Biology Reference
In-Depth Information
(exploitative), and only 12 were harmful to both species (antagonistic).
Although this was not a rigorous statistical test of facilitation, it showed
that interactions that benefited one or both alien species were very
frequent.
Ricciardi (2001) performed a more rigorous test for aquatic species
introduced to the North American Great Lakes. Reviewing articles deal-
ing with the 162 alien species now known from the lakes, he identified
101 interspecific interactions that could be categorized. Of these, 17 were
either mutualistic or commensal, 73 were exploitative, and only 11 were
antagonistic or amensal (detrimental to one but harmless to the other). Of
the exploitative interactions, more than one-third were instances in which
a later invader exploited a previously established alien. Ricciardi (2001)
found, furthermore, that the number of invaders per decade had increased
in a straight-line manner with time since the early 1800s. Thus, invaders
tended to show a degree of synergism, although whether this effect was
stronger than for relations among native species was still not clear.
Global Climate Change and Alien Invasions
The magnitude of worldwide invasion of alien species is itself a phenom-
enon unique in the history of life on earth. Superimposed on rapid spread
of species to new geographical areas, however, is global climate change, a
phenomenon also occurring at an unprecedented rate. Global climate
change is both directly and indirectly promoting alien invasions and their
evolutionary consequences.
Little doubt now exists that the global environment is changing.
Global change involves several components. Global climatic warming, as
a consequence of increasing concentrations of carbon dioxide and other
anthropogenic greenhouse gases, is the primary driver of climate change.
The overall climate of the earth has warmed by about 0.6°C during the
past century, with the increase being much greater at high than at low lat-
itudes (Walther et al. 2002). Other components of global change include
the fertilization effect of increased atmospheric carbon dioxide; increases
in ultraviolet radiation due to weakening of the stratospheric ozone layer;
discharges of acids, oxides, heavy metals, and other products of fossil fuel
combustion; environmental pollution by industrial and agricultural
chemicals; and increased mobilization of bioactive elements such as mer-
cury, selenium, and many others. Stratospheric depletion of ozone over
the south polar region may also be a major contributor to altered atmos-
pheric circulation and surface warming of the Antarctic Peninsula and
