Frogs & Global Warming

By | August 29, 2008

Back in 1995, schoolchildren in Minnesota discovered a number of frogs that had more than their normal complement of hind appendages. Further investigations revealed other abnormalities: missing limbs, twisted jaws and declining populations all around the globe. Was it checkout time for toads? The end of the world for amphibians? A harbinger of even more terrible things about to descend on the rest of the biosphere? As described in our Editorial of 1 June 1999, no speculation was too great for the fertile imaginations of scientists and journalists alike.

The resultant and ever-so-popular frog-as-canary-in-a-coal-mine hypothesis linked the observations of deformed and dying swamp things to all sorts of environmental perturbations: to the widespread pollution of lakes and rivers caused by pesticides, to enhanced and more-hazardous ultraviolet radiation caused by presumed CFC-induced thinning of the ozone layer, to even the has-to-be-happening warming of the globe believed to be driven by mankind’s CO2 emissions.

Stepping back from this particular problem, it is almost axiomatic that when knowledge is lacking in any given area, hypotheses abound; and a theory may be as great a prod to remedial action as are facts. That is why a little knowledge is a dangerous thing; it can mistakenly point one in the wrong direction, giving an urgent sense of righteous zeal to a course of action that in a more enlightened environment might even be realized to be inimical to one’s own welfare.

Fortunately, there is a proven means for dealing with such problems: science. Plodding along, one observation after another, experiment after experiment, and meticulous measurement after meticulous measurement, its trained practitioners slowly but surely acquire new facts that either buttress or bulldoze initial ideas relative to perceived problems. And in the case of the frenzy over frogs, new information began to constrain the types of hypotheses that might logically be offered as explanations for the creatures’ sorry state.

In a story in the 30 April 1999 issue of Science, for example, contributing correspondent Virginia Morell (1999) asked "Are Pathogens Felling Frogs?" Her comprehensive survey of scientific studies being conducted at various sites around the world answered this question in the affirmative. Based upon work in the United States, Central America, and Australia, Morell noted that, although "massive frog die-offs [had] for years been linked to environmental conditions," that hypothesis was beginning to look less and less tenable. Instead, she reported that "new data from Australia suggest that the real killer may be a deadly fungus." This particular pathogen — Batrachochytrium dendrobatidis — was recognized as a lethal disease only nine months prior to the publication of Morell’s report and was not even given a name until the month before her story appeared in print. However, it had been proven to kill healthy frogs in the laboratory; and by studying preserved specimens, it had been implicated "in some of the very die-offs that first raised the amphibian alarm in the United States."

So what about its link to global warming? That possibility soon seemed pretty tenuous, especially in view of the fact that both Australian and American laboratory studies had shown that the "chytrid," as it is called, is hard to grow above 30°C, and that it normally wrecks its havoc in cold and wet habitats. In fact, the decline of the once-common lowland leopard frog in Arizona remained a mystery for many years, until researchers extended their normal summer studies into the winter, when the then-unnamed pathogen was found to be decimating whole populations of the species.

No, global warming did not appear to be the cause of the dwindling frog and toad numbers it was once opined to be; and other contemporary studies also lessened the likelihood that it could be the cause of the amphibian deformities observed around the world. In another News Focus story that appeared in the very same issue of Science, for example, news writer Jocelyn Kaiser (1999) recounted how trematodes, a type of parasitic flatworm, were vying with various types of environmental change as the prime suspects in this case as well, based upon two research reports also published in the 30 April 1999 issue of Science.

In the first of these studies (Johnson et al., 1999), the researchers demonstrated that the kinds of limb abnormalities and other deformities seen in frogs in natural settings could be precisely duplicated by infecting tadpoles with the trematode parasite; while the second study (Sessions et al. 1999) demonstrated that the pattern of duplicated limbs found in five species of frogs from twelve different localities in California, Oregon, Arizona and New York was consistent with a purely mechanical effect that had been induced in the laboratory and shown to be nearly identical with the physical perturbation caused by the presence of trematode infestation.

So was that the end of the attempt to blame global warming for the problems of frogs around the world? No, it wasn’t; for two weeks earlier, in fact, in the 15 April 1999 issue of Nature, two groups of scientists — Still et al. (1999) and Pounds et al. (1999) — published a pair of papers dealing with an extremely complex subject: the cause of major decreases in frog and toad populations in the highland forests of Monteverde, Costa Rica, as described in our Editorial of 21 Nov 2001. These diebacks — in which 20 of 50 local species totally disappeared — had occurred over the preceding two decades, decades that climate alarmists described as having experienced "unprecedented warming."

