Here's how the hypothesis had developed. Still et al. ran a global climate model 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, which periodically shrouded 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 (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 CO2-induced global warming was indeed the culprit behind the observed change in environmental conditions (essentially more dry days), which were believed to be responsible for the changes in amphibian populations 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 scientists looked pretty strong. In fact, to many it was compelling. Then, however, came the study of Lawton et al. (2001), in which was 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 expose by noting that the trade winds that reach the Monteverde cloud-forest ecosystem flow across approximately 100 km of lowlands in the Rio San Juan basin, and that deforestation proceeded rapidly in the Costa Rican part of the basin over the past century. By 1992, in fact, only 18% of the original lowland forest remained; and the four researchers astutely noted that this conversion of forest to pasture and farm land altered the properties of the air flowing across the landscape. The reduced evapotranspiration that followed deforestation, for example, decreased the moisture content of the air mass; and regional atmospheric model simulations suggested there should be reduced cloud formation and higher cloud bases over such deforested areas, which would cause there to be fewer and higher-based clouds than would otherwise have been the case when the surface-modified air moved into the higher Monteverde region.

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

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 remained largely intact -- providing a striking contrast to the mostly-deforested lands in neighboring Costa Rica -- they used 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." Consequently, they demonstrated by direct observation that the effects predicted by the theory they espoused 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 cloud effects.

Two years later, Nair et al. (2003) 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 said it suggested 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 relieved anthropogenic CO2 emissions of any blame whatsoever for the decreases in frog and toad populations that had 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.

So global warming got a reprieve as far as declines in amphibian populations were concerned; but it was rather short-lived, for as time progressed, and as cases of amphibian mass mortality were reported throughout the world, climate alarmists once again grabbed center stage, with Parmesan (2006) and Pounds et al. (2006) pointing accusing fingers at CO2 and claiming that global warming was promoting the spread of Batrachochytrium dendrobatidis (Bd, a non-hyphal zoosporic fungus that was the immediate cause of the amphibian declines) and triggering outbreaks of chytridiomycosis via what came to be known as the climate-linked epidemic hypothesis (CLEH).

Investigating this concept within the Penalara Natural Park in the Sierra de Guadarrama of Central Spain, Bosch et al. (2007) looked for relationships between 20 different meteorological variables and the development of chytridiomycosis infection in the area's amphibian populations, focusing on "two time periods according to the lack (1976-1996) or presence (1997-2003) of observed chytrid-related mortalities." This work revealed, as they described it, "a significant association between change in local climatic variables and the occurrence of chytridiomycosis," leading them to conclude that "rising temperature is linked to the occurrence of chytrid-related disease."

Being careful to not be too adamant about what their data implied, however, Bosch et al. noted that "associations between climate and disease do not necessarily imply causation." They also stated that "chytrid-related declines are probably the result of a complex web of interaction, and the effects of climate will be conditional on other factors such as host density, amphibian community composition, microbial competitors and zooplankton predators, to name but a few [our italics]." And in order to disentangle this network and break it down into its key compartments, they said it would be necessary "to collect seasonal data on amphibian densities, contemporary and historical measurements of the prevalence and intensity of infection, seasonal mortalities, and fine-scale meteorological conditions from a range of sites that represent altitudinal clines," as well as to conduct "molecular epidemiological analyses." Consequently, in light of the many complexities they listed, it was clear that the last word on the subject was yet to be written; and, in fact, several additional studies appeared in print the following year.

Lips et al. (2008) 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, noting in this regard that "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."

In discussing their findings, Lips et al. (2008) said they reveal "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 concluded 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 four U.S. scientists made 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 showed 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.

Next up was Laurance (2008), who tested the hypothesis -- put forward by Pounds et al. (2006) -- that "the dramatic, fungal pathogen-linked extinctions of numerous harlequin frogs (Atelopus spp.) in upland rainforests of South America mostly occurred immediately following exceptionally warm years, implicating global warming as a likely trigger for these extinctions." This he did "using temperature data for eastern Australia, where at least 14 upland-rainforest frog species [had] also experienced extinctions or striking population declines attributed to the same fungal pathogen, and where temperatures [had] also risen significantly in recent decades." This work, in Laurance's words, provided "little direct support for the warm-year hypothesis of Pounds et al." Instead, he wrote that he "found stronger support for a modified version of the warm-year hypothesis," where frog declines were only likely to occur following three consecutive years of unusually warm weather; and he added that such was observed "only at tropical latitudes, where rising minimum temperatures were greatest."

