Biology Reference
In-Depth Information
A few populations of native songbirds do survive, however, at low ele-
vations (Shehata et al. 2001). One of these is the O'ahu 'amakihi
(
Hemignathus flavus
), which occurs at the Lyon Arboretum, where it is in
close association with many alien birds, mosquito vectors, and the
Plas-
modium
parasite. Closely related 'amakihi species on other islands are often
infected by
Plasmodium
and suffer lower survival rates than infected
species of alien birds. Using molecular genetic techniques, Shehata et al.
(2001) examined native and introduced birds at the Lyon Arboretum for
a gene specific to the malaria parasite as an indication of active infection.
Of 13 species of alien birds examined, 11 showed some malaria infections,
the average incidence being about 11.5%. None of 42 individuals of the
'amakihi showed infections. Thus, the O'ahu 'amakihis have apparently
evolved some mechanism of resistance to avian malaria.The resistance of
O'ahu 'amakihis has probably evolved over some 125-170 generations,
depending on the exact time that both the mosquito vector and the dis-
ease agent became established. Resistance in such cases can be expected
to appear in 5-50 generations (Dobson and May 1986), so this case is well
within expectation.A more limited degree of resistance also seems to exist
for the Hawai'i 'amakihi (
Hemignathus virens
) (Jarvi et al. 2001).
Another example of adaptation by native organisms to alien diseases
involves plague (
Yersinia pestis
), a bacterial disease of mammals, including
humans. It is known from more than 200 mammal species (Biggins and
Kosoy 2001). Plague can be transmitted from one animal feeding on
another or by bites of fleas carried by infected animals. The plague bac-
terium apparently evolved in Central Asia some time within the past
20,000 yr and was introduced to North America and many other parts of
the world by infected rats transported by ocean ships. That plague is a
newcomer to North America is evident from its relative homogeneity and
the absence of strains adapted to specific animals.
North American mammals now show varying degrees of sensitivity to
plague. The primary hosts of plague are rodents, both in Asia and North
America. Most North American carnivores appear to be resistant to the
effects of plague. Rodents, on the other hand, vary greatly in resistance,
most being highly susceptible. Populations of prairie dogs (
Cynomys
spp.),
in particular, appear to be highly susceptible to plague, and local popula-
tions have been extirpated by plague outbreaks. A few rodent species
show high resistance, apparently in some measure in response to whether
or not the disease has recently affected their populations.
Clear evidence that evolutionary resistance is fostered by exposure to
plague exists for at least some species (Biggins and Kosoy 2001). Offspring
