http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v427/n
6970/full/427107a_fs.html
Nature 427, 107 - 109 (08 January 2004); doi:10.1038/427107a
Ecology: Clouded futures
J. ALAN POUNDS AND ROBERT PUSCHENDORF
J. Alan Pounds is at the Golden Toad Laboratory for Conservation,
Monteverde Cloud Forest Preserve and Tropical Science Center, Santa
Elena, Puntarenas 5655-73, Costa Rica.
e-mail: [EMAIL PROTECTED]
Robert Puschendorf is in the School of Biology, University of Costa
Rica, San Pedro de Montes de Oca, Costa Rica.
e-mail: [EMAIL PROTECTED]
Global warming is altering the distribution and abundance of plant
and animal species. Application of a basic law of ecology predicts
that many will vanish if temperatures continue to rise.
Evidence that climate change is affecting life on Earth continues to
mount1, 2. But how great is the threat to biodiversity? On page 145
of this issue, Thomas et al.3 show that global warming, projected to
the year 2050, could sharply increase extinction probabilities for a
sample of 1,103 species representing terrestrial regions from Mexico
to Australia. If temperatures follow middle-of-the-road projections,
the study suggests, about one-quarter of these species may disappear
- a loss that would exceed that expected from habitat destruction.
Thomas et al. assume that each species can persist only under a
particular set of climatic conditions. This 'climate envelope',
assessed by modelling current geographical distribution in relation
to climatic gradients, serves to predict future distribution. As
warming alters these gradients, many species are shifting towards the
poles or to higher elevations, their ranges often contracting as the
area of climatically suitable habitat declines1, 2.
To predict the outcome, Thomas et al. turn to one of ecology's few
ironclad laws: the species-area relationship. Basically, smaller
areas support fewer species. In 1859, the year in which Darwin's
Origin of Species appeared, H. C. Watson demonstrated this law for
Britain's vascular flora by comparing sampling areas ranging from a
square mile to all of England4. Plotting the logarithm of the number
of species as a function of the logarithm of area, he found a linear
relationship (Fig. 1). This pattern, we now know, is typical of
regional scales, where a power-law equation usually describes the
relationship between number of species and area4. At these scales,
the species-area curve probably reflects the configuration of
species' ranges and thus the history of speciation, dispersal and
extinction.
Figure 1 H. C. Watson's demonstration of the species-area law
for Britain's vascular flora. Full legend
High resolution image and legend (21k)
Using this relationship to assess climate-related extinction risk,
Thomas et al. explore three approaches. Their conclusions are the
same with each of them. They consider change in area summed for the
various species, proportional loss of area averaged across species,
and change in area for each species individually. Averaging the
results of these methods, applied under two dispersal scenarios, they
estimate the extinction probability for different amounts of warming
as 18% (0.8-1.7 ¡C), 24% (1.8-2.0 ¡C) and 35% (above 2.0 ¡C).
These estimates might be optimistic. The risk of extinction increases
as global warming interacts with other factors - such as landscape
modification, species invasions and build-up of carbon dioxide - to
disrupt communities and ecological interactions. Furthermore, the
models might not capture some key climatic changes. One pillar of
Thomas and colleagues' analysis is the modelling of range
contractions in the tropical rainforests of northeast Australia5. But
the authors of that study5 emphasize that it considers only the
effects of rising temperatures and that other changes could magnify
the impacts. In the highlands, for example, an increase in the
altitudes at which clouds form6-8 could affect communities that
require frequent immersion in clouds and mist.
Changes in cloud cover might also be important. Warming accelerates
evaporation and increases the air's capacity to hold water, thereby
increasing its content of water vapour9. Cloud formation, however,
depends on relative humidity, which varies inversely with
temperature, so warming may reduce cloudiness over some regions. In
contrast, where air masses cool sufficiently - for instance where
they ascend mountain slopes - increased water vapour should translate
into enhanced cloud formation, even if condensation begins at
increased altitudes. Accordingly, widespread increases in cloud cover
are under way9.
Are changing cloud patterns already contributing to the extinction of
species? Thomas et al.3 refer to amphibian declines and
disappearances in the mountains of Costa Rica as the one example in
which recent warming has been implicated in such losses (Fig. 2).
Various biological changes in these mountains are associated with
unusually dry weather attributed to an increase in heights of cloud
formation6. Understanding amphibian extinctions is crucial, given
that they are taking place in highlands around the world10, 11. For
example, most of the 70-odd members of the harlequin frog genus
Atelopus, endemic to Central and South America, have vanished or
declined markedly (E. La Marca, personal communication).
Figure 2 Absent amphibians. Full legend
High resolution image and legend (48k)
Nevertheless, few studies have examined how climatic changes might be
linked to the immediate causes of these declines12, 13. The
climate-envelope concept championed by Thomas et al. might help to
shed light on one such cause - outbreaks of the chytrid fungus
Batrachochytrium dendrobatidis14, 15. This lethal parasite of
amphibian skin thrives under cool, moist conditions. In culture, it
grows at 6-28 ¡C but dies at higher temperatures. Experiments with
the Australian frog Litoria chloris16 show that elevated body
temperatures, reached naturally by basking in the sun or seeking warm
microenvironments, can rid the frogs of this fungus. The low humidity
typical of warm microsites might likewise enhance frog survival.
Both increased cloud cover and unusually dry weather might hamper
these defences. In highland tropical forests, ambient air
temperatures generally lie within the climate envelope of
Batrachochytrium. But these forests include shaded and sunlit
microhabitats. Under clear skies, temperatures in the latter can
quickly exceed 30 ¡C, so an amphibian can 'escape' from this
envelope. Under cloudy skies, however, microhabitat temperatures
mirror ambient temperatures, making escape difficult. Dry conditions
may have similar consequences: with limiting moisture, an amphibian
might have to stay in cool, damp places.
Although exploring these potential links between climate and recent
extinctions is essential, the patterns implicating global warming in
such losses attest to the urgency of Thomas and colleagues' principal
recommendation3. Reducing the concentrations of greenhouse gases -
and reducing them soon - could minimize this warming and hence the
number of extinctions. The threat to life on Earth is not just a
problem for the future. It is part of the here and now.
References
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12. Kiesecker, J. M., Blaustein, A. R. & Belden, L. K. Nature
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13. Pounds, J. A. Nature 410, 639-640
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14. Berger, L. et al. Proc. Natl Acad. Sci. USA 95, 9031-9036
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15. Burrowes, P. A., Joglar, R. L. & Greene, D. E. Herpetologica
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16. Woodhams, D. C., Alford, R. A. & Marantelli, G. Dis. Aquat.
Org. 55, 65-67 (2003). | PubMed | ISI |
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