Native snail numbers in both our monitoring sites have been decreasing rapidly over the last three years. Two species of carnivorous land snails (Powelliphanta hochstetteri and Rhytida oconnori) are found in the park, and both are at risk of local extinction.
Initially, snail numbers at the sites at Canaan and Wainui looked promising. With rat numbers decreasing, population sizes were large and high recruitment rates suggested a recovering population. However, this picture has changed dramatically with growing weka numbers. Numbers of Powelliphanta hochstetteri snails at our Wainui plot have dropped from 450 to 73 in three years. Current projections show the population could be reduced to 16 by this season.
Project Janszoon scientist Ruth Bollongino says monitoring over the last four years points to weka as the main reason for a decline in snail numbers. The snails are surveyed at night in 70 m × 70 m plots, with snails marked and recaptured year on year to gauge population size, mortality and growth rates.
High weka predation at both the Wainui and Canaan sites has coincided with two exceptionally dry summers. Ruth adds that “growing numbers of shells without signs of predation are a sign that drought is also having a major impact”.
Project Janszoon and DOC are considering the possibility of erecting weka enclosure fences in the park to protect the snails from weka predation. Ongoing monitoring will provide more answers about the impact of weka and drier summers, and if karst habitat provides some protection from weka.
