Abstract
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.
Highlights
The human-mediated spread of marine species from their natural ranges to new habitats is occurring with increasing frequency (Seebens, Gastner & Blasius 2013) and resulting invasions have been reported from most regions (Molnar et al 2008)
Whilst S. ornata occurred in sandy areas covering underlying rock and in S. maritima beds in 2001, in 2013 it was no longer present in the S. maritima beds but was found instead in Nanozostera capensis beds and attached to rocks resting on fossilised oyster beds
This study aimed to provide an updated baseline on the status and distribution of S. ornata, to assess the diet of this alien anemone and to investigate its impact on the structure of the sandy-shore communities of the West Coast National Park (WCNP)
Summary
The human-mediated spread of marine species from their natural ranges to new habitats is occurring with increasing frequency (Seebens, Gastner & Blasius 2013) and resulting invasions have been reported from most regions (Molnar et al 2008). The increase in the number of ships and ballast tank size (Carlton 1996), accompanied by an increase in transoceanic travel (Ruiz et al 1997) over the last century, has led to an increase in the introduction and rate of establishment of invasive marine species around the world (Wonham et al 2000) This is cause for concern as these alien species can pose a threat to indigenous biota (Gurevitch & Padilla 2004; Molnar et al 2008; Strain & Johnson 2013) and can have negative economic impacts (Cinar 2013; Pimentel, Zuniga & Morrison 2005). Most species alien to South African waters have been introduced via hull fouling and ballast water (Mead et al 2013)
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