A Framework for Future Frog Recovery Efforts

As I’ve mentioned in previous posts, the state Endangered Species Act (ESA) listing process for the mountain yellow-legged frog is underway, with a Status Review document due to the California Fish and Game Commission this October. In addition, the U.S. Fish and Wildlife Service (USFWS) recently sent a letter to state and federal agencies stating their intention to begin the “final rule” process for listing the mountain yellow-legged frog in the Sierra Nevada under the federal ESA. Given the steep decline of both species of the mountain yellow-legged frog in the Sierra Nevada (Rana muscosa, Rana sierra), it seems likely that both the state and federal processes will end up listing both species as either threatened or endangered. These listings will accelerate the development and implementation of recovery actions across the range of the species.

To date, these recovery efforts have involved the removal of nonnative trout from critically-important lake and stream habitats, and have been undertaken primary by the California Department of Fish and Game, National Park Service, and U.S. Forest Service. These projects have set the stage for marked increases in affected mountain yellow-legged frog populations. However, they have been undertaken without much in the way of coordination between agencies. Given that these recovery projects currently affect only a tiny fraction of the Sierra Nevada, this coordination hasn’t been critical. But with an escalation of recovery efforts, close coordination between recovery projects will be essential to ensure that implementing agencies plan recovery efforts based on a mutually agreed-upon prioritization of recovery locations, and that recovery projects are uniformly based on the best available science and utilize a consistent set of site selection criteria, implementation methods, and monitoring protocols.

So, what mechanisms exist to coordinate these recovery activities? The most likely is the development of a mountain yellow-legged frog Conservation Strategy. The USFWS recently requested that all state and federal agencies with jurisdiction over mountain yellow-legged frogs and their habitat appoint a representative for inclusion on such a team, but it remains uncertain what the time frame for this Conservation Strategy will be. Ideally, the Conservation Strategy would be in place within one year, allowing coordinated recovery projects to be implemented either during the ESA listing processes or soon thereafter. This plan would identify the locations of recovery actions, prioritize these actions, and outline protocols for implementing the actions and monitoring their effects. Such a coordinated effort, with a centralized data base in which all monitoring data from recovery projects is maintained, would allow rapid assessment of the degree to which projects are meeting recovery goals.

In addition to providing a coordinating framework within which recovery projects are implemented, it is important that the Conservation Strategy be transparent to all stakeholders. This transparency will provide all interested parties with information on the criteria used to identify and prioritize recovery projects, and the locations of recovery projects. There is always a reluctance to provide the public with the exact locations of trout removal efforts due to concerns that disgruntled individuals could sabotage projects, but it is time to take this important step. It is my hope that any risks will be outweighed by the benefits that will come from fully involving the public in the task of recovering mountain yellow-legged frogs across the Sierra Nevada.

Only time will tell whether the Conservation Strategy meets these important goals.

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The Changing View of Pesticides as a Driver of Frog Declines

Pesticides have long been hypothesized to be drivers of amphibian declines in the Sierra Nevada. In the early 1970s, Lawrence Cory and colleagues published a study showing that DDT residues could be detected in mountain yellow-legged frogs throughout the Sierra Nevada, and they suggested that this was the result of DDT applications in the Central Valley. Since then, a myriad studies have detected many additional pesticides in numerous media in the Sierra Nevada, including water, lake sediments, air, snow, rain, fish, and frogs. 

The hypothesis that pesticides could be the cause of frog declines is rooted in the observation that declines observed during the 1980s and 1990s were most severe close to the Central Valley but populations in the High Sierra seemed to be relatively healthy. In an effort to test the pesticide hypothesis, in the mid-1990s, Gary Fellers and colleagues moved southern mountain yellow-legged frogs (Rana muscosa) from Sixty Lake Basin near the Sierra Nevada crest (where frogs were still abundant) west to the Tablelands area of Sequoia National Park where frogs had disappeared a decade earlier due to unknown causes. During this study, pesticide concentrations made in Sixty Lakes Basin and Tablelands indicated that levels were generally higher in the Tablelands area. The frog populations translocated to the Tablelands declined rapidly to extinction, a result at least consistent with the pesticide hypothesis. 

