The beaver (Castor canadensis), well known for altering ecosystems, may be more influential than originally suspected. Living along streams and rivers across the United States, many beavers encounter different varieties of cottonwoods. In a study published in the March issue of Ecology, "Beavers as molecular geneticists: a genetic basis to the foraging of an ecosystem engineer," researchers from Northern Arizona University and University of Wisconsin theorized that genetic differences in pure and hybrid cottonwoods would lead to changes in the chemistry of the trees, affecting the foraging preferences of beavers.

Cottonwoods often find themselves food for the beavers. At Weber River near Ogden, Utah, beavers live amongst Fremont cottonwoods and narrowleaf cottonwoods whose distributions overlap, leading to hybridization. Beavers practice selective herbivory, essentially choosing what they do and do not eat, a behavior that can affect plant types in the community.

The researchers first observed beavers in the wild, recording their preference for different cottonwoods. They then experimentally tested the beavers by laying out branches from different cottonwoods in pure and hybrid cottonwood areas, a sort of "cafeteria" of Fremont, narrowleaf, and various cottonwood hybrid branches.

The beavers preferred Fremont cottonwoods to narrowleaf and hybrid cottonwoods. As a result, narrowleafs probably have a better chance of surviving to reproduce than do Fremont cottonwoods. The beavers not only prefer Fremont trees, but as researchers discovered, they also eat less narrowleaf if that is the only variety in their territory.

The group took another look at the chemicals in the branches chosen, and discovered the beavers preference fell in-line with lower levels of tannins in the Fremont and close hybrids as opposed to the high tannin levels in narrowleaf cottonwoods. The more Fremont gene markers present in the wood, the l
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Contact: Annie Drinkard
annie@esa.org
Ecological Society of America
31-Mar-2004

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