The causes of extinction are (mostly) fairly obvious, and we touched on one of the largest ones all ready - habitat destruction and conversion. But let's spend a little time reminding ourselves what the causes of extinction are, because knowing those causes helps us better to understand their consequences.
- Overexploitation
Whales
- A record three year cruise in the 19th century killed fewer than one hundred whales.
- In 1933 almost 30,000 whales were killed, yielding 2.5 million barrels of whale oil.
- By 1967 60,000 were killed, but they yielded only 1.5 million barrels of oil - because the larger species, e.g., Blues and Fins, had been hunted virtually to extinction.
- Analyses of genetic diversity suggest pre-exploitation population sizes 6-20 times greater than current estimates [10].1
Atlantic cod
Wildlife trade, orchids and succulents
Predator control - until 1952 the Bald Eagle had a price on its head
- Development - urbanization, agriculture, & mining
Cape flora
- 6,000 species of native vascular plants; 1,200 threatened; 36 are extinct
- More than 60% of the area previously occupied by the Cape flora has been replaced by farms, plantations, roads, dams, towns, etc.
- More than 80% of the $4 million earned in the cut-flower industry is extracted directly from the wild flora
Dams: Pacific northwest salmon fishery has declined dramatically in the last 50 years. Many runs are now listed (or proposed for listing) under the Endangered Species Act.
Deforestation (Table 1) is just one aspect of habitat conversion, although it is probably among the most extreme.
- Invasive exotics
Zebra mussel
Chestnut blight
Wilcove, Rothstein, Dubow, Phillips, and Losos [16] surveyed recovery plans for species listed under the United States Endangered Species Act and categorized the threats they identified into one of five categories: habitat degradation/loss, alien species, pollution, overexploitation, and disease. Table 2 shows the percentage of listed species for which each of these five factors was mentioned as a cause contributing to endangerment.2
In short, many species are going extinct for reasons very different from those that caused their extinction in the past. Those causes make it clear that we are responsible for the elevated rates of extinction, but they don't tell us how many species are going extinct.
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