The alien invasive land snail Theba pisana in the West Coast National Park: Is there cause for concern?

Abstract

The distribution, abundance, size distribution and diurnal activity patterns of invasive land snails, Theba pisana, in the West Coast National Park (WCNP), South Africa, were investigated. The park was divided into 1 km2 grids, within each of which five 1 m2 quadrat counts of live snails were recorded. Of 106 grids sampled, 19% contained live snails. The average density of snails was 4.04 m-2 ± 24.9, significantly lower than in disturbed habitats adjacent to the park (57 m-2 ± 96.25), but very high densities were recorded at two sites. Snails were most abundant along roadsides and densities decreased dramatically with distance from roads. T. pisana in the WCNP appear to have an annual lifecycle, breeding in autumn to winter and growing to adult size of about 14 mm diameter by the end of the following summer. Snails were observed on a wide variety of endemic and introduced plant species and appeared to have a catholic diet. They are active mostly at night and especially during periods of high humidity, irrespective of temperature. Given the very high densities that T. pisana can attain at some sites, plus their apparently catholic feeding habits, their potential impact on the vegetation of the park is cause for concern and should be further investigated. Control of the main colonies should also be considered.