Extinction dynamics under extreme conservation threat: The flora of St. Helena

The flora of the island of St Helena provides an amplified system for the
study of extinction by reason of the island’s high endemism, small size,
vulnerable biota, length of time of severe disturbance (since 1502), and
severity of threats. Endemic plants have been eliminated from 96.5% of St
Helena by habitat loss. There have been eight recorded extinctions in the
vascular flora since 1771 giving an extinction rate of 581 extinctions per
million species per year (E/MSY). This is considerably higher than
background extinction rates, variously estimated at 1 or 0.1 E/MSY. We
have no information for plant extinctions prior to 1771 but applying the
same extinction rate to the period 1502 to 1771 suggests that there may be
around ten unrecorded historical extinctions. We use census data and
population decline estimates to project likely extinction forward in time.
The projected overall extinction rate for the next 200 years is somewhat
higher at 625 E/MSY. However, our data predict an extinction crunch in the
next 50 years with 4 species out of the remaining 48 likely to become
extinct during this period. It is interesting that during a period when
the native plant areas dropped to 3.5% of the original, the extinction
rate appears to have remained shallowly linear with under 30% of the
endemic flora becoming extinct.