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Rana aurora: Red-legged intruder

Posted: Sunday, June 07, 2009

It all started as an innocent class science project: order frog eggs from the biological supply catalog; keep the eggs in an aquarium in the classroom; observe as they metamorphose into tadpoles and then into frogs; release the juvenile red-legged frogs into a small pond near Kennel Creek, on Freshwater Bay, northeast Chichagof Island.

Courtesy Ellen Anderson
Courtesy Ellen Anderson

Fast-forward about 25 years, and now look at the map. As of 2008, this is the known distribution of the non-native red-legged frog (Rana aurora) - approximately 20,000 acres of infestation and likely growing. DNA tests have shown these frogs originated in the Columbia River area of Oregon.

So, why should we care about invading red-legged frogs? Currently the only native amphibian species known on Chichagof Island are the well distributed western toad (Bufo boreas), and one record of northwestern salamander (Ambystoma gracile) on northwest Chichagof. Native species have developed a more or less balanced association with their environment and fellow denizens. This balance can be perturbed in several ways. A new disease, for which they are not resistant, may be introduced. New predators may arrive. An introduced competitor may gobble up resources and leave the native impoverished or worse. Disturbances ranging from large-scale climate change to small scale local physical environmental changes may alter the neighborhood enough to put the viability of the native in jeopardy unless it is able to adapt.

Western toad populations have suffered significant losses in the Lower 48, and have disappeared from broad areas of their former range, mirroring rapid worldwide declines in amphibians in general. Long-time residents in Southeast Alaska have reported sharp reductions, even in relatively pristine environments. The cause for these downward trends is uncertain, but combinations of threats mentioned above are suspected.

Western toad and red-legged frog egg masses and adults have been recorded co-habiting ponds in the infested area. It is unknown at this time if the frogs are preying on the toads. They could certainly be competing for food and suitable rearing habitat. Many amphibian declines worldwide are associated with a particular type of fungus which is known to exist in Southeast Alaska.

In 2006 toads tested at three locations on Admiralty Island were infected; toads and frogs from six locations on Chichagof were not infected. The results on samples tested in 2008 are expected soon. If the fungus is found to be present on the island, the rapid dispersal of frogs into neighboring habitats could likely spread the fungus more quickly than would naturally have occurred.

A recent ecological niche model on habitat suitability and projected effects of climate change related to the red-legged frog on Chichagof came to some interesting conclusions. The eastern half of the island, including the current infested area, contains the most suitable habitat, although it appears to be of marginal quality compared to its native range.

This habitat is predicted to become less suitable under global warming. If the frogs don't adapt to the changing conditions, they are likely to decline. If they do adapt, they could expand their niche even beyond the conditions found in their native range, similar to what the introduced cane toad has done in Australia, much to the distress of native birds and small mammals.

So what can be done about this situation? Eradication is likely to be very expensive and have a low likelihood of success considering the remoteness of the landscape. Another option is to monitor the frogs and evaluate range changes and potential effects on native species. Removal of egg masses could help control their proliferation and spread, especially along the edges of the infestation. The goal would be, as long as they are not damaging native species, to keep them restricted to Chichagof and not transport them to other areas. The niche model suggests the suitable habitat in Glacier Bay may be especially at risk of invasion. Please do not transport or release any amphibians anywhere.

With a touch of irony, this species is declining in southern British Columbia, and even extirpated in parts of Oregon and California where it is native. Maintaining the Chichagof Island population may act as insurance against future extinctions.

• Ellen Anderson is a Forest Service botanist and can be reached at ellenanderson@fs.fed.us. An invasive species is one that is not native to an ecosystem, and is likely to cause harm to human health, the economy, or the environment. This column will address plant and non-plant invasives.



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