More About Climate Change and Amphibian Declines

The role of climate change as a driver of amphibian declines is a controversial topic. As I described in my 3/27/2008 post, an article by Pounds et al. in Nature suggested that climate warming in South America was responsible for the emergence of the amphibian chytrid fungus (Batrachochytrium dendrobatidis) and subsequent catastrophic amphibian declines on that continent. These results have been challenged on several grounds, and in my view remain conjectural at best.

An article just published in the Proceedings of the National Academy of Sciences provides much clearer evidence that recent climatic anomalies have resulted in declines of amphibian populations. The authors report that six decades of climate data for Yellowstone National Park shows a trend of increasing temperatures and decreasing precipitation. These changes in temperature and precipitation have caused many of the ponds in the study area to dry up. This, in turn, has resulted in marked decreases in the number of sites occupied by three species of amphibians: blotched tiger salamander (Ambystoma tigrinum melanostictum), boreal chorus frog (Pseudacris triseriata maculata), and Colombia spotted frog (Rana luteiventris). These results are certainly of concern, but it remains to be seen whether these patterns hold up over the longer term.

Direct effects of climate change on amphibians due to consequent changes in habitat are likely to increase in severity in coming decades. Even the mountain yellow-legged frog, a species that requires permanent water bodies for successful breeding and overwintering, will likely be affected as what were once permanent lakes, ponds, and streams become ephemeral. Such changes could spell extinction for many populations of this already imperiled species.

Back to The Mountain Yellow-legged Frog Site.

Are Frogs Becoming Resistant to the Amphibian Chytrid Fungus?

The arrival of a new disease into a naive animal or human population usually results in strong impacts initially, but the severity of these effects often subsequently declines as the host animals develop resistance to the disease. Recent observations of amphibians persisting following die-offs caused by the amphibian chytrid fungus (Batrachochytrium dendrobatidis - "Bd") has raised speculation that these post-crash amphibian populations, including those of the mountain yellow-legged frog, are resistant to Bd. Development of resistant populations would dramatically increase the likelihood of population persistence despite the presence of Bd, and would provide ideal source populations for reintroduction efforts.

However, are amphibians really developing resistance to Bd? Unfortunately, as of yet there is no evidence of this in mountain yellow-legged frogs or in other species. Mountain yellow-legged frogs taken from populations that are persisting with Bd and introduced into nearby lakes often experience Bd-caused die-offs within a couple years. Furthermore, frogs from persistent populations succumb to Bd when held in the laboratory under a wide range of environmental conditions. Both results indicate that frogs from persistent populations are clearly not resistant to Bd. So, if frogs aren't developing resistance, how do we explain the persistence of some populations following Bd outbreaks?

There are at least three explanations in addition to frog resistance to Bd. First, it is possible that Bd strains differ in their virulence and that infection by a less virulent strain allows persistence while more virulent strains cause extinction. Second, habitat conditions (e.g., water temperature) may influence the amphibian-Bd interaction, either by changing Bd virulence or by changing the amphibian immune response. For example, in warm climates where air and water temperatures stay in a range that is detrimental to Bd (above 30 degrees C), the effect of Bd may be much less severe compared to in cooler climates that are more favorable to Bd. Third, persistence of amphibian populations in the presence of Bd may simply be a consequence of density-dependent disease dynamics. If higher frog densities allow higher Bd densities ("positive density dependence"), then after a frog population crash Bd densities should decline. Under these conditions, it is possible that frogs would experience Bd infections of lower intensity, allowing frog persistence. This persistence may be short lived, however, because if frog density subsequently increase so too will Bd, likely resulting in another frog population crash.

All four of these explanations for frog persistence are plausible, but all remain speculative. We're working to understand the role that each of these might play in allowing frog persistence but with numerous field and laboratory experiments either underway or planned in the near future, we are likely several years away from definitive answers. With Bd having already caused massive declines of mountain yellow-legged frogs across their entire range, time is definitely not on our side.

