Distribution, abundance and size distribution of the alien invasive pulmonate, Theba pisana, in the West Coast National Park, South Africa: Is there cause for concern?

The distribution, abundance and size distribution of the invasive land snail, Theba pisana, in the West Coast National Park (WCNP), South Africa was investigated. The Park was divided into 1 km2 grids within each of which five 1 m2 quadrat counts, taken at 50 m intervals, of both dead and alive snails were recorded and mapped. Of a total of 106 grids (535 quadrat samples), 19 % and 46 % of the study area contained live and dead snails respectively. However, densities were generally low and only 18% of grids contained more than zero but less than 10 live snails m-2. The average density of live snails within the park (4.04 m-2 ± 24.91) was significantly lower than both the density of dead snails (8.77 m-2 ± 21.25) and the density in more disturbed habitats outside the park (57 m-2 ± 96.25). As alien species do well in disturbed habitats, the near pristine environment of the park could have limited their distribution. Also, while most snails (dead or alive) were found along roads, densities decreased dramatically with distance from the road. Even though recent summer temperatures of the WCNP are consistent with the norm for the region, the average rainfall is considerably less than normal, which could also have contributed to the present restricted distribution of T. pisana within the Park. In early autumn, the field population consisted of two size classes, large snails of about 14 mm and small snails of 6 mm diameter. By the end of winter, most snails were much smaller (2-7 mm) with very few snails exceeding 10 mm. During spring, the snails grew rapidly and a larger cohort (11 mm) replaced the smaller one. Thus, T. pisana in the WCNP probably has an annual lifecycle. The Western Cape is characterised by a Mediterranean climate and thus T. pisana is well adapted for this region. Based on its present and past distribution, its invasion into the WCNP should not be taken lightly and more studies concerning the threat to the fynbos biome, is necessary to conserve the rich biodiversity of the region. (** See published paper at https://ir.sun.ac.za/cib/handle/123456789/718)