Impacts of alien ‘ecosystem engineers’ overwhelm interannual and seasonal shifts in rocky-shore community composition on Marcus Island, South Africa

Abstract

Of the 89 alien species recorded from South African marine waters (Robinson et al. 2016), three have become widespread along the open coast: the Mediterranean mussel Mytilus galloprovincialis, from the Mediterranean Sea and eastern Atlantic; the Pacific barnacle Balanus glandula from the American west coast; and the bisexual mussel Semimytilus algosus from Chile (Branch and Steffani 2004;Laird and Griffiths 2008;de Greef et al. 2013;Assis et al. 2015;Robinson et al. 2015). In 1980, prior to any marine invasions, a study of intertidal rocky-shore community composition was undertaken on Marcus Island, Saldanha Bay, to assess prey availability for the endemic African black oystercatcher Haematopus moquini (Robinson et al. 2007). These data were later used as a historical baseline against which to consider changes in community composition associated with the introduction of M. galloprovincialis (Robinson et al. 2007) and the later arrival of B. glandula and S. algosus (Sadchatheeswaran et al. 2015). Adult M. galloprovincialis are recognised as an example of 'ecosystem engineers': species that can transform environments and modulate resource availability to an ecosystem, particularly the benthic communities (sensu Jones et al. 1994;McQuaid and Arenas 2009). Because of limitations inherent in the historical data, there were restrictions on the analyses presented by Robinson et al. (2007) and Sadchatheeswaran et al. (2015). The most notable limitation was that the studies were all snapshots in time, set 10 to 20 years apart, and thus failed to consider either interannual or seasonal changes. Therefore, changes in community composition ascribed to the arrival of these alien invasive species could not be distinguished from natural shifts that might have arisen seasonally or from year to year, such as those recorded by Dye (1998). An assessment of the magnitude of seasonal and interannual changes at Marcus Island was thus required. The second challenge associated with the historical data was that the datasets considered only invertebrates, because the original 1980 survey was focused on the availability of prey for H. moquini (Robinson et al. 2007). Furthermore, the exclusion of algae is problematic, as multiple studies have demonstrated a variety of roles for

Status of a decennial marine invasion by the bisexual mussel Semimytilus algosus (Gould, 1850) in South Africa

“…In particular, M. galloprovincialis has modified intertidal rocky shores and has impacted native mussel species. For example, on the west coast, it reduces the abundance of the ribbed mussel Aulacomya atra in intertidal habitats, and it restricts the distribution of the black mussel Choromytilus meridionalis (F. Krauss, 1848) (Griffiths et al 1992;Hockey and van Erkom Schurink 1992;Branch and Steffani 2004;Robinson et al 2007;Reimers et al 2014;Alexander et al 2015b;Sadchatheeswaran et al 2018). On the south coast, M. galloprovincialis interacts intensely with the native P. perna, and the two species exhibit partial habitat segregation based on differences in settlement, recruitment and adult growth rates McQuaid 2006, 2009) as well as complex interactions including competition and facilitation (McQuaid et al 2015).…”

Coastal Invasions: The South African Context

“…Many of the impacts associated with S. algosus are similar to those of M. galloprovincialis, as both species dominate previously open rocky surfaces. Notably, both species elevate the structural complexity of invaded rocky shores (Sadchatheeswaran et al 2015), ultimately elevating diversity and altering community structure (Robinson et al 2007b;Sadchatheeswaran et al 2018). As they also form an abundant prey resource, these alien mussels have also altered the foraging landscape of native predators.…”

A four‐component classification of uncertainties in biological invasions: implications for management

“…The same invasive mussel upon which the South African mussel industry is founded and which has benefitted African black oystercatchers has dramatically altered the structure of rocky shore communities in the region (Sadchatheeswaran et al. ).…”

“…The rainbow trout (Oncorhynchus mykiss), native to the United States, was introduced to many countries for recreational fishing, and is viewed positively by recreational anglers, but has detrimental effects on native biodiversity (Shackleton et al 2019). The same invasive mussel upon which the South African mussel industry is founded and which has benefitted African black oystercatchers has dramatically altered the structure of rocky shore communities in the region (Sadchatheeswaran et al 2018).…”