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Historic Mass Extinctions
Adapted from Raup 1991.
In the history of life on earth there have been numerous mass extinctions.
A mass extinction is recognized as an interval of one to several million years where an unusually high number of
"unrelated groups from a number of habitats, terrestrial as well as marine" become extinct.
No one knows what actually causes mass extinctions, although there is
a great deal of speculation. There is a consensus among many paleontologists that extra-terrestrial objects, like
meteorites and comets, have played a significant role in past extinction events. For example, often cited for the
demise of the dinosaurs is the impact of a 6-mile (10-km) wide meteorite near the Yucatan, Mexico. There is substantial
evidence (iridium anomalies, craters, and shattered quartz fragments) to support such theories and there is good
reason to believe that such an impact could create conditions (shock waves, tsunamis, forest fires, acid rain,
darkness lasting months or years, global cooling or warming) to eliminate a large portion of the world's species.
Other leading theories to causes of mass extinctions include: global climate change, changes in sea level, chemical
poisoning of the atmosphere and/or oceans, variation in solar radiation, and extreme volcanic activity.
Two thousand years ago there were more species on Earth than ever before.
Earth's biodiversity has reached a peak and now it is declining into what many ecologists and biologists are calling
the sixth great extinction. Today species are being driven to extinction at a rate higher than any time in the
past.
Extinction Event
Years Before Present (millions)
Families Lost
Recovery Time (millions of years)
Cretaceous-Tertiary
66
15% of 650
20
Triassic
213
20% of 300
**
Permian
250
30% of 400
100
Devonian
360
22% of 450
30
Ordovician
440
22% of 450
25
Recovery time refers to the amount of time necessary for the number of species/families to reach the level that
existed prior to the event.
**The Triassic and Permian extinction events are often combined as the
Permo-Triassic event because of the short duration between the two extinctions. During this period 76-96% of species
died out.
"Rainforest" is used interchangeably with "rain forest" on this site. "Jungle" is generally not used.
Recent news
Beef consumption fuels rainforest destruction (02/16/2009)
Nearly 80 percent of deforestation in the Brazilian Amazon results from cattle ranching, according to a new report by Greenpeace. The finding confirms what Amazon researchers have long known – that Brazil's rise to become the world's largest exporter of beef has come at the expense of Earth's biggest rainforest.
How to save the Amazon rainforest (01/04/2009)
Environmentalists have long voiced concern over the vanishing Amazon rainforest, but they haven't been particularly effective at slowing forest loss. In fact, despite the hundreds of millions of dollars in donor funds that have flowed into the region since 2000 and the establishment of more than 100 million hectares of protected areas since 2002, average annual deforestation rates have increased since the 1990s, peaking at 73,785 square kilometers (28,488 square miles) of forest loss between 2002 and 2004. With land prices fast appreciating, cattle ranching and industrial soy farms expanding, and billions of dollars' worth of new infrastructure projects in the works, development pressure on the Amazon is expected to accelerate. Given these trends, it is apparent that conservation efforts alone will not determine the fate of the Amazon or other rainforests. Some argue that market measures, which value forests for the ecosystem services they provide as well as reward developers for environmental performance, will be the key to saving the Amazon from large-scale destruction. In the end it may be the very markets currently driving deforestation that save forests.
Amazon rainforest damage surges 67% in 2008 (12/20/2008)
The area of rainforest in the process of being deforested — razed but not yet cleared — surged in the Brazilian Amazon during 2008, according to new figures released by Brazil's National Institute for Space Research (INPE). The announcement comes shortly after the Brazilian government reported a 4 percent increase in forest clearing for the year. Using an advanced satellite system that tracks changes in vegetation cover INPE found that 24,932 square kilometers of Amazon forest was damaged between August 2007 and July 2008, an increase of 10,017 square kilometers -- 67 percent -- over the prior year.
Cutting deforestation can fight climate change, reduce poverty and conflict (09/24/2008)
Forest conservation can play a critical role in efforts to reduce greenhouse gas emissions and alleviate poverty, said a prominent group of politicians, development experts, and environmental NGOs meeting in New York City to discuss U.S. climate policy.
Future threats to the Amazon rainforest (07/31/2008)
Between June 2000 and June 2008, more than 150,000 square kilometers of rainforest were cleared in the Brazilian Amazon. While deforestation rates have slowed since 2004, forest loss is expected to continue for the foreseeable future. This is a look at past, current and potential future drivers of deforestation in the Brazilian Amazon.
