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| The Sixth Mass Extinction |
Should we consider, as some environmental scientists have, that the current biodiversity crisis is the start of a sixth mass extinction? Regardless of how one answers that question, it is clear that we are losing species at rates that, while exceedingly difficult to calculate, are above the background extinction rate and far exceed the speciation rate. Estimates are that 100,000-500,000 species of insects will go extinct in the next 300 years. The higher end of that estimate is comparable to the magnitude of the loss of species during the previous mass extinction episodes. Even the lower estimate represents a considerable loss of biodiversity. Moreover, 300 years is much shorter than the duration of those mass extinction periods.
The current biodiversity crisis stems from several causes. The two major contributors are habitat destruction and global climate change, both of which are largely due to human activity. As discussed earlier, much of the (largely unexplored) biodiversity lies in the tropics and, in particular, tropical rain forests. Tropical forests are being lost at an alarming rate. Conservative estimates place the loss of rain forests during the 1980s and 1990s at about 0.8 percent per year. This is in large part due to changes in the way the land has been used. For quite a long time, many areas had practiced slash and burn agriculture. In recent decades, however, the practice of cutting and clearing has been used increasingly for grazing or timber harvest, resulting in the loss of the tropical forest habitat. As a consequence, countless thousands of species (most of which are unknown to humans) are imperiled.
Global climate change has also impacted biodiversity. During the twentieth century, the mean temperature has increased by slightly more than one degree Fahrenheit (0.6 degree Celsius), and most of that change occurred between 1970 and 2000. Projections vary between x and y degrees Fahrenheit increase by mid-century. These changes do not appear great in the context of daily and seasonal temperature fluctuations, but they are large in comparison with prehistoric climate changes. While the magnitude of these changes is not beyond the range of historical variation, the rate at which the change has taken place appears to be so. The climate change is human-induced, due mainly to increases in carbon dioxide and other "greenhouse gases" that have appeared since the Industrial Revolution and accelerated during the twentieth century.
The human-induced global climate change is coupled with other climate cycles of various temporal and spatial scales. For example, the eastern United States had a cold winter in 2002-3 after several mild winters. In contrast, the western United States had a milder than normal winter that year. The pattern in 2002-3, most likely due to El Niño, does not invalidate the global upward climb in temperatures over a decades-long timespan. In addition to a mean increase in temperature, human-induced global warming is also likely to cause increased variation in climate. Some climate models suggest that the global warming may actually cause the northeastern United States to be cooler. The reason for this seemingly paradoxical possibility is that warming of the oceans could cause the Gulf Stream to be diverted south and east. Were this to happen, it would cause the Atlantic coast to be cooler. Regardless of the specifics of the local changes, more extreme weather will likely exacerbate already fragile ecosystems.
A paper published by Terry Root and her colleagues in 2003 shows that many species have altered their geographic ranges, presumably as a result of global climate change.5 Of those species that had altered their range, eighty percent were in the direction predicted by climate change models. The mean change of movement was about six kilometers per decade. In addition, many bird species have started laying eggs earlier in the spring. This study shows that forces of small, sustained change can be powerful over long enough time scale. But what about species that are unable to move? What will happen as their habitat changes due to human-induced global climate change?
Because of human-induced climate change and habitat destruction, we face a grave and growing crisis. Biodiversity is being lost at alarming but unknown rates. Moreover, if the diversity-stability hypothesis is true, loss of some species may trigger the loss of others, leading to a vicious circle. Although our knowledge about biodiversity and the extent to which it is lost is meager, the consequences are too grave to continue in ignorance.
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| ||End Notes |
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- Wilson, E. O. 1994. Naturalist. Washington, D. C.: Naturalist Warner Books, 359.
- Levin, S. A. 1999. Fragile dominion: Complexity and the commons. Cambridge, MA: Perseus Books, 6.
- McCann, K. S. 2000. The diversity-stability debate. Nature 405:228-33.
- Root, T. L., J. T. Price, K. R. Hall, S. H. Schneider, C. Rosenzweig, and J. A. Pounds. 2003. Fingerprints of global warming on wild animals and plants. Nature 421:57-60.