We cannot discuss the ecology of islands without making
a few disparaging comments on goats. These creatures
must be the true embodiment of the devil for a plant lover.
(Koopowitz and Kaye 1990, p. 72)The biodiversity crisis is nowhere more apparent and in
need of urgent attention than on islands. Approximately
90% of all bird extinctions during historic times have
occurred on islands...
(Paulay 1994, p. 139)11.1 Current extinctions in context
Throughout our discussions of island ecology and
evolution, we have mostly set to one side the sig-
nificance of humans as shapers of island biogeogra-
phy. As this chapter will show, this is a questionable
practice, as human societies have had a profound
influence on island biodiversity in both recent his-
torical and prehistorical time (Morgan and Woods
1986; Johnson and Stattersfield 1990; Groombridge
1992; Pimm et al. 1995). This influence has been
manifested in many ways, but the most profound
must surely be the extinction of numerous island
races and species.
Of all the species that have ever lived on the
Earth, only a small fraction are alive today. Natural
catastrophes, such as volcanic eruptions, meteorite
impacts, climate changes, marine transgressions
and regressions, in combination with biotic forcing,
have been key agents of species extinction in the
past. Extinction rates have varied through time,
with five phases recognized as particularly intense
and widespread events in the fossil record, the so
called mass-extinctions events. The most recent of
these was the end-Cretaceous event, which resulted
from a bolide impact approximately 65 Ma
(Groombridge and Jenkins 2002). Whether the com-
parison is entirely apposite is a moot point, but
many argue that a sixth mass extinction event is
now under way, which uniquely is being driven by
the activities of a single species.
Primack and Ros (2002) suggest that more than
99% of recent species extinctions can be attributed
to human activities. Extinctions of vertebrate
species over the last 400 years average 20–25 per
century, a rate between 20 to 200 times the back-
ground extinction rate (i.e. the natural rate exclud-
ing mass extinction events) (Groombridge and
Jenkins 2002). This anthropogenic episode began
some 100 000 years ago with the expansion of mod-
ern humans from Africa throughout the world, and
has been particularly intense in Australia and
North and South America. However, many of the
less recent continental extinctions occurred concur-
rent with episodes of pronounced climate change,
and the extent to which humans caused many of
the past losses has been much debated (a well-
balanced evaluation is provided by Lomolino et al.
2005). During this anthropogenic period, some of
the most dramatic and convincing illustrations of
extinctions due to human action come from islands
worldwide (Diamond 2005).
Estimating the extent of the biodiversity crisis is
a hard task, because we still have a remarkably
poor knowledge of the total diversity of the planet
(the so-called Linnean shortfall), and of how that
diversity is distributed (the Wallacean shortfall).290
CHAPTER 11