The biology of Placostylus ambagiosus (Pulmonata: Bulimulidae) in New Zealand: Part 2. Population changes, growth, mortality and life expectancy

Abstract

The large land snail Placostylus ambagiosus (Pulmonata: Bulimulidae) was studied in northernmost New Zealand from 1988 to 2004. At Cape Maria van Diemen (CMvD), more juveniles than adults were found, although estimates showed adults as the most abundant. A cohort, hatched after rodent control commenced in 1990, began maturing in 1995, but 89% died by 1997 (partly through competition with garden snails, Cornu aspersum), before numbers partially recovered by 2003. At Surville Cliffs, adults were estimated to be more abundant than juveniles but few juveniles were observed. Numbers under 12 food plants fluctuated between 425 and 657. Many snails dispersed after horses (Equus caballus) partially defoliated their food plants but returned once these recovered. A translocated population increased after rodent control commenced in 1999, but declined when rodent control ceased in 2002. Another translocated population declined when rodents were present and never recovered even after rodent control commenced in 2002. Snails took 2.8–11.7 years to mature from half-grown juveniles depending on location. Estimated adult life expectancies were 5.2 years at CMvD and 10.0 years at Surville Cliffs. Small juvenile snails experienced the highest mortality, and snails that moved between food plants experienced increased mortality. These results are discussed in relation to conservation management.

Keywords:

• long-term monitoring
• demography
• developmental rate
• lifespan

Acknowledgements

We thank the following who assisted with many days of field work: Tristan Armstrong, Paul Barrett, Suzanne Bassett, Andrea Booth, Murray Efford, Ian Flux, Diane Gleeson, Elizabeth Grant, Katrina Hansen, Tracy Harris, Ruth Henry, Leigh Honnor, Maree Hunt, Carrie Jane, Jens Jorgensen, Cathy Lake, Gary Land, Megan McLean, Peter MacGregor, Nigel Miller, Kahori Nagakawa, Carol Nicholson, Richard Montifiore, Graeme Murtagh, Richard Overwijk, Gerard Pilon, Tony Porteous, Stephanie Prince, Christine Reed, Tim Shaw, Michele Salas and Catherine Yong.

The field staff of the Department of Conservation provided extensive practical support and included Whiti Abahams, Mike Aviss, Trevor and Gail Bullock, Nik Conrad, Francis Fitzpatrick, Rod Green, Simon Job, Alan and Willie McCrae, Bill McLeod, DJ Neho, Siebert Rohiopa, Eddie Smith and Aileena Sucich. We thank Don Newman and Gary Barker for constructive criticism of the text; Maheswaran Rohan for statistical help; and Sandra Pakkarli for preparing the figures. This research was supported by DOC (Science Investigation No. 2386) and Massey University.