Pathways to the Ponds

Beaver canals are illustrated on this aerial photo of test ponds in the study. Image courtesy of Nils Anderson.

Beavers are well known for damming streams and creating ponds, forging new habitat for a wide range of wildlife. In the absence of streams, however, these ecosystem engineers often dig networks of canals emanating from ponds. A wildlife biologist for the Canadian province of Alberta has found that these canals are used by amphibians as travel corridors to and from the forest. Nils Anderson observed that beavers and wood frogs often share space in the same combination of pond and forest habitat, and he wondered how the frogs use the habitat modifications of the beavers.

“Beavers feel safest in the water, so the farther they can stretch the water toward their food in the forest, the better it is for them,” Anderson said of the canals beavers construct to reach their food resources. “It’s easier for them to float logs to their lodge than it is to drag them across the land.”

At his study site at Miquelon Lake Provincial Park in Alberta, beavers have dug canals that average a meter deep and a meter wide, and are up to 200 meters long. Walking the natural shorelines of 14 ponds and along their affiliated beaver-constructed canals, Anderson found nine times as many wood frogs along the canals as along shorelines that were not modified by beavers. And in late summer, after placing drift nets at varying distances from the canals, the number of young-of-the-year wood frogs he encountered was highest at the canals and declined farther away from the canal edge.

“Wood frogs are vulnerable to dehydration as they move away from their pond after metamorphosis, so they tend to move along moist corridors that connect the pond to the forest,” Anderson said. “As long as there was water in the canal, that’s where most of the frogs were.” After a canal went dry, however, frog abundance was no longer linked to the canal. “Perhaps the surface water was providing some security to the frogs,” he added.

Anderson speculates that the frogs were concentrated on the beaver canals not only for the safety of the water but also because the edges of the canals provide good places for basking and overhanging vegetation provides places for them to hide. He still wonders, though, whether the canals may be doing more harm than good for the frogs: the canals proved attractive not only to the frogs, but also to garter snakes, raccoons, herons, and other species that prey upon frogs.

In addition to wood frogs, Anderson encountered several other species of amphibians using the beaver canals, including boreal chorus frogs and tiger salamanders, but they were much less conspicuous and found in much lower densities than wood frogs, so he could not draw conclusions about their preference for the canals.

Anderson concluded that by linking ponds to adjacent forest, beavers are providing a boost to amphibian populations. “Ultimately, this research is a great illustration of how the impact of an ecosystem engineer like a beaver can vary so widely from one place to another,” Anderson concluded.


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