The frog and toad declines had also been accompanied by changes in bird and lizard populations that made the composition of the cloud-forest fauna look a lot more like that of forests further down slope; and the ecological mystery surrounding these changes captured the attention of a large sector of a public already conditioned to hearing all sorts of bad things attributed to the rising CO2 content of earth’s atmosphere. Thus, it was perhaps only to be expected that in a popular article describing the mystery’s putative solution, Holmes (1999) noted that the authors of the Science reports made "a convincing case blaming global climate change for these ecological events," which, of course, they truly did.

Here’s how the theory had developed. Still et al. ran a global climate model simulation for a doubled atmospheric CO2 concentration, finding — after what Holmes said "might seem like a lot of hand waving" — that the absolute humidity required to create and maintain the clouds that periodically shroud the Monteverde mountain tops shifted upwards in response to this perturbation (CO2-induced global warming, which was supposedly manifest in increasing sea surface temperatures), especially during the winter dry season when the forests there rely most heavily on the moisture they receive directly from the clouds. At the same time, the climate modelers noted an increase in a parameter they termed the "warmth index," which change implied a greater concurrent demand for evapotranspiration; and it was the combination of these two changes, i.e., an implied reduction in the amount of cloud contact with the mountain-top forest and the forest’s increased need for water, that led the modelers to believe that (presumed) CO2-induced global warming was indeed the culprit behind the observed change in environmental conditions (essentially more dry days) that were believed to be responsible for the changes in animal life documented by Pounds et al.

At the time of the publication of the two Nature papers, and for a year or more thereafter, the explanation put forth by the two groups of researchers looked pretty strong. In fact, to many it was compelling. Then, however, came the study of Lawton et al. (2001) that suggested something quite different, in which the authors presented what they called "an alternative mechanism — upwind deforestation of lowlands — that may increase convective and orographic cloud bases even more than changes in sea surface temperature do."

Lawton et al. began their analysis of the situation by noting that the trade winds that reach the Monteverde cloud-forest ecosystem flow across approximately 100 km of the lowlands of the Rio San Juan basin, and that deforestation proceeded rapidly in the Costa Rican part of this basin over the past century. By 1992, in fact, only 18% of the original lowland forest remained. The four scientists noted that this conversion of forest to pasture and farm land significantly altered the properties of the air flowing across the landscape. The reduced evapotranspiration that follows deforestation, for example, decreases the moisture content of the air mass; and regional atmospheric model simulations suggest (quite logically) that there should be reduced cloud formation and higher cloud bases over such deforested areas, which would also cause there to be fewer and higher-based clouds than there would otherwise be when the surface-modified air moves into the higher Monteverde region.

At this point, we thus had two theories from which to choose a candidate mechanism for the environmental changes that had altered the Monteverde cloud-forest ecosystem: one that was global (CO2-induced warming) and one that was local (upwind lowland deforestation). So how was one to pick the winner?

Lawton et al. chose an approach that pretty much proved their case. Noting that the lowland forests north of the San Juan River in southeastern Nicaragua remain largely intact — providing a striking contrast to the mostly-deforested lands in neighboring Costa Rica — they used Landsat and Geostationary Operational Environmental Satellite imagery to show that "deforested areas of Costa Rica’s Caribbean lowlands remain relatively cloud-free when forested regions have well-developed dry season cumulus cloud fields," noting further that the prominent zone of reduced cumulus cloudiness in Costa Rica "lies directly upwind of the Monteverde tropical montane cloud forest." Hence, they demonstrated by direct observation that the effects predicted by the theory they developed did indeed occur in the real world, and that they occurred right alongside a "control" area that was identical in all respects but for the perturbation (deforestation) that produced the noted effects.

