THE AGENCIES OF DESTRUCTION 301been among the native peoples of islands, particu-
larly on first contact with Europeans. This factor, in
combination with more purposeful persecution, led
to the decimation of many island peoples, such that
in some cases little or no trace of them remains
(Kunkel 1976; Watts 1987; Diamond 2005).
The most commonly cited example of exotic dis-
ease afflicting native island birds is from Hawaii.
Hypotheses to explain why so many species of
birds became extinct in the period since Cook’s
landfall in AD1778 include habitat destruction,
hunting, competition with introduced birds, and
introduced predators. Undoubtedly, each has had
its role in the degradation of Hawaii’s ecosystems.
Early work by Warner (1968) identified avian
malaria (and avian pox), carried by the introduced
mosquitoCulex quinquefasciatus, as an important
factor. Warner proposed an ‘imaginary line’ at
about 600 mASL, above which there were no mos-
quitoes and below which native birds were not
found because they had succumbed to the disease.
However, van Riper et al. (1986) suggest that avian
malaria probably took some time to build up a large
enough reservoir and that it did not have a major
impact upon the numbers of Hawaiian birds until
the 1920s, some time after the extinction pulse of
the late nineteenth century. Van Riper et al. (1986)
also report that the malarial parasite Plasmodiumis
found up to treeline elevations, i.e. well above
Warner’s 600 m threshold. The parasite is nonethe-
less concentrated in the mid-elevational ranges,
where introduced bird species and native birds
have the greatest distributional overlap. The intro-
duced species have been found to be less suscepti-
ble to malaria and thus to act as vectors for the
disease. Although a number of native bird species
have developed immunogenic and behavioural
responses reducing the impact of the parasite, van
Riper and colleagues concluded that avian malaria
is currently a major limiting factor, restricting both
abundance and distribution of the endemic avi-
fauna.
The omao (Myadestes obscurus) is one of four sur-
viving species of thrushes on the Hawaiian islands.
It occupies no more than 30% of its former range,
and although locally abundant in rain forests in
particular parts of the Big Island of Hawaii, it is
absent from other areas in which it was once com-
mon. Its disappearance from the Kona and Kohala
districts during the early part of the twentieth cen-
tury remains an enigma. A plausible hypothesis is
that it failed to develop resistance swiftly in these
parts of its range to avian malaria, but in other parts
of its range, such as the Puna district, it did develop
resistance, thereby allowing its present pattern of
coexistence with both the mosquitoes and the
malarial parasites in that area (Ralph and Fancy
1994).
Given the possibilities of local variations in the
development of resistance to disease, it is clearly
going to be difficult to be certain about the role of
disease in the demise of Hawaii’s birds. Added to
which, it needs to be remembered that most species
were lost before European contact, and that a host
of other changes have occurred over the past
200 years or so. The pattern of post-European con-
tact losses is bimodal, being concentrated between
1885 and 1900, and 1915 and 1935. The former
phase of losses included many species historically
confined to higher altitudes ( 600 mASL) and
therefore contradicting Warner’s (1968) original
malarial scenario. The explanation for this phase of
losses may relate to another introduced disease,
avian pox, or to the impact of habitat modifications
and introduced predators. However, the second
period does appear to relate to the arrival and
spread of malaria. Birds which succumbed during
this period were found in the mid-elevational
forests in which the highest prevalence of avian
malaria was found during studies around 1977–80
(van Riper et al. 1986). As the introduced tropical
mosquitoCulex quinquefasciatusslowly adapted to
cooler elevations, the die-off of native bird species
slowly advanced up the mountains, reaching 600 m
by the 1950s and 1500 m by the 1970s, sweeping
away several mid-elevation species in the process,
and restricting the survivors to ever higher refugia.
This process is ominously described by Pratt (2005)
as ‘the third and ongoing wave.’
Two things emerge. First, disease has played and
continues to play a part in the problem, causing
range reductions and precipitating extinctions.
Secondly, we are dealing with a complex system
involving opportunities for the spread of exotics