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FREQUENTLY ASKED QUESTIONS

What killed the Monteverde Golden Toad?
The toad was probably killed directly by the chytrid fungus, Batrachochytrium dendrobatidis. However, our data indicate that the extinction occurred during one of the driest periods of the last century at Monteverde, which was caused by the 1986-1987 El Nino event and was associated with an extensive and warm Western Hemisphere Warm Pool. We don't detect a long-term trend in our isotope record of Monteverde hydroclimate. Rather, the record is dominated by year-to-year and decade-to-decade variability.

How did El Nino contribute to the extinction of the Golden Toad?
We hypothesize that the dry conditions during 1986 and 1987 caused the majority of the population of the toads to congregate around a few moist microhabitats, where they would have been readily infected. We're not the first to suggest this mechanism -- Alan Pounds and Marty Crump wrote about this in the 1990s, and the phenomenon has been observed elsewhere. As we say in the paper,

The decline of amphibians in El Yunque Forest in Puerto Rico is believed to be a consequence of a change in their behavior during dry periods, with populations moving from a dispersed distribution to a few protected microsites on the landscape, increasing their vulnerability to contagion [Burrowes et al. 2004]. Similar patterns were observed at Monteverde prior to the multispecies population crash and extinction of the Golden Toad in 1987 [Pounds and Crump 1994, Pounds et al. 1997, Pounds et al, 1999, Crump 2000].

Does this mean anthropogenic climate change doesn't cause extinctions or won't cause them in the future?
Absolutely not. Both the rate and magnitude of ongoing and future climate change are very likely to put additional stresses on ecosystems. In combination with land use change, introduced pathogens, pollution, and other related ecological changes, anthropogenic climate change will undoubtably play a role in future extinctions.

For instance, Karen Lips and her colleagues also describe another fascinating occasion where climate changes may have interacted with amphibian populations and the fungus to result in an extinction event:

A straightforward example of how climate change might drive amphibian extinctions from invasion of [Batrachochytrium dendrobatidis] into new environments was described for the Peruvian Andes [Seimon et al. 2007]. Glacial retreat produced new meltwater ponds that were colonized by three species of frogs [Seimon et al. 2007] and later by [Batrachochytrium dendrobatidis]. This event was followed by die-offs of adults, and declines in metamorphic juveniles and tadpoles. Regional climate analyses [Ron 2005] indicated this area of frozen ice and snow was not within the appropriate thermal envelope for [Batrachochytrium dendrobatidis], but measurements of water temperature indicated that solar heating was capable of warming ponds to levels tolerable by [Batrachochytrium dendrobatidis].

How do you get climate information out of tropical trees without annual rings?
We use a technique we've called 'tropical isotope dendroclimatology'. We take increment cores from tropical trees, and take microscopic slices (a few tens to hundred of a micrometer) with a rotary microtome at continuous, regular intervals along the growth radius of the tree. We then turn these small wood slices into nearly-pure cellulose [PDF]. The cellulose is weighed (a few hundred micrograms) and wrapped in silver capsules, then introduced into a molybdenum reactor in a high temperature generator [PDF] at 1500C. In the absence of oxygen (the samples are bathed in helium gas) this converts the cellulose to carbon monoxide gas (CO). We then measure the isotopic composition on a mass spectrometer (to be specific, a continuous flow isotope ratio mass spectrometer). We made over 2000 measurements of samples and standard materials just for this article!

Because of this lengthy process of extracting chronological and climate information from tropical trees without rings, our technique and its uncertainties make it more similar to paleoclimatology using corals or speleothem, as opposed to traditional dendrochronology where large sample replication and robust crossdating are possible and fundamental. Improved instrumentation and protocols, however, are already improving our ability to develop longer and better replicated records. Stay tuned.

What are the major uncertainties in your approach and conclusions?
The largest uncertainties are associated with replication -- ours is a study from a single, albiet important, site. The next step will be to perform this type of study at other cloud forests and at other locations in Costa Rica. In particular, we can't be certain about the cause of the apparent multidecadal changes in our isotope series. For instance, was the change toward a larger amplitude seasonal cycle in the early 1970s the consequence of forest clearance, large-scale climate variability, or local conditions at Monteverde? We hope to start to answer these questions in the future.

Have other questions? Send an email ( ) to Kevin and ask!

Citation

Anchukaitis, K.J. and M.N. Evans, Tropical cloud forest climate variability and the demise of the Monteverde Golden Toad, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.0908572107, 2010. [BibTeX] [DOI]

Data Sources

Data files in MATLAB .mat format as well as plain ASCII are available here. These data will shortly also be mirrored in the official public database of the NOAA World Data Center of Paleoclimatology.

MATLAB and NCAR Command Language (NCL) Code

Program code for reproducing the figures in this article is written for MATLAB (Figure 1 and 3) and the NCAR Command Language (NCL, Figure 2). The scripts that create the figures (and can be used to access the data and do calculations) are here. Two subdirectories contain the data and some user-written functions. If you copy the complete directory structure, and have the necessary MATLAB and NCL setup on your system, you should be able to reproduce the figures from the article by just calling the 3 script files.