Genes make the marsh

The role between plant genotypes and beavers in building ecosystems

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 lower the tannin levels, suggesting the “tastier wood” that the beavers preferred was due to a specific genotype.

“In contrast to the idea that plant genetics effects become weaker at the population, community, and ecosystem level, these results suggest that beavers may indirectly influence many aspects of cottonwood-stands, including genotype, age structure, and the distribution of cottonwood habitat,” said Joseph Bailey, lead author of the study.

Bailey and the other researchers note that “Our study shows that beavers discriminate at the level of closely related cottonwoods species and their natural hybrids. These points emphasize that terrestrial and aquatic ecosystems may be linked by genetically based plant traits that can directly and indirectly affect population-, community-, and ecosystem-level processes.”

The authors involved in this study included: Joseph Bailey, Jennifer Schweitzer, Gregory Martinsen, and Thomas Whitham from Northern Arizona University and Brian Rehill (currently at United States Naval Academy) and Richard Lindroth from the University of Wisconsin.