5 Non-Native Species in Aquaculture
165
In South Africa, the Mediterranean mussel Mytilus galloprovincialis was
accidentally introduced in ~1979 and is now grown commercially (Robinson
et al. 2005) with over 6,100 t produced in 2005 (FAO 2006a). M. galloprovincialis has become the dominant intertidal mussel along the west coast, where it has
considerably modified the natural community composition by dominating rock
surfaces (Robinson et al. 2005). M. galloprovincialis forms dense, multi-layered
structures and supports a higher biomass per m
2 than the single layered beds of
the indigenous mussels Choromytilus meridionalis and Aulacomya ater (Robinson
et al. 2005). The increased vertical range of M. galloprovincialis, due to a
greater dessication tolerance, higher fecundity and faster growth rates than the
native species (Van Erkom Schurink and Griffiths 1990, 1991; Hockey and van
Erkom Schurink 1992; Van Erkom Schurink and Griffiths 1992), has led to a
massive increase in non-native mussel biomass along the South African west
coast (Griffiths et al. 1992).
The introduction of certain non-native species to a region can considerably
modify the system, as shown by the introduction of C. gigas, S. alterniflora
and M. galloprovincialis. Predicting the impact that non-native species will
have on habitat structure and, as a consequence, existing food webs and community composition is inherently difficult. A greater understanding of how
these non-native “ecosystem engineers” alter energy flow, ecological processes, biogeochemical cycles and ecosystem function is critical in determining
the impact that these species will have on the ecosystem as a whole.
5.3.2 Changes in Ecosystem Functioning
Ecosystem services are a set of ecosystem functions that are useful to humans and
many are critical to our survival (climate regulation, air purification, pollination,
nutrient recycling) while others enhance it (aesthetics) (Kremen 2005). Ecosystem
functioning is intrinsically linked to biodiversity and changes in biodiversity and
community structure can cause drastic changes in ecosystem function and hence in
the provision of ecosystem services.
The majority of studies of ecosystem function have concentrated on biodiversity
loss due to extinctions; however, many biological invasions have resulted in a net
gain at the local or regional level (Sax and Gaines 2003). This causes a net increase
in diversity at the ecosystem level and an important consideration is how these
species additions affect ecosystem functioning (Stachowicz and Tilman 2005). Few
studies have been undertaken to specifically address this question, although it is clear
that invasive species can affect ecosystem structure and function (Stachowicz and
Tilman 2005). For example, Levin et al. (2006) showed that invasion by a Spartina
hybrid in San Francisco Bay (USA) shifted the system from an algae based to a
primarily detrital-based system. Furthermore, the Spartina hybrid canopy changed
the hydrodynamic regime causing drastic and multiple changes in the physical,
chemical and biological properties in the benthic system (Neira et al. 2006). These
165
In South Africa, the Mediterranean mussel Mytilus galloprovincialis was
accidentally introduced in ~1979 and is now grown commercially (Robinson
et al. 2005) with over 6,100 t produced in 2005 (FAO 2006a). M. galloprovincialis has become the dominant intertidal mussel along the west coast, where it has
considerably modified the natural community composition by dominating rock
surfaces (Robinson et al. 2005). M. galloprovincialis forms dense, multi-layered
structures and supports a higher biomass per m
2 than the single layered beds of
the indigenous mussels Choromytilus meridionalis and Aulacomya ater (Robinson
et al. 2005). The increased vertical range of M. galloprovincialis, due to a
greater dessication tolerance, higher fecundity and faster growth rates than the
native species (Van Erkom Schurink and Griffiths 1990, 1991; Hockey and van
Erkom Schurink 1992; Van Erkom Schurink and Griffiths 1992), has led to a
massive increase in non-native mussel biomass along the South African west
coast (Griffiths et al. 1992).
The introduction of certain non-native species to a region can considerably
modify the system, as shown by the introduction of C. gigas, S. alterniflora
and M. galloprovincialis. Predicting the impact that non-native species will
have on habitat structure and, as a consequence, existing food webs and community composition is inherently difficult. A greater understanding of how
these non-native “ecosystem engineers” alter energy flow, ecological processes, biogeochemical cycles and ecosystem function is critical in determining
the impact that these species will have on the ecosystem as a whole.
5.3.2 Changes in Ecosystem Functioning
Ecosystem services are a set of ecosystem functions that are useful to humans and
many are critical to our survival (climate regulation, air purification, pollination,
nutrient recycling) while others enhance it (aesthetics) (Kremen 2005). Ecosystem
functioning is intrinsically linked to biodiversity and changes in biodiversity and
community structure can cause drastic changes in ecosystem function and hence in
the provision of ecosystem services.
The majority of studies of ecosystem function have concentrated on biodiversity
loss due to extinctions; however, many biological invasions have resulted in a net
gain at the local or regional level (Sax and Gaines 2003). This causes a net increase
in diversity at the ecosystem level and an important consideration is how these
species additions affect ecosystem functioning (Stachowicz and Tilman 2005). Few
studies have been undertaken to specifically address this question, although it is clear
that invasive species can affect ecosystem structure and function (Stachowicz and
Tilman 2005). For example, Levin et al. (2006) showed that invasion by a Spartina
hybrid in San Francisco Bay (USA) shifted the system from an algae based to a
primarily detrital-based system. Furthermore, the Spartina hybrid canopy changed
the hydrodynamic regime causing drastic and multiple changes in the physical,
chemical and biological properties in the benthic system (Neira et al. 2006). These