Biodiversity of Freshwaters - Freshwater Ecology: Concepts and Environmental Applications - page 195

invasions are far greater than natural rates, although natural invasions of
species influence how evolution of new species relates to community for-
mation. Numerous case studies illustrate potential problems associated with
unwanted species introductions; I will present only a few here.
Why do species invade successfully? Predicting which species will in-
vade and the effects of the invasion is difficult (Fuller and Drake, 2000).
A sound knowledge of the natural history of the invader and the commu-
nity it has entered allows for the best predictive assessment of the influence
of an invading species. Qualitative techniques may be used to assess the
risks associated with freshwater species introductions (Li et al., 2000).
Moyle and Light (1996) proposed a conceptual model of species inva-
sions and establishment based on case studies of species of invading fish.
Their points appear useful and are generalized here with additional obser-
vations provided by Kolar and Lodge (2001). As with any conceptual eco-
logical model, exceptions exist, and the model is presented mainly as a
method to approach the problem. First, most invaders fail to establish.
Most failed invasions are never documented; thus, the success rate of inva-
sions is not well-known. Second, most successful invaders are integrated
without major effects on the ecosystem or community, although some have
major effects. Third, all aquatic systems can be invaded. Fourth, major com-
munity effects are observed most often in low-diversity systems, including
island and highly disturbed habitats. Fifth, top predators that invade suc-
cessfully are more likely to have strong community effects than successful
invaders at lower trophic levels. Sixth, species must have physiological and
morphological characteristics suited to the environment to invade success-
fully. Seventh, invaders are most likely to become established when native
assemblages are disturbed. Natural or anthropogenic disturbance increases
susceptibility to invasion. Eighth, success of invaders can depend on envi-
ronmental variability. Invasion and establishment in an extremely harsh and
variable environment may be difficult unless invaders are preadapted to the
variation (see the sixth point). Variation can also play a role in the seventh
point. Ninth, very stable systems may be susceptible to invasion, although
the data are weak for this generalization. Tenth, the greater the number of
invading individuals and times they are introduced, the greater the proba-
bility they will become established. Finally, species with a history of being
invasive are most likely to invade other habitats.
The previous model does not follow more traditional treatments of
community assembly theory that depend on the sequence of invasion and
species interactions. For example, early attempts at understanding success
of invasions led to suggestions that alien species are more likely to colo-
nize cropland and disturbed habitats, and by extension, more diverse com-
munities are less susceptible to invasions (Elton, 1958). This idea of diverse
communities being resistant to invasion has since been challenged (Moyle
and Light, 1996; Levine, 2000). Many treatments of invasion success have
revolved around competition and predation as determinants (Roughgar-
den, 1989).
The Great Lakes of North America have experienced a tremendous
number of invasive species (Sidebar 10.1), including the zebra mussel
(Dreissena polymorpha), which has had considerable economic and eco-
logical impacts. The spread of the zebra mussel is one of the more pro-