In 2007, Carlos Davidson and I published a paper showing that the distribution of mountain yellow-legged frogs across the central and southern Sierra Nevada was correlated with a distance-weighted metric describing the amount of pesticides applied upwind. This provided additional suggestion that pesticides might be involved in the decline of mountain yellow-legged frogs, but the evidence was correlative and based on the untested assumption that the variable describing the amount of pesticides applied upwind was in fact a good predictor of contaminant levels across the high country. As such, the role of pesticides in causing the mountain yellow-legged frog's decline remained largely a possibility based on circumstantial evidence.

During the past year, David Bradford and colleagues have published three papers that provide for the first time detailed field measurements of concentrations of dozens of pesticides across Sequoia-Kings Canyon National Park. Collectively, these papers cast doubt on the idea that pesticides are an important driver of amphibian declines in the high elevations of the Sierra Nevada. First, concentrations of all detected chemicals were extremely low, averaging in the low parts-per-billion in lake sediments and tadpoles, and 10 picograms/m3 in the air. For reference, one part-per-billion is roughly equivalent to a single drop in an Olympic-sized swimming pool, and a picogram is one-trillionth of a gram! Second, there was no relationship between measured pesticide concentrations and distance to the Central Valley, indicating that the pesticide metric that Carlos and I used in our paper was likely not a good predictor of pesticide exposure. Third, there was no correlation between the occurrence of mountain yellow-legged frogs and measured pesticide concentrations. 

So, these latest studies suggest that pesticides are unlikely to be playing an important role in amphibian declines in the high elevation portions of the Sierra Nevada. Whether such effects could be occurring in lower elevation habitats that are closer to the Central Valley and that therefore receive higher levels of airborne contaminants remains possible, although still largely untested. 

Bradford, D. F., E. M. Heithmar, N. G. Tallent-Halsell, G.-M. Momplaisir, C. G. Rosal, K. E. Varner, M. S. Nash, and L. A. Riddick. 2010. Temporal patterns and sources of atmospherically deposited pesticides in alpine lakes of the Sierra Nevada, California, U.S.A. Environmental Science and Technology 44:4609-4614.

Bradford, D. F., K. Stanley, L. L. McConnell, N. G. Tallent-Halsell, M. S. Nash, and S. M. Simonich. 2010. Spatial patterns of atmospherically deposited organic contaminants at high elevation in the southern Sierra Nevada mountains, California, USA. Environmental Toxicology and Chemistry 29:1056-1066.

Bradford, D. F., R. A. Knapp, D. W. Sparling, M. S. Nash, K. A. Stanley, N. G. Tallent-Halsell, L. L. McConnell, and S. M. Simonich. 2011. Pesticide distributions and population declines of California, USA, alpine frogs, Rana muscosa and Rana sierrae. Environmental Toxicology and Chemistry 30:682-691 (PDF).

Back to The Mountain Yellow-legged Frog Site.

Disappearing Frogs and Shifting Baselines

I've studied mountain yellow-legged frogs for the last 15 years, all in the High Sierra, the last stronghold of these beasts. As a consequence of my experiences, when I think of mountain yellow-legged frog habitat in the Sierra Nevada I think of lake basins in the alpine and subalpine zones. But scientists who came before me, including Joseph Grinnell and Richard Zweifel, would have had a very different view. Back in their time, they found abundant mountain yellow-legged frog populations in streams at 6000' in the Ponderosa Pine belt of the western Sierra, in the expansive meadows of the unglaciated Kern Plateau, in the lakes, ponds, and streams in the Lake Tahoe watershed, in streams in Nevada's Carson Range, and in the far northern reaches of the historic range, in streams as low as 2000' on the Plumas National Forest. 

Most of these mountain yellow-legged frog populations are gone now, pushed aside by reservoirs, water diversions, introduced trout, and disease. The people who remember frogs in those places are mostly gone too. Luckily, they left us with field notes containing detailed descriptions of what they saw and collections of frogs in jars on museum shelves. Those relics serve as indelible reminders of the places these frogs once occupied and provide an unchanging reference point against which to judge the current situation and the success of future conservation efforts. 

