to 180 meters above sea level, researchers hope to model the true complexity of our planet's biodiversity.

Test bed.

Extensive, long-term studies of Madidi National Park are testing biodiversity theories.

CREDIT: JEAN FRIEDMAN-RUDOVSKY

Rainforests may be the conservationist's poster child, but they fall short as models of the true complexity of our planet's biodiversity. So says Peter Jørgensen, a botanist at the Missouri Botanical Garden in St. Louis, whose long-term project in Bolivia promises to shake up our understanding of the distribution of tree species in tropical South America. The Madidi Project (see main text) charts the changes in tree communities growing along an elevation gradient that plunges from above 6000 meters in the mountains to 180 meters above sea level. By identifying all trees at least 10 centimeters in diameter within hundreds of research plots along this gradient, Jørgensen and his colleagues have built a database that allows them to examine spatial patterns of diversity in ways not possible from studies limited to rainforests, which tend to be comparatively homogeneous because they are confined to the lowlands.

The Madidi Project focuses on comparing the rate of turnover of species in a sample of plots—a concept known as beta diversity. “It's about trying to understand why a certain species lives in one area and not in another nearby,” says Brad Boyle, a biologist at the University of Arizona in Tucson. To gauge beta diversity, researchers count the number of species in various plots. Then they ask: How many species are common to the plots? How many are different? The higher the number of species found in only one plot, the greater an area's beta diversity.

Researchers want to know why beta diversity is higher in some areas than others. Why do some species thrive in two different areas while some drop out? “There are many different factors,” says Nathan Kraft, a biologist at the University of Maryland, College Park, including climatic tolerance, soil quality, rainfall, and the differences in the dispersal abilities of species. “The really important next step is determining the relative importance of these factors and how they interact.”

His work shows that the high beta diversity along elevation gradients arises because so many species are capable of living in these areas that by chance each plot has numerous species not found in another nearby (Science, 23 September 2011, p. 1755). But Jørgensen and his colleagues suspect that chance is not the most important factor. They think that high beta diversity in tropical mountain forests like those of Madidi is driven by a special mix of environmental factors. They hope an analysis under way will prove them right. “The value of [the Madidi Project] is in testing diversity theories of a landscape with real actual data, which is very rare,” says Thomas Lovejoy, biodiversity chair at the Heinz Center in Washington, D.C. “It speaks to the importance of matching theories and what happens in nature.”

Kraft is eager to see the results: “The strength of the Madidi data set is that it's intensively sampled and spans a broad elevation gradient. It will be a great step forward.”

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