|
ScienceWeek
EXTINCTION: CLIMATE CHANGE AND FROGS
The following points are made by A.R. Blaustein and A. Dobson (Nature 2006 439:143):
1) One of the worries about global climate change is that it will raise the transmission rates of infectious diseases[1]. New work[2] provides compelling evidence that anthropogenic climate change has already altered transmission of a pathogen that affects amphibians, leading to widespread population declines and extinctions.
2) According to the Global Amphibian Assessment (GAA)[3], approximately a third of amphibian species (1856) are classified globally as "threatened". The tenuous hold these animals have on life is especially evident in tropical America, where, for example, 67% of the 110 species of harlequin frog (Atelopus) endemic to the region have died out in the past 20 years[3]. A pathogenic chytrid fungus, Batrachochytrium dendrobatidis, is implicated as the primary cause of Atelopus population crashes and species extinctions[4,5]. Now, Pounds et al[2] offer a mechanistic explanation of how climate change encourages outbreaks of B. dendrobatidis in the mountainous regions of Central and South America: night-time temperatures in these areas are shifting closer to the thermal optimum of B. dendrobatidis, and increased daytime cloudiness prevents frogs from finding "thermal refuges" from the pathogen.
3) Pounds et al[2] defined an "extinction" as the time when a frog species was last observed by professional teams of herpetologists working in these regions. Most extinctions (78% to 83%) occurred in years that were unusually warm across the tropics. The likelihood that this correlation arose by chance is less than one in a thousand. Moreover, the observed patterns of extinction vary with altitude -- as do the effects of climate change. Montane Atelopus species that live between 1000 and 2400 meters show higher rates of extinction than do those that live only in the lowlands (where extinctions are rare) or just in the highest elevations. Pounds et al[2] propose that this is because the extreme sites afford thermal refuges, with temperatures being either too high or too low for optimal growth of the pathogen. Mid-elevation Atelopus communities are not only the hardest hit by extinction, but they also harbour the most species, so biodiversity in these areas is in double jeopardy.
4) These results corroborate the GAA findings[3] for a broad array of amphibians that the percentage of extinct or threatened species is largest at middle elevations. This is contrary to the expectation that higher-elevation species would be more prone to extinction because they generally have smaller environmental ranges over which they can survive. Although the little-known Batrachochytrium fungus was proposed to be potentially the sole reason for declines in amphibian populations in the tropics[4,5], no one had come up with an explanation for the sudden emergence of this pathogen. Moreover, although chytrid disease was a common condition in many areas experiencing declines, it was not clear whether Batrachochytrium was directly responsible or whether the infection was a secondary effect associated with dead or dying animals. Previous attempts to explain the prevalence of the disease in terms of climate change had been stymied by the so-called "climate chytrid paradox", because the climatic conditions favoring chytrid growth seemed to be the very opposite of those created by current climate trends.
References (abridged):
1. Dobson, A. P. & Carper, E. R. Lancet 342, 1096 1099 (1993)
2. Pounds, J. A. et al. Nature 439, 161 167 (2006)
3. http://www.globalamphibians.org (2004)
4. Daszak, P. et al. Emerg. Infect. Dis. 5, 735 748 (1999)
5. Berger, L. et al. Proc. Natl Acad. Sci. USA 95, 9031 9036 (1998)
Nature http://www.nature.com/nature
--------------------------------
Related Material:
ECOLOGY: EXTINCTION PATTERNS AND ECOSYSTEMS
The following points are made by David Raffaelli (Science 2004 306:1141):
1) The accelerated extinctions of species and changes in biodiversity are no longer disputed issues. Much effort has gone into quantifying biodiversity loss rates for particular animal and plant groups (1). Less clear, however, is the impact of such losses on ecosystems, especially when many different kinds of species of plants and animals are lost simultaneously (2). Yet policy-makers urgently need guidance on the effects of multispecies losses if they are to plan for and advise on the societal consequences of biodiversity changes. The ecological research community has been highly active in attempting to provide such guidance (3,4), but many challenges remain. Foremost among these is that most real extinction events are nonrandom with respect to species identity -- some species are more likely to go extinct than others -- whereas research studies often assume that extinctions are random.
2) Solan et al (5) and Zavaleta and Hulvey (6), reporting on work in two very different types of ecosystem, reveal that the impact of nonrandom species extinctions on ecosystems is markedly different from that predicted by scenarios where extinctions are random. These studies bring us a step nearer to understanding the impact of nonrandom species losses on ecosystems and should help to provide policy-makers with a firmer basis for decision-making.
3) The two studies examine very different habitats (marine versus terrestrial), each with different kinds of organisms (sea-bed invertebrates versus grassland plants), different ecosystem processes (sediment biogeochemistry versus resistance to invasion by exotic species), and different types of experimental approaches (data analysis and modeling versus controlled experimentation). So it is all the more interesting, for scientists and policy-makers alike, that both papers arrive at the same conclusion: Nonrandom extinction events have impacts on ecosystems that are quite different from those predicted by scenarios that assume species extinctions occur at random.
