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ScienceWeek
ECOLOGY: ON THE CONSEQUENCES OF EXTINCTION
Notes by ScienceWeek:
In this context, the term "food web" refers in general to a description of who eats whom in an ecosystem.
The following points are made by Peter Kareiva (Current Biology 2004 14:R627):
1) The "global extinction crisis" has become a focus of concern and activism for conservationists [1]. We are currently in the middle of the sixth major extinction event in geologic history --this one almost entirely human induced. Current extinction rates are estimated to be 100 to 1000 times higher than pre-human extinction rates [2]. This rapid loss of species has spurred researchers to examine what might be the consequences of losing such a large proportion of our biodiversity.
2) Although ecosystems clearly would not function if all species went extinct, no one can really say what might be the impact of losing 80% of the species as opposed to only 20% of the species. In fact, even though we have seen many conspicuous species go extinct before our eyes, we know precious little about the consequences of those extinctions [3]. Recently, community ecologists have manipulated experimental communities by either removing one or two species or assembling communities of differing species richness [4]. These experiments teach us about the role of biodiversity and predation or competition, but have not provided a compelling picture of the consequences of extinction. The limitation of these targeted removals is their small scale and short duration.
3) The weakness of our empirical insight regarding extinction has caused ecologists to rely heavily on metaphors and models. The purpose of these models is to anticipate what might happen if the predictions of massive species loss hold true [5]. Models that consider the consequences of extinction have tended to focus on either the reliability or the stability of ecosystems. Reliability models emphasize that the loss of species eliminates redundancy, so that at some point ecosystems may end up with only one or two species filling some critical function -- such as nitrogen sequestration or primary production -- leaving the ecosystems vulnerable to any catastrophe or stress that harms these now irreplaceable species. Stability models adopt a more traditional population dynamics framework, and ask how the loss of species alters either the ability to recover from disturbances, or the tendency towards fluctuations in the face of randomly varying environments. The general message of these many theoretical explorations of extinction is that species loss impairs both stability and reliability. But the theory is in no way complete: in particular, very few models consider food webs and highly structured trophic communities.
References (abridged):
1. Gibbs, W.W. (2001). On the termination of species. Sci. Am. 285, 40-49
2. Pimm, S.L., Russell, G.J., Gittleman, J.L. and Brooks, T.M. (1995). The future of biodiversity. Science 269, 347-350
3. Simberloff, D. (2003). Community and ecosystem impacts of single-species extinctions. In The Importance of Species. Kareiva, P. and Levin, S. eds. (Princeton: Princeton University Press), pp. 221-234
4. Wootton, J.T. and Downing, A.L. (2003). Understanding the effects of reduced biodiversity: a comparison of two approaches. In The Importance of Species. Kareiva, P. and Levin, S. eds. (Princeton: Princeton University Press), pp. 85-103
5. Pounds, J.A. and Puschendorf, R. (2004). Clouded futures. Nature 427, 107-108
Current Biology http://www.current-biology.com
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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
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Related Material:
EVOLUTIONARY BIOLOGY: ON EXTINCTION
The following points are made by David Jablonski (Nature 2004 427:589):
1) Extinction is a fundamental part of nature -- more than 99% of all species that ever lived are now extinct. Whereas the loss of "redundant" species may be barely perceptible, more extensive losses of whole populations, groups of related species (clades) or those that share particular morphologies (for example, large body sizes), or functional attributes such as feeding mechanisms, can have profound effects, leading to the collapse of entire ecosystems and the extermination of great evolutionary dynasties. The challenge is to understand both the causes -- particularly the biological attributes that govern species' vulnerability --and the consequences of extinction.
2) These challenges are of more than academic interest. Today's biota is beset by many stresses, such as habitat destruction and fragmentation, over-exploitation and invasive species. In addition, species are susceptible to chain reactions that can destabilize them from the top down (by removing predators and other consumers) or from the bottom up (by removing or replacing primary producers). The most daunting obstacle to assessing and responding to these problems is the lack of anything close to a full accounting of present-day biodiversity: the 1.75 million known species probably represent less than 10% of the true inventory, and the figure is surely less than 1% for genetically distinct populations.
3) Attempts to estimate global extinction from rates of habitat loss may eventually be verified, but a more effective strategy has been to analyse extinction in groups where the (approximate) size of the species pool is known, as in North American birds, tropical palms, or Australian mammals. Such analyses have generally found, first, that the extinction or endangerment of species and populations is proceeding at an alarming pace, and second, that selectivity of extinction or decline tends to match theoretical expectations. For example, species with slow population growth rates, low population densities, or narrow geographic ranges tend to be more extinction-prone.
4) However, empirical data on extinction risk do not always follow neat theoretical lines. For example, large body size is associated with vulnerability in primates and birds, but is unimportant in carnivores, reptiles and marine mollusks. What emerges as an important issue is the covariation among the many traits that affect extinction risk -- species with high population densities tend to have short generation times, small body sizes and so on -- so that indirect effects may underlie apparent patterns. Further, this covariation is complex, with relationships among traits often defining polygonal fields rather than linear trends (for example, small-bodied forms may be widespread or spatially restricted). And the impact of different factors may depend on the extinction mechanism, such as habitat loss versus introduced predators. We are only beginning to get to grips with such problems.
References:
1. Erwin, D. H. Proc. Natl Acad. Sci. USA 98, 5399-5403 (2001)
2. Jablonski, D. Proc. Natl Acad. Sci. USA 98, 5393-5398 (2001)
3. Purvis, A., Jones, K. E. & Mace, G. M. BioEssays 22, 1123-1133 (2000)
4. Lawton, J. H. & May, R. M. (eds) Extinction Rates (Oxford Univ. Press, Oxford, 1995)
Nature http://www.nature.com/nature
ScienceWeek http://scienceweek.com
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