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R.A. Hufbauer and M.E. Torchin
species can have significant negative ecological and environmental effects
while having positive economic effects (e.g., cows). Third, it is important to
have a baseline against which changes associated with invasion can be judged.
We argue that comparing the ecology of an introduced species to that of populations
in its native range will set the most relevant baseline by which to
measure changes in ecology resulting from translocation. If we first understand
the causes of demographic variation among populations within the
native range, then we can compare introduced populations to this gradient
(Torchin et al. 2001).
For some species, population demographics may be similar between the
native and novel ranges, and mechanisms of their success may be no different.
Such species may be quite benign when introduced, or if they cause economic
problems in their native range, then often they will in their introduced range
as well (e.g., insect pests such as Western corn root worm, Diabrotica virgifera,
which is a native pest in North America, and an invasive pest in Europe;
Chap. 2). If a species is more abundant, dense, or widespread in the novel
range than in the native range, then – by definition – something has fundamentally
changed in its ecology or perhaps evolution. Because invasiveness is
a combined function of the invaded community and the invader, the changes
leading to greater success in the new range can be extrinsic changes in the
environment that favor the invading species, or they can be intrinsic to the
invading species. We refer to species that experience significant positive
demographic changes that contribute to invasion success as strong invaders,
and those with no change or significant negative changes as weak invaders.As
Hierro et al. (2005) point out, there are remarkably few comparative data
between the native and novel ranges of species documenting whether the
demography of invaders changes (but see Torchin et al. 2001).
Introduced species that are weak invaders can be important economically
and environmentally, and also give rise to interesting and urgent ecological
questions. However, weak invaders are less useful in helping us address questions
regarding fundamental ecological and evolutionary changes that appear
to underlie the most damaging invasions (e.g., tamarisk and zebra mussel in
North America). To further an understanding of the causes of biological invasion,
it is vital to know whether or not most species considered to be “invasive”
are strong invaders that have experienced dramatic demographic
changes relative to their native range.
Herein, we propose a metric to quantify the continuum from weak
invaders to strong invaders. Response ratios are used to compare the means
of experimental treatments (X E ) and controls (X C ), where R=X E /X C (Hedges
et al. 1999). If R>1, then the experimental treatment is larger than the control,
and if R