Monitoring Powelliphanta land snails: an assessment of the current technique and the development of a new mark-recapture technique

Abstract

There are currently no proven techniques for monitoring populations of land snails in the genus Powelliphanta. The Department of Conservation has developed a method for establishing an index of abundance. Although this method is routinely used, its reliability remains unclear. Many Powelliphanta species and sub-species are currently listed as threatened. Improving estimates of population trends and assessing the effectiveness of conservation management is therefore critical. The main aim of this research is to improve the techniques used for assessing and monitoring populations of these animals. The study is split in to three main parts: an assessment of the current monitoring technique, a description of a newly developed mark-recapture technique and an analysis of the ancillary data that can be collected when utilising a mark-recapture method. The second two chapters focus on Powelliphanta augusta, a species that was translocated from its natural habitat to make way for coal mining. It was found that a large proportion of snails are overlooked when using the current monitoring technique. On average, approximately 30% of snails were observed during a standard monitoring event. However, the proportion of snails overlooked at each monitoring plot was highly variable, ranging from 10% to over 50%. It is recommended that this method is used with extreme caution. An alternative mark-recapture technique is described, which was developed to monitor the critically endangered snail P. augusta. To have confidence in the results obtained from the monitoring, it was important to establish if the method was reliable and, in particular, if the mark-recapture assumptions underpinning the statistical analysis were being met. It was concluded that the method is reliable and is a practical alternative to the standard method for monitoring Powelliphanta snails. Recommendations are made for further improvement and refinement of this mark-recapture technique. Abundance estimates were produced for P. augusta at all sites in which they currently occur. The abundance estimates showed that most of the populations are stable and may even be growing. As a species that is relatively new to science, there were many important aspects of the ecology of P. augusta that were completely unknown. An advantage of using a mark-recapture technique for monitoring is the possibility of developing a richer understanding of the ecology and behaviour of the animal being studied by estimating other parameters. It was found that annual survival is in excess of 80% in most locations, which is higher than previously estimated. It was previously unknown how fast P. augusta grows in the wild. Average shell growth was found to be approximately 2.6mm over a year. It is estimated that the average age of reproductive maturity is eight years old, younger than previously thought. Other parameters are explored, such as population structure and recruitment. A model of population persistence suggests that most of the P. augusta populations appear to be able to persist at present. Further recommendations are made regarding the management of the species and the management of Powelliphanta in general.