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

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.