11.8 Summary
As a result of human action, island birds have been
40 times more likely to become extinct within the
past 400 years than continental birds. In the same
period, the proportion of documented animal
extinctions from islands varies from about 60% for
mammals to 95% for reptiles. Moreover, the losses
caused by humans in prehistoric times exceed those
of the past 400 years. This is most clearly estab-
lished for birds, but applies to other animal taxa
and to islands across the world. Island plants have
also been subject to a disproportionate rate of loss.
The scale of environmental degradation on certain
Pacific islands, exemplified by Easter Island and
Henderson Island, has been shown to have been so
great that it led to complete cultural collapse and, in
the latter case, human abandonment of the island.
The available evidence indicates that the key turn-
ing point in the transformation of island ecologies
is typically the first colonization by humans, but
that successive waves of contact with the outside
world, and of innovation in resource use, can con-
tinue to ramp up the pressure on island biodiver-
sity, sometimes generating multiple phases of
extinction.
The major causes of local and global extinction
from islands have been: (1) predation by humans,
(2) the introduction of exotics, (3) the spread of dis-
ease, and (4) habitat loss, of which habitat loss and
the introduction of alien species currently give
greatest overall cause for concern. Predation
remains a critical driver of extinction threat for a
subset of threatened species and islands. The intro-
duction of exotic species, in some cases a single
exotic species, can cause the local extinction of
numerous native island species. The shrub Miconia
calvescenson Tahiti and the brown tree snake (Boiga
irregularis) on Guam provide two classic examples.
However, case details provided in this chapter from
the Pacific, Indian Ocean, Caribbean, and Atlantic
illustrate that while some losses can indeed be
attributed to a sole major cause, many species
losses have resulted from synergisms between
several causes, such as habitat alteration allowing
the invasion of exotic plants, which in turn are
spread by exotic birds, which in turn carry exotic
diseases. The term trophic cascadeis used to
describe the situation where the loss of one or a
small number of species triggers the disruption of
ecosystem processes more generally. Such knock-on
effects can work through predatory relationships
and, as just indicated, through other forms
of ecological networks, such as pollination or
dispersal networks.
While the big problem for continental organisms
is the increasing insularization of their habitats, the
problem for oceanic island biotas in recent history
and today is the increasing breakdown of the insu-
larity of theirs. As a result, many oceanic islands
now have a high representation of exotic species. It
does not necessarily follow that island habitats,
if undisturbed, are always more invasible than
continental habitats. However, exotic invaders
have had a significant impact on many island
ecosystems, and this susceptibility to exotic impact
can be related both to peculiarities of human use of
islands and to evolutionary features of the island
biotas, especially to their evolution in the absence
of terrestrial mammalian browsers and predators.322 ANTHROPOGENIC LOSSES AND THREATS TO ISLAND ECOSYSTEMS