An investigation into declining skink populations and their behavioural responses to introduced mammalian predators
Thesis DisciplineBiological Sciences
Degree GrantorUniversity of Canterbury
Degree NameMasters of Science in Ecology
New Zealand provides an excellent example of the effect of exotic predators on native reptile populations. Prior to human arrival, reptiles evolved in the absence of mammalian predators but are now sympatric with 11 introduced mammalian predators. New Zealand’s reptile populations have declined over the past millennium because they have few defensive behaviours against this new predator guild. However, relatively few studies have investigated the effects of introduced mammalian predators on skinks. In this thesis, I studied the life history of several skink species and the long-term demographic changes in these species to evaluate population trends. I evaluated the effectiveness of mammalian predator control in the Rotoiti Nature Recovery Project (RNRP) for restoring skink populations and I investigated the potential sub-lethal effects of mammalian predators on skinks. Finally, I tested whether two skink species had developed behaviours to avoid the scent of introduced rats or hedgehogs. I estimated size at sexual maturity, birthing season and habitat preferences for speckled skinks (Oligosoma infrapunctatum) in the Nelson Lakes area. My research shows that skink populations are declining both inside and outside of the RNRP. The largest declines are seen in the rarer species and even within the predator-controlled area of the RNRP the speckled skink is nearing extirpation. In addition, the proportion of female northern grass skinks (O. polychroma) and larger individuals of both sexes has decreased since 1970; suggesting females and larger individuals are more vulnerable to predation. An investigation of three fitness surrogates (body condition, parasite load and prevalence of caudal autotomy) showed that for both northern grass and glossy brown skinks (O. zelandicum), body condition was significantly lower in populations with mammalian predators than without. This has serious conservation implications because it shows that lizard populations may not only be in decline from direct predation, but also additional stresses associated with predation that may lead to reduced reproductive output. Neither parasite load, nor the prevalence of caudal autotomy, appears to be good indicators of fitness for northern grass or glossy brown skinks. I found no evidence of substantial avoidance behaviours in glossy brown and northern grass skinks to either rat or hedgehog odour. Two hypotheses are suggested to explain this. Firstly, there may not have been enough time for these species to evolve avoidance behaviours, and secondly, there may be insufficient selection pressure due to the high efficiency of the alien predators, or because rats and hedgehogs are active foragers and thus scent gives the prey limited information on immediate predation pressure. This lack of evidence for the evolution of antipredator behaviour, in addition to reduced body condition and population decline in areas with mammalian predators present, highlights the importance of intensive mammalian predator control for the continued survival of skink species on mainland New Zealand.