Life under one tree's rule?

The evidence comes in a series of experiments with cottonwood trees in the lab and in the field. Cottonwoods are of particular interest because they represent the dominant species in ecosystems along the banks of streams and rivers in the western US. Only about 3 percent of the these habitats are left, Whitham says. These threatened habitats are "hot spots of biodiversity."

In February, researchers led by Jennifer Schweitzer of Northern Arizona University (NAU) published results of a study that looked at the effect of tannin concentrations on leaf-decomposition in soils. Tannins represent a genetically based trait that offer trees a defense against disease, foraging, and environmental stress.

Working with two types of cottonwoods and their hybrids along Utah's Weber River, the group found that differing tannin concentrations in fallen leaves from each type of cottonwood could explain up to 65 percent of the variation they saw in the amount of nitrogen returned to the soil. The results suggested that tannin levels had an effect on the microorganisms breaking down the leaves.

A month later, NAU researcher Joseph Bailey and colleagues published results of work along the Weber showing that beavers had a marked preference for cottonwood varieties based on their tannin concentrations - the lower the tannin levels, the tastier the wood. Because beavers can reengineer an ecosystem by building dams, a single genetic trait in a dominant species could in effect indirectly sculpt a landscape.

Last month, a group led by University of Maryland entomologist Gina Marie Wimp published the results of work that looked at the range of genetic diversity in cottonwood stands and how tightly that diversity correlated with genetic diversity in the ecological community the stands inhabited. The results suggested that the greater the genetic diversity in individual cottonwoods, the greater the diversity in the insect community linked to a given tree. The team found that variations in genetic diversity in the trees accounted for 60 percent of the variation in genetic diversity in the attendant bug population.

Based on these and other experiments, Whitham and his colleagues are taking the next step - to see if they can map entire associations within cottonwood- dominated communities and trace them back to a particular gene or part of the tree's genome. "That's where it gets contentious," Lindroth notes.

Beyond the feasibility of such a mapping effort, the notion that a community can evolve genetically raises eyebrows.

"Historically, in community biology, a lot of the previous research has assumed that communities are just assemblages of organisms and that natural selection can only work at the individual level. At higher levels of organization things just fall together as a result of individual selection," says Stephen Shuster, a zoologist and colleague of Whitham's at NAU.

Here, the team hopes to tease out evidence that genetic interactions among species can evolve distinct characteristics within a natural community.

Given the sheer scale of the experiment, the results "may not turn out the way we hypothesize," Lindroth says. Even so, the work done so far leads to several take-home messages, he says. The first: "Individual genetic factors can have a more pronounced effect on a community than we ever dreamed they could."

This, he says, leads to another, cautionary observation. With genetically modified organisms, "if you insert a gene, the fact that the gene is one of many millions in a complex ecosystem doesn't necessarily mean it's not going to have a profound effect. What we are showing is that certain genes can have very significant effects."

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