Pacific Chorus Frogs As Disease Carriers?

Last week, the journal PLoS ONE published an interesting paper by Natalie Reeder and colleagues that suggested a potential role for the Pacific chorus frog (Pseudacris regilla) as a carrier of the amphibian chytrid fungus (Batrachochytrium dendrobatidis; "Bd"). Numerous media outlets covered the paper (e.g., San Francisco Chronicle), which in essence just adds a few details to a phenomenon that was already reasonably well-known. That is, chorus frogs can carry high-intensity Bd infections while showing few symptoms of disease (i.e., chytridiomycosis). As such, they appear to be more tolerant of chytridiomycosis than other amphibian species. For example, mountain yellow-legged frogs typically die when their Bd infection intensities approach 10,000 zoospore equivalents (2010 paper by Vredenburg and colleagues). In contrast, most chorus frogs in the Reeder study maintained infections at this level but showed few of the typical disease symptoms (excessive skin sloughing, lethargy, etc.) that result from Bd-caused changes to the frogs' skin. 

The apparent ability of chorus frogs to carry high-level Bd infections without suffering significant negative effects could make this species an effective carrier of Bd. That is potentially important in the Sierra Nevada, where chorus frogs and mountain yellow-legged frogs often exist in the same habitats. Could chorus frogs be responsible for spreading Bd to formerly uninfected mountain yellow-legged frog populations? This remains a distinct possibility, but it would still be only part of the story. For example, I've documented numerous successful invasions of Bd into mountain yellow-legged frog populations despite mountain yellow-legged frogs being the only amphibian species present. Without any chorus frogs in the vicinity, how did Bd disperse into these populations? We don't know, but clearly Bd is able to do so in the absence of chorus frogs. 

It also remains unknown how chorus frogs are able to tolerate these intense Bd infections. But the Reeder paper does provide an interesting observation. That is, some of the heavily infected frogs had highly localized Bd infections in which most of the skin surface was uninfected and a few areas were highly infected. That should allow normal skin functions to continue, with little impact on frog physiology. It remains to be seen how general this result is in chorus frogs because only a few of the study frogs showed this pattern of infection, but it does suggest interesting avenues for future research.

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The Origin of the Amphibian Chytrid Fungus

The amphibian chytrid fungus (Batrachochytrium dendrobatidis: Bd) is the cause of the most spectacular loss of vertebrate biodiversity in recorded history. To date, at least 200 species have been driven extinct and hundreds more have suffered major declines. Even amphibians within the world's best protected ecosystems have been hard-hit, including California's mountain yellow-legged frog (Rana muscosa, Rana sierrae). This amphibian pathogen appears to have emerged in just the last 50 years, subsequently spreading around the world at lightning speed. 

So, where did Bd come from and what allowed its recent emergence? These are questions that researchers have asked since its description in 1999. Using the best available methods, molecular biologists from around the world have slowly but surely been zeroing in on the answers. In 2003 and 2007, studies by Morehouse et al. and Morgan et al., respectively, used evidence that Bd had little genetic variation to suggest that Bd was a recently emerged clone, not a pathogen with a long evolutionary history with amphibians. Results published in 2009 by James et al. supported these interpretations and suggested that the emergence of Bd may have been caused by a single hybridization event.

A just-published paper by Farrer et al. now advances this story even further. Using sequences of entire Bd genomes, Farrer et al. found evidence of multiple distinct Bd strains with apparently non-overlapping distributions. However, they also found a single lineage that was globally distributed, more virulent than the geographically isolated strains, and associated with worldwide frog die-offs. Based on this evidence, they suggest that contact between two previously isolated strains produced a hypervirulent strain that subsequently spread globally, causing amphibian declines and extinctions in its wake. They further postulate that the global amphibian trade was likely responsible for bringing these genetically isolated strains into contact with each other.

Another research group is using similar methods to provide an even more detailed view of the emergence of Bd as an amphibian pathogen, and will hopefully publish their results in the near future. I suspect that we haven't yet heard the final word of this evolving story. Given the likely role of human commerce in driving the emergence of Bd, there are important lessons here for biodiversity conservation in the Anthropocene. Namely, as our increasingly global economy moves goods around the world we will inevitably also move less desirable things, including invasive animals and plants but also invisible things like pathogens. The spread of introduced pathogens from their new introduction points will often be impossible to control, and decimation of naive animal and plant populations into which they come into contact is all but guaranteed. Bd provides a sobering example of what is to come.  

