Abstract
Understanding factors that influence the abundance of native lizards in ecosystems modified by invasive species is important, as today these ecosystems are globally abundant. Relationships between vegetation composition, livestock grazing and indicators of mammalian predation were examined for the native jewelled gecko, Naultinus gemmeus (Diplodactylidae) on the Otago Peninsula, South Island, New Zealand. Livestock grazing was expected to prevent the proliferation of rank grass (which is often associated with high rodent densities) and thereby reduce the frequency of predation by introduced rodents on jewelled geckos. Thus, activity of rodents was predicted to be lower and density of jewelled geckos higher at sites grazed by livestock. This was tested for two habitat types: shrubland dominated by Coprosma spp. and coastal forest dominated by kānuka (Kunzea ericoides). Abundance, density, detectability and incidence of tail loss in jewelled geckos were estimated using visual searches, photography and mark-recapture (POPAN formulation). Tracking tunnel surveys were undertaken at several sites to estimate the activity of introduced rodents. Selected habitat characteristics that were predicted to influence the density of jewelled geckos were measured and related to density using an information-theoretic approach.
The activity of rodents (Rattus sp. and Mus musculus) was significantly higher at ungrazed compared to grazed sites in both Coprosma shrubland and kānuka forest. Density of jewelled geckos in Coprosma was significantly (over four times) higher at grazed sites, although there was no significant difference in density between grazed and ungrazed kānuka forest. Incidence of tail loss in jewelled geckos was significantly higher in ungrazed Coprosma shrubland, possibly due to a greater number of predation attempts by rodents. Density of jewelled geckos decreased with increasing grass height in Coprosma shrubland. Jewelled geckos favoured dense divaricating shrubs (particularly Coprosma spp., Corokia cotoneaster and Helichrysum glomeratum) and used these plants significantly more often than other plants relative to their availability.
On Otago Peninsula, livestock may facilitate the persistence of high-density populations of jewelled geckos in Coprosma by reducing the abundance of their major predators. As such, controlled grazing can provide a useful means for conserving jewelled geckos. Predator control (particularly in the absence of grazing), controlled grazing (to reduce rodent abundance and therefore predation) and increasing the area of suitable jewelled gecko habitat (including the formation of habitat corridors) are considered priorities for gecko conservation.
In New Zealand, the fencing-off of bush fragments is often advocated due to the potential botanical benefits (e.g. increased regeneration) associated with livestock removal. However, this research indicates that in the absence of on-going, long-term rodent control it may often be beneficial to maintain grazing at Coprosma sites in order to preserve jewelled gecko populations. This study increases our understanding of the factors that influence the abundance of native lizards in ecosystems modified by invasive species and outlines the potential value of controlled grazing regimes as a means of predator control for conservation benefit. The effects of stock removal on the mammalian predator guild and on the native species present should be a concern in all conservation planning in New Zealand.