species likely reflects the lack of rocky shores in these sandy protected areas, as most
alien species known from South Africa require hard substrata like rocks or artificial
infrastructure for attachment (Mead et al. 2011a). The most widespread species was
M. galloprovincialis, which occurred in 13 of the protected areas spanning the region
between Langebaan Lagoon on the west coast and Bird Island on the south coast.
Other notable occurrences included the first reports of the ascidians Microcosmus
squamiger and Diplosoma listerianum, the hydrozoans Obelia dichotoma and
Obelia geniculata and the bryozoan Cryptosula pallasiana in natural habitats. This
highlights that although most marine alien species are known from harbours, natural
habitats are susceptible to regional spread. This may be of particular
conservation concern in protected areas. A strong link exists between yachts and the
local spread of alien species along the South African coast (Peters et al. 2017). As such,
MPAs that are situated close to harbours, or that are visited by yachts, may be at
elevated risk of invasion by alien species and should be prioritised for monitoring.
9.6 Impacts of Dominant Intertidal Invaders
Because they are easily accessed and offer habitat to spatially dominant alien
species, the ecological impacts of non-native biota on rocky shores have been well
studied in South Africa. Three species have extensively invaded rocky shores along
the open coast, M. galloprovincialis, Semimytilus algosus and Balanus glandula
(Fig. 9.6). It is notable that all three invasions emanated on the west coast, with
species spreading south before crossing the biogeographic break of Cape Point and
dispersing onto the south coast.
Mytilus galloprovincialis was first noted in Saldanha Bay in the late 1980s. It is
now the dominant intertidal species between northern Namibia and East London on
the south coast (Assis et al. 2015). This accounts for approximately 2800 km of the
South African coast. Within this range, it has proliferated at the expense of various
native taxa (Branch and Steffani 2004; Robinson et al. 2007b). Along the west coast,
this dominance has been driven primarily by the alien mussel’s superior growth rate,
reproductive output and tolerance to desiccation when compared to the native
mussels Choromytilus meridionalis and Aulacomya atra (van Erkom Schurink and
Griffiths 1991, 1993; Hockey and van Erkom Schurink 1992). As a result, since the
arrival of M. galloprovincialis, mussel beds in this region have extended further up
shore (Hockey and van Erkom Schurink 1992). Along the south coast
M. galloprovincialis co-exists with the native mussel Perna perna, through partial
habitat segregation that sees the alien mussel excluded from the low-shore by a
combination of wave exposure and interspecific competition with P. perna (Rius and
McQuaid 2006). Through its ability to preclude other species from occupying
primary rock space, M. galloprovincialis has also displaced native limpets. Through
this mechanism the abundance of the Granular Limpet Scutellastra granularis has
declined on bare rock but, interestingly, overall abundance has increased as
M. galloprovincialis shells offer a favourable recruitment substratum for juvenile
limpets (Hockey and van Erkom Schurink 1992; Branch et al. 2010). Thus, the mean
240
T. B. Robinson et al.
alien species known from South Africa require hard substrata like rocks or artificial
infrastructure for attachment (Mead et al. 2011a). The most widespread species was
M. galloprovincialis, which occurred in 13 of the protected areas spanning the region
between Langebaan Lagoon on the west coast and Bird Island on the south coast.
Other notable occurrences included the first reports of the ascidians Microcosmus
squamiger and Diplosoma listerianum, the hydrozoans Obelia dichotoma and
Obelia geniculata and the bryozoan Cryptosula pallasiana in natural habitats. This
highlights that although most marine alien species are known from harbours, natural
habitats are susceptible to regional spread. This may be of particular
conservation concern in protected areas. A strong link exists between yachts and the
local spread of alien species along the South African coast (Peters et al. 2017). As such,
MPAs that are situated close to harbours, or that are visited by yachts, may be at
elevated risk of invasion by alien species and should be prioritised for monitoring.
9.6 Impacts of Dominant Intertidal Invaders
Because they are easily accessed and offer habitat to spatially dominant alien
species, the ecological impacts of non-native biota on rocky shores have been well
studied in South Africa. Three species have extensively invaded rocky shores along
the open coast, M. galloprovincialis, Semimytilus algosus and Balanus glandula
(Fig. 9.6). It is notable that all three invasions emanated on the west coast, with
species spreading south before crossing the biogeographic break of Cape Point and
dispersing onto the south coast.
Mytilus galloprovincialis was first noted in Saldanha Bay in the late 1980s. It is
now the dominant intertidal species between northern Namibia and East London on
the south coast (Assis et al. 2015). This accounts for approximately 2800 km of the
South African coast. Within this range, it has proliferated at the expense of various
native taxa (Branch and Steffani 2004; Robinson et al. 2007b). Along the west coast,
this dominance has been driven primarily by the alien mussel’s superior growth rate,
reproductive output and tolerance to desiccation when compared to the native
mussels Choromytilus meridionalis and Aulacomya atra (van Erkom Schurink and
Griffiths 1991, 1993; Hockey and van Erkom Schurink 1992). As a result, since the
arrival of M. galloprovincialis, mussel beds in this region have extended further up
shore (Hockey and van Erkom Schurink 1992). Along the south coast
M. galloprovincialis co-exists with the native mussel Perna perna, through partial
habitat segregation that sees the alien mussel excluded from the low-shore by a
combination of wave exposure and interspecific competition with P. perna (Rius and
McQuaid 2006). Through its ability to preclude other species from occupying
primary rock space, M. galloprovincialis has also displaced native limpets. Through
this mechanism the abundance of the Granular Limpet Scutellastra granularis has
declined on bare rock but, interestingly, overall abundance has increased as
M. galloprovincialis shells offer a favourable recruitment substratum for juvenile
limpets (Hockey and van Erkom Schurink 1992; Branch et al. 2010). Thus, the mean
240
T. B. Robinson et al.