Biology Reference
In-Depth Information
aliens becomes blurred. Many temperate zone species will likely spread to
areas much farther north, and desert species into areas that are becoming
warmer and drier.
Because climatic zones are likely to shift faster than long-lived species
can track through reproduction and dispersal, communities in disequilib-
rium may result, with conditions that indirectly favor weedy species,
many of which are likely to be aliens. Dispersal is hindered, as well, by the
high degree of fragmentation of many natural habitats due to human land
use. In Britain, for example, an analysis of changes in distribution of 46
species of butterflies that reach their northern limits within the islands
showed that most had declined over the past 30 yr despite the expansion
of areas of favorable climate (Warren et al. 2001). Those species that did
expand as expected were mostly habitat generalists.
Habitat fragmentation can seriously impede plant migration. In Bel-
gium, where forest habitat is highly fragmented, Honnay et al. (2002)
examined the colonization of suitable new forest patches by forest plant
species. The average distance from source to target patches in the area
studied was 210 m. The required migration rates of these species under
projected climate change are 3-5 km per year (Davis and Shaw 2001).
The rates observed in the Belgian study were only a few meters per year,
and some species showed essentially no ability to colonize new patches
over a period of about 40 yr. Interestingly, migration rates were highest
for plant species dispersed by birds and mammals and lowest for those dis-
persed by wind.
Whether or not invasive species will benefit more than noninvasive
species by global change is uncertain (Dukes 2000). Many invasive plants
show considerable growth stimulation by elevated carbon dioxide and
increased nitrogen availability, but their responses do not appear to be
greater than those of related noninvasive species. Nevertheless, other char-
acteristics of many invasive species, particularly the dispersal capability and
opportunistic response to disturbance, means that they are well adapted to
changing and fragmented habitats (Dukes and Mooney 1999; Barrett
2000). In Sweden, several nonnative trees, including Norway maple (
Acer
platanoides
), Swiss stone pine (
Pinus cembra
), and lodgepole pine (
Pinus con-
torta
), have spread upward to near the treeline formed by native species
(Kullman 2002).
Many invasive animal species are likely to invade new areas where cli-
mate has warmed. In North America, several widely introduced warm-
water fish, including largemouth bass (
Micropterus salmoides
), bluegill sun-
