Comparing Model Projections with Observations: Other Studies

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Results of other studies also suggest the model-based species extinction hypothesis is unlikely to occur. In a review paper published in Current Biology, for example, Erwin (2009) explored past epochs and the myriad nooks and crannies of contemporary Earth, all in a search for the primary trigger of speciation. His conclusion? Warmth is the fire that fuels the process by which species originate, whereas cold tends to destroy what warmth produced.

Headquartered in the Department of Paleobiology at the National Museum of Natural History in Washington, DC (USA), Erwin writes, “some of the best evidence for a link between biodiversity and climate comes from latitudinal gradients in diversity, which provide an avenue to explore the more general relationship between climate and evolution.” In reviewing that evidence, he indicates “among the wide range of biotic hypotheses, those with the greatest empirical support indicate that warmer climates [1] have provided the energetic foundation for increased biodiversity by fostering greater population size and thus increased extinction resistance, [2] have increased metabolic scope, [3] have allowed more species to exploit specialized niches as a result of greater available energy, and [4] have generated faster speciation and/or lower extinction rates.” He states “in combination with geologic evidence for carbon dioxide levels and changing areas of tropical seas, these observations provide the basis for a simple, first-order model of the relationship between climate through the Phanerozoic and evolutionary patterns and diversity,” and he adds “such a model suggests that we should expect greatest marine diversity during globally warm intervals,” as is typically also found to be the case for terrestrial diversity.

Erwin notes “the three best-studied mass extinction events are associated with sharp changes in climate and support the contention that rapid shifts in climate can reduce global diversity,” which sounds much like the mantra of the IPCC with respect to global warming. However, the climate shifts Erwin cites consist mostly of cooling, and it is not only the shift to cooling but stagnating in a cool state that bodes badly for Earth’s biodiversity. As Erwin describes it, “the long interval of stagnant evolution during the Permo-Carboniferous glaciation is consistent with studies of modern-day latitudinal diversity that [indicate] rates of evolutionary innovation and diversification are higher in high-energy climates than in low-energy climates.”

In further explanation of this conceptual framework, Erwin notes “contemporary studies suggest a positive relationship between high-energy climates and [1] increased diversification rates, [2] increased number of niches because of increased metabolic scope, and [3] more specialized niches, and possibly because of [4] niche construction.” Indeed, he states “studies showing that the tropics are a cradle of diversity, pumping clade representatives into higher latitudes, as well as evidence of increased ordinal level originations in the tropics, and of the sudden appearance of several mammalian groups during the Paleocene-Eocene Thermal Maximum suggest an asymmetric pattern of innovations associated with high-energy climate regimes.”

Erwin’s parting comment in this regard is his statement, “there is an intriguing possibility that diversity does not track climate, but rather builds up during warm intervals but without falling by proportional amounts when climates turn cooler,” with the result that “warmer climates may serve as an evolutionary diversification pump with higher diversity persisting [throughout following cooler periods], at least for a time.”

Whatever the details may be, two generalizations clearly can be made: warmth typically begets speciation, whereas cold tends to lead to species extinctions.

Independent support for this thesis was provided in two papers by Jaramillo et al. (2010) and Willis et al. (2010). Jaramillo et al. (2010) examined the Paleocene-Eocene Thermal Maximum (PETM), which occurred some 56.3 million years ago, at sites in Venezuela and Columbia. The PETM provides an interesting analog to the Current Warm Period, and it is postulated that at that time, carbon dioxide rose rapidly over a period of 10,000 to 20,000 years and global temperatures were elevated approximately 5 °C for 100,000 to 200,000 years.

The PETM was clearly identifiable in terms of temperature, where it was between 31 and 34 °C during the peak of global warmth. Using pollen and other organic materials, the authors were also able to identify various plant species that existed during this period. In spite of the high temperatures and possibly less-moist conditions, it was found the number of drought-tolerant plants did not increase, nor were moisture-requiring plants shown to decrease during the PETM. These facts led the authors to speculate that either rainfall or water use efficiency increased because of higher levels of atmospheric carbon dioxide, which would have compensated for the higher temperatures.