I was recently assembling a data set of historic frog localities, and it was that data set that reminded me of just how ubiquitous mountain yellow-legged frogs once were. These records provide an amazing glimpse into a nearly-forgotten past. For example, in 1911 Joseph Grinnell and his survey team worked their way up the South Fork Kern River, describing the vertebrate fauna that they encountered and collecting the specimens that now reside at the U.C. Berkeley Museum of Vertebrate Zoology. In Monache Meadow, field notes record the presence of mountain yellow-legged frogs and also note the impacts caused by intensive livestock grazing. As they moved upstream, they saw mountain yellow-legged frogs in abundance in Templeton and Ramshaw Meadows, and at Big Whitney Meadow, at the headwaters of Golden Trout Creek, they observed Swainson's Hawks feeding on the seemingly ubiquitous frogs. At the nearby Rocky Basin Lakes, Grinnell had this to say:

"There are great numbers of apparently full-grown frogs around the shores of the lakes. The conspicuous thing about them is their extreme wariness. They jump into the water and dive quickly into the deepest holes within reach when one is yet fully 10 yards from them. There is a shower of frogs in advance of a person as he walks along the beach. They must have some nimble and persistent enemy."

That is a sight I would have loved to see, but the point of this isn't to romanticize the past. Instead, it is to recognize how quickly we forget what once was and replace it with a view based on our own experiences. Every human generation replaces what the previous generation knew to be true with their own version of reality, producing a series of shifting baselines through time. It is only the field notes and collections of those who came generations ago that remind us of what the world really looked like 100 years past. As someone who has hiked all over the High Sierra studying frogs, I thought I knew this frog well. It took Grinnell's journals to once again remind me of how narrow my own understanding is and of how much we've lost.
 
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Lessons from the Plight of Frogs in Southern California

Researchers and managers from California and Nevada held their annual Amphibian Populations Task Force meeting January 6-7, 2011, this time in Yosemite Valley. With more than 100 attendees, the meeting once again provided a great opportunity to hear about the status of myriad frog conservation projects and catch up with colleagues. For me, the most insightful talk was that by Adam Backlin, the USGS scientist who with Robert Fisher (also with USGS), has been leading efforts to restore southern mountain yellow-legged frog populations (Rana muscosa) in the Transverse and Peninsular Ranges in southern California. This group of populations was listed as endangered under the U.S. Endangered Species Act in 2002.

Starting with only 150 frogs scattered across several populations and mountain ranges, the efforts to keep southern California R. muscosa from going extinct have begun to produce promising results. The removal of non-native rainbow trout from a reach of Little Rock Creek has allowed the resident R. muscosa population to begin to expand. The recovery of this population is still in its early stages but the fact that this population has increased to approximately 50 frogs from just a handful since fish removal is a very promising start. 

Thanks to the efforts by staff at the San Diego Zoo, researchers now have access to captive-bred R. muscosa for use in reintroductions. Reintroductions using this captive stock were conducted for the first time in 2010 so it is still too early to know the outcome of these efforts. But just having the captive colony available to allow reintroductions is a huge step forward. 

The insight from Adam's talk that really hit home for me was the necessity of active management to restore R. muscosa. A hands-off approach of protecting habitat and hoping for the best wasn't sufficient to stem the decline of this group of populations. This decline was slowed only by very intensive interventions. As we try to halt the decline of mountain yellow-legged frogs in the Sierra Nevada, there is an important lesson to be learned from the southern California efforts. That is, we can't rely simply on a hands-off approach to accomplish our goal. Unfortunately, it has been my experience that the hands-off approach is increasingly the one being relied on. 

As the mountain yellow-legged frog in the Sierra Nevada has disappeared from more and more of its native range, the response by managers has been to increase the protections afforded the remaining populations. In some cases, that has meant that populations are off-limits for research. I understand and generally support these protections but they aren't enough. For example, it is critically important that we also continue to test reintroduction methods and conduct experiments to improve our understanding of the frog-chytrid fungus interaction. I fear that with the continuing decline of the mountain yellow-legged frog and its eventual listing under the state and federal Endangered Species Act, our ability to conduct this critical research will be increasingly restricted. 

I hope we can learn an important lesson from the southern California experience.

Back to The Mountain Yellow-legged Frog Site.

