Abstract
This study examined population dynamics of jewelled geckos at the Eve1y Scientific Reserve (ESR), Otago Peninsula, New Zealand. Research was conducted between November 1998 and December 1999. Based on mark-recapture I estimated population size in 1998 and 1999. By comparing population size of 1998 to an estimate of 1994 (Shaw, 1994) I determined a population decline of about 50%. The aim of this study was to assess whether avian predation or habitat degradation are reasons of the observed decline.
To assess habitat quality in the ESR I compared survival and home range size with an unmanaged population at the Portobello Peninsula. To estimate effects of avian predation, I tested whether experimental exclusion of birds increases survival of jewelled geckos. Birds were partially excluded using birdnetting and survival was estimated using mark-recapture techniques. Estimates of home range size were based on location data obtained by radiotelemetry and mark-recapture. For the first time radio tracking of jewelled geckos was successfully implemented.
During the netting experiment between May and December 1999 I sampled 82 geckos: of these, 32 (39%) individuals were collected in the ESR, 50 (61%) individuals were members of the Portobello population, 28 (34%) geckos were found in netted habitats without birds and 54 (66%) geckos were found in control habitats with birds.
Gecko survival in the ESR was 0.88 (95%CI 0.82-0.92) and did not significantly differ from survival of geckos at the Portobello Peninsula 0.90 (95%CI 0.85-0.93). Survival in control areas (Portobello: 0.95 (95% Cl 0.86- 0.98), ESR: 0.89 (0.81- 0.94) was significantly lower than survival in netted habitats (Portobello: 0.61 (95%CI 0.37 - 0.88), ESR: 0.87 (95% Cl 0.73 0.93). Results show that the population decline in the ESR is unlikely to be a consequence of bird predation. They also suggest that gecko survival differs between microhabitats.
Between May and December 1999 a total of 675 location fixes (Portobello: 247, and ESR: 428) have been obtained to estimate home range size. Location data included location fixes of 17 untagged geckos (Portobello: 13, ESR: 14). Additionally, analysis incorporated location data of 11 radio tagged geckos (ESR: 6, Portobello: 5). Individual geckos were radio tracked between 10 to 29 days in November and December 1999.
Median home range sizes for geckos in the ESR were significantly larger than median home range sizes in Portobello. Results suggest that resource supply in the ESR is low. Low resources may be the result of increased habitat fragmentation. A suspected increase of energetically costly and risky interpatch movements might have increased mortality or emigration rates. Therefore, it is likely that sparse vegetation cover and related resource availability contributed to the population decline at the ESR.
Since increased vegetation cover may increase resource availability and therefore habitat quality in the ESR, planting of native vegetation should be of high priority. Mice and rabbit densities might be negatively affect vegetation cover and resource availability. Consequently mice and rabbit densities should be monitored and minimised.
To increase management efficiency, it is crucial to gain more knowledge regarding the requirements for food and vegetative cover. To facilitate immediate action, planting of vegetation should involve an experimental set-up, comparing different vegetation densities with mice, rabbits and gecko densities.