An assessment of invasive predatory marine crabs and the threat they pose along the South African coastline

Swart, Cheruscha (2017-12)

Thesis (MSc)--Stellenbosch University, 2017.


ENGLISH ABSTRACT: Invasions by marine alien species are occurring at an unprecedented rate and are known to negatively impact upon society and biodiversity. Due to the weak regulatory forces exerted by native predators, South African intertidal systems could be considered vulnerable to the invasion by predatory crabs. As this group has been suggested as one of the most successful marine invasive taxa and can have negative ecological impacts in recipient regions, mitigating their potential establishment is important. As such the main aim of this study was to review global invasions by predatory crabs, assess their ecological impacts and finally create a watch-list of species that could establish along the South African coastline under both current and predicted future temperature regimes. As part of the review, a list was developed of all predatory crab species reported as alien. Additionally, their global occurrence, vectors and potential traits associated with their successful establishment were documented. In total, 56 alien crab species were recorded with more than half these being documented in the last two decades. The majority of species originated from the North West Pacific, while the Mediterranean received more alien crabs (33 species) than any other bioregion. Shipping, specifically ballast water, has been responsible for the majority of introductions. Unexpectedly, no biological or ecological traits could be identified as good predictors of establishment success in crabs. While this work identified the most important vectors and most invasive crab families, it emphasises the need for more studies considering the basic biology of these crabs so as to improve our understanding of the traits governing their invasion. The Environmental Impact Classification for Alien Taxa (EICAT) was used to assess the impacts of the species identified in the review. It was found that impacts had been quantified for only 9% of the 56 alien crab species. Thus, only five species could be allocated EICAT ratings due to the data deficiency of the remaining 51 species. The Japanese shore crab Hemigrapsus sanguineus was rated as having Major impacts, while impacts of the remaining four species, the Chinese mitten crab Eriocheir sinensis, European shore crab Carcinus maenas, Indo-Pacific swimming crab Charybdis hellerii and brush-clawed shore crab Hemigrapsus takanoi were rated as Moderate. To create an ordered watch-list for South Africa, species that could be expected to reach the region, on account of the pathways they are associated with, were identified. Their realised temperature ranges were compared to that of each of the four South African marine ecoregions and finally they were ranked based on their EICAT rating. In total, 28 alien crab species had pathways to reach South Africa, with shipping highlighted as the most important pathway. At least 26 species could survive along the South African coast under both present and predicted future temperatures, with warm water species being excluded from the cool west coast and temperate species excluded from the warm east coast. Three species, H. sanguineus, E. sinensis and H. takanoi were placed on the top of the watch-list due to their negative ecological impacts identified by the EICAT scheme. This study provides the first South African horizon scanning assessment to identify and prioritise potential marine alien species. This watch-list can be used to support at-border management enabling the fast response to new arrivals, ultimately minimising chances of establishment of these alien crabs along South African shores. This thesis has provided a detailed global review of predatory marine crab invasions. It has highlighted that despite few studies quantifying impacts of these invaders, it is clear that they can have notable ecological impacts in recipient regions. Nonetheless, there is a dire need for more research into their impacts so as to support evidence based management. Until such evidence becomes available it is suggested that a precautionary approach be applied when managing alien crabs.

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