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E.J. Cook et al.
5.3 Ecological Consequences of Intentional
and Unintentional Introduction of Non-Native Species
The intentional and unintentional introduction of non-native cultured species
represents a “biological introduction”, which are human mediated movements of
organisms to regions where they did not evolve. Biological introductions are
widely recognised as a major threat to species diversity (CBD 1992; Worm et al.
2006) arising from habitat modification, changes in ecosystem functioning,
extinction of native fauna and flora, disease transfer and genetic effects such as
hybridisation with native congeners (Lovei 1997; Ruiz et al. 1997; D’Antonio
et al. 2001; Jonsson and Jonsson 2006). Regions supporting high levels of
endemic species are particularly vulnerable to these introductions.
5.3.1 Habitat Modification
The accidental or intended introductions of exotic species can cause significant
changes to ecosystems (Ruesink et al. 2006). However, the response of natural
communities to the introduction of a non-native species is complex, and impacts
can have positive, negative or negligible, depending on the species, location,
age, or type of habitat considered (Neira et al. 2005; Gribben and Wright 2006).
To highlight the potential effects of introduced species on habitat structure, two
case-studies will be considered: the first in Willapa Bay, Washington USA
(Ruesink et al. 2006) and the second on the South African coast (Robinson
et al. 2005).
In Willapa Bay, USA the Pacific oyster Crassostrea gigas was introduced in 1928
for aquaculture purposes due to the overexploitation of the native oyster Ostreola
conchaphila (Ruesink et al. 2006) and is now the main oyster species cultivated.
The Pacific oyster naturally recruits to uncultivated regions of the bay and forms
dense intertidal hummocks of shell and live oysters (Ruesink et al. 2006). This
recruitment has also been observed in a number of other countries where C. gigas
has been introduced (Orensanz et al. 2002; Nehring 2003; Diederich et al. 2005).
The importation of C. gigas and the development of the oyster industry also
unintentionally brought the invasive smooth cordgrass Spartina alterniflora to
Willapa Bay in the form of packaging material for transplanted Crassostrea virginica in 1890 (Townsend 1893, 1896; Feist and Simenstad 2000).
Species that can change habitat structure and modify the local environment are
known as ecosystem engineers (Crooks 2002). C. gigas and S. alterniflora can
substantially re-engineer a habitat to provide biogenic structures which provide
substrate for fish, invertebrate and macroalgal recruitment and sediment accumulation (Ruesink et al. 2006). The expansion of culture sites and biogenic reefs formed
by oysters can also cause significant changes in sediment porosity, bioturbation
activity and have an effect on biogeochemical cycling (Ruesink et al. 2006).
E.J. Cook et al.
5.3 Ecological Consequences of Intentional
and Unintentional Introduction of Non-Native Species
The intentional and unintentional introduction of non-native cultured species
represents a “biological introduction”, which are human mediated movements of
organisms to regions where they did not evolve. Biological introductions are
widely recognised as a major threat to species diversity (CBD 1992; Worm et al.
2006) arising from habitat modification, changes in ecosystem functioning,
extinction of native fauna and flora, disease transfer and genetic effects such as
hybridisation with native congeners (Lovei 1997; Ruiz et al. 1997; D’Antonio
et al. 2001; Jonsson and Jonsson 2006). Regions supporting high levels of
endemic species are particularly vulnerable to these introductions.
5.3.1 Habitat Modification
The accidental or intended introductions of exotic species can cause significant
changes to ecosystems (Ruesink et al. 2006). However, the response of natural
communities to the introduction of a non-native species is complex, and impacts
can have positive, negative or negligible, depending on the species, location,
age, or type of habitat considered (Neira et al. 2005; Gribben and Wright 2006).
To highlight the potential effects of introduced species on habitat structure, two
case-studies will be considered: the first in Willapa Bay, Washington USA
(Ruesink et al. 2006) and the second on the South African coast (Robinson
et al. 2005).
In Willapa Bay, USA the Pacific oyster Crassostrea gigas was introduced in 1928
for aquaculture purposes due to the overexploitation of the native oyster Ostreola
conchaphila (Ruesink et al. 2006) and is now the main oyster species cultivated.
The Pacific oyster naturally recruits to uncultivated regions of the bay and forms
dense intertidal hummocks of shell and live oysters (Ruesink et al. 2006). This
recruitment has also been observed in a number of other countries where C. gigas
has been introduced (Orensanz et al. 2002; Nehring 2003; Diederich et al. 2005).
The importation of C. gigas and the development of the oyster industry also
unintentionally brought the invasive smooth cordgrass Spartina alterniflora to
Willapa Bay in the form of packaging material for transplanted Crassostrea virginica in 1890 (Townsend 1893, 1896; Feist and Simenstad 2000).
Species that can change habitat structure and modify the local environment are
known as ecosystem engineers (Crooks 2002). C. gigas and S. alterniflora can
substantially re-engineer a habitat to provide biogenic structures which provide
substrate for fish, invertebrate and macroalgal recruitment and sediment accumulation (Ruesink et al. 2006). The expansion of culture sites and biogenic reefs formed
by oysters can also cause significant changes in sediment porosity, bioturbation
activity and have an effect on biogeochemical cycling (Ruesink et al. 2006).