The Golden Toad’s Tale

Spread the love

From Climate Scepticism


Sometimes, a dried-up, dessicated, perhaps even mummified cynic can still be disappointed by ecologists. Ecology, as I am fond of saying, is mostly common sense. But common sense is all too often lacking.

“Teh Science” is not science at all, but a preferred explanation that is impervious to facts. Sometimes its answer might be true. Sometimes it isn’t. But the average punter cannot know the difference, thanks to the way alternative explanations bounce off Teh Science’s iron umbrella.

What has brought on this sudden attack of bitterness, you ask. Well, I finally decided to start to delve, timorously, into the IPCC’s AR6. One of the first things I wanted to look at was what AR6 said about the extinction of the golden toad from the Monteverde cloud forest preserve in Costa Rica (and, because this was its only home, the world). This was a tale I covered very briefly in Denierland. I set out the allegation as follows:

“Climate claims the golden toad” – BBC, 1999.

“Even with temperature rises of 0.7C, some animals have already been lost – such as the golden toad that lives in the cool mountains of Costa Rica…” – Suzanne Goldenberg, The Guardian, 12 Feb 2010.

Last seen in the 1980s high in Costa Rica’s cloud forest, declining from 1500 individuals to 1 in a single year at the known location. At the time, ecologists knew the golden toad was exceptionally rare, but there was still hope that it would be re-found in some out-of-the-way spot. Such hopes have now faded.

The key year of 1987 was exceptionally dry, leading to the toad’s demise.

The real cause I described thusly:

A paper in 2010 identified no trends in rainfall in the affected site. Instead, the blame for the toad’s demise was placed on an introduced fungus, Batrachochytrium. In dry years, the concentration of amphibians in fewer pools would have enabled the fungus to spread easily.

“Our results indicate that the chytrid pathogen emergence in plethodontid salamanders in Mexico and Guatemala, and in salamanders and frogs at Monteverde, Costa Rica, is coincident with the amphibian community collapse and extinctions that occurred at these localities.” – Cheng et al, 2011.

“…those responsible for collecting amphibians, their equipment and associated activities would have to be the means of [Batrachochytrium] introduction.” – Fisher & Garner, 2007.

It was a case of case closed (as well as a big fat “ouch”) as far as I was concerned. So how did AR6 describe the golden toad’s demise?

Two terrestrial and freshwater species have become extinct in the wild, with climate change implicated as a key driver. The cloud forest-restricted golden toad (Incilius periglenes) was extinct by 1990 in a nature preserve in Costa Rica, driven by successive extreme droughts. This occurred in the absence of chytridiomycosis infection, caused by the fungal pathogen Bd [Batrachochytrium], verified during field censuses of golden toad populations in the process of extinction as well as genetic analyses of museum specimens, although Bd was present in other frog species in the region (medium evidence, high agreement) (Pounds et al., 1999; Pounds et al., 2006; Puschendorf et al., 2006; Richards-Hrdlicka, 2013).AR6 WGII Chapter 2, p220

Never mind Batrachochytrium dendrobatis, it was all about the climate-induced droughts. I was somewhat sceptical that the extinction of the golden toad had been characterised accurately in AR6, to say the least. Of course there is always the possibility that I have an iron umbrella myself, impervious to the data that says it was climate what dunnit. So I decided to look at some of the literature, including recent stuff.

The first article I read was a 2020 piece by Jeffrey Coleman in the Bulletin of the Chicago Herpetological Society. Coleman goes through all the different hypotheses relating to the golden toad’s extinction, until:

…the fungal disease known as chytridiomycosis, as well as changing climate, is currently considered the most probable cause of the declines of A. varius and I. periglenes…Coleman 2020

So climate change has not been let off the hook, but it seems that the evidence has to be twisted into pretzels in order to keep it on the charge sheet; it might have driven the getaway car, but it didn’t fire the gun: that was done by the fungal pathogen called Bd by those too lazy to type it out or to remember what it stands for.

