Land snails associated with limestone outcrops in northern Australia - a potential bioindicator group

Limestone outcrops and their associated monsoon vine thickets (dry rainforest) comprise a distinctive but poorly-known ecological community in northern Australia. Currently, most outcrops are poorly protected and lack adequate conservation management, the fire-sensitive rainforest vegetation is threatened by increased levels of landscape burning, and many outcrops are in need of restoration. Land snails obligatorily associated with this ecological community are particularly susceptible and have been identified as a potential bioindicator group for monitoring environmental health and for biodiversity conservation. These invertebrates are characterised by high levels of narrow-range endemism and beta-diversity. In the Tindall Limestone Formation at Katherine, NT, at least seven species of camaenid land snails have been recorded, of which five are endemic to the area. We describe a quantitative sampling protocol based on nocturnal counts to assess snail abundance in this formation at locations currently subjected to varying management regimes. Our preliminary observations indicate that snail assemblages in this area may be affected by an unbalanced grass-fire cycle driven by an increase in fire frequency and abundance of Sorghum macrospermum, an annual grass which is endemic to the Katherine region. At one site, for which snail abundance had been reported 30 years previously, it appears that this grass-fire cycle may have led to a dramatic loss of the understorey monsoon vine thicket habitat and the concomitant decline in abundance of a highly localised species of land snail. We conclude that the endemic land snails can be used as bioindicators for developing conservation management strategies of limestone-monsoon vine thicket associations, and recommend that this ecological community be better managed to minimise the incidence and intensity of fire.