Direct and Indirect Effects of Grazing by Introduced Mammals on a Native, Arboreal Gecko (Naultinus gemmeus)

Abstract

We examined relationships between livestock grazing and the activity of introduced rodents and population density of Jewelled Geckos, Naultinus gemmeus (Diplodactylidae) on the Otago Peninsula, South Island, New Zealand. We expected livestock grazing to prevent the proliferation of rank grass (which is often associated with high rodent densities) and thereby reduce the frequency of predation by rodents on Jewelled Geckos. Thus, we predicted activity of rodents to be lower, and density of Jewelled Geckos higher, at sites grazed by livestock. We tested this for two habitats: Coprosma spp. shrubland and regenerating kānuka (Kunzea ericoides) forest. We quantified density of Jewelled Geckos using visual searches, photography and mark–recapture (POPAN formulation). We used tracking tunnel surveys to estimate the activity of introduced rodents. The activity of rodents (Rattus spp. and Mus musculus) was significantly higher at ungrazed compared to grazed sites in both habitats. Density of Jewelled Geckos in Coprosma was over four times higher at grazed sites and decreased with increasing rodent activity; however, in kānuka, we found a contrasting result with density being significantly higher at the ungrazed sites. We infer that in ungrazed Coprosma, rank grasses support high rodent densities and, consequently, high predation of Jewelled Geckos. Thus, grazing by introduced livestock may promote the survival of Jewelled Geckos and the persistence of high density populations in Coprosma shrubland. In contrast, grazing in regenerating kānuka may impact on Jewelled Geckos negatively, possibly by removing thermal refugia.