Over the Hump: Through Network Science, USU Ecologist Challenges Model

For decades, ecologists have toiled to nail down general principles explaining why some habitats have so many more plant and animal species than others. Much of this debate is focused on the idea that the number of species is determined by the productivity of the habitat. Shouldn’t a patch of prairie contain a different number of species than, say, an arid steppe or an alpine tundra?

Maybe not, says an international team of scientists that pooled its resources to re-evaluate the relationship between species numbers and habitat productivity. Their innovative, standardized global sampling effort of 48 sites on five continents yielded an unprecedented data set.

“Our study shows no clear relationship between productivity and the number of plant species in small study plots,” says Utah State University plant ecologist Peter Adler.

Adler and his colleagues’ findings, which represent a significant advance in ecological thought, appear in the Sept. 23, 2011, issue of the journal Science.

“We challenged a prevailing model developed in the early 1970s by British ecologist J. Philip Grime,” says Adler, lead author on the paper. “He proposed that the number of species rises then declines with increasing productivity.”

Though hotly debated, Grime’s “hump-shaped” model has remained a textbook standard for nearly four decades.

In an “Emperor’s New Clothes” moment, Adler remembers skeptical observations about the hump-shaped model made by graduate students in his classroom.

“‘Why do ecologists spend so much time on this model when the evidence to support it is so weak?’ they asked me,” he says. “That was the kick in the pants I needed to pursue this question.”

The challenge was daunting. Existing, disparate case studies couldn’t conclusively support Grime’s unimodal pattern. Inconsistencies in data collection methods further hampered efforts to distill reliable empirical evidence to support the hump-shaped model.

So, five years ago, Adler and fellow ecological researchers formed Nutrient Network or “NutNet,” a cooperative research initiative dedicated to investigating biodiversity and ecosystem processes in grasslands around the world. Based at the University of Minnesota, coordination of the network is currently funded by a National Science Foundation grant to network organizers and UM researchers Elizabeth Borer and Eric Seabloom.

“Our work not only sheds light on this classic question, it also demonstrates the power of our network approach,” Borer says. “NutNet data are poised to inform many pressing ecological questions. Similar global, grassroots collaborations could help settle other longstanding scientific debates.”

“Our data emphasize the need to consider many factors to explain patterns of diversity — not just productivity alone,” Adler says.

For the ecological community, he says, NutNet’s findings should spur ecologists to focus on other important factors regulating biodiversity, such as evolutionary history, disturbance and resource supply.

“It’s time to remove outdated models from our textbooks and concentrate on more sophisticated approaches,” Adler says. “That will improve our ability to predict the effect of environmental change on biodiversity.”

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Contact: Peter Adler, 435-797-1021, peter.adler@usu.edu

Writer: Mary-Ann Muffoletto, 435-797-3517, maryann.muffoletto@usu.edu