The causes of extinction

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 [13].²

  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  • More than 60% of the area previously occupied by the Cape flora has been replaced by farms, plantations, roads, dams, towns, etc.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. Nearly 30% of the ca. 1400 historical populations are extinct (14% of populations from coastal regions, 55% from interior regions) [4].

  Deforestation (Table 1) is just one aspect of habitat conversion, although it is probably among the most extreme.

• Invasive exotics

  Zebra mussel

  Chestnut blight

Table 1: Proportion of old-growth forest remaining in selected temperate forest countries (from Table 11.2-6 [16]).

Country/Region	% of old growth forest
USA	15
Washington & Oregon	13
Canada	52
British Columbia	40
Europe
Western Europe	1
Scotland	1
Sweden	1
Finland	2
Norway	3
Oceania
New Zealand	25
Australia	5-21
Asia
China	1

Wilcove, Rothstein, Dubow, Phillips, and Losos [19] 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.³

Table 2: Causes of endangerment mentioned in species recovery plans [19].

	All	Vertebrates	Invertebrates	Plants
Cause	$(n=1880)$	$(n=494)$	$(n=331)$	$(n=1055)$
Habitat degradation	85%	92%	87%	81%
Alien species	49%	47%	27%	57%
Pollution	24%	46%	45%	7%
Overexploitation	17%	27%	23%	10%
Disease	3%	11%	0%	1%

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.