The Hot Zone

Sixth extinction almost here, but not quite

Posted by Alison Hawkes on March 3, 2011
Category : Climates of the Past, The man made climate

Scientists define a mass extinction as when the Earth loses more than 75 percent of its species in short geological time, within 2 million years. This hasn’t happened very often — only five times in the last 540 million years. Is it happening now again?

The “sixth extinction” has been discussed by biologists for decades. In a paper published this week in the journal Nature, University of California at Berkeley-led biologists take stock on the status of the the Earth’s current mass extinction.

In comparing extinction rates in the past to the modern day, the researchers analyzed “extreme diversity loss.” By this they mean not just species dieing out, but the rate of die-out relative to the formation of new species. This rate of die-off is crucial because a species typically takes  hundreds of thousands of years or longer to evolve. When new species don’t have time to form and fill the spots of extinct ones, the Earth experiences a net loss in biodiversity.

The scientists in the study compared modern day extinction to the fossil record and “background levels”; in other words, the normal rate of extinction that would have occurred had humans not accelerated the process. They determined that the rate of extinction in the past thousand years is far above the average fossil rate and is higher even than pre-human averages.

The hawksbill turtle is listed a critically endangered species. It's loss would help propel the sixth extinction. Photo: Wikipedia

But the scientists don’t believe the “sixth extinction” is here just yet. Humans have undoubtedly wrought havoc on species through out-competing for resources, fragmenting habitats, introducing non-native species and pathogens, slaughtering them directly, and instigating climate change. Nevertheless, only a few percent of assessed species have been lost (although estimates are somewhat inexact since we have no way of knowing about losses of species that have never been discovered).

But they say that losing species in the “critically endangered category” would propel the Earth into the next mass extinction. If that happens to species listed as “endangered” or “vulnerable,” a mass extinction could happen within a few centuries. The conditions would be similar to past mass extinctions: multiple and atypical stressors on ecosystems of the type caused by climate change and highly elevated CO2 levels.

The authors write: “It is encouraging that there is still much of the world’s biodiversity left to save, but daunting that doing so will require the reversal of many dire and escalating threats.”

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