Two years later, as indicated in our Editorial of 22 Nov 2006, Nair et al. (2003) further demonstrated that the reduced evapotranspiration that followed on the heels of prior and ongoing deforestation upwind of the Monteverde cloud forest decreased the moisture contents of the air masses that ultimately reached the tropical preserve, while regional atmospheric model simulations they conducted indicated there should also have been reduced cloud formation and higher cloud bases over these areas than there were before the deforestation began; and three years after that, in a study that extended the work of Lawton et al. and Nair et al. while exploring in more detail the impact of deforestation in Costa Rican lowland and premontane regions on orographic cloud formation during the dry season month of March, Ray et al. (2006) used the mesoscale numerical model of Colorado State University’s Regional Atmospheric Modeling System to derive high-spatial-resolution simulations that were "constrained by a variety of ground based and remotely sensed observations," in order to "examine the sensitivity of orographic cloud formation in the Monteverde region to three different land use scenarios in the adjacent lowland and premontane regions," namely, "pristine forests, current conditions and future deforestation."

This observation-constrained modeling work revealed, in the researchers’ words, that historic "deforestation has decreased the cloud forest area covered with fog in the montane regions by around 5-13% and raised the orographic cloud bases by about 25-75 meters in the afternoon." In addition, they say it suggests that "further deforestation in the lowland and premontane regions would lead to around [a] 15% decrease in the cloud forest area covered with fog and also raise the orographic cloud base heights by up to 125 meters in the afternoon." These findings clearly relieve anthropogenic CO2 emissions of any blame whatsoever for the decreases in frog and toad populations that have been experienced in the highland forests of Monteverde, Costa Rica, while placing the blame squarely on the shoulders of those responsible for the felling of the adjacent lowland forests.

Three years later, Parmesan (2006) tried once again to resurrect the idea propounded years earlier by Pounds et al. (1999), stating that "many cloud-forest-dependent amphibians have declined or gone extinct on a mountain in Costa Rica (Pounds et al., 1999, 2005)," and that "among harlequin frogs in Central and South American tropics, an astounding 67% have disappeared over the past 20-30 years," citing Pounds et al. (2006) as authority for this latter contention. In carefully reviewing these claims, however, they appear to be far from conclusive, as we report in our Editorial of 29 Nov 2006.

In the first place, all of the extinctions and disappearances of the amphibian species to which Parmesan refers appear to have nothing at all to do with "rapid loss of habitable climate space" at the tops of mountains. In fact, as noted by Pounds et al. (2006), the loss of these species "is largest at middle [our italics] elevations, even though higher-elevation species generally have smaller ranges." In addition, as noted in an earlier review of the subject by Stuart et al. (2004), many of the amphibian species declines "took place in seemingly pristine [our italics] habitats," which had not been lost to global warming nor even modestly altered. Last of all, the extinctions and species disappearances appear not to be due to rising temperatures per se, but to the fungal disease chytridiomycosis, which is caused by Batrachochytrium dendrobatidis, as noted by both Stuart et al. (2004) and Pounds et al. (2006).

In a final attempt to circumnavigate these several dilemmas, Pounds et al. (2006) strove mightily to implicate global warming as the cause of Batrachochytrium’s increased virulence in recent years. However, so convoluted and tenuous was their reasoning that they repeatedly referred to their view of the subject as being but a hypothesis. Also, in their paper’s Supplementary Information they say their goal was merely "to stimulate thought and generate ideas concerning the altitudinal patterns of thermal environments, the recent temperature shifts, and the interactions between Batrachochytrium and its amphibian hosts," with the hope that "future experimental studies should examine these ideas, while also considering the influence of other climatic changes such as shifts in precipitation and humidity." Last of all, and most damaging to their thesis, was the almost unbelievable fact, as reported by Bosch et al. (2007), that "Pounds et al. (2006) did not focus on showing whether the pathogen was present, or causing disease, in the species studied, raising questions as to whether infection by B. dendrobatidis [was] actually involved in the observed species declines."

Clearly, the last word on this subject has yet to be written; but Pounds et al. (2006) nevertheless stated as factual that "with climate change promoting infectious disease and eroding biodiversity, the urgency of reducing greenhouse-gas concentrations is now undeniable [our italics]," which suggests they are totally unwilling to even entertain the possibility that a different point of view might have merit, and, we might add, a different point of view that has been substantiated multiple times with observational data.

