Using Mark-recapture to provide population census data for use in Red Listing of invertebrates: the rare terrestrial snail Prestonella bowkeri as a case study

Invertebrates in general, and terrestrial molluscs in particular, are greatly affected by the current biodiversity extinction crisis, and accurate assessment of these organisms for inclusion in Red Lists is essential for the conservation of these taxa. However, obtaining accurate population counts of molluscs for this purpose can be challenging. Using Prestonella bowkeri, a terrestrial snail up to 23 mm in length and 14 mm in width found in mesic rocky cliff face habitats on the Great Escarpment of southern Africa as an example, we show that the use of Mark-recapture techniques to obtain population census data is appropriate to terrestrial molluscs of this size and larger. While currently listed as vulnerable, our results indicate that there are as many as 4,108 individuals of this species in one population, which suggests that the population size (sensu the IUCN definition) is probably >20,000, and the species could be down-listed and classified as near threatened. However, when considering the problems associated with applying the IUCN Red List criteria to invertebrates, and the very small number of populations and hence area of occupancy of this species, down-listing would be premature.