Understanding How Science Works

Non-scientists are often understandably frustrated by the conflicting results commonly reported in the media on any one of a myriad of science-related topics. It seems that hardly a day goes by that we don't hear another story about how a previous study showed one thing but the newest study now refutes those results. With all of this conflicting information I can understand when some people throw up their hands and conclude that scientists don't know what the heck they are talking about.

This lack of clarity can be partly attributed to the superficial coverage provided by most media outlets in this sound-bite age of ours. When a relatively complex issue is distilled down to a one minute story there are bound to be lots of important facts lost in the process. However, to a large extent these conflicting results are inherent in how science works. Scientific progress is not a linear path. Instead, it is a process that zigs and zags and over time closes in on something resembling the truth. This non-linear progression may be frustrating but it is an unavoidable consequence of the fact that complex questions have complex answers (and are there any simple questions these days?) and different approaches to these questions will often provide different answers.  

The amphibian decline literature provides some excellent examples of this non-linear scientific progression. For example, starting in the mid-1990s Andrew Blaustein at Oregon State University published a series of papers in high-profile scientific journals showing that ultraviolet (UV) radiation had a range of negative effects on amphibians and that increasing UV radiation could be responsible for global amphibian declines.This hypothesis was compelling because it fit with known increases in UV radiation resulting from thinning of the ozone layer and it potentially explained the global nature of these declines. As a consequence, the UV hypothesis received lots of attention from the media and these many stories had the effect of solidifying in the public's mind that UV radiation was in fact a cause of amphibian declines. In fact, the science to test that hypothesis in different ecosystems and using different methods had just begun.

As with any novel idea Blaustein's hypothesis caught the attention of other scientists and in the following 15 years resulted in a plethora of additional scientific studies focused specifically on testing the idea. That research has provided a much more comprehensive picture of the effects of UV radiation on amphibians. In large part, the current thinking is generally that global UV radiation levels have in fact increased during the last 30 years but these increases have been relatively modest (~5% in North America) and have leveled off since the mid-1990s. For several reasons these increases, although obviously of concern, don't necessarily translate into impacts to amphibians. First, UV radiation has been a selective force on amphibians since amphibians first evolved more than 300 million years ago. As a consequence many amphibians have adaptations that effectively protect them from UV exposure. Second, water is a strong attenuator of UV radiation and can block UV from even reaching amphibian life stages. So, increased UV radiation may be having some effects in localized areas but this hypothesis is no longer seen as providing a general explanation for global amphibian declines. 

So, science worked the way it almost always does: someone puts out an idea and then over subsequent years that idea is subjected to many tests by different groups of people and we eventually arrive at a more complete understanding of the phenomenon in question. To understand the full story you just need to read beyond the splashy headlines.

For more information on UV radiation and its impacts on amphibians check out the following papers (you can find PDFs of most of these by conducting your searches using Google Scholar):

Adams, M. J., B. R. Hossack, R. A. Knapp, P. S. Corn, S. A. Diamond, P. C. Trenham, and D. B. Fagre. 2005. Distribution patterns of lentic-breeding amphibians in relation to ultraviolet radiation exposure in western North America. Ecosystems 8:488-500.

Blaustein, A. R., P. D. Hoffman, D. G. Hokit, J. M. Kiesecker, S. C. Walls, and J. B. Hays. 1994. UV repair and resistance to solar UV-B in amphibian eggs:  a link to population declines? Proceedings of the National Academy of Sciences, USA 91:1791-1795.

Blaustein, A. R. and D. B. Wake. 1995. The puzzle of declining amphibian populations. Scientific American 272:52-57.

Herman, J. R. 2010. Global increase in UV irradiance during the past 30 years (1979–2008) estimated from satellite data. Journal of Geophysical Research 115: D04203.

Kiesecker, J. M., A. R. Blaustein, and L. K. Belden. 2001. Complex causes of amphibian population declines. Nature 410:681-684.

Palen, W. J. and D. E. Schindler. 2010. Water clarity, maternal behavior, and physiology combine to eliminate UV radiation risk to amphibians in a montane landscape. Proceedings of the National Academy of Sciences 107:9701-9706.

