Habitat use of Tradescantia fluminensis by Powelliphanta traversi

This study was initiated owing to concern that removal of the invasive ground cover, Tradescantia fluminensis, might adversely effect the rare, giant predatory land snail, Powelliphanta traversi. Of 18 sites surveyed in the Horowhenua District, seven sites provide habitat for P. traversi and do not contain Tradescantia. Five colonies of P. traversi are affected by Tradescantia, these are: Waiopehu Scenic Reserve, Prouse’s Bush, Kimberley Scenic Reserve, Ohau River Bush, and Hillas Covenant. Tradescantia infestation at these sites varies from heavy (Prouse’s Bush) to forest edge only (Hillas Covenant). Overall, Tradescantia affects a small proportion of snail habitat in the Horowhenua District. Our study of P. traversi at Prouse’s Bush, using harmonic radar to follow long-term movements and cotton tracks for short-term movements, indicates that this snail commonly occurs under Tradescantia, sometimes exclusively, and that many of the snails move regularly between leaf litter and this weed. Moreover, Tradescantia provides an important refuge for juvenile snails at Prouse’s Bush. Hence, removing Tradescantia from this site would have a detrimental impact on P. traversi. Graduated control of Tradescantia and concomitant replacement with native ground cover could be of mutual benefit to P. traversi and other ground-dwelling invertebrates.