Biology Reference
In-Depth Information
Columbia and Alberta, Canada (Benkman 1999). Red squirrels were
absent from this hill area until their introduction in 1950. Red crossbills
probably colonized the Cypress Hills within the past 6,800 yr. The pop-
ulation of red crossbills there evolved larger, deeper bills, while the lodge-
pole pines (
Pinus murrayana
) evolved larger cones with more seeds and
thicker distal scales. Again, the red crossbill population apparently became
extinct following introduction of red squirrels. This pattern of coevolu-
tion is also evident in other isolated mountain ranges in Idaho and Mon-
tana (Benkman et al. 2003).
A more complex example of the organization of an animal group
invading an area in post-glacial time, for the most part, is provided by the
land birds of Tasmania (Keast 1970). Located about 225 km south of the
state of Victoria,Australia,Tasmania is an island about 66,560 km
2
in area.
During the last Pleistocene glaciation, about 2,000 km
2
of the island were
covered by glacial ice and the climate was very severe.Timberline lowered
in elevation to about 90-115 m, and bird habitats were much less diverse
than those at present. Because of lowered sea level, however, Tasmania
possessed a land connection with mainland Australia. Some bird species
that colonized Tasmania before the last glacial period may have survived
throughout glaciation. At the end of the glacial period, about 12,000 yr
ago, however, the land connection was broken and warming conditions
brought rapid habitat change that made Tasmania favorable for coloniza-
tion by many new species. Nevertheless, in spite of the current wide range
of bird habitats, the land bird fauna is impoverished compared to adjacent
mainland Australia. Only 43 species of small passerine birds occur in Tas-
mania, compared to 89 in southern Victoria.
This less diverse land bird fauna of Tasmania, however, shows many
ecological and evolutionary adjustments to the modern environment
(Keast 1970). Tasmania, for example, has only two common species of
owls, which are conspecific with two of the six owl species that occur in
southern Victoria.The body sizes of the Tasmanian owls differ from those
of birds inVictoria.The smaller Tasmanian species is reduced in body size,
whereas the larger species has become still larger and shows greater sex-
ual dimorphism in body size, compared to the same two species in Victo-
ria.These differences are presumed to reflect the altered way that the Tas-
manian species partition prey by body size.
The passerine birds of Tasmania show many differences in habitat rela-
tions, feeding behavior, and morphology, as well (Keast 1970). Several
species occupy a greater range of habitats and show more diverse feeding
patterns than equivalent species in Victoria. The pink robin (
Petroica
