Biology Reference
In-Depth Information
to survive. More detailed analyses of morphological difference among
introduced birds on Hawaii and other oceanic islands further supported
the conclusion that differentiation of feeding ecology is prerequisite to
survival (Moulton and Pimm 1987; Lockwood et al. 1993, 1996).
Short-term Evolutionary Responses
As we have seen, many recent invaders trigger rapid evolutionary
responses, as well as ecological responses, by native species (chapters 12,
13, and 14). Areas where this occurs are termed evolutionary hot spots
(Parchman and Benkman 2002) and show that evolutionary adjustments
may begin quickly but continue for long periods of evolutionary time. In
spite of many years of presence, however, some invasive species and the
natives with which they interact in other locations show little coevolu-
tionary adjustment, in a sense representing evolutionary cold spots.Thus,
on a geographic scale, a mosaic of coevolved relationships, varying in the
species involved and in the strength of the interaction, is expected
(Thompson 1999c).
Examples of how invading species can adapt to new biotic environ-
ments are also provided by species that have colonized regions released
from the most recent phase of Pleistocene glaciation. Parchman and
Benkman (2002), for example, showed that coevolution has occurred
between black spruce (
Picea mariana
) and red crossbills (
Loxia curvirostra
) in
Newfoundland, Canada, following release of this island from glaciation
about 9,000 yr ago.These two species colonized Newfoundland, but the
red squirrel (
Tamiasciurus hudsonicus
), a major predator on black spruce
seeds, did not. Under these circumstances, black spruce reduced cone
defenses that functioned against red squirrels and increased those that
functioned against crossbills. In Newfoundland, black spruce cones exhib-
ited greater numbers of seeds and a higher ratio of seed mass to cone mass
than in mainland Canada, evidently as a result of reduced red squirrel pre-
dation. In contrast, cone scales were thicker and the force required to pry
them open to expose the seeds was greater than in mainland Canada, cor-
related with high seed predation by crossbills. The crossbill itself showed
a larger, deeper bill than crossbills of mainland Canada and was consid-
ered to be a distinct subspecies (
Loxia curvirostra percna
). Unfortunately, this
crossbill race declined to extinction following the introduction of red
squirrels to Newfoundland in 1963.
The pattern of coevolution of red crossbills and conifers in New-
foundland is similar to that in the Cypress Hills, on the border of British
