University of Michigan
Department of Ecology and Evolutionary Biology

Network patterns in the Serengeti food web reflect habitat structure

The relationships among organisms in an ecosystem can be described by a food web, a network representing who eats whom. Food web organization has important consequences for how populations change over time, how one species extinction can cause others, and how robustly ecosystems respond to disturbances. A paper published in December 2011 by Ed Baskerville, EEB graduate student (first author) and colleagues, presents a computational method to analyze how species are organized into groups based on their interactions.

“We apply this method to the plant and mammal food web from the Serengeti savanna ecosystem in Tanzania, a pristine ecosystem increasingly threatened by human impacts,” he writes in the author summary. “This web is unusually detailed, with plants identified down to individual species and corresponding habitats. Our analysis, which differs from the compartmental studies typically done in food webs, reveals that functionally distinct groups of carnivores, herbivores, and plants make up the Serengeti web, and that plant groups reflect distinct habitat types. Furthermore, since herbivore groups feed across multiple plant groups, and carnivore groups feed across multiple herbivore groups, energy represents a wider range of habitats as it flows up the web. This pattern may partly explain how the ecosystem remains in balance.”

In addition, their approach, based on Bayesian modeling, can be easily applied to other kinds of networks, such as gene interaction networks, and can be modified to find other patterns, such as relationships between species traits and group structure.

Co-authors include Andy Dobson, professor of EEB, Princeton University; Trevor Bedford, a former U-M EEB postdoctoral fellow, currently a Newton International Fellow at the University of Edinburgh; Stefano Allesina, a former U-M EEB postdoctoral fellow, now an assistant professor at The University of Chicago; Michael Anderson, assistant professor of biology, Wake Forest University; and Rosemary Grant Collegiate Professor of EEB Mercedes Pascual.

Acid rain poses a previously unrecognized threat to Great Lakes sugar maples

The number of sugar maples in Upper Great Lakes forests is likely to decline in coming decades, according to Professor Don Zak and his colleagues, due to a previously unrecognized threat from a familiar enemy: acid rain.

Over the past four decades, sugar maple abundance has declined in some regions of the northeastern United States and southeastern Canada, due largely to acidification of calcium-poor granitic soils in response to acid rain.

Sugar maple forests in the Upper Great Lakes region, in contrast, grow in calcium-rich soils. Those soils provide a buffer against soil acidification. So sugar maple forests here have largely been spared the type of damage seen in mature sugar maples of the Northeast.

But now, a U-M-led team of ecologists has uncovered a different and previously unstudied mechanism by which acid rain harms sugar maple seedlings in Upper Great Lakes forests. The research is the subject of Sierra Patterson’s thesis, then a master's student at SNRE and a former graduate student instructor in General Ecology and Woody Plants.

The scientists have concluded that excess nitrogen from acid rain slows the microbial decay of dead maple leaves on the forest floor, resulting in a build-up of leaf litter that creates a physical barrier for seedling roots seeking soil nutrients, as well as young leaves trying to poke up through the litter to reach sunlight.

The thickening of the forest floor has become a physical barrier for seedlings to reach mineral soil or to emerge from the extra litter," said Zak, professor of ecology and evolutionary biology, natural resources and environment and co-author of an article published online Dec. 8, 2011 in the Journal of Applied Ecology.

"What we've uncovered is a totally different and indirect mechanism by which atmospheric nitrogen deposition can negatively impact sugar maples," Zak said.

The new findings are the latest results from a 17-year experiment at four sugar maple stands in Michigan's lower and upper peninsulas. By the end of this century, nitrogen deposition from acid rain is expected to more than double worldwide, due to increased burning of fossil fuels.

U-M News Service press release