Pretzel Logic

Below I am going to deal with the first explanation how climate change wiped out the golden toad. That is perhaps now a straw man, because the explanation has evolved. You might say it has wriggled. No longer is it “Carbon dioxide emissions caused climate change, which caused El Niño, which caused a drought and the toads died.” Now it is “Carbon dioxide emissions caused climate change, which caused the sudden appearance of a deadly new pathogen to wipe out the golden toad, when in the absence of climate change but the presence of the pathogen, it would have been fine, so there, shut up denier!” Or words to that effect.

It turns out that far from warmer temperatures favouring the pathogen as we might expect by the enthusiasm for the climate-Bd nexus theory, they actually kill it. Bd instead thrives when it is cool:

Temporal (seasonal) and spatial studies documenting the prevalence and intensity of Bd and localities of disease outbreaks are in general agreement with the hypothesis that Bd has a predilection for cooler temperatures. However, although Bd is known in the tropical Andes at altitudes of 5348 m, where temperature minima fall to −13.5 ◦C and diurnal variability may reach 30 ◦C, it has been hypothesized that at extreme altitudes where temperature maxima fall beneath the recognized envelope for optimal Bd growth, amphibians may be afforded a refuge from fatal chytridiomycosis.Fisher et al 2009 (references removed for clarity).

But warm and dry weather drives amphibians into a more “contagious” distribution (clumped, that is), which encourages the fungus’s spread. I will quote directly from Coleman 2020 to explain the pretzel logic version:

The chytrid-thermal-optimum hypothesis, put forth in light of the pattern of A. varius [Atelopus varius, the harlequin frog] declines, posits that the lowlands are frequently too warm for the fungus during the day and the highlands too cool for it at night. Vertical thermal profiles have suggested a moist adiabatic lapse rate whereby the drop in temperature with increasing elevation had lessened in the years leading to this species’ most intense declines. This daytime cooling in tandem with nighttime warming increases disease development rate, and most extinctions have had occurred in places where the minimum temperature is shifting toward the growth optimum for Bd.Coleman 2020

The nights are warmer and the days are cooler. Bd loves it, is the theory. Climate change has led to a sort of Goldilocks‘ porridge. Now let’s go back to the IPCC’s position, which, remember, deletes Bd from the theory altogether. The extinction

…occurred in the absence of chytridiomycosis infection, caused by the fungal pathogen Bd, verified during field censuses of golden toad populations in the process of extinction…

The extinction occurred in or before 1990. The causal agent was described… in 1999 by Longcore et al. (It was first identified in 1997.) How are these field censuses to have detected the undescribed fungus? Visible signs of infestation, you might think, along the lines of The Last of Us. But toads afflicted by Bd don’t end up with a sporocarp growing out of their heads. In fact, there are few external signs until infection is severe, and post-mortems don’t reveal how the fungus killed the host. The infection is skin deep, but that can be enough for it to be lethal.

Chytridiomycosis is notable for an overall lack of disease pathologies: Metamorphosed amphibians infected with Bd typically exhibit epidermal hyperplasia and hyperkeratosis, and possibly upregulated skin shedding, but only rarely exhibit any lesions visible to the naked eye.Fisher et al 2009

The authors of AR6 also note the analysis of museum specimens. They cite four studies in support of their paragraph, of which the most recent is Richards-Hrdlicka (2013). What did Richards-Hrdlicka do? She found that preserved specimens of the golden toad collected between 1967 and 1982 all tested negative for Bd. Now I’ll just let you cogitate on that for the space of a carriage return.

How does Richards-Hrdlicka explain her results? Was it that the fungus was below the detection limit, or was it that

…Bd was absent from the samples, because the specimens were collected prior to the presumptive emergence and documentation of Bd in Monteverde.Richards-Hrdlicka 2013

To put it another way: is it surprising that the specimens collected in 1982 tested negative, when the sudden extinction occurred after 1987? In fact, had these specimens tested positive, it would have thrown out the Bd theory altogether, and accompanied it with a shout of “And don’t come back, loser!”

Richards-Hrdlicka’s results are 180 degrees from the way the AR6 authors have described them. They support the Bd hypothesis, not the “climate” hypothesis. It’s as if I allege a murder victim has been poisoned by strychnine, and your expert witness re-analyses a stored blood sample to prove an absence of strychnine in the victim five years before the murder.

