THE AGENCIES OF DESTRUCTION 295species grows on oceanic islands, but one in three of
all known threatened plant species are island
endemics. The degree to which the remaining island
endemic birds are believed to be threatened is quan-
tified in Table 11.3. Although IUCN data show that
the threat of extinction is switching significantly
towards the continents (Groombridge 1992), statis-
tics such as these underline the continuing impor-
tance of focusing conservation efforts on islands.
11.4 The agencies of destruction
There are four major reasons why island species are
reduced by human action: (1) direct predation; (2)
the introduction of non-native species; (3) the
spread of disease; and (4) habitat degradation or
loss. In a global context, the loss of habitat is com-
monly seen as the greatest problem for biodiversity
(Lawton and May 1995), but on particular islands
other factors from this list may be more pressing.
For instance, on the Galápagos, introduced animals
and plants have been described as: ‘...absolutely
the most insidious, silent, and dangerous scourge
that exists in the archipelago’ (Galápagos Newsletter
1996). Commonly, native endemic species are
caught in a pincer action, reduced by habitat loss to
a rump, attacked by disease or predation, and fac-
ing competition with exotic species. The syner-
gisms between these forces result in the loss of
species which conceivably could cope with and
adjust to one force alone. Similarly, there may sub-
sequent trophic cascade effects (above), such that
the loss of prey species might prejudice the survival
of native predators. For example, the very large
extinct New Zealand eagle, Harpagornis moorei, was
probably dependent on moas and other large and
now extinct bird species (Milberg and Tyrberg
1993).
Predation by humans
Humans hunt island species to eat and also for
other reasons, such as the societal values attached
to brightly coloured feathers. In the case of the
extinct Hawaiian mamo (Drepanis pacifica), it has
been estimated that the famous royal cloak worn by
Kamehameha I would have required some 80 000
birds to be killed (Johnson and Stattersfield 1990).
Trade in such biological resources has a long his-
tory, and was an important feature of the period in
which the Polynesians spread across and held sway
over the Pacific. Present-day trade in fruit bats (fly-
ing foxes) provides an example of how hunting, not
primarily for local consumption, but for export, can
reduce oceanic island populations to crisis point.
The critical role that these bats play as pollinators
and as dispersers of plants may mean that their loss
will have significant knock-on impacts, i.e. they act
in effect as ‘keystone’ species (Rainey et al. 1995;
Vitousek et al. 1995). Hunting for collections contin-
ues to be problematic. Today, the trade is driven
mostly by individual collectors, but in the past
hunting for museum collections was common and
is, for example, believed to have dealt the final
blow to the spectacular New Zealand bird, the huia
(Heteralocha acutirostris) (Johnson and Stattersfield
1990).Introduced species
As humans have spread across the islands of the
world we have taken with us, either purposefully
or inadvertently, a remarkable array of plant and
animal species (see Box 11.2). These anthropogenic
introductions are variously called exotic, alien or
non-native species. Some species persist simply as
domesticated (animal) or cultivated (plant) species,
while others become naturalized, i.e. they form self-
sustaining populations within modified habitats
(Hendersonet al. 2006). Of these, some become feral
or invasive, expanding into intact or semi-intact
habitats. Of this subset, a few species cause serious
ecological impacts. These species are termed
ecosystem transformers (Henderson et al. 2006).
Based on a review of numerous case studies,
Williamson (1996) has suggested that only about
10% of introduced species become established, with
in turn about 10% of these species achieving pest
status.
The impact of non-native species can take
numerous forms. These include predation and
grazing impacts that are often strikingly apparent,
while the changes can also be more cryptic, but
nonetheless profound, such as alterations to