More recently, as described in our Editorial of 11 June 2008, Lips et al. (2008) began their analysis of the possible role of historical climate change in triggering disease outbreaks of chytridiomycosis — an emerging infectious disease of amphibians caused by the fungal pathogen B. dendrobatidis (Bd for short) — with the statement that "amphibian populations are declining across the globe at an alarming rate, with over 43% of species in a state of decline." Noting that the role of Bd in these population declines "has been linked to interactions with climate change" via the climate-linked epidemic hypothesis (CLEH) of Pounds et al. (2006) and Bosch et al. (2007), they indicate they have some serious reservations about this idea, because, as they continue, "both field studies on amphibians (Briggs et al., 2005; Lips et al., 2006) and on fungal population genetics (Morehouse et al., 2003; Morgan et al., 2007) strongly suggest that Bd is a newly introduced invasive pathogen." Consequently, and "from an ethical standpoint," as they put it, they cite as the primary reason for their further study of the subject, the "need to understand, as quickly as possible, the global patterns and causes of amphibian declines to prevent further losses of biodiversity."

In pursuit of the basic knowledge required to achieve this important goal, the four researchers evaluated data pertaining to population declines of frogs of the genus Atelopus, as well as similar data from other amphibian species, in Lower Central America and Andean South America, based on their own work and that of others recorded in the scientific literature, seeking to determine if the documented population declines were more indicative of an emerging infectious disease or a climate-change-driven infectious disease.

In discussing their findings, Lips et al. (2008) said they revealed "a classical pattern of disease spread across native populations, at odds with the CLEH proposed by Pounds et al. (2006)," emphasizing that their "analyses and re-analyses of data related to the CLEH all fail to support that hypothesis." Quite to the contrary, they say their analyses "support a hypothesis that Bd is an introduced pathogen that spreads from its point of origin in a pattern typical of many emerging infectious diseases," reemphasizing that "the available data simply do not support the hypothesis that climate change has driven the spread of Bd in our study area."

Although the U.S. scientists make it clear that disease dynamics are indeed "affected by micro- and macro-climatic variables," and that "such synergistic effects likely act on Bd and amphibians," their work clearly demonstrates that the simplistic scenario represented by the CLEH — which posits, in their words, that "outbreaks of chytridiomycosis are triggered by a shrinking thermal envelope" — paints an unrealistic picture of the role of global climate change in the much more complicated setting of real-world biology, where many additional factors may play even greater roles in determining amphibian wellbeing. Before concluding this Summary, therefore, we highlight the results of one such study of the subject that was published by Skelly et al. (2007) and described in our Editorial of 20 Feb 2008.

This group of seven scientists from the United States, Canada and Australia critiqued the common technique of using the "climate-envelope approach" to predict extinctions, citing as their primary reason for doing so the fact that this approach "implicitly assumes that species cannot evolve in response to changing climate," when in numerous cases they can do so very effectively. Stating that "many examples of contemporary evolution in response to climate change exist," they report that "in less than 40 years, populations of the frog Rana sylvatica have undergone localized evolution in thermal tolerance (Skelly and Freidenburg, 2000), temperature-specific development rate (Skelly, 2004), and thermal preference (Freidenburg and Skelly, 2004)," while noting that "laboratory studies of insects show that thermal tolerance can change markedly after as few as 10 generations (Good, 1993)." Adding that "studies of microevolution in plants show substantial trait evolution in response to climate manipulations (Bone and Farres, 2001)," they go on to say that "collectively, these findings show that genetic variation for traits related to thermal performance is common and evolutionary response to changing climate has been the typical finding in experimental and observational studies (Hendry and Kinnison, 1999; Kinnison and Hendry, 2001)."

Although evolution will obviously be slower in the cases of long-lived trees and large mammals, where long generation times are the norm, Skelly et al. say that the case for rapid evolutionary responses among many other species "has grown much stronger (e.g., Stockwell et al., 2003; Berteaux et al., 2004; Hariston et al., 2005; Bradshaw and Holzapfel, 2006; Schwartz et al., 2006; Urban et al., 2007)." As a result, they write that "on the basis of the present knowledge of genetic variation in performance traits and species’ capacity for evolutionary response, it can be concluded that evolutionary change will often occur concomitantly with changes in climate [our italics and bold] as well as [with] other environmental changes (e.g., Grant and Grant, 2002; Stockwell et al., 2003; Balanya et al., 2006; Jump et al., 2006; Pelletier et al., 2007)." And frogs, as noted above, are no exception to this general rule.


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Last updated 13 August 2008