Back to The Mountain Yellow-legged Frog Site.

Frog Fable #3 - Fish Removals Cause Frog Disease Outbreaks

I recently read a letter that stated that fish removals, far from benefiting mountain yellow-legged frogs, actually cause outbreaks of the amphibian chytrid fungus (Batrachochytrium dendrobatidis - "Bd"). The purported basis for this is essentially that fish removal causes frog populations to grow and that the resulting increased frog density boosts the likelihood of Bd outbreaks. Furthermore, by removing fish populations from clusters of lakes, frog dispersal between adjacent habitats is increased and this enhances Bd spread.

Ecological theory suggests the plausibility of the above-outlined scenario. However, a decade of field research in the Sierra Nevada provides absolutely no support for the idea that fish eradications cause Bd outbreaks.

This idea seems to have its origin in two sources, the first an article written by Tom Stienstra - the outdoor columnist for the San Francisco Chronicle - published August 17, 2003, and the second a scientific paper written by me and two colleagues published in 2007 (PDF). Both sources mention cases in which initial frog population increases following fish removal were followed by frog population crashes. So, let's look at both of these in turn.

To quote the key section of Tom Stienstra's piece, "The father, Don Vachini of Novato, and sons Jason and Matt had hiked eight miles over 11,000-foot Piute Pass and into Humphreys Basin, where several gorgeous lakes are set in granite pockets. They set up a base camp at Lower Desolation Lake, set at the timber line at 11,375 feet, then trekked another two miles cross-country to their favorite lakes at the foot of towering Mount Humphrey (sic), looming overhead at 13,986 feet. These lakes are cobalt blue with pristine clarity. On windless mornings, there is a perfect mirror image of the mountain on the lakes' surfaces. The surrounding landscape is stark batholithic granite, with pockets of ice and snow and a few scraggly pines. But Vachini shook his head, a hollow look in his eyes. 'Can you believe all the trout are gone from this magnificent water?' he asked. 'After 20 years of hiking into Humphreys Basin, it breaks my heart.' In a program that could become the prototype for hundreds of high Sierra lakes, all the trout have been netted out and killed at Vichini's favorite lake.... But last month, Roland Knapp of the Sierra Nevada Aquatic Research Laboratory made his own shocking discovery in Humphreys Basin: at Vachini's favorite lake, where all the trout were cleaned out to protect the frogs, all the frogs were suddenly gone, too. They were wiped out last winter by chitrid (sic) fungus, Knapp said."

Quite a dramatic outcome, isn't it? Unfortunately, Tom Stienstra fabricated this story. Following the removal of fish from several small lakes in this part of Humphreys Basin, the frog population has thrived and in marked contrast to Tom Stienstra's vivid imagination, to date there has been no Bd-caused population crash. During my visit to this area in early September of this year, I counted more than 8000 tadpoles and 1500 adult and juvenile frogs, counts that continue the trend of a steep increase following fish removal. The above photo and the photo accompanying my October 10 post were taken immediately following this survey.

Which brings me to the article we published in 2007 (Knapp, R. A., D. M. Boiano, and V. T. Vredenburg. 2007. Removal of nonnative fish results in population expansion of a declining amphibian (mountain yellow-legged frog, Rana muscosa). Biological Conservation 135:11-20). This article focused primarily on the recovery of mountain yellow-legged frogs following fish removal in Humphreys Basin (John Muir Wilderness) and LeConte Canyon (Kings Canyon National Park). It also mentioned the important role played by Bd in limiting frog populations and in particular, the Bd outbreak in 60 Lake Basin (Kings Canyon National Park) a few years after fish removal and subsequent frog population increases in several lakes. This is decidedly not a case of fish removal causing the Bd outbreak, however. In fact, Bd is spreading across Sequoia-Kings Canyon National Park as a west-to-east moving front. Having arrived at the basin just west of 60 Lake Basin in the mid-1990s, I expected its arrival in the adjacent 60 Lake Basin by 2000. It took a bit longer than that, but in 2004 Bd outbreaks began in 60 Lake Basin and in the next four years spread across the entire watershed, reducing the mountain yellow-legged frog population to a fraction of its pre-Bd size. The first Bd outbreaks actually began in a portion of the basin that was most distant from the fish removal sites. Furthermore, Bd caused high mortality in all frog populations in this basin, regardless of frog numbers or density, providing another suggestion counter to the notion that larger frog populations are more susceptible to Bd outbreaks.

