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ScienceWeek
PALEONTOLOGY: ON LATE PLEISTOCENE EXTINCTIONS
Notes by ScienceWeek:
The Pleistocene epoch is the time-frame 1.64 million years ago to 10,000 years ago (the beginning of the present Holocene).
The following points are made by A.D. Barnosky et al (Science 2004 306:70):
1) Fifty thousand years ago, continents were populated with more than 150 genera of megafauna (animals weighing more than 44 kg) [1-4]. By 10,000 years ago, at least 97 of those genera were gone [5]. Prevailing explanations include human impacts [1,2,5], environmental changes [1,2], and a combination of both [1,3,4]. If humans caused the extinctions, it will profoundly influence our thinking about what is "natural", how ecosystems respond to different scales and kinds of environmental change, how long extinctions take, and conservation of species and ecosystems [2].
2) Anthropogenic extinction models, including overkill, blitzkrieg (rapid overkill), and sitzkrieg (fire, habitat fragmentation, and the introduction of exotic species and diseases), have been considered plausible because large animals were preferentially affected [1,2,5]. Species with low reproductive rates, with which large body size correlates, were hit hardest. Almost all of the slow-breeding survivors in Australia, Eurasia, the Americas, and Madagascar are nocturnal, arboreal, alpine, and/or deep-forest dwellers, which is consistent with overkill models of extinction but hard to explain by environmental change alone. Survival of large, open-country, slow-breeding animals in Africa is an exception to this pattern that must be factored into extinction explanations.
3) On islands, humans cause extinctions through multiple, synergistic effects, including predation and sitzkrieg [1,2]. Only rarely have island megafauna been demonstrated to go extinct because of environmental change without human involvement. Incontrovertible impacts of humans on islands have been cited as a potent argument that prehistoric humans also caused extinctions on continents [1,2], but extrapolation of extinction mechanisms from islands to continents is often disputed [4].
4) In summary: One of the great debates about extinction is whether humans or climatic change caused the demise of the Pleistocene megafauna. Evidence from paleontology, climatology, archaeology, and ecology now supports the idea that humans contributed to extinction on some continents, but human hunting was not solely responsible for the pattern of extinction everywhere. Instead, evidence suggests that the intersection of human impacts with pronounced climatic change drove the precise timing and geography of extinction in the Northern Hemisphere. The story from the Southern Hemisphere is still unfolding. New evidence from Australia supports the view that humans helped cause extinctions there, but the correlation with climate is weak or contested. Firmer chronologies, more realistic ecological models, and regional paleoecological insights still are needed to understand details of the worldwide extinction pattern and the population dynamics of the species involved.
References (abridged):
1. P. S. Martin, R. G. Klein, Eds., Quaternary Extinctions: A Prehistoric Revolution (Univ. of Arizona Press, Tucson, 1984)
2. R. D. E. MacPhee, Ed., Extinctions in Near Time: Causes, Contexts, and Consequences (Kluwer/Plenum, New York, 1999)
3. A. J. Stuart, Biol. Rev. Camb. Philos. Soc. 66, 453 (1991)
4. S. Wroe, J. Field, R. Fullagar, L. S. Jermin, Alcheringa 28, 291 (2004)
5. J. M. Diamond, J. Archaeol. Sci. 16, 167 (1989)
Science http://www.sciencemag.org
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DINOSAURS, DRAGONS, AND DWARFS: THE EVOLUTION OF MAXIMAL BODY SIZE
The following points are made by G.P. Burness et al (Proc. Nat. Acad. Sci. 2001 98:14518):
1) The size and taxonomic affiliation of the largest locally present species ("top species") of terrestrial vertebrate vary greatly among faunas, raising many unsolved questions. Why are the top species on continents bigger than those on even the largest islands, bigger in turn than those on small islands? Why are the top mammals marsupials on Australia but placentals on the other continents? Why is the world's largest extant lizard (the Komodo dragon) native to a modest-sized Indonesian island, of all unlikely places? Why is the top herbivore larger than the top carnivore at most sites? Why were the largest dinosaurs bigger than any modern terrestrial species?
