Survival of Grand and Otago Skinks Following Predator Control

Abstract

Both grand (Oligosoma grande) and Otago skinks (Oligosoma otagense) are threatened with extinction. Predation by introduced mammalian predators, in particular feral cats, is thought to be the major cause of decline. Survival of 5 grand and 2 Otago skink populations was monitored for 3 years before and 3 years during intensive predator control, targeting feral cats but also culling ferrets. In addition, 1 Otago and 2 grand skink populations served as controls. For both species, capture–recapture modeling of skink survival failed to find evidence that survival was significantly influenced by the removal of predators, but retrospective power tests showed uncertainty in this result was high. It is possible the residual predator populations were sufficient to depress skink survival during the treatment interval. Seasonal modulation of apparent survival was evident for both species. Grand skink survival was higher over the winter months and summer survival was characterized by episodic lows, the cause of which remains a mystery. By contrast, Otago skink survival was higher in summer than in winter and did not exhibit the severe lows recorded for grand skinks. My study exemplifies the difficulty in conducting an experiment aimed at guiding management, in a complex predator- species-rich mainland environment.