A Hopeful Find

In this age of declines in global amphibian populations, signs of hope are often hard to come by. On my recent research trip into Yosemite National Park, I made one of those rare hopeful finds. My field crew and I were hiking through a forested low elevation portion of the Park and approached a small stream flowing through a meadow. As I neared the stream, a Sierra Nevada yellow-legged frog jumped from the shore into a nearby pool and disappeared. I'd crossed the stream at this very spot numerous times before and had never seen an amphibian. Had I imagined the frog? A quick search upstream and downstream turned up numerous other adults, subadults, and tadpoles. Had they been here all along? If so, why had we never found them previously? If they hadn't been here all along, where did they come from? In all of our survey work, we'd found no other Sierra Nevada yellow-legged frog populations anywhere in the vicinity. Regardless of the answers to these questions, finding a new frog population is always a rare and welcome discovery. It certainly made my day.

More in a week when I'm back from my next trip into the backcountry.

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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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The Increasing Inadequacy of the Endangered Species Act

The Endangered Species Act (ESA) was signed into law in 1973, and is responsible for bringing numerous species back from the brink of extinction. However, 35 years after its signing the ESA is increasingly showing its age. In 1973, most species endangerment was the result of local impacts that were relatively easy to identify and the resolution of which was generally straightforward. For example, Brown Pelicans were driven into decline by increased DDT concentrations in the environment. DDT was banned in the U.S. in 1972, and Brown Pelican populations have rebounded dramatically.

Although the current declines of many species have similarly identifiable causes, an increasing number have complex causes that are challenging the relevance of the ESA. The polar bear and the mountain yellow-legged frog provide two examples of the problems posed by this complexity. The polar bear was recently listed as "threatened" under the ESA due to current and projected declines resulting from global warming. Unfortunately, the ESA was designed to deal with local threats such as habitat destruction, not global threats like climate change, and therefore lacks the tools necessary to deal with challenges as complex and far-reaching as global warming. Even if the ESA contained tools that would aid in reducing carbon dioxide emissions in the U.S., the ESA of course lacks the ability to regulate emissions in other countries.

The mountain yellow-legged frog (Rana muscosa, Rana sierrae) that inhabits the mountains of California presents a related but different challenge for the ESA. These two species have declined by more than 90% during the past century. Once the most common vertebrate across much of their range, they are now one of the rarest. R. muscosa in southern California was listed as "endangered" in 2002 and the listing of R. muscosa and R. sierrae in the Sierra Nevada was found to be "warranted" in 2003. The primary threat to these species through the 1970s was likely the introduction of nonnative trout into historically fishless lakes and streams. That threat is now been compounded by the emergence of the disease, chytridiomycosis, that is decimating mountain yellow-legged frog populations once thought to be secure because of the local absence of introduced fish.

If introduced fish were the only threat to mountain yellow-legged frogs, a recovery plan presumably would identify water bodies from which fish should be removed, and those removals would allow frog populations to rebound. Simple, right? Given the known importance of disease in driving current declines, what tools does the ESA give us to deal with this threat? Very few, if any. The biggest challenge caused by chytridiomycosis is that we don't even know what caused the emergence of this disease. Was it human-caused (e.g., climate change) or was it caused by the natural evolution of a more virulent strain? Disentangling "natural" from human-caused stressors is already extremely challenging, and will become much more so in coming decades as subtle effects of climate change, contaminants, and ecosystem changes become ever-more pervasive.

In the case of chytridiomycosis, even if disease emergence was human-caused, we currently have no means of eradicating the disease. Therefore, many species being pushed toward extinction by chytridiomycosis are doomed, and yet the ESA provides no mechanism for triage. Alternatively, if chytridiomycosis is the result of a natural evolutionary event (which will be very difficult to show conclusively), should we try to counteract the effect of that natural event? If yes, how should we allocate scarce resources between species being driven to extinction by anthropogenic causes versus apparently natural causes?

Our inability to distinguish between natural and human-caused species declines and questions on how to respond to these types of declines will increasingly compromise our ability to halt the impending flood of species extinctions. In our ever more complex world, our only hope of slowing the rate of human-caused extinctions may be an international "Endangered Ecosystems Act". More about that pipe dream some other time.

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