Biomass link to plant diversity questioned

IOWA STATE (US) — New research calls into question a decades-old theory about the relationship between how much biomass plant species produce and how many species can co-exist.

A 1970s study claimed that as plant productivity increased, so did plant richness (the number of plant species) but only to a point, with maximum species richness occurring at the point of intermediate productivity.  After that, the number of species was thought to decline, creating a hump curve when plotted on a graph.

New research published in the journal Science doesn’t support the relationship, researchers say.

“This hump pattern that everyone thought was true . . . it just isn’t there,” says Stanley Harpole, assistant professor of ecology, evolution, and organismal biology at Iowa State University. “This hump was the hypothesis for a long time, but it just isn’t supportable.

“Ecologists have long been interested in this relationship between how many plants there are and how much they produce,” says Harpole. “For years they (scientists) have been plotting correlations looking at the relationship of biomass to species richness.”

Harpole believes the original work that led to the predictions for a hump shape was good research, and showed a correlation between richness and productivity. But it didn’t show cause-and-effect relationships.

Peter Adler from Utah State University is lead author of the Science paper. He and Harpole formed Nutrient Network (NutNet), a worldwide, ecological research group of more than 70 scientists on five continents.

For the study, Harpole says NutNet researchers used “the same experiment, the same design, the same measurements were taken, the species were counted in the same way, and the biomass was clipped in the same way. It is important that you do everything in the same way.”

When the results from NutNet’s 65 research sites came in, the results were clear—only one of the sites showed a hump shape. “And that is supposed to be the ‘true’ pattern?” he says.

The NutNet group wasn’t trying to prove anyone wrong, but just hoped for clearer understanding, Harpole says.

“This is exciting science to me. We are just trying to figure out what is going on. How the world works. That is what we really wanted to know.”

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