Frog Fable #2 - Declines are Caused by Disease not Trout

The causes of global amphibian declines are hotly debated within the scientific community, but one of the best documented causes is the role of nonnative trout introduced into historically fishless lakes in the Sierra Nevada. (If you'd rather not take me on my word on this one, check out a 2003 review of the topic by Kats and Ferrer - Diversity and Distributions 9:99-110.) The conclusion that predatory trout play an important role is based on surveys of thousands of lakes across the Sierra Nevada (likely the most extensive lake survey ever conducted anywhere) and on experiments in which trout were removed from lakes and frog populations subsequently increased rapidly (click here for details). Collectively, the results from these studies are irrefutable: introduced trout have severe negative effects on mountain yellow-legged frogs and are directly responsible for the extinction of many frog populations.

The story of the mountain yellow-legged frog's decline became considerably more complex with the recent discovery that the extremely virulent amphibian chytrid fungus (Batrachochytrium dendrobatidis - "Bd") was spreading across the Sierra Nevada (and through other habitats around the world) and was causing widespread extinctions of mountain yellow-legged frog populations. Soon after these studies were published, I began to hear the misconception that these results "proved" that trout were in fact not responsible for the decline of the mountain yellow-legged frog. I beg to differ. The important role of Bd in causing declines in no way changes the obvious negative effect that nonnative trout have on mountain yellow-legged frogs. We know for certain that trout and Bd BOTH play important roles in driving this decline. For example, Bd is ubiquitous in Yosemite National Park but despite this surveys of all 3000+ of the Park's lakes and ponds showed a clear negative effect of nonnative trout on mountain yellow-legged frogs and other amphibians (Knapp 2005).

The current "state of the science" regarding the relative importance of Bd versus trout in causing frog declines isn't complete, but it does provide critical insights to guide restoration actions. Here is what I wrote in a recent peer-reviewed article (Knapp et al. 2007):

"... there are several reasons why fish eradication is likely to remain a critically important tool for restoring R. muscosa populations. First, patterns of B. dendrobatidis spread remain enigmatic and it is not clear that B. dendrobatidis will eventually spread to all R. muscosa populations. Second, although the arrival of B. dendrobatidis may reduce the benefits to R. muscosa populations conveyed by fish eradications, our understanding of B. dendrobatidis and its effects on R. muscosa is still far from complete, and fish eradications may in fact confer long-term benefits. For example, the presence of nonnative fish has relegated many R. muscosa populations to marginal habitats that only support small frog populations or have increased the degree of R. muscosa population isolation. As a result of small population sizes and isolation, these populations may have a lower likelihood of surviving stochastic events such as disease outbreaks. If some R. muscosa populations do persist following a disease outbreak, this creates the potential for host-pathogen evolution that may over time favor more resistant frogs and/or a less virulent strain of B. dendrobatidis.... As such, the eradication of fish populations is likely to be a key element of any attempt to conserve and restore amphibian populations inhabiting these montane lentic environments, regardless of whether other anthropogenic factors also exert a controlling influence on these populations."

The only way to determine with certainty whether fish eradications can in fact benefit Bd-infected mountain yellow-legged frog populations is to conduct experimental fish removals in areas where Bd is known to be present. Precisely such experiments are currently being conducted in Yosemite and Kings Canyon National Parks, but several years of frog surveys will be necessary before strong conclusions are possible.

In closing, we need to move beyond overly simplistic views ("Bd is the cause, not fish") of the complex situation regarding the decline of the mountain yellow-legged frog. Instead, all of us interested in the future of frogs and fish in the Sierra Nevada need to work together to craft solutions that are based on the best available science. Twisting the science to fit particular perspectives does not advance this cause.

Note: I'm currently away from my office so the posting of any comments submitted by readers will be delayed by several days. Thanks in advance for your patience.

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