And now for an ecology lesson. Remember, ecology is common sense most of the time. What do we know about amphibians that return to pools and ponds to spawn? Why do they do what they do? Why grow legs and lungs, shed tails and gills, and abandon their watery nursery? Why not stay whre they are, like fish? Well, we can use our own common or garden frog as an archetype of our amphibian friends. Frogs become terrestrial so that they can move. Ponds and pools are isolated in the landscape to various degrees, and they always end up becoming unsuitable eventually. They silt up, become choked with leaves, shaded by trees, acidic and anoxic, and finally dry up altogether. If our froggy friends did not leave and find pastures new, they would dry up altogether too. As well as the perennial worry that “it might dry up,” frogs have to contend with the chance that when rivers flood, ponds can be colonised by fish – and that effectively ends the survival chances of tadpoles. In some years, ponds dry up altogether, only to be replenished by winter rains. Sometimes they retain water all year. Sometimes they dry up before tadpoles have had the chance to grow two pairs of legs. In the lowlands, ponds that never dry up often end up with fish in them. This means that ponds that sometimes dry out are actually more suitable for frogs than ponds that never dry up.

But back to the hero of this tale: the golden toad. Restricted to a single locality, apparently measuring 4km2, discovered in 1963, gone by 1990. Here’s how Pounds & Crump, authors of the climate theory of the toad’s destruction describe the situation:

In 1987, the golden toad (Bufo periglenes; Bufonidae), endemic to elfin cloud forest in Costa Rica’s Cordillera de Tilaran, appeared safe in the Monteverde Cloud Forest reserve. During April-May,more than 1500 toads gathered to mate in temporary pools at Brillante, the principal known breeding site. This spectacle had occurred annually since at least 1972, when the preserve was founded. But in 1988 and again in 1989, only a single toad appeared at Brillante, and a few others gathered 4-5 km SE. A variety of other amphibians in the area, including the harlequin frog (Atelopus varius; Bufonidae) and members of six other families, became scarce at the same time. During 1990-1992, despite our intensive surveys, no golden toads or harlequin frogs were found.Pounds & Crump 1994 (references removed for clarity).

The picture that is painted is of a toad that was locally abundant but in an extremely restricted area. It was a species that was clinging on to life, and that drew tourists every spring to gawp at it when it gathered in temporary pools to mate.

And then nothing; gone.

In 1986-7 there was an El Niño, which dried pools out before tadpoles could metamorphose. But an El Niño is not climate, however much we want it to be, and – as explained above – pools drying out at inopportune times is a regular hazard for many amphibians. The ponds drying out could halt recruitment for the year, but it could not kill the adult toads, which would be expected to return a year later to try again.

Except that didn’t happen, did it? The toads gathered as usual in 1987; they dispersed; they were never seen again. Here’s what Fisher et al said about Bd in 2007:

Mesoamerica has witnessed a rapid wave-like front of expansion from an apparent origin in Monteverde, Costa Rica, southward at estimated rates of between 17 and 43 km per year, and has recently crossed the Panama Canal.Fisher et al, 2009

The apparent point-inoculation at Monteverde – the only place ever known to have had a population of golden toads – leads Fisher et al to place the blame for the outbreak on… scientists and amphibian-collectors bringing spores from their travels elsewhere (see also the quote included in Denierland above). However, I don’t know if that’s what happened. There may well be a “natural” vector for the fungus (see Prahl et al 2020), although the human trade in amphibians certainly spread Bd around the world (overview in Fisher et al 2009), from which its subsequent expansion was perhaps more “natural.”

Here is figure 3 from Cheng et al, 2011, a picture that paints, well, several thousand words:

Cheng et al say:

Whereas our results do not rule out a role for climate change, we have found no necessary connection between climate change and the particular extinction events at our study sites. We suggest that the emergence and spread of a pathogen into naïve host populations can explain amphibian declines in the neotropics as other studies have also posited.Cheng et al, 2011

What we have here is a pathogen introduced to an area where one particular species of toad had already declined to a very confined population in the low thousands before it was even discovered by humans in the 60s (see Savage, 1966 for the tale of its discovery). That population had no immunity to Bd. It was wiped out, not by climate change, but by human carelessness. The gun was fired in the early twentieth century when toads were being traded around the world. They took Bd with them. Where the fungus was seeded, it extirpated naïve populations and spread inexorably. The bullet reached Monteverde and hit the golden toad in 1987, and Bd is still spreading around the world today. And even if there had been no Bd, and if the golden toad was finished off by drought, to blame it on climate change you have to believe that all the factors that drove it into its final stronghold were no longer acting.