In summary, despite many years of data collection at numerous sites across the southern Sierra Nevada, there is no evidence that frog population recovery following fish removal causes Bd outbreaks. The long-term fate of the recovering frog populations in now-fishless lakes in 60 Lake Basin and elsewhere will provide the ultimate test of whether fish removal can benefit mountain yellow-legged frogs despite the presence of Bd. Until those results are in, let's stick to the facts as we currently know them. Wild extrapolation from these facts only results in the loss of credibility on the part of the person making the extrapolation.

Back to The Mountain Yellow-legged Frog Site.

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.

Back to The Mountain Yellow-legged Frog Site.

Frog Fable #1 - Frog Disease is Spread by Researchers

Due to an extremely busy summer field season, it has been several months since I've had the luxury of a few days off. As regular readers have surely noticed, that lack of time has seriously limited my ability to continue with my usual weekly posts. But now, with fall in the air, my field season has concluded and I can once again return to a more sane schedule. So, you can look forward to weekly posts once again.

As an ecologist involved in efforts to restore mountain yellow-legged frog populations, I have the opportunity to read lots of comment letters submitted by members of the public in response to various frog-related agency projects. These letters are an important part of any democratic process, but some of the statements contained in these letters take undue liberty with the facts as we currently understand them. Given my firm belief that sound decisions designed to conserve biodiversity (and mountain yellow-legged frogs in particular) absolutely depend on decision-makers and the general public understanding the current "state of the science", I'll use my next several blog posts to clarify some of the most common misunderstandings.

As detailed on the Mountain Yellow-legged Frog Site, chytridiomycosis (the disease caused by the amphibian chytrid fungus, Batrachochytrium dendrobatidis) is associated with declines and extinctions of amphibians all around the world. Mountain yellow-legged frogs are highly susceptible to this disease and many recent disappearances of frog populations in the Sierra Nevada are attributable to chytridiomycosis outbreaks (click here for details). Some people, perhaps concerned that the steady decline of the mountain yellow-legged frog will result in calls for additional removals of nonnative trout, have sought to pin the blame for this decline on the scientists studying the problem. Scientists, the argument goes, have been spreading the chytrid fungus from frog population to frog population during the course of their research.

On the surface, this seems a reasonable argument but it ignores several important facts. First, research in Australia, Central and South America, and California's Sierra Nevada indicates that the chytrid fungus is spreading as a distinct front and is moving into new areas at a very predictable rate. This uniform spread is not what would be expected if researchers (whose movements could perhaps best be described as haphazard) themselves were the agent of spread. Second, in the Sierra Nevada and elsewhere chytridiomycosis has spread into many areas that were never visited by amphibian researchers. And third, amphibian scientists around the world (including those of us working in the Sierra Nevada) adhere to a strict protocol of chemically disinfecting all of their field research gear between each visited water body or watershed.

A far more likely vector of the chytrid fungus is infected frogs moving between water bodies. In addition, adult stages of aquatic insects fly in great numbers from one water body to the next, and could be effective agents of disease spread. The half-baked notion that researchers themselves are spreading the amphibian chytrid fungus provides a convenient scapegoat but it doesn't hold up under even the most basic scrutiny.

Next week I'll discuss the common misconception that the discovery of the important role played by the amphibian chytrid fungus in causing mountain yellow-legged frog declines exonerates nonnative trout as a major cause of these declines.

Back to The Mountain Yellow-legged Frog Site.