4) Solan et al (5) combine into a model a well-documented data set of invertebrate communities in marine sediments off the coast of Galway, Ireland. This fusion, facilitated by the BIOMERGE initiative, enables the authors to predict what will happen to the cumulative effects of the small-scale sediment disturbances (bioturbation) caused by the movement, feeding, and respiration activities of all 139 species of clams, worms, sea urchins, brittle stars, and shrimps present in this system if species are lost through impacts such as overfishing, habitat destruction, and pollution. The authors scored each species for its body size, mobility, and mode of sediment mixing to calculate an index of bioturbation potential for different species combinations and for different degrees of species richness. In their model, either extinction scenarios could be random or losses could be ordered with respect to the sensitivity of species to environmental stress, body size, and abundance, traits that in turn reflect different kinds of impact.
References (abridged):
1. See www.royalsoc.ac.uk/events
2. See www.millenniumassessment.org/en/index.aspx
3. M. Loreau et al., Science 294, 804 (2001)
4. M. Loreau, S. Naeem, P. Inchausti, Biodiversity and Ecosystem Functioning (Oxford Univ. Press, Oxford, 2002)
5. M. Solan et al., Science 306, 1177 (2004)
6. E. S. Zavaleta, K. B. Hulvey, Science 306, 1175 (2004)
Science http://www.sciencemag.org
--------------------------------
Related Material:
PALEONTOLOGY: ON ICE-AGE EXTINCTIONS
The following points are made by J. Pastor and R.A. Moen (Nature 2004 431:639):
1) Sabre-toothed tigers, mastodons, woolly mammoths -- these and many other spectacular large mammals are generally thought to have become extinct about 10,000 years ago, at the end of the Pleistocene epoch, otherwise known as the last ice age. But it's becoming clear that some of these species clung on close to the present day. Thomas Jefferson's instruction to Meriwether Lewis and William Clark to search for live woolly mammoths in the American West in 1804 was perhaps a little optimistic. But the species survived on Wrangel Island in the northeastern Siberian Arctic until some 4000 years ago(1), making it contemporaneous with the Bronze Age Xia Dynasty in China. Stuart et al(2) have reported that another charismatic ice-age mammal that was thought to have become extinct 10,000 years ago -- the giant deer or Irish elk (Megaloceros giganteus) -- survived in western Siberia to the dawn of historic times. The finding lends weight to the idea that there is no one explanation for the so-called Pleistocene extinctions.
2) The Irish elk must have cut an impressive figure, standing more than two meters high at the shoulder -- about the same as a bull moose, the largest living member of the deer family. But when and why did it become extinct? In their investigation, Stuart et al(2) began by carrying out radiocarbon dating of five skeletal specimens, including a complete skeleton of an antler-bearing male. By combining this information with maps of the specimens' locations, they demonstrated that Irish elk were widespread in Europe -- from Ireland to Russia, and from Scandinavia to the Mediterranean -- before 20,000 years ago. But by the last glacial maximum 15,000 years ago, they may have been restricted to refuges in the shrub steppes of central Asia. From there, Irish elk apparently recolonized northwestern Europe following the retreat of the Alpine and Scandinavian ice sheets during a period of climatic warming. The European population made a last stand in the British Isles before dying out 10,500 years ago, but the Siberian population persisted for another 3000 years.
3) What caused the extinction of so many large mammals 10,000 or so years ago? Human hunting(3), changes in climate or vegetation, or both(4), are often proposed to be causal factors. But the "ragged" nature of these Late Pleistocene extinctions, with isolated pockets of populations surviving for longer, suggests that the extinctions have a complex ecology, with no single mechanism responsible for the demise of every species in every location.
4) Theories for both the expansion and the extinction of Irish elk populations, for instance, often focus on the animals' huge antlers, which weighed 40 kilograms and spanned 3.5 meters, making them 30% larger than those of modern moose. It has been suggested(5) that female Irish elk selected males with large antlers, as this might have signified an ability to find sufficient food to support building and shedding a rack each year. This ability would then be passed on to their male progeny. But the large antlers, which contained as much as 8 kilograms of calcium and 4 kilograms of phosphate, would have posed a large annual nutritional burden on bulls. The antlers would also be physically unwieldy in dense forests. So both physical and nutritional constraints probably restricted the Irish elk to productive open environments, with relatively tall willow and birch shrubs that could be navigated but still supply sufficient calcium and phosphate for antler growth. An inability to balance sexual selection for large antlers with nutritional selection pressures for smaller antlers may have led to the Irish elk's demise in the British Isles, particularly as the climate cooled rapidly and caused the vegetation to change to short-statured and unproductive tundra.
References (abridged):
1. Vartanyan, S. L., Garrut, V. E. & Sher, A. V. Nature 362, 337-340 (1993)
2. Stuart, A. J., Kosintsev, P. A., Higham, T. F. G. & Lister, A. M. Nature 431, 684-689 (2004)
3. Martin, P. S. in Quaternary Extinctions: A Prehistoric Revolution (eds Martin, P. S. & Klein, R. G.) 364-403 (Univ. Arizona Press, Tucson, 1984)
4. Stuart, A. J. Biol. Rev. 66, 453-562 (1991)
5. Geist, V. Nat. Hist. 95, 54-65 (1986)
Nature http://www.nature.com/nature
ScienceWeek http://scienceweek.com
|