Vredenburg, V. T., J. M. Romansic, L. M. Chan, and T. Tunstall. 2010. Does UV-B radiation affect embryos of three high elevation amphibian species in California? Copeia 2010:502-512.

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Frog Recovery - An Emerging Story

I've spent a large portion of the last 10 years describing the spread of the amphibian chytrid fungus (Batrachochytrium dendrobatidis - "Bd") across the Sierra Nevada. Given the hundreds of frog population extinctions that have resulted from the arrival of Bd it is sometimes easy to forget about the other side of this story - that there is also evidence of frog recovery following Bd-caused population crashes. 

To read more about what is happening in the Sierra Nevada and Australia check out this recent story in the New Scientist. The writer somewhat exaggerates the recovery angle but it is a worthwhile read nonetheless. The reality in the Sierra Nevada is that recovery of populations to a pre-Bd population size is very rare and has happened at just a handful of sites. But the fact that it is happening at all is important.

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Pesticide Harmful to Amphibians is Banned in U.S.

Last week the U.S. Environmental Protection Agency announced that it would ban the use of endosulfan. Endosulfan is an organochlorine pesticide that was developed in the 1950s. Although it is an effective compound against many crop pests it comes with serious health risks to humans and wildlife, including acute neurotoxicity and endocrine disruption. As a consequence endosulfan is banned in more than 63 countries, including the European Union.

Several studies indicate that because endosulfan persists in the environment for years it can be transported long distances from application sites. For example, endosulfan is commonly detected in the Sierra Nevada despite the fact that it is not applied anywhere nearby. The idea that endosulfan detected in the Sierra Nevada originates from applications in upwind agricultural areas is supported by the fact that endosulfan concentrations in the Sierra Nevada correspond very closely with application rates in the Central Valley, with a lag time of 1-2 weeks.

The fact that endosulfan can disperse widely and persist in the environment has long raised concerns about its effects on wildlife. Research on its impacts on amphibians in the Sierra Nevada indicates that endosulfan is detectable in Pacific treefrog populations (Pseudacris regilla) throughout the range and is toxic to amphibians at extraordinarily low concentrations. However, a just-published study indicates that concentrations in frog tissues are generally well below levels that would cause direct or indirect impacts. 

Overall, the effects of endosulfan on Sierran amphibians is likely relatively minor compared to those caused by trout introductions and the amphibian chytrid fungus (Batrachochytrium dendrobatidis). But given its broad toxicity to wildlife and humans the ban on endosulfan use was nonetheless long overdue. 

The following publications provide additional information on the effects of pesticides on amphibians in the Sierra Nevada:

Bradford, D. F., E. M. Heithmar, N. G. Tallent-Halsell, G.-M. Momplaisir, C. G. Rosal, K. E. Varner, M. S. Nash, and L. A. Riddick. 2010. Temporal patterns and sources of atmospherically deposited pesticides in alpine lakes of the Sierra Nevada, California, U.S.A. Environmental Science and Technology 44:4609-4614.

Bradford, D. F., K. Stanley, L. L. McConnell, N. G. Tallent-Halsell, M. S. Nash, and S. M. Simonich. 2010. Spatial patterns of atmospherically deposited organic contaminants at high elevation in the southern Sierra Nevada mountains, California, USA. Environmental Toxicology and Chemistry 29:1056-1066.

Davidson, C. and R. A. Knapp. 2007. Multiple stressors and amphibian declines:  dual impacts of pesticides and fish on yellow-legged frogs. Ecological Applications 17:587–597.

Davidson, C., H. B. Shaffer, and M. R. Jennings. 2002. Spatial tests of the pesticide drift, habitat destruction, UV-B, and climate-change hypotheses for California amphibian declines. Conservation Biology 16:1588-1601.

McConnell, L. L., J. S. Lenoir, S. Datta, and J. N. Seiber. 1998. Wet deposition of current-use pesticides in the Sierra Nevada mountain range, California, USA. Environmental Toxicology and Chemistry 17:1908-1916.

Sparling, D. W. and G. Fellers. 2009. Toxicity of two insecticides to California, USA, anurans and its relevance to declining amphibian populations. Environmental Toxicology and Chemistry 28:1696–1703.

Back to The Mountain Yellow-legged Frog Site.