Must ecologists account for time to understand biodiversity in space?

June 20, 2006

Ecologists typically study biodiversity in "snapshots"--single-time surveys conducted in many locations--and try to understand why some habitats have more species than others, or why larger areas contain more species than smaller ones. But what are ecologists missing if they ignore the reality that communities are not snapshots but continually in flux?

By studying how plants in three hyper-diverse grasslands change annually over a decade, ecologists Jason Fridley (University of North Carolina, Chapel Hill), Robert Peet (University of North Carolina, Chapel Hill), Eddy van der Maarel (University of Groningen), and Jo Willems (Utrecht University) show how one crucial property of ecosystems--the species-area curve, describing the relation of area and number of species--cannot be fully understood unless annual changes in the species composition of local communities are taken into account.

Reporting in The American Naturalist, Fridley and colleagues demonstrate, for the first time, that "local" species-area curves (those confined to one community) and those of large regions can be linked if one considers that the species composition of small areas changes faster than that of larger areas.

"It is increasingly clear," says Fridley, "that plant communities are dynamic entities in which variation in space and time are inextricably linked."

Indeed, ecologists have argued for decades over why species-area curves measured locally do not seem to match predictions derived from larger areas. This study shows that smaller surveys are heavily constrained by the poor sample size of individuals in any given year. Over time, as individuals die and are replaced by others from the surrounding area, the sample size increases and the community begins to more resemble its region--but in a manner that strictly follows the region's species-area curve.

This novel connection of local and regional biodiversity patterns extends the generality of the species-area relationship to very small areas, and thus allows ecologists to explicitly link processes that drive biodiversity across scales.
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Founded in 1867, The American Naturalist is one of the world's most renowned, peer-reviewed publications in ecology, evolution, and population and integrative biology research. AN emphasizes sophisticated methodologies and innovative theoretical syntheses--all in an effort to advance the knowledge of organic evolution and other broad biological principles.

Jason D. Fridley, Robert K. Peet, Eddy van der Maarel, and Jo H. Willems, "Integration of local and regional species-area relationships from space-time species accumulation." The American Naturalist 167:7.

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