Biology Reference
In-Depth Information
Figure 7.1.
Hypothesized interaction of diversity of native and alien species across
local environments that vary in resource richness and native biotic diversity of
mature communities. For each group of species (such as the group with open
circles), environmental conditions and availability of resources are assumed to be
the same, and as native species diversity increases, exotic species diversity declines.
With more favorable conditions and more abundant resources, the diversity of both
groups generally increases, as suggested by the groups of species farther along the
number axes (Reprinted with permission from K. Shea and P. Chesson. 2002.
Community ecology theory as a framework for biological invasions.
Tr ends in
Ecology and Evolution
17:170-176. © 2002 Elsevier Publishing Company.)
Vulnerability of Insular Communities
That insular communities with low species diversity are so vulnerable to
biological invasions also suggests that the degree of community organiza-
tion and complexity influences the probability of invader success—the
so-called biotic resistance hypothesis. Ecological release, the expansion of
habitat and resource use seen in many recent island invaders, is an indica-
tion that the organization of most insular communities is weaker than that
of comparable continental regions. The absence of particular animal
groups, such as large terrestrial herbivores and predators, from many
oceanic islands also means that constraints to the establishment of many
kinds of plants and animals are almost completely lacking.
Alien plant species exceed 20% of the flora of many oceanic islands,
and in several cases, exceed 80% (Rejmánek 1996).The highest percent-
