Environmental Engineering Reference
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FIGURE 5.16 Species extinction rates, expressed as extinctions per 1,000 species per 1,000 years. The fossil
record tells us that in the distant past less than one mammal went extinct every 1,000 years. In the recent
past the number of extinctions has increased 3 orders of magnitude higher than that of the fossil record.
SOURCE: May (2010).
longer replacing themselves. These species are called functionally extinct.
When that assistance is not possible, there is still a time lag due to humans
not being comfortable declaring a species extinct. Indeed, there is a “50-year
rule” that says a species cannot be said to be extinct until it has not been
found for 50 years. Even so, current rates of extinctions can be calculated
at least for well-studied species, i.e., mammals, birds. and amphibians. The
number of species in these three taxa is around 21,000, and over the past
century ~100 species in those taxa are known to have gone extinct, giv-
ing an extinction rate of ~50 E/MSY (MEA, 2005). Adding the functionally
extinct species, the rate of “extinction” increases to ~200 (May, 2010; see
Figure 5.16).
What about future extinctions? For many decades now we have known
the importance of three factors in contributing to a species becoming a
concern with regard to extinction (Rabinowitz et al., 1986): (1) The overall
range size is small enough for a major perturbation to influence greatly all
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