79
Introduced riverine species are non-native plants and animals accidentally or
deliberately introduced to a river corridor. Invasive introduced species are those that
dramatically increase in population densities and geographic range after introduction. Introduced species can prey on or compete with native species for habitat and
resources, leading to declines in native species (Townsend and Crowl 1991).
Introduced species can also alter form and process within river ecosystems, as when
introduced riparian vegetation stabilizes channel banks, traps sediment, and causes
channel narrowing (Dean and Schmidt 2011), or introduced beavers in South
America cause persistent flooding of riparian zones that kills native vegetation
(Pastur et al. 2006).
Invasive species are of greatest concern, particularly if they cause physical and/
or biological changes in river ecosystems. Widely publicized examples of invasive
introduced species in North America include the algae Didymosphenia geminata,
which forms thick layers on stream beds and alters macroinvertebrate community
structure and function (Gillis and Chalifour 2010), and zebra mussels (Dreissena
polymorpha), which reach enormous numbers, consume large quantities of plankton and thus reduce food sources for other aquatic animals that consume plankton,
and outcompete native filter-feeders such as bivalves (Strayer et al. 1999). Other
invasive exotics in North America include bighead (Hypophthalmichthys nobilis)
and silver carp (H. molitrix), which are Asian species that consume planktonic
organisms on which most species of native fish in the Mississippi River basin
depend (Chick and Pegg 2001). Among sport fisheries, introduced species include
brown trout (Salmo trutta) native to Europe, brook trout (Salvelinus fontinalis)
introduced to western North America from the eastern part of the continent, and
rainbow trout (Oncorhynchus mykiss) introduced from the Pacific coast of North
America to the Rocky Mountains and eastern North America. Each of these introduced trout species has the potential to alter consumption of aquatic and terrestrial
insects and thus change food webs within river corridors (Baxter et al. 2007).
Invasive exotic species including willows (Salix spp.) (Cremer 2003), carp
(Cyprinus carpio) (Shearer and Mulley 1978), and water hyacinth (Eichhornia
crassipes) (Perna and Burrows 2005) have also strongly affected Australian river
ecosystems. Although Eurasian and African river ecosystems also host non-native
species, far more research and management have focused on non-native species in
North America, Australia, and New Zealand (e.g., Townsend and Crowl 1991).
The invasive riparian woody plant tamarisk (Tamarix spp.) grows in such dense
thickets along river banks and floodplains that it excludes many native riparian
plants and alters flow resistance, bank stability, sedimentation, and channel form
(Graf 1978; Patten 1998) (Fig. 3.12). Tamarisk exemplifies some of the uncertainties surrounding mitigation of invasive species. Among the concerns initially
expressed regarding the spread of tamarisk in the United States were the potential
for increased transpiration and associated declining stream flows and the potential
loss of habitat for native songbirds and other riparian species. Massive campaigns
using herbicide, cutting, and burning were undertaken to limit the spread of tamarisk (Hultine et al. 2010) and the tamarisk beetle (Diorhabda elongata) was introduced as a non-native biological control agent capable of tamarisk defoliation
3.2 Direct Alterations of River Networks and River Corridors
Introduced riverine species are non-native plants and animals accidentally or
deliberately introduced to a river corridor. Invasive introduced species are those that
dramatically increase in population densities and geographic range after introduction. Introduced species can prey on or compete with native species for habitat and
resources, leading to declines in native species (Townsend and Crowl 1991).
Introduced species can also alter form and process within river ecosystems, as when
introduced riparian vegetation stabilizes channel banks, traps sediment, and causes
channel narrowing (Dean and Schmidt 2011), or introduced beavers in South
America cause persistent flooding of riparian zones that kills native vegetation
(Pastur et al. 2006).
Invasive species are of greatest concern, particularly if they cause physical and/
or biological changes in river ecosystems. Widely publicized examples of invasive
introduced species in North America include the algae Didymosphenia geminata,
which forms thick layers on stream beds and alters macroinvertebrate community
structure and function (Gillis and Chalifour 2010), and zebra mussels (Dreissena
polymorpha), which reach enormous numbers, consume large quantities of plankton and thus reduce food sources for other aquatic animals that consume plankton,
and outcompete native filter-feeders such as bivalves (Strayer et al. 1999). Other
invasive exotics in North America include bighead (Hypophthalmichthys nobilis)
and silver carp (H. molitrix), which are Asian species that consume planktonic
organisms on which most species of native fish in the Mississippi River basin
depend (Chick and Pegg 2001). Among sport fisheries, introduced species include
brown trout (Salmo trutta) native to Europe, brook trout (Salvelinus fontinalis)
introduced to western North America from the eastern part of the continent, and
rainbow trout (Oncorhynchus mykiss) introduced from the Pacific coast of North
America to the Rocky Mountains and eastern North America. Each of these introduced trout species has the potential to alter consumption of aquatic and terrestrial
insects and thus change food webs within river corridors (Baxter et al. 2007).
Invasive exotic species including willows (Salix spp.) (Cremer 2003), carp
(Cyprinus carpio) (Shearer and Mulley 1978), and water hyacinth (Eichhornia
crassipes) (Perna and Burrows 2005) have also strongly affected Australian river
ecosystems. Although Eurasian and African river ecosystems also host non-native
species, far more research and management have focused on non-native species in
North America, Australia, and New Zealand (e.g., Townsend and Crowl 1991).
The invasive riparian woody plant tamarisk (Tamarix spp.) grows in such dense
thickets along river banks and floodplains that it excludes many native riparian
plants and alters flow resistance, bank stability, sedimentation, and channel form
(Graf 1978; Patten 1998) (Fig. 3.12). Tamarisk exemplifies some of the uncertainties surrounding mitigation of invasive species. Among the concerns initially
expressed regarding the spread of tamarisk in the United States were the potential
for increased transpiration and associated declining stream flows and the potential
loss of habitat for native songbirds and other riparian species. Massive campaigns
using herbicide, cutting, and burning were undertaken to limit the spread of tamarisk (Hultine et al. 2010) and the tamarisk beetle (Diorhabda elongata) was introduced as a non-native biological control agent capable of tamarisk defoliation
3.2 Direct Alterations of River Networks and River Corridors