The citation for the latest paper is as follows: Farrer, R. A., et al. 2011. Multiple emergences of genetically diverse amphibian-infecting chytrids include a globalized hypervirulent recombinant lineage. Proceedings of the National Academy of Sciences, USA 108:18732-18736. [link]

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New Insights Into the Cause of Recent Amphibian Declines

During the last decade, Karen Lips and colleagues have documented the spread of the amphibian chytrid fungus (Batrachochytrium dendrobatidis: "Bd") southward through Costa Rica and Panama, and its devastating impacts on amphibian populations. However, it has remained unclear whether Bd was also the cause of declines in areas north of the region studied by Lips et al., including Mexico, Guatemala, and the Monteverde region of Costa Rica. Throughout this region, a host of amphibian species either disappeared or markedly declined in the 1970s and 1980s, and the cause of these declines and extinctions has until now been largely speculative. 

In a new study published this week in the Proceedings of the National Academy of Sciences, Cheng and colleagues used museum specimens collected in Mexico and Central America during the period 1967-1987 to test for the presence or absence of Bd. Their results provide compelling evidence that since the 1970s Bd has been spreading south through southern Mexico and Central America (red dots with stars in figure). This timeline dovetails nicely with that for Costa Rica and Panama posited by Lips et al. (red dots in figure). Across their study sites, Cheng et al. also show a repeated pattern of Bd being absent from amphibians collected in the early years of their analyses followed by initial detections of Bd and the coincident decline of multiple amphibian species. 

So, it is now quite clear that Bd was a major cause of enigmatic amphibian declines that occurred in this region during past decades. The molecular technique developed by Cheng et al. to detect Bd on preserved museum specimens should open the door to a host of future Bd-related studies that take advantage of additional preserved amphibian specimens stored in museums around the world. I suspect that these specimens have many a tale to tell regarding the global spread of Bd and its impacts on amphibians. 

Further reading:
Lips, K. R. 1998. Decline of a tropical montane amphibian fauna. Conservation Biology 12:106-117.

Lips, K. R. 1999. Mass mortality and population declines of anurans at an upland site in western Panama. Conservation Biology 13:117-125.

Lips, K. R., F. Brem, R. Brenes, J. D. Reeve, R. A. Alford, J. Voyles, C. Carey, L. Livo, A. P. Pessier, and J. P. Collins. 2006. Emerging infectious disease and the loss of biodiversity in a Neotropical amphibian community. Proceedings of the National Academy of Sciences, USA 103:3165-3170.

Lips, K. R., J. R. Mendelson III, A. Munoz-Alonso, L. Canseco-Marquez, and D. G. Mulcahy. 2004. Amphibian population declines in montane southern Mexico: resurveys of historical localities. Biological Conservation 119:555-564.

Rovito, S. M., G. Parra-Olea, C. R. Vásquez-Almazán, T. J. Papenfuss, and D. B. Wake. 2009. Dramatic declines in neotropical salamander populations are an important part of the global amphibian crisis. Proceedings of the National Academy of Sciences 106 3231-3236.

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Mitigating the Effects of Chytridiomycosis - Part 2

Chytridiomycosis is the disease of amphibians caused by the fungal pathogen, Batrachochytrium dendrobatidis. Given the severe impacts of this disease on the world's amphibian biodiversity, there is a lot of interest in mitigating these impacts to the extent possible. I wrote about some of those efforts in a previous post. Doug Woodhams and colleagues have just published a paper that reviews the progress made to date in this important field (pdf). Having been involved in research that tested some mitigation strategies, I suspect that it will be quite a while before we have effective mitigation measures that can be effectively applied in the wild. 

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Mitigating the Effects of Chytridiomycosis

For the last couple of years our research group has conducted several field experiments aimed at understanding the extent to which we can change the outcome of outbreaks of the amphibian chytrid fungus (Batrachochytrium dendrobatidis - "Bd") from frog population extinction to persistence. This summer and last we treated frogs at die-off sites with anti-fungal drugs, something I never in my wildest nightmares imagined that I'd ever be doing. My field crews and I put everything we had into those experiments but so far success remains elusive. 

