Biology Reference
In-Depth Information
interactions of vertebrates introduced to island archipelagos such as
Hawaii (Moulton and Pimm 1983). More species of songbirds have been
introduced to Hawaii than to any other location in the world. On the six
main islands of Hawaii, 47 species of songbirds were introduced between
1871 and 1960 to one or more islands, for a total of 145 introductions.
These species have largely occupied the disturbed urban and agricultural
habitats that predominate at lower elevations, although some do invade
native forest and other natural communities.
About 36% of the songbirds (passerines and members of the pigeon
order) introduced to Hawaii failed after several to many years (Moulton
and Pimm 1983). Nevertheless, 9-20 species of aliens have survived, cre-
ating a community of largely alien species in the Hawaiian lowlands. For
the islands of Oahu, Kauai, Maui, and Hawaii, the number of surviving
species of alien birds has apparently reached a plateau. For Molokai and
Lanai, such a plateau is not yet evident. Between 7 and 15 extinctions
were noted on the four islands on which the number of extant species
reached a plateau. As the number of established alien species increased,
so did the rate of extinction of species that had managed to prosper for
some time. In fact, the extinction rate for the archipelago as a whole
increased at a significantly faster rate than the increase in number of
species, strongly indicating that interactions among species were con-
tributing to the extinctions. In several cases, the establishment of an eco-
logically similar species was associated with the extinction of a previously
established species.Thus, on four of the islands, an equilibrium number of
species appeared to have become established, such that the establishment
of new species tended to be balanced by the extinction of previously
established species.
The success of introduced birds in Hawaii also relates to their appar-
ent competitive relationship with other introduced species (Moulton
1985).Although the size and shape of the beak are far from giving a com-
plete picture of the pattern of food resource use by a bird, major differ-
ences are usually related to feeding ecology. In some 18 cases, congeneric
songbirds were introduced to the main islands of Hawaii in such a fash-
ion that two species were present in similar habitat at the same time
(Moulton 1985). In three cases, neither species survived, and in nine cases,
one of the two failed to survive.The six pairs that did survive on average
showed more that twice the percentage difference in bill length as did the
members of pairs for which only one member survived. In turn, mem-
bers of pairs in which one member survived showed greater percentage
difference in bill lengths than did members of pairs both of which failed
