Extinctions

Earth is now entering its sixth period of mass extinction, defined as a major decrease in the number of species in a relatively short period of time. Extinctions are a normal part of the evolution of ecosystems. New species evolve, other species die out, and over millennia the diversity of life on the planet has increased.

The previous five mass extinction events are believed to have been caused by prehistoric meteor impacts and/or significant climatic changes (themselves caused by a variety of factors). The fifth event occurred 65 million years ago and wiped out an estimated 50% of Earth's species, including the dinosaurs. In the current time, a similar percentage of Earth's species are again threatened with extinction but this time as a result of the actions of the human species. The main causes include (1) habitat loss, (2) overexploitation, (3) introduced species, and (4) extinction cascades. Through these combined factors, around 70 species are believed to die out every single day; the rate of extinction is approximately 100 to [Page 1070]1,000 times greater than normal levels. In the coming years, anthropogenic climate change is also expected to add increasing pressure to struggling species and ecosystems, further accelerating the rate of extinctions.

Although extinction is a global event, the geographical distribution of extinctions is patchy; critical zones with high rates of extinction are unevenly distributed around the world. In addition, as the 2005 Millennium Ecosystem Assessment points out, the human experience of extinction effects falls disproportionately on rural communities and the poor. A geographical perspective brings a regional specificity to this global event and highlights the interconnections that link and amplify the extinction process. Four brief case studies will draw out connections between the diverse causes of extinctions and provide examples of the way in which a spatial analysis deepens our understanding of the loss and disturbance experienced when species die.

Human/Nonhuman Entanglements in Place

Even before a species becomes extinct, the role that it plays in an ecosystem may be severely impaired by dwindling numbers (called functional extinction). A good example is the case of Gyps vultures in India and the surrounding regions. Over the past decade, approximately 95% of these vultures have died as a result of poisoning caused by eating cattle treated with an anti-inflammatory drug called diclofenac. With vultures in decline, the millions of cattle carcasses they traditionally “cleaned up” have become a new source of food for growing populations of stray dogs and others. The increase in dog numbers is believed to be a significant cause for the spread of rabies and other diseases to humans, domestic animals, and potentially to some other threatened species. In addition, the vulture deaths are affecting people whose cultural and economic lives are directly connected with the birds, for example, the Parsi community, which traditionally exposes its dead to be consumed by vultures in “sky burials,” and “bone collectors,” who gather vulture-cleaned cattle bones for the fertilizer industry.

An Oriental white-backed vulture (Gyps bengalensis) that was brought injured to the Sanjay Gandhi National Park stands inside a cage at the park in Borivili, a suburb of Mumbai, India, September 23, 2008. Several vulture species in Asia have been driven to the brink of extinction after eating cattle carcasses tainted with an anti-inflammatory painkiller given to sick cows. The drug has led to global population declines as high as 99% in slender-billed and other vulture species, especially in India.

Source: AP Photo/Gautam Singh.

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