Environmental Engineering Reference
In-Depth Information
monly represented strategies among species now established are competitive (C) or
competitive-ruderal (CR) combinations of traits.
Marchetti et al. (2004) take up where the parrot and plant studies leave off. They
compiled information on invasions and failed introductions of fi sh in catchment
areas throughout California and separate the invasion process into the three fi nal
stages of our scheme: (3) establishment (of a self-sustaining population); (4) spread
(assessed as number of catchment areas occupied); and (5) impact (assessed as
average abundance attained). Their analysis includes a variety of species traits, from
physiological tolerance and adult size to prior invasion success. The process involves
a comparison of all conceivable models to predict species 'success' at the three stages,
on the basis of the species traits. The importance, or weight, of particular traits was
estimated across all models, and the larger the weight, the more important the
variable in predicting the response of interest (Table 3.3).
All eight traits in the analysis are important for predicting species establishment.
This is not really surprising as the traits were chosen, according to fi rst principles,
because they might be expected to be infl uential. Establishment is more likely for
larger species (more competitive), those showing broad physiological tolerances and
diets, parental care, a large native range (broader niches), a nearby native source, a
record of successful invasions elsewhere and where large numbers were involved in
introduction attempts.
Prediction of the propensity to spread, on the other hand, depends mainly on
physiological tolerance and adult size, as well as the effort put in to establishing the
species in the fi rst place. Once again, and unlike the pine species, successful fi sh
invaders do not fall consistently at one end of the
r
/
K
continuum. Competitive status
(large size) seems to play a role, but the overriding ecological variable is broad
physiological tolerance, probably increasing the likelihood, in the probabilistic
Table 3.3
Weight of
evidence to suggest that
a particular response
variable is important in
predicting 'success' at
each stage of invasions
of freshwater fi sh in
Californian river
catchment areas. The
larger the 'weight', the
more important is the
variable in predicting
invasion across all
possible models created;
maximum weight is 1.0.
(After Marchetti et al.,
2004.)
Establishment
Response
(self-sustaining
Spread (number
Impact (average
variables
population)
of catchment areas)
abundance)
Propagule pressure
1.00
0.83
0.34
(numbers involved
in introduction
attempts)
Physiological
1.00
0.82
0.08
tolerance
Adult maximum size
1.00
0.38
0.09
Distance from
1.00
0.23
0.02
source
Size of native range
1.00
0.10
0.07
Prior invasion
1.00
0.10
0.76
success
Trophic st atus
1.00
0.00
0.00
Parental care
1.00
0.00
0.00
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