In further discussing his findings, Laurance states that many researchers "remain unconvinced that ongoing disease-linked amphibian declines are being widely instigated by rising global temperatures or associated climatic variables, as proposed by Pounds et al." He notes, for example, that "chytrid-linked amphibian declines have been documented on several continents and at varying times," and that to date, "no single environmental stressor has been identified that can easily account for these numerous population crashes." In his personal view, as he continues, "it stretches plausibility to argue that the chytrid pathogen is simply an opportunistic, endemic microparasite that has suddenly begun causing catastrophic species declines as a consequence of contemporary global warming."

Also in the same year, Rohr et al. (2008) provided a rigorous test of the two competing hypotheses by evaluating "(1) whether cloud cover, temperature convergence, and predicted temperature-dependent Bd growth are significant positive predictors of amphibian extinctions in the genus Antelopus and (2) whether spatial structure in the timing of these extinctions can be detected without making assumptions about the location, timing, or number of Bd emergences." And after completing their research, the five scientists reported that "almost all of our findings are contrary to the predictions of the chytrid-thermal-optimum hypothesis," even noting that "not all of the data presented by Pounds et al. (2006) are consistent with the chytrid-thermal-optimum hypothesis." Most tellingly, in this regard, they say "there was no regional temperature convergence in the 1980s when extinctions were increasing, and that convergence only occurred in the 1990s when Atelopus spp. extinctions were decreasing, opposite [our italics] to the conclusions of Pounds et al. (2006) and the chytrid-thermal-optimum hypothesis." On the other hand, they report "there is a spatial structure to the timing of Atelopus spp. extinctions but that the cause of this structure remains equivocal, emphasizing the need for further molecular characterization of Bd."

Last of all, we come to the study of Alford et al. (2009), who -- no longer feeling any need to address the repudiated climate-linked epidemic hypothesis -- quantified four movement characteristics of three groups of radio-tracked cane toads (Bufo marinus) at three different places in Australia: (1) a location where the toads had been established for some fifty years at the time of their sampling, (2) a location where the first toads arrived about six months before sampling began in 1992 and 1993, and (3) a location where sampling occurred for a period of thirteen months, starting at the time of the toads initial arrival in 2005. The results of this exercise revealed that for all of the movement parameters they studied, "toads from the current invasion front differed dramatically from animals in the long-established population, while toads from the earlier invasion front were intermediate between these extremes."

So just how dramatically did the movement parameters differ? The five researchers report that "cane toads are now spreading through tropical Australia about 5-fold faster [our italics] than in the early years of toad invasion." As for why this is so, they say that "the current invasion-front animals achieved these [high invasion speeds] by rarely reusing the same retreat site two days in succession, by travelling further each night when they did move, and by moving along straighter paths." Therefore, as they describe it, the toad invasion front "advances much more rapidly than would occur if the toads retained ancestral behaviors (less frequent relocation, with shorter movements, and fewer toads using straight paths)." And because of the fact that "invasion-front toads in 1992 were more dispersive than origin-population toads in the same year, but that invasion-front toads have continued to evolve heightened dispersal ability and dispersed even more effectively in 2005 than they did in 1992," they say these observations suggest that "as long as toads continue to invade suitable new habitat, dispersal ability will be selected upwards."

In discussing their findings, Alford et al. write that the rapidity and magnitude of the shifts in cane toads "are truly remarkable," having been accomplished in only 50 generations (about 70 years); and they state that "such a major shift over such a brief period testifies to the intense selective pressure exerted on frontal populations of range-shifting species." This development, in their words, "not only has implications for our understanding of the rates of invasion by non-native species, but also for the rate of range-shift in native taxa affected by climate change," the implication being, in our view, that most species will do what they have to do (by evolving how they need to evolve) to meet whatever challenge a rapidly changing climate might place before them.

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