Given our attempts at changing disease outcomes I was excited to attend a disease mitigation workshop last week in Zurich, Switzerland and learn what other researchers from around the world have been trying. Over a two day period we learned of each others myriad efforts to confront the Bd plague and were sobered to realize that all of the attempts at changing Bd disease outcomes attempted so far have failed. 

At a Bd-positive site in Mallorca (Spain) Jaime Bosch and his colleagues removed all of the amphibians, treated them in the laboratory with an anti-fungal drug until they were Bd-free, and then completely dried the pond. When winter rains refilled the pond the treated tadpoles were released back into the site. Much to everyone's surprise by spring the tadpoles were once again infected with Bd. Given that Bd apparently does not have a resistant stage that could survive dessication it remains a mystery how Bd survived at the site or reinvaded so quickly (additional details are provided here). 

A graduate student at the University of Zurich, Corina Geiger, recently conducted an experiment in which she established frog populations in large outdoor tanks which she subsequently infected with Bd. Once the frogs showed evidence of chytridiomycosis she treated the entire tanks with antifungal drugs. For six weeks following treatment frogs were uninfected, but then Bd reappeared and reinfected the frogs. These results mirror our own results to date in the Sierra Nevada. 

Other mitigation efforts are currently ongoing, including an experiment using frog skin bacteria that have strong anti-Bd properties (additional details provided here), and another in which amphibian densities are being temporarily reduced to assess the effect on disease dynamics.

The ineffectiveness of the anti-Bd treatments attempted to date is obviously disappointing and illustrates just how incomplete our understanding of Bd still is. If we do stumble across an effective mitigation strategy it seems it will be almost entirely a stroke of luck. And yet, with Bd spreading into new uninfected populations with every passing month, we don't have the luxury of waiting for better information.

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Two New Papers Detail Impact of Disease on Sierran Frogs

Two papers by our research group were published today in the journal, Proceedings of the National Academy of Sciences (PNAS). Both papers are open access and can be downloaded from the PNAS web site (Vredenburg et al. paper; Briggs et al. paper). These papers represent more than a decade of research by our group on the role of the amphibian chytrid fungus (Batrachochytrium dendrobatidis - Bd) in driving the decline of mountain yellow-legged frogs in the Sierra Nevada.

The first paper (by Vredenburg, Knapp, Tunstall, and Briggs) describes the frog-Bd dynamics in detail for the first time and provides a critical insight into how Bd outbreaks might be controlled. Bd was absent from the three study basins at the inception of the research but invaded the basins in 2004-2005. Following its initial arrival Bd spread through the basins at 0.6 km per year, eventually infecting all mountain yellow-legged frog populations. 

Within 1-2 years of its first detection in a frog population, the population began to show evidence of severe chytridiomycosis (the disease caused by Bd). Frog die-offs and population crashes occurred when infection intensities (amount of Bd on a frog's skin) reached a critical threshold. The fact that a disease threshold exists is important because it provides a target for intervention strategies, strategies designed to prevent infection intensities from surpassing this threshold. One strategy, clearing frogs of Bd using an anti-fungal drug, was tested in two of the study basins in 2009 and preliminary results are promising. We'll be conducting additional tests in 2010, one designed to investigate whether clearing frogs of Bd early in an epidemic can actually prevent the epidemic from occurring.


Although most frog populations are extirpated following Bd epidemics, a few persist despite ongoing chytridiomycosis. Scientists have suggested that this persistence could be a consequence of selection for reduced susceptibility of frogs or reduced virulence of Bd. In the Briggs et al. paper, we use a mathematical model to demonstrate that neither of these changes are necessary to explain the long-term persistence of infected frog populations in the presence of Bd. Instead, this outcome could be solely the result of density-dependent host-pathogen dynamics. Under this scenario, epidemics are the result of Bd invading naive frog populations existing at their naturally high densities. This allows the production of vast numbers of zoospores (the infective stage of Bd) that overwhelm any frog defenses and causes massive frog die-offs. Most populations are extirpated but frogs in a few populations survive by chance alone. In these populations, zoospore density is kept low by low frog numbers, and frogs are able to tolerate the resulting low Bd infection intensities. As a consequence frog populations persist at low densities with Bd over the long term. 


This model assumes no role for an adaptive immune response by frogs against Bd. Although this is consistent with our current knowledge of the frog-Bd interaction, a series of experiments we are currently conducting will provide clearer insights into the existence of any such response. 