It was also noted the PETM brought a radical increase in diversity, measured in both absolute terms (number of species/taxa) and in terms of evenness (relatively more even species percentages). This was achieved by addition of new taxa. The origination rate for new taxa during the PETM was two to six times higher than during the periods before and after, but extinction rates were not unusual. The species/taxa that originated, and the overall diversity, continued after the PETM. Thus the gains in plant species richness during the PETM were relatively persistent rather than transient.

It is believed that tropical forests of today are currently growing in climates near the maximum temperature the plants can tolerate (~27.5 °C), and that future global warming in tropical zones may stress plants and cause a reduction in forest growth, perhaps leading some species to extinction. However, as shown by Jaramillo et al., temperatures of 31 to 34 °C seem not only to have been tolerated but to have caused a burst of speciation in the PETM, including major taxa that persist today. The high temperatures of the PETM were not detrimental to the tropical moist forests examined in this study.

In the study by Willis et al. (2010), past historical periods were identified in which climate was similar to that projected by global climate models for the next century or so, or in which the rate of temperature change was unusually rapid. These real-world periods were examined to see if any real-world climate-related extinctions had occurred.

The first period they examined was the Eocene Climatic Optimum (53–51 million years ago), when the atmosphere’s CO2 concentration exceeded 1,200 ppm and tropical temperatures were 5–10°C warmer than current values. Far from causing extinctions of the tropical flora (where the data are best), the four researchers report, “all the evidence from low-latitude records indicates that, at least in the plant fossil record, this was one of the most biodiverse intervals of time in the Neotropics.” They also note “ancestors of many of our modern tropical and temperate plants evolved ... when global temperatures and CO2 were much higher than present, ... indicating that they have much wider ecological tolerances than are predicted based on present-day climates alone.”

The second period they examined consisted of two rapid-change climatic events in the Holocene—one at 14,700 years ago and one at 11,600 years ago—when temperatures increased in the mid- to high-latitudes of the Northern Hemisphere by up to 10°C over periods of less than 60 years. There is evidence from many sites for rapid plant responses to rapid warming during these events. The authors note, “at no site yet studied, anywhere in the world, is there evidence in the fossil record for large-scale climate-driven extinction during these intervals of rapid warming.” On the other hand, they report extinctions did occur due to the cold temperatures of the glacial epoch, when subtropical species in southern Europe were driven out of their ecological tolerance zone.

The study by Willis et al. also makes use of recent historical data, as in the case of the 3°C rise in temperature at Yosemite Park over the past 100 years. Comparing surveys of mammal fauna conducted near the beginning and end of this period, they detected some changes, but no local extinctions. Thus they determined for all of the periods they studied, with either very warm temperatures or very rapid warming, there were no detectable extinctions.

Taking a slightly different approach to the subject were D’Odorico et al. (2008), who write, “recent climate change predictions indicate that, in addition to trends in the mean values of climate variables, an increase in interannual variability is expected to occur in the near future.” Also noting “environmental fluctuations are usually believed to play a ‘destructive role’ in ecosystem dynamics,” they explore the validity of this latter assumption in the context of current climate model predictions, asking, “Can environmental variability have only ‘negative’ effects on ecosystem dynamics?”

After lengthy mathematical analysis, D’Odorico et al. determined “opportunities for species existence/coexistence are found to increase with ‘moderate’ values of the variance of environmental fluctuations, while they decrease when these fluctuations are relatively strong.” This outcome constitutes what has come to be known as the “intermediate disturbance hypothesis,” which grew out of the work of Connell (1978)—who, in the words of D’Odorico et al., suggested “coral reefs and rain forests maintain high levels of diversity only in a non-equilibrium state”—and Huston (1979), who noted “most communities have relatively high levels of diversity because environmental variability maintains them in a non-equilibrium state.”

