Dark extinction: the problem of unknown historical extinctions

Abstract

The extinction of species before they are discovered and named (dark extinction, DE) is widely inferred as a significant part of species loss in the 'pre-taxonomic' period (approx. 1500-1800 CE) and, to some extent, in the 'taxonomic period' (approx. 1800-present) as well. The discovery of oceanic islands and other pristine habitats by European navigators and the consequent introduction of destructive mammals, such as rats and goats, started a process of anthropogenic extinction. Much ecosystem change happened before systematic scientific recording, so has led to DE. Statistical methods are available to robustly estimate DE in the 'taxonomic period'. For the 'pre-taxonomic period', simple extrapolation can be used. The application of these techniques to world birds, for example, suggests that approximately 56 DEs occurred in the 'taxonomic period' (1800-present) and approximately 180 in the 'pre-taxonomic period' (1500-1800). Targeting collection activities in extinction hotspots, to make sure organisms are represented in collections before their extinction, is one way of reducing the number of extinct species without a physical record (providing that collection efforts do not themselves contribute to species extinction).

Keywords: St Helena; dark extinction; extinction debt; extinction rate; island extinction; oceanic island.

Figure 1.

(a) Plot of cumulative numbers of named species of birds based on the HBW checklist [14]. A similar plot for is given by Pimm et al. [15] based on earlier data. (b) Plot of cumulative numbers (log scale) of bird preserved specimen collections in museums based on records in the Global Biodiversity Information Facility (gbif.org).

Figure 2.

‘Extinction diagram’ for birds 1500–present: plots of cumulative extinction of birds since 1500 against time (calendar year, CE), showing the effect of DE. (line a) The lowest line shows the cumulative extinctions from 1500 to present that we know about (E = 199), plotted over the time period (1800–present) for which the data are most credible. This line has an intercept at 1728. (line b) The middle line shows the addition of DE estimated for the taxonomic period by the program seux (increment ii, approximately 56). This line has an intercept at 1710. (line c) Adding an additional 180 extinct species (increment iii) to the numbers (pre-taxonomic DE = 180) gives an intercept at 1500 (see the text). Here, the points are shown on the line to indicate that this is not a new regression but simply an upward displacement of the line below. Inset: Conceptual models of pre-1800 extinction. (line A) Slow-start model (no DE): this is equivalent to the actual known extinctions post-1500. Solid line: post-1800 known extinction; dotted line: pre-1800 with no DE (rather few extinctions are known from this period). (line B) Constant extinction (linear) model. Cumulative post-1500 extinction curve with the addition of pre-1800 DE (pre-1800 DE = 100). (line C) Filter model (see the text): high early DE filters out susceptible species (pre-1800 DE = 200).