For the first time in several years I feel like we are making substantial progress toward understanding and potentially controlling chytridiomycosis. Let's hope our advances haven't come too late.

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The Origin of the Amphibian Chytrid Fungus

One of the most pressing questions related to the amphibian chytrid fungus (Batrachochytrium dendrobatidis - "Bd") is where this pathogen originated from. Unfortunately, we still don't know, but a recently published paper adds an interesting wrinkle to what we do know. Until recently, the available data suggested an African origin. The "Out of Africa" hypothesis was based on the fact that the earliest known record of Bd was from South African amphibians collected in 1938. Under this hypothesis, Bd was spread around the world as a consequence of the large-scale export of African clawed frogs (infected with Bd) from Africa for medical research starting in the 1940s.


Now Goka and colleagues present detailed information on the distribution and genetic structure of Bd in Japan, the first such study for any Asian country. In addition to showing that Bd is widespread in amphibian populations across Japan, the authors also state that Bd was detected on amphibian specimens collected as early as 1902. Furthermore, one of the species infected with Bd is the Japanese giant salamander (Andrias japonicus - shown in photograph), an ancient species that reaches a length of more than 1 m (39") and is apparently not negatively affected by Bd infection. This species was infected with unique Bd strains that were not found on any other Japanese amphibians, suggesting that the Bd-Andrias relationship is the result of a long-term coevolution between pathogen and host. These findings led Goka et al. to suggest that Bd originated in Japan, not Africa.

Additional research will be necessary to validate this conclusion, but the possibility that Bd originated in Asia certainly has shaken up our current thinking on the origin of this pathogen. For now, all we can say with some certainty is that Bd originated somewhere in the world and was subsequently moved around the globe by human activities. 

For details on the Japanese study: Goka, K., J. Yokoyama, Y. Une, T. Kuroki, K. Suzuki, M. Nakahara, A. Kobayashi, S. Inaba, T. Mizutani, and A. D. Hyatt. 2009. Amphibian chytridiomycosis in Japan: distribution, haplotypes and possible route of entry into Japan. Molecular Ecology 18:4757-4774.


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Chytridiomycosis in Asia

Of the Earth's amphibian-harboring continents (all but Antarctica), Asia was until recently unique in not yet having been invaded by the amphibian chytrid fungus (Bd). That now seems to be changing quickly. During the last few years ago Bd was reported from Japan, South Korea, and Indonesia, all countries around the periphery of this amphibian-rich continent. In the last few weeks Bd was reported from the Philippines, another country on Asia's edge. For a description of the amphibian fauna of the Philippines, check out Herpwatch Philippines.

Given that elsewhere in the world, including Australia, Central America, and California's Sierra Nevada, Bd spread as a distinct wave it seems likely that these initial occurrences of Bd in Asia represent the first outbreaks in what will become a wave of infections. Given the high amphibian species diversity in Asia and high levels of endemism, the spread of Bd across this continent will almost certainly result in hundreds of species extinctions.

Will we find a way to stop the spread of this disease or will our only role be to describe Asia's amphibian extinctions?

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Another Disease-caused Frog Conservation Crisis

The mountain chicken, Leptodactylus fallax, is one of the world's largest frogs, with adults sometimes weighing up to one kilogram (>2 lbs). This frog was once common on some Caribbean islands but has been decimated by the amphibian chytrid fungus (Bd). In recent years the frogs were found only on the island of Montserrat, an island that until recently was free of Bd. During the last few weeks, these frog populations have suffered severe Bd outbreaks that have killed many hundreds of animals. To ensure that these frogs are not driven to extinction frogs have been flown to three zoos in Europe where they will be used to start a captive breeding population. For more information, check out this story: http://www.wildlifeextra.com/go/news/mountain-chickens472.html#cr

The challenges posed by Bd to the conservation of the world's amphibians would have been unthinkable only a few years ago. This stuff gets more challenging by the day.

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Amphibian Decline Movie - Sadness and Hope

I watched the film, "Frogs: The Thin Green Line", last night. Filmmaker Allison Argo and her team did an amazing job of bringing the amphibian decline issue to a general audience. Despite how dire the situation is I was left with a feeling of hope that collectively we just might make a difference. As usual, time will tell.... The film can now be viewed online on the PBS web site.