The disgrace is that we are supposed to believe that, had we reined in our emissions of carbon dioxide ***before 1987***, the golden toad would still be alive. That is misinformation, or disinformation, or perhaps more accurately, a lie.

I do not have the time or inclination to delve in great depth into the truth or otherwise of the bland pro-alarm statements that I read in AR6. I hardly see how I can trust any of it, when the first item I look at in detail is as good a manifestation of “Teh Science” as you could hope to find. Falsus in uno, as they say. I have low expectations of climate science, but I have been disappointed again.


There are three hypotheses before us for what caused the demise of the golden toad. I will leave it to the reader to discriminate between them.

  1. The Batrachochytrium hypothesis. A tiny population restricted to a tiny area encountered a novel pathogen that they had no resistance to. They died.
  2. The Pretzel hypothesis. A tiny population restricted to a tiny area encountered a novel pathogen that they had no resistance to. Human-caused climate change made them vulnerable to the pathogen, whereas without it, they would have resisted it. They died.
  3. Teh Science. A tiny population restricted to a tiny area experienced a drought which would not have happened, or would not have been as severe, in the absence of human-caused climate change. They died.

Featured image

A common or garden toad Bufo bufo snapped a few years ago in Norwich and coloured golden by the author. By the way, according to reports golden toads did not vocalize, so my whimsical subtitle should be treated with the respect it deserves. Note also that the golden toad has gone through a number of monickers over its brief career: Bufo, Cranopsis, Incilius all refer to the same beast.


AR6: [You’re looking for Chapter 2.]

The remainder are all obtainable at Google Scholar if you search for them there.

Cheng, T. L., Rovito, S. M., Wake, D. B., & Vredenburg, V. T. (2011). Coincident mass extirpation of neotropical amphibians with the emergence of the infectious fungal pathogen Batrachochytrium dendrobatidisProceedings of the National Academy of Sciences108(23), 9502-9507.

Coleman, J. L. (2020). Anthropogenic Drivers and Chytridiomycosis: Untangling the Disappearances of the Golden Toad and Costa Rican Variable Harlequin Toad and Addressing Amphibian Decline. Bulletin of the Chicago Herpetological Society55(8), 157-162.

Fisher, M. C., & Garner, T. W. (2007). The relationship between the emergence of Batrachochytrium dendrobatidis, the international trade in amphibians and introduced amphibian species. Fungal biology reviews21(1), 2-9.

Fisher, M. C., Garner, T. W., & Walker, S. F. (2009). Global emergence of Batrachochytrium dendrobatidis and amphibian chytridiomycosis in space, time, and host. Annual review of microbiology63, 291-310.

Longcore, J. E., Pessier, A. P., & Nichols, D. K. (1999). Batrachochytrium dendrobatidis gen. et sp. nov., a chytrid pathogenic to amphibians. Mycologia91(2), 219-227.

Pounds, J. A., & Crump, M. L. (1994). Amphibian declines and climate disturbance: the case of the golden toad and the harlequin frog. Conservation Biology8(1), 72-85.

Prahl, J., Wilson, T. P., Giles, D., & Craddock, J. H. (2020). An overview of research regarding reservoirs, vectors and predators of the chytrid fungus Batrachochytrium dendrobatidisActa Herpetologica15(1), 39-45.

Richards-Hrdlicka, K. L. (2013). Preserved specimens of the extinct Golden Toad of Monteverde (Cranopsis periglenes) tested negative for the amphibian chytrid fungus (Batrachochytrium dendrobatidis). Journal of Herpetology47(3), 456-458.

Savage, J. M. (1966). An extraordinary new toad (Bufo) from Costa Rica. Revista de Biologia Tropical14(2), 153-167.