Browsing by Author "Swart, Cheruscha"

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    An assessment of invasive predatory marine crabs and the threat they pose along the South African coastline
    (Stellenbosch : Stellenbosch University, 2017-12) Swart, Cheruscha; Robinson-Smythe, Tammy; Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology.
    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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    Distribution and impact of the alien anemone Sagartia ornata in the West Coast National Park
    (AOSIS Publishing, 2015-03) Robinson, Tamara B.; Swart, Cheruscha
    Sagartia ornata is an alien anemone that occurs intertidally within the West Coast National Park (WCNP). Whilst baseline distributional data was gathered in 2001, the range and abundance of this alien has not been reassessed. The present study aimed to determine the current status and distribution of this anemone, to assess its diet so as to establish the role it may play as predator and to investigate its impact on sandy-shore communities. Sagartia ornata was found to be restricted to the WCNP, where it occurred in densities of up to 508 ± 218 individuals per m2 . Within the park the distribution of this anemone had changed. Populations were recorded in Nanozostera capensis seagrass beds for the first time and this alien was absent from two areas in which it had previously occurred. Diet analysis revealed indigenous polychaetes and amphipods as the dominant prey items consumed by S. ornata. This alien was found to significantly alter sandy-shore community structure, with differences caused primarily by increases in the abundance and biomass of the tanaid Anatanais gracilis and the polychaete Orbinia angrapequensis. Additionally, invaded areas supported significantly greater invertebrate diversity, density and biomass. It is concluded that whilst this anemone negatively affects native biota, its current dependence on restricted habitats precludes widespread impacts with the park. Conservation implications: With regard to conservation implications, this invasion should be routinely monitored outside the WCNP as in its native range S. ornata occurs on rocky shores and kelp holdfasts, suggesting a potential for spread along the west coast of South Africa.
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    Patterns and traits associated with invasions by predatory marine crabs
    (Pensoft Publishers, 2018-08-28) Swart, Cheruscha; Visser, Vernon; Robinson, Tamara B.; Ruiz, G.
    Predatory crabs are considered amongst the most successful marine invasive groups. Nonetheless, most studies of these taxa have been descriptive in nature, biased towards specific species or regions and have seldom considered traits associated with invasiveness. To address this gap in knowledge, this study presents a global review of invasions by this group and applies biological trait analysis to investigate traits associated with invasion success. A total of 56 species belonging to 15 families were identified as having spread outside their native ranges. The family Portunidae supported the highest number of alien species (22). Most crabs had their origin in the North West Pacific IUCN bioregion while the Mediterranean Sea received the most species. No traits associated with successful establishment were identified, but this finding may reflect the paucity of basic biological knowledge held for many species. This lack of foundational knowledge was unexpected as crabs are large and conspicuous and likely to be well studied when compared to many other groups. Addressing this knowledge gap will be the first step towards enabling approaches like biological trait analysis that offer a means to investigate generalities in invasions.