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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An Unnatural Quiet

This summer I returned again to one of the Sierra's most remote lake basins to resurvey the amphibian populations there. This basin holds a special place in my heart due to its spectacular beauty, isolation, and at one time, an incredible Sierra Nevada yellow-legged frog population. The first time I visited this basin was in 1997 when my assistants and I conducted our initial surveys of its frogs, fish, and invertebrates. Back then, the sheer number of frogs was overwhelming. 

I remember counting 10,000 tadpoles in a single lake. The frantic swimming of these tadpole masses at my approach turned the tranquil water surface to a muddy froth. Adult frogs seemed to occupy every possible bit of water; nearly every lake, pond, stream, and puddle was full of them. As I walked along one lake shore, a frog dove off of a damp ledge ten feet above my head and with legs splayed wide splashed into the water at my feet. The abundant bears in this basin seemed just as fond of the frogs as I was. In several spots where tadpoles aggregated by the thousands, I saw where bears had waded out into the water to catch them. One evening I watched a Clark's Nutcracker land at the waters edge and spear a tadpole. With the tadpole in its beak it flew to a nearby snag, snipped off the tadpole's tail, and fed the body to a begging fledgling. Transfixed, I sat down and watched the show repeat itself several more times. When the adult and fledgling had flown away I walked to the snag and saw that the branch the fledgling had been perched on was covered with 50 or more tadpole tails, some dry and shriveled, some still wet. What an incredible web of life these frogs were a part of!

Back then the world was unaware of the amphibian chytrid fungus (Batrachochytrium dendrobatidis - "Bd") and its devastating effects on amphibians around the globe. I took it for granted that given the paucity of trout in this basin the frogs would always be there, for the bears, for me. A few years later Bd arrived in the basin, causing the collapse of one frog population after another. Within five years the frog numbers had decreased by 95% but I still held out the hope that maybe a few would survive to repopulate the lakes. 

Our resurveys this summer showed that even that modest hope was too optimistic. Most water bodies were now entirely frogless. A few contained only a single adult frog. We found some tadpoles, but too few to give us much hope that the population would still be there the next time we did our resurveys. 

Sitting on a lakeside boulder eating my lunch in the summer sun, I couldn't help notice how quiet the basin had become. No more frothing masses of tadpoles. No frogs jumping into the water by the 10s and 100s in front of me as I walked along the shoreline. Bear sign, once so common, was far less in evidence, although I did see one set of bear tracks that went into the lake where a few tadpoles still basked in the warm shallows, but the paucity of animals would hardly have made a snack much less a meal. No Clark's Nutcrackers eating tadpoles. The mayflies, caddisflies, and beetles were still there, but the once-dominant frogs and the web of life they once supported were gone.

When I first happened upon this place it was defined by the frogs, frogs as predators, frogs as prey, frogs as inhabitants. Now it was different. In the span of less than a decade, a shangri la had become a place of ghosts.


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Back from a Summer in the Mountains

It has been months since I've added any posts. My only excuse is that I've been in the mountains almost continuously since mid-June conducting research. I've been impressed at the number of people who emailed me during my absence from the blogosphere to make sure I hadn't abandoned my practice of regular posts related to the mountain yellow-legged frog. Rest assured that I have not. Now that my field season is over I'll do my best to again provide updates on a more regular basis. 

Of the many frog-related happenings that occurred this summer, perhaps the most important was a decision on September 15 by the California Fish and Game Commission to designate the mountain yellow-legged frog (both Rana sierrae and Rana muscosa) as a "candidate" species under the California Endangered Species Act (CESA). This action was taken in response to a petition submitted earlier this year by the Center for Biological Diversity. The California Department of Fish and Game now has one year to develop a formal status review that will recommend whether or not the mountain yellow-legged frog should be listed. 


During the one-year review period, the CESA requires that candidate species be treated as if they were already listed. This is an important difference between the Federal and California Endangered Species Acts. Under the Federal ESA, a candidate species has no formal status until it is actually listed. Because of the potential for the new "candidate" status of the mountain yellow-legged frog to immediately impact a wide variety of ongoing activities the Commission adopted a series of "take" exemptions (i.e., exemptions for actions that could potentially result in harassment or mortality of mountain yellow-legged frogs). 

Exempted actions include those for the Department's ongoing fish stocking programs (no surprise there), timber harvest, reservoir operations, and scientific research. Although some have expressed their displeasure with these exemptions, I support them because they will provide individuals and agencies with some breathing room to continue already-permitted activities during the one-year review period. The negatively publicity that could result from an immediate shutting down of these existing projects (especially fish stocking) would likely have more long-lasting negative effects on frog conservation efforts than these projects would. 


So, by next October we'll get an up-or-down vote by the Commission on the listing status of the mountain yellow-legged frog. If the frog is listed under CESA I suspect that efforts to conserve and restore populations of this imperiled species will ramp up to a level not yet seen.


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