Rates from known extinctions

It may come as a surprise to you to learn that we can actually get reasonable estimates of current extinction rates from examining documented extinctions in groups that are reasonably well-studied. In the United States alone, for example, 45 vertebrates (over half of which are birds), 347 invertebrates, and 147 plants are either presumed or possibly extinct [9].⁴ By calculating the fraction of known species that have gone extinct in historical times, we get a direct estimate of extinction rates. The calculations that follow are based on the data and the approach described in [10,15].

• 100 documented extinctions of birds and mammals worldwide in the last century out of $\approx 14,000$ total. That's a rate of $7 \times 10^{-3}$ yr$^{-1}$.

• The average life span of bird and mammal species in the fossil record is about $1 \times 10^6$ years. This is equivalent to an extinction rate of about $1 \times 10^{-6}$ y$^{-1}$.

• So the recent historical rate of vertebrate extinctions is a little over 7,000 times greater than the background rate of extinction.

The rates calculated by these very different approaches are reasonably comparable, given the great uncertainties involved. They suggest that contemporary rates of extinction are 100 to 1,000 (and possibley 10,000) times higher than at any time in the last 65 million years. The figures in Table 4 show how many documented extinctions have occurred in the last 400 years, along with an estimate of the corresponding median lifetime of taxa in each group (derived from [15]).

Table 4: Extinction in major taxa since 1600.

		Extinct	Threatened	Extant	Time to 50% extinction
Animals	Molluscs	191	354	$10$$^5$	60,000
	Crustaceans	4	126	$4\times10$$^3$	--
	Insects	61	873	$10$$^6$	--
	Vertebrates	229	2,212	$4.7\times10$$^4$	600
	Fishes	29	452	$2.4\times10$$^4$	900
	Amphibians	2	59	$3\times10$$^3$	3,000
	Reptiles	23	167	$6\times10$$^3$	2,000
	Birds	116	1,029	$9.5\times10$$^3$	350
	Mammals	59	505	$4.5\times10$$^3$	250
	Total	485	3,565	$1.4\times10$$^6$	15,000
Plants	Gymnosperms	2	242	758	--
	Dicotyledons	120	17,474	$1.9\times10$$^5$	1,000
	Monocotyledons	462	4,421	$5.2\times10$$^4$	1,700
	Palms	4	925	2820	70
	Total	584	22,137	$2.4\times10$$^5$	1,100