Monday, October 11, 2010

This Can’t Be Good: Squamate Diversity and the Evapotranspiration Rate

The following is a composite of the articles listed at the bottom of the post. As well as a press release from Oregon State University about the Jung et al. study and carried by Eureka Alert.

Earlier this year David Bickford and colleagues summarized the impact of climate change on the herpetofauna of Southeast Asia. They predicted the following impacts as the temperature increases: desiccation of frog eggs laid in leaf litter and soil; changes in tadpole behavior due to low dissolved oxygen followed by an increase in tadpole mortality; an increase in susceptibility to diseases; loss of freshwater turtle habitat; a decrease in mass or population size due to increased metabolism; skewed sex rations in TSD species; elimination of males or females in TSD species and more species of reptiles showing single sex populations; reduced population size of fossorial species because of soil desiccation; loss of marine turtle prey base and subsequent population declines; and reduced diversity at low and high elevations; all of these alterations increase competition and changes in community composition.

Additionally, Christy McCain (2010) used 25 elevational gradients of reptile diversity from temperate, tropical and desert mountains in both hemispheres that spanned the latitudes from 10.3° N to 46.1° N. She found reptile richness and, specifically snake, as well as lizard, richness on mountains showed four distinct patterns: decreasing, low-elevation plateaus, low-elevation plateaus with mid-elevation peaks, and mid-elevation peaks. Reptile richness at various elevations was most strongly correlated with temperature. The temperature effect was mediated by precipitation; reptile richness was more strongly tied to temperature on wet gradients than on arid gradients. Area was secondarily important, while the mid-domain effect was not strongly associated with reptile diversity on mountains. Montane reptile diversity patterns did not follow the predicted temperature–water effect, as all diversity patterns were found on both wet and dry mountains. However, precipitation’s effect on temperature most likely reflects reptiles using basking opportunities that are more abundant on arid mountains than wet mountains because of lower humidity, sparser vegetation and less cloud cover at low and intermediate elevations.
Now, a study lead by Martin Jung from the Max Planck Institute for Biogeochemistry in Germany report that the soils in large areas of the Southern Hemisphere, including major portions of Australia, Africa and South America, have been drying up for the past decade. This is the first major study to examine evapotranspiration on a global basis.

Most climate models have suggested that evapotranspiration (the movement of water from the land to the atmosphere) would increase with global warming. Jung et al’s study published online this week in the journal Nature, found that's exactly what happened from 1982 to the late 1990s.

However, in 1998, this significant increase in evapotranspiration – which had been seven millimeters per year – slowed dramatically or stopped. In large portions of the world, soils are now becoming drier than they used to be, releasing less water and offsetting some moisture increases elsewhere.

Due to the limited number of decades for which data are available, scientists say they can't be sure whether this is a natural variability or part of a longer-lasting global change. But one possibility is that on a global level, a limit to the acceleration of the hydrological cycle on land has already been reached.

Jung et al suggest the trend could reduced terrestrial vegetation growth, reduce carbon absorption, and reduce the natural cooling mechanism created by evapotranspiration. The results would produce increased heating of the land’s surface, more intense heat waves, and a feedback loop that could intensify global warming.

Regions that show most severe drying include southeast Africa, much of Australia, central India, large parts of South America, and some of Indonesia. Most of these regions are historically dry, but some are actually tropical rain forests.

The rather abrupt change from increased global evapotranspiration to a near halt in this process coincided with a major El Nino event in 1998, the researchers note in their report, but they are not suggesting that is a causative mechanism for a phenomenon that has been going on for more than a decade now.
Greater evapotranspiration was expected with global warming, because of increased evaporation of water from the ocean and more precipitation overall. And data indeed show that some areas are wetter than they used to be. However, other huge areas are now drying out. This could lead to increased drought stress on vegetation and less overall productivity. The result could be less carbon absorbed, less cooling through evapotranspiration, and more frequent or extreme heat waves.

Evapotranspiration returns about 60 percent of annual precipitation back to the atmosphere, in the process using more than half of the solar energy absorbed by land surfaces. This is a key component of the global climate system, linking the cycling of water with energy and carbon cycles. Long term studies will be needed to determine if these changes are part of decade trend or a longer-term shift in global climate, the researchers said.

While changes in the evapotranspiration rate are likely to favor some species of squamates at some localities, others will be negatively impacted, thus radical changes in the species composition of communities seems likely.

Bickford, D., S, D. Howard, J. J. Ng, and J. A. Sheridan. 2010. Impacts of climate change on the amphibians and reptiles of Southeast Asia. Biodiversity and Conservation 9:1043-1062.
McCain, C. M. 2010. Global analysis of reptile elevational diversity. Global Ecology and Biogeography 19:541-553.

Jung, M., M. Reichstein, P. Ciais, S.I. Seneviratne, J. Sheffield, M.L. Goulden, G. Bonan, A. Cescatti, J. Chen, R. de Jeu, A.J. Dolman, W. Eugster, D. Gerten, D. Gianelle, N. Gobron, J. Heinke, J. Kimball, B.E. Law, L. Montagnani, Q. Mu, B. Mueller, K. Oleson, D. Papale, A.D. Richardson, O. Roupsard, S.W. Running, E. Tomelleri, N. Viovy, U. Weber, C. Williams, E. Wood, S. Zaehle, K. Zhang. 2010. A recent decline in the global land evapotranspiration trend due to limited moisture supply. Nature xxxx: xxx-xxx. DOI 10.1038/nature09396.

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