As I mentioned recently on The Mountain Yellow-legged Frog Site, the April issue of Fly Rod & Reel magazine has an article by renowned conservation writer, Ted Williams, on the lawsuit over fish stocking in California that was recently won by the Center for Biological Diversity and the Pacific Rivers Council. This is far and away the best article written so far on this issue. I've been hoping that this article would soon be posted on the Fly Rod & Reel web site, but that hasn't happened yet. So, here is a scanned copy of the article (PDF). Additional details on this topic are available on the California Department of Fish and Game web site and in my 4/18/08 and 11/21/08 Frog Blog posts.

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Global Amphibian Declines - The Movie

During the past year filmmaker Allison Argo and her team have been traveling around the world to film declining amphibians and find out from scientists what can being done about it. In addition to filming in Panama and the southeastern U.S., Allison's team also spent a few days with my research group while we conducted research on Sierra Nevada yellow-legged frogs in Yosemite National Park. The resulting film, "Frogs: The Thin Green Line" will be shown for the first time on April 5 (this Sunday) on PBS. Allison's previous films on amphibian declines (The Last Frog), declining shorebirds (Crash: A Tale of Two Species), and other wildlife-related topics were extraordinarily good, so I suspect that this latest film will be well worth watching. Hopefully Allison didn't include the footage of me falling into the pond.... On 3/31,the film's creators posted a podcast from "behind the scenes". Check it out.

If you do watch the film, feel free to post your reviews and thoughts here.

Now to the task of finding a friend who owns a TV....

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New National Geographic Story on Amphibian Declines

Writer Jennifer Holland and photographer Joel Sartore have created a masterful story on the global amphibian decline phenomenon, just published in National Geographic magazine. The situation surrounding the mountain yellow-legged frog is described in the story, and Joel's photographs of live and dead Rana muscosa from Sixty Lake Basin bring this animal's plight into a sad focus. Check it out.

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One Fewer Frog in the Forest

Research by Karen Lips and others on amphibian declines in Central America has made clear that the scope of these declines is unprecedented in modern times. In recent years dozens and perhaps hundreds of Central American amphibian species are thought to have been driven to extinction by the amphibian chytrid fungus. The following article provides a glimpse at one of those amphibians, the Panamanian golden frog. This species is a national icon in Panama but is now believed to be extinct in the wild (link). The only individuals still in existence are those housed in a captive breeding facility in Central America. For additional information about this iconic species, check out www.ranadorada.org.

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Once Upon a Time....

I remember with crystal clarity the day I realized the impact that the amphibian chytrid fungus (Batrachochytrium dendrobatidis - Bd) was having on mountain yellow-legged frogs in the Sierra Nevada. It was July 2000 and my field crew and I were surveying lakes and ponds along the Monarch Divide and Cirque Crest for amphibians and fish. I'd been as excited as a little kid to get into this area and see some of the most remote lakes in the Sierra, places where several years earlier a backcountry ranger had made observations of many mountain yellow-legged frog populations. When the trip was over 20 days later I felt like I'd been hit over the head with a hammer. The future of frog restoration in the Sierra Nevada had just gotten a lot more complicated than I ever imagined it would be.

Up until that trip the available data on Bd and its impacts was still sparse and remained an abstraction to me. The observations made by Dave Bradford 20 years earlier of frog die-offs in western Sequoia National Park were worrisome but were surrounded by enough uncertainty regarding the cause that I wasn't paying those observations much attention. Afterall, for the past five years I'd surveyed hundreds of lakes and the effects of fish were as clearcut as could be. But there were a few lakes that I'd run across during all of those surveys that did give me pause. There were those lakes near Pavilion Dome that contained fabulous frog habitat but in which I found either no frogs or only the decaying carcasses of frogs that had died the previous winter. And that lake between Lake Basin and Dumbbell Basin where I'd seen lots of frogs during a backpack trip in the early 1990s but by the time we surveyed that site there wasn't a frog to be found. These observations nagged at me but still lacked any broader context. In contrast, my recent analyses of survey data from more than 2000 lakes had clearly shown the negative effect of fish on frogs, and early results from two fish removal experiments conducted by me and by Vance Vredenburg showed dramatic increases in the frog populations at these sites just a few years after fish removal. Restoration of frog populations wasn't going to be easy because fish were so ubiquitous but at least we had the tools to reverse the decline of the mountain yellow-legged frog. To my eyes, the future looked bright.

