Invasive Species - Ecological Systems - page 169

(e.g., filtration rate of mussels, functional response of predators, rate of habitat
conversion for ecosystem engineers). Data on per-capita effects are often scarce,
but inferences regarding the magnitude of impact may be drawn from abundance,
which has been shown to be a useful predictor of impact [ 61 ]. Range size, in
contrast, may not necessarily be a good predictor. Beyond the trivial expectation
that the impacts of an invading species accumulate as it occupies more territory,
there is no statistical correlation between the invasion success of a species (i.e., its
rate of establishment success or spread) and the magnitude of its impact [ 98 ]. Even
relatively poor invaders can have strong local impacts on native populations (e.g.,
the Asian clam Potamocorbula amurensis ; Atlantic salmon Salmo salar ), whereas
highly successful colonizers do not necessarily displace native species (e.g., fresh-
water jellyfish Craspedacusta sowerbyi ). One generalization that has emerged from
numerous case studies is that high-impact invaders often represent novel life forms
in the invaded system. They acquire and use resources differently than resident
species, possess defense mechanisms and “weapons” that are foreign to the invaded
community [ 99 ], and may have predatory capabilities to which residents are poorly
adapted. Such species tend to belong to taxonomic or functional groups that were
not present in the ecosystem prior to invasion [ 100 - 102 ]. As such, the phylogenetic
distinctiveness of the invader in its novel environment might be an indicator of its
impact potential [ 101 , 102 ].
A major challenge to prediction is context-dependent variation generated by
site-specific environmental factors [ 60 , 61 ]. The best predictor of the colonization
success and impact of an introduced plant or animal is its invasion history [ 20 , 61 ].
Although impacts vary across a heterogeneous environment, models may be devel-
oped to predict the impact (or abundance) of a species with a well-documented
impact history [ 61 ], but the predictive power of such models is diminished at sites
that have been highly invaded. Nonnative species can interact in multiple ways to
produce unpredictable effects [ 12 , 75 ], sometimes by facilitating each other's
spread and impact (i.e., invasional meltdown [ 103 ]).
Prevention
Given the growing frequency of invasions, their profound impacts, and the sub-
stantive resources required to control rapidly spreading species after they become
established, the most cost-effective management strategy is prevention [ 14 ]. Argu-
ably, invasions warrant similar investments in preparedness and response planning
as natural disasters; despite being slower in their onset, invasions have more
persistent impacts and a greater scope of ecological and economic damage than
natural disasters [ 104 ].
Prevention involves controlling either species entry or establishment. Preventing
entry of nonnative species begins with the identification and control of dominant
transportation vectors and pathways [ 14 ]. The effectiveness of vector-control policies
requires rigorous inspection, enforcement, evaluation, and - where necessary -
refinement, as has been demonstrated by the evolution of a management program