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.
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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.
Beaver canal
Beaver canal
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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BEAVERS HELP OUT YOUNG FROGS
Beavers are a boon to the environment: Their dams create ponds that provide homes for birds, amphibians, and other critters. Now scientists have found that beavers also aid their wetland companions by digging canals that young frogs use to hop from ponds to forests.
The canals, which allow beavers to transport branches and hide from predators, can stretch over hundreds of meters. But “the effect of canals on wetland ecosystems has received little study,” the researchers write in Animal Conservation. If the canals help beavers move around, they wondered, do they also help amphibians? For instance, wood frogs are born in ponds, but they must find their way through meadows to forests where they can spend the winter.
The team investigated the question at Miquelon Lake Provincial Park in Alberta, Canada, where beavers have dug canals more than 200 meters long. The researchers installed fences by canals at six ponds and checked them for wood frogs during the summers of 2011 and 2012. They also visited 14 ponds and scanned the areas with and without beaver canals for wood frogs.
The fences caught 4,473 young frogs and 279 adult frogs, the study authors report. The closer a fence section was to the canal, the more young frogs they found. In their surveys of 14 ponds, the researchers also spotted six to nine times more young wood frogs on canals than along pond shorelines without canals.
Wood frogs probably like the canals because they provide a source of water and a refuge from predators. The researchers noticed frogs taking cover under the plants on the canal banks, and they observed other amphibians such as western tiger salamanders near the canals as well. By connecting patches of habitat, the team says, beavers could be valuable allies in amphibian conservation. —Roberta Kwok | 30 October 2014
Source: Anderson, N.L., C.A. Paszkowski, and G.A. Hood. 2014. Linking aquatic and terrestrial environments: can beaver canals serve as movement corridors for pond-breeding amphibians? Animal Conservation doi: 10.1111/acv.12170.
Image © John A. Davis | Shutterstock
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