The first lake I surveyed on that Monarch Divide trip was at the head of the Gorge of Despair. Looking down on the lake from above I could only smile. There should be gobs of tadpoles in that warm cove on the north side of the lake, I thought, and adult frogs will be sunning by the hundreds on those granite slabs near the outlet. An hour later I had finished the survey and could only shake my head. I'd found only four mountain yellow-legged frog tadpoles and had not seen a single adult. Even more alarming was that the tadpoles were clearly sick. We didn't have an accurate method of detecting Bd in those days but my notes on the datasheet that I filled out that day had an ominous tone: "All four tadpoles were large (~3 years old). The one individual I was able to catch had only a partial beak and no toothrows. Skin had a mottled appearance. Chytrid fungus?". Every subsequent day produced similar observations. In lake after lake where frogs had been abundant just a few years earlier we found either no mountain yellow-legged frogs or only a few tadpoles, all with mouthpart deformities suggestive of Bd infection.

Twenty days later, surveys completed and out of food, we hiked down the Copper Creek trail to Cedar Grove. On that long hot descent I struggled to put all of the pieces together. Frog die-offs in western Sequoia National Park in the 1970s and 1980s, healthy frog populations throughout the eastern portions of Sequoia and Kings Canyon National Parks, and now evidence of recent and ongoing die-offs along the Monarch Divide and Cirque Crest.... Could it be that Bd had been spreading from west to east across this part of the Sierra Nevada since the 1970s and that even those huge frog populations in places like Barrett Lakes Basin and Sixty Lake Basin might someday also succumb to this plague? The thought sent shivers down my spine and haunted me for years.

I called Dave Bradford the next day and before I could describe to him what we'd seen on our trip I somewhat immodestly blurted out, "I know what wiped out your frogs on the Tablelands - the chytrid fungus!". I don't know what he thought of my remark or the subsequent description of lake after lake with hardly a frog to be found, but I had seen the future of the mountain yellow-legged frog and it filled me with a profound sense of dread. All that I knew was about to change and the frogs that to me had become as much a part of the Sierra Nevada as the towering granite peaks were soon to be pushed aside by an unseen and unstoppable force.

And so it has come to be. Many of the lake basins where I counted thousands of frogs in the late 1990s are now frogless. When I walk along a lake shoreline no frogs jump into the water from their grassy hiding places. The shallows where tadpoles used to congregate by the thousands on warm afternoons, thrashing frantically in their retreat to deeper water when I approached, are calm and placid now. The mayflies and beetles are still in abundance in the nearshore waters and on the surface the lakes are as beautiful as ever, but these lakes are profoundly changed, perhaps forever. The same could be said of that idealistic young biologist....

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Save the Frogs Day - April 28 2009

April 28, 2009 is the first annual Save the Frogs Day. Check out www.savethefrogs.com/day for more information.

The San Francisco Chronicle had a story on efforts to combat the amphibian chytrid fungus in their 2/19/2009 edition. The battle continues....

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Salamanders Declining in Latin America

My research on amphibian declines focuses almost exclusively on mountain yellow-legged frogs. As such, it is easy to forget the fact that the amphibian decline phenomenon has a global reach and that these declines affect a huge diversity of amphibian species. Compared to the large body of evidence showing declines in frogs and toads, evidence for salamander declines has been sparse but a new paper in the Proceedings of the National Academy of Sciences (available here) brings these declines into startling focus. Rovito et al. describe the disappearance of multiple salamander species from montane sites in Mexico and Guatemala (figure from paper showing these declines is shown to left). Surveys conducted at the study sites in the 1970s turned up more than 70 salamanders per visit but surveys in 2005-2007 at the same sites turned up fewer than five salamanders per visit. Several of the most common species during the 1970s were not found during any of the recent surveys and may be extinct.

Given the very high diversity of salamanders in the Neotropics these declines suggest that many salamander species may be at much higher risk of extinction than has been appreciated. Unfortunately the cause of the declines described by Rovito et al. remains obscure. The amphibian chytrid fungus was detected on some salamanders at the study sites and could be a contributing factor as could climate change.

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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.

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Another Day, Another Frog Die-off

I'm out in the field this week and won't be able to post anything substantive as a result. However, this photo gives you an indication of what we found on our first trip - lots of dead frogs killed by chytridiomycosis. I've seen these die-offs many times, but still get a knot in my stomach whenever I stumble across another one. More next week.

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