Look back to the "Big Five", they were all physically caused events so that they might seem to have little or even nothing to tell us about the Sixth mass extinction event -- which is related to human activities [1]. Obviously, human activities do a strong impact to the ecosystem which may directly lead to species destruction in the modern world. For instance, transformation of the landscape, pollution, overexploitation of species, etc.
In order to compare the sixth mass extinction(refer to human impact) with the previous events, scientists have identified a "background rate" of species extinctions from the fossil record[2]. A widely-used metric is E/MSY (extinctions per million species years)
Previous extinctions
It is estimated from the fossil extinctions that the background rates took place in million-year-or-more timeframes. Therefore, it is difficult to compare the current rate of extinctions-- just occurring over periods of decades to centuries. According the research，the average E/MSY over the fossil record is about 1.8 which means there were fewer than 2 species became extinct every million species-years on average. To break the time into less than 1,000 years the most common E/MSY is zero.
Current Extinction Rate
Over the past 1,000 years, it has been found the average extinction rate is 24 E/MSY that is about 13 times larger than the background rate. If the data is broken into 1-year bins, the maximum extinction rate over that period is bout 693 E/MSY which is 385 times larger than the background rates.
To sum up, human activities are the most different thing between the 6th mass extinction and the "big five". Their influences are causing faster rate extinctions that may lead to an acceleration for the coming 6th mass extinction. However, there is another hypothesis that the extinction event has been already arrived.
Reference:
1. http://www.actionbioscience.org/newfrontiers/eldredge2.html.
2. http://www.skepticalscience.com/sixth-mass-extinction.html
No comments:
Post a Comment