Biology Reference
In-Depth Information
Great Plains has exposed many new populations of songbirds to nest par-
asitism by cowbirds. In southern California, the cowbird first appeared as
a breeding species in about 1920. Cowbird nest parasitism is now a major
cause of nest failure for the California gnatcatcher (
Polioptila californica
).
Analysis of clutch completion dates for California gnatcatchers from 1880
through 1959 showed that since 1920 a significant shift toward earlier
clutch completion has occurred (Patten and Campbell 1998). The mean
date of clutch completion between 1920 and 1959 was 9 days earlier than
for the period prior to 1920. Egg laying by cowbirds tends to be concen-
trated in late spring and early summer. It is likely that other songbirds
have made similar adjustments to cowbird parasitism.
In Hawaii, another bird, the i'iwi (
Vestiaria coccinea
) has apparently
undergone a reduction in bill length correlated with the considerable
decline in plants of the genus
Lobelia
(Smith et al. 1995).The long, curved
bill of the i'iwi was adapted to the tubular corollas of lobelias, which pro-
vided the birds with abundant nectar. Most of the low-elevation native
forest has disappeared and introduced ungulates have decimated lobelia
populations in the understory of the remaining forest at higher elevations
(Scott et al.1986). As a result, the i'iwi has shifted much of its feeding to
the ohia tree (
Metrosideros polymorpha
), which has flowers lacking a corolla
and accessible to short-billed birds. In this case, the evolutionary response
of the i'iwi is an indirect effect of the impact of alien animals on native
plants.
Adaptation of Native Animals to Introduced Diseases
Alien diseases often impose very strong selective pressures on native ani-
mals, and many examples now exist of evolutionary adaptation to alien
diseases.The whirling disease parasite (
Myxobolus cerebralis
), native to Eura-
sia, was introduced to North America in the late 1950s. This parasite has
a life history alternating between two hosts, salmonid fish and tubificid
worms. Spores from dead fish enter the stream water and are ingested by
tubifex worms (
Tubifex
spp.), small annelid worms that live in muddy
stream bottoms. Clusters of a new spore stage develop in the worms.
These clusters have hooks that enable them to attach to the gills or other
internal surfaces of fish. The spores then enter the circulatory system of
the fish. The parasite attacks developing cartilage tissues of young fish,
eventually disrupting tissues associated with the cranial balance organs. As
a result, the fish swims erratically and has difficulty in feeding and avoid-
ing predators. Severe infections cause high rates of mortality of young