2) A useful starting point is the observation of Marquet and Taper (1998), based on three data sets (Great Basin mountaintops, Sea of Cortez islands, and the continents), that the size of a landmass's top mammal increases with the landmass's area. To explain this pattern, they noted that populations numbering less than some minimum number of individuals are at high risk of extinction, but larger individuals require more food and hence larger home ranges, thus only large landmasses can support at least the necessary minimum number of individuals of larger-bodied species. If this reasoning were correct, one might expect body size of the top species also to depend on other correlates of food requirements and population densities, such as trophic level and metabolic rate. Hence the authors assembled a data set consisting of the top terrestrial herbivores and carnivores on 25 oceanic islands and the 5 continents to test 3 quantitative predictions:
a) Within a trophic level, body mass of the top species will increase with land area, with a slope predictable from the slope of the relation between body mass and home range area.
b) For a given land area, the top herbivore will be larger than the top carnivore by a factor predictable from the greater amounts of food available to herbivores than to carnivores.
c) Within a trophic level and for a given area of landmass, top species that are ectotherms will be larger than ones that are endotherms, by a factor predictable from ectotherms' lower food requirements.
3) The authors point out that on reflection, one can think of other factors likely to perturb these predictions, such as environmental productivity, over-water dispersal, evolutionary times required for body size changes, and changing landmass area with geological time. Indeed, the database of the authors does suggest effects of these other factors. The authors point out they propose their three predictions not because they expect them always to be correct, but because they expect them to describe broad patterns that must be understood in order to be able to detect and interpret deviations from those patterns.
Proc. Nat. Acad. Sci. http://www.pnas.org
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Related Material:
ON GIANT MAMMALS AND ROTTING FRUITS
The following points are made by Connie Barlow (citation below):
1) The Age of Great Mammals ended long before chain saws and internal combustion engines evolved. In Europe and nontropical regions of Asia, it petered out in steps between 50 and 15 thousand years ago, when the straight-tusked elephants, woolly mammoths, rhinos, and other great beasts of the Pleistocene epoch vanished. Throughout that vast continental mass, a quarter of all genera of animals regarded as megafauna -- those weighing more than a 100 pounds, or 45 kilograms -- were lost to extinction. Europe lost all 6 species of herbivores weighing more than 1000 kilograms.
2) In Australia, the Age of Great Mammals ended sometime between 40,000 and 30,000 years ago, when giant kangaroos, enormous wombats, and rhino-like marsupials (as well as the most formidable crocodiles, lizards, and snakes) were purged from the landscape. This extinction catastrophe stripped Australia of all but one of its 16 genera of megafauna.
3) In the Western Hemisphere, the Age of Great Mammals came to an abrupt end 13,000 years ago, when the mastodons and mammoths, the ground sloths and glyptodonts, the native horses and large camels, and a beaver and an armadillo both as big as a bear all disappeared forever. North America lost 68 percent of its generic richness of Pleistocene megafauna (32 of 47 genera), and South America lost 80 percent (47 of 59 genera). Outlying islands were hit even harder, though several thousand years later, following advances in sailing technologies. Not until 7000 years ago, for example, did Cuba lose its half dozen species of sloth, including a ground dweller as big as a black bear.
4) Just 4000 years ago, while the Egyptians were building pyramids, the last mammoths on Earth expired on an island off Siberia. Madagascar lost all of its biggest lemurs within the past 2000 years, along with both native species of hippopotamus, a strange carnivore, and the giant elephant birds unique to that island.
5) The Age of Great Mammals may be over, but the plants have not yet caught on. Those that depended upon mammals to swallow big fruits, as well as those that deployed armaments to deter soft snouts from stripping foliage, are still doing what they have always done. Fruit rotting on the ground is the most obvious sign.
6) The big beasts are gone, but the fruits remain. Year after year in the American tropics (and temperate climes too), trees and vines produce fruits that make little sense today. Some fruits simply rot on the ground beneath the parent plant. Others are raided by seed predators or plundered by pulp thieves. Whether rotted, raided, or plundered, viable seeds are rarely dispersed. The plants not only remember the great mammals of the Pleistocene and before, they expect gomphotheres, ground sloths, toxodons, and their ilk to show up any day now. Thirteen thousand years is not enough time for plants to notice and genetically respond to the loss.
Adapted from: Connie Barlow: The Ghosts of Evolution: Nonsensical Fruit, Missing Partners, and other Ecological Anachronisms. Basic Books 2000, p3. More information at: http://www.amazon.com/exec/obidos/ASIN/0465005519/scienceweek
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