Noting numerous real-world indications (Chapin et al., 1997; Steneck et al., 2002; Bengtsson et al., 2003; Elmqvist et al., 2003; Bellwood et al., 2004; Folke et al., 2004) that “biodiversity may enhance ecosystem resilience,” the four researchers provide a theoretical basis for this phenomenon, demonstrating the validity of the ecological equivalent of the common dictum of physical exercise gurus: no pain, no gain. In light of this exercise, it would appear what the IPCC looks upon as unfavorable may be just what is needed to (1) increase the diversity and resilience of myriads of terrestrial and aquatic ecosystems and (2) propel them to higher levels of activity in the brave new world of our CO2-enriched future.

Finally, in an overview of a symposium titled “Molecules to Migration: Pressures of Life”— held in Africa on the Maasai Mara National Reserve of Kenya—Fuller et al. (2010) write the theoretical approach most commonly used to predict future species distributions in a CO2-enriched and warmer world (the “climate envelope” approach) assumes “animals and plants can persist only in areas with an environment similar to the one they currently inhabit.” This approach “typically ignores the potential physiological capacity of animals to respond to climate change,” and they go on to explain how “behavioral, autonomic, and morphological modifications such as nocturnal activity, selective brain cooling, and body color may potentially serve as buffers to the consequences of climate change.”

The six scientists begin by noting all organisms “have the capacity to adapt to changing environmental conditions both by phenotypic plasticity within a life span and by microevolution over a few life spans.” In the latter instance, they note, “there is evidence that microevolution—that is, heritable shifts in allele frequencies in a population (without speciation)—has occurred in response to climate warming,” citing Bradshaw and Holzapfel (2006, 2008). And in the first case, they state phenotypic plasticity “is likely to represent the first response of individual organisms,” and they report “adaptive changes in phenotype induced by climate change have been documented, for example, in the morphology and phenology of birds (Charmantier et al., 2008) and mammals (Reale et al., 2003; Linnen et al., 2009; Maloney et al., 2009; Ozgul et al., 2009).”

Fuller et al. cite the work of Pincebourde et al. (2009), who “showed that intertidal sea stars can behaviorally regulate their thermal inertia by increasing their rate of water uptake during high tide on hot days,” which is “a response that affords protection against extreme aerial temperatures during subsequent low tides.” Next they note “exposure of humans to hot conditions on successive days induces an increase in sweat capacity (Nielsen et al., 1993).” And they state “other adaptations also ensue, including plasma volume expansion and decreased electrolyte content of sweat,” such that “a typical unacclimatized male, who can produce about 600 ml of sweat per hour, can double that output with heat acclimatization (Henane and Valatx, 1973),” which “phenotypic adaptation (in this case, heat acclimatization) can alter physiological tolerance (the risk of heat illness).”

The Australian, South African, and U.S. scientists also cite several studies—Zervanos and Hadley (1973), Belovsky and Jordan (1978), Grenot (1992), Hayes and Krausman (1993), Berger et al. (1999), Dussault et al. (2004), Maloney et al. (2005), and Hetem et al. (2010)—of large herbivores that “increase nocturnal activity in the face of high diurnal heat loads.” And they state “another adaptation that may enhance plasticity in response to aridity that is available to oryx and other artiodactyls, as well as members of the cat family (Mitchell et al., 1987), is selective brain cooling,” whereby cooling the hypothalamus and the temperature sensors that drive evaporative heat loss “inhibits evaporative heat loss and conserves body water (Kuhnen, 1997; Fuller et al., 2007),” which “is likely to be particularly valuable to animals under concurrent heat stress and dehydration.” Finally, they suggest maintaining genetic diversity for a trait such as fur or feather color that adapts various organisms to different thermal environments “may provide important plasticity for future climate change,” citing Millien et al. (2006), and they add “there is already evidence that, over the past 30 years as the climate has warmed, the proportion of dark-colored to light-colored Soay sheep has decreased on islands in the outer Hebrides,” citing Maloney et al. (2009).

Clearly, much of Earth’s animal life is well-endowed with inherent abilities to cope, either through behavior or otherwise, with climate changes over a period of a few generations, a single generation, or even in real time.


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