Environmental Engineering Reference
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Fig. 2.5 Plot of results
of canonical corre-
spondence analysis
(CC A) show i ng native
species of fi sh (black
circles), introduced
invader species (open
triangles) and fi ve
infl uential environmen-
tal variables (arrows
represent the correla-
tion of the physical
variables within the
two dimensions of the
graph). Species with
similar niches appear as
points close together on
the graph. Note how
the native and exotic
species occupy different
regions of niche space.
(From Begon et al.,
2006, after Marchetti &
Moyle, 2001.)
2
Temperature
1
Conductivity
Discharge
0
Shade
Pools
-1
-2
-2
-1
0
1
2
CCA axis 1
most important of the niche dimensions (see Ter Braak & Smilauer, 1998, for
details). Species with similar niches appear as points close together on the graph,
while infl uential environmental factors appear as arrows indicating their direction
of increase within the two dimensions of the graph. Marchetti and Moyle (2001)
used an ordination method called canonical correspondence analysis to describe
how a suite of fi sh species, 11 native and 14 invaders, are related to environmental
factors at sites in a regulated stream in California (Figure 2.5).
The native and invasive species clearly occupy different parts of multidimensional
space: most of the native species occur in places associated with higher mean dis-
charge (m 3 s −1 ), good canopy cover (higher levels of % shade), lower concentrations
of plant nutrients (lower conductivity,
S), cooler temperatures (˚C) and less pool
habitat in the stream (i.e. greater % of fast-fl owing, shallow riffl e habitat). This
combination of variables refl ects the natural, undisturbed state of the stream and
encompasses the niches of the native species.
The pattern for introduced species is generally the opposite; invaders are favored
by the present combination of conditions where water regulation has reduced dis-
charge and increased the representation of slower-fl owing pool habitat, riparian
vegetation has been removed leading to higher stream temperatures, and nutrient
concentrations have been increased through agricultural and domestic runoff. We
do not know what proportion of the native species would be successful in areas
occupied by the exotic species if the latter were absent (or vice versa). In other words
we cannot be sure whether the researchers are dealing with realized or fundamental
niches; experimental removal studies would be needed to establish this. However,
the general conclusion of Marchetti and Moyle (2001) holds good - restoration of
more natural fl ow regimes (providing for the niche requirements of native fi sh) is
needed to limit the advance of invaders and halt the continued downward decline
of native fi sh in this part of the western USA.
It should not be imagined, however, that invaders inevitably do less well in
'natural' fl ow regimes. Invasive brown trout ( Salmo trutta ) in New Zealand streams
do better in the face of fl oods than native fi sh such as Galaxias anomalus ; indeed
river reaches whose discharge has been substantially reduced because of water
abstraction for land irrigation seem to provide refuges for the natives in the face of
predation by brown trout (Leprieur et al., 2006). The low discharge sites would
µ
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