Furthermore, the researchers argue, the new evidence raises doubts that a mass extinction
on land even happened.
According to the International Union for Conservation of Nature, about 41 percent of all amphibian species and 26 percent of all mammals are threatened with extinction
The author's purpose in this section of the text is to use a concrete example--a wave of amphibian extinctions
in Central America that was first noticed in the early 1990s--to introduce the central idea that a "Sixth Extinction
" may be occurring on Earth because of human activities.
The revised dates clear up lingering confusion over whether the impact actually occurred before or after the extinction
, which was characterized by the almost overnight disappearance from the fossil record of land-based dinosaurs and many ocean creatures.
Princeton-led researchers found that a trail of dead plankton spanning half a million years provides a timeline that links the mass extinction
to large-scale eruptions of the Deccan Traps, a primeval volcanic range in western India that was once three-times larger than France.
It is far harder to count extinctions
since that requires a judgment that the last individual has died.
amp;nbsp;"If you look only at the critically endangered mammals -- those where the risk of extinction
is at least 50 percent within three of their generations -- and assume that their time will run out, and they will be extinct in 1,000 years, that puts us clearly outside any range of normal, and tells us that we are moving into the mass extinction
realm," said Anthony D.
Habitat destruction is a major cause of species loss and has accelerated rapidly in recent years, especially in the world's most species-rich environments--about half the original extent of tropical moist forest has been lost, for example, most of it in the last 50 years--so it's likely that many extinctions
have not yet had time to occur.
The average extinction
rate is now some 1,000 to 10,000 times faster than the rate that prevailed over the past sixty million years.
Focusing on the mass extinction
event that killed the dinosaurs, at the Cretaceous/Tertiary boundary 65,000,000 years ago, he found that recovery rates in North America and Europe were very different even though they are at roughly the same latitude.
may solve an old puzzle: Why did the asteroid impact cause more extinctions
in North America than anywhere else?
Long before causing today's animal losses, people may have been a primary cause of extinctions
during the last Ice Age.
Now, a team led by scientists at the California Institute of Technology (Caltech) has created a framework for weighing the factors that might have led to mass extinction
and has used that framework to determine that the majority of extinctions
were caused by habitat loss due to falling sea levels and cooling of the tropical oceans.
To think it is unprecedented deprives us of the opportunity to learn from past extinctions
what we may expect of this one.
I think we can accept that certain extinctions
are a natural part of evolution, but the rate of 50,000 species a year is an inordinate loss.