Extinction spasmgreenspun.com : LUSENET : Theology and Creation : One Thread
Unit 2 page 15 What is an extinction spasm? I've read the very interesting article but have I missed something?
Help needed Thanks
-- Anonymous, March 02, 2002
Because of the interrelated character of species in an ecology (who gets to eat whom and under what conditions) and because of the varied dependence of the different species on the environment, change can result not only in the disappearance of one species but of many. There have been catastrophic extinction spasms in the distant (prehuman) past of this planet, the most famous being the disappearance of the dinosaurs (about 65 million years ago). Here is a quote about our present situation:
"An extinction spasm: Best estimates indicate that current extinction rates are 1,000 to 10,000 times faster than average extinction rates over geological time, leaving 1 in 4 mammal and 50% of some plant species threatened with extinction. The main cause of this extinction spasm is habitat loss, compounded by unsustainable harvesting levels. For example, 28% of 8,600 threatened tree species worldwide are declining because of over- felling. A widely recognised instance of trade pressure leading to species decline is capture fisheries, with 22% of commercial world fish stocks overexploited or depleted. Another major direct cause of species extinctions is the widespread introduction of alien species, cited as the single most common cause (about 40%) of documented causes of mammal extinctions. Recent habitat and ecosystem losses - Some 37% of wildlands in developing countries were lost in the 20 years between 1960 and 1980. - Up to 20% of tropical forests have disappeared in the 30 years between 1960 and 1990." (taken from the internet site: http://wcpa.iucn.org/pubs/pdfs/biodiversity/biodiv_brf_19.pdf)
-- Anonymous, March 02, 2002
Here is a list not just of "extinction spasms" but of the TOP FIVE MASS EXTINCTIONS:
Permian-Triassic. Date: About 250 million years ago. Death Toll: 84 percent of marine genera; 95 percent of marine. species; 70 percent of land species. Possible Causes: Asteroid or comet impact; severe volcanism; dramatic fluctuations in climate or sea level.
Cretaceous-Tertiary: Date: About 65 million years ago. Death Toll: Up to 75 percent of marine genera; 18 percent of land vertebrates, including dinosaurs. Possible Causes: Impact; severe volcanism.
Late Ordovician: Date: About 440 million years ago. Death Toll: 60 percent of marine genera. Possible Cause: Dramatic fluctuations in sea level.
Late Devonian: Date: About 365 million years ago. Death Toll: 55 percent of marine genera. Possible Causes: Global cooling; loss of oxygen in oceans; impact.
Late Triassic: Date: About 200 million years ago. Death Toll: 52 percent of marine genera. Possible Causes: Severe volcanism; global warming.
-- Anonymous, April 06, 2002