Relative importance of sugar resources to endemic gecko populations in an isolated island ecosystem

In many ecosystems food-web dynamics are driven by spatial and temporal variation in the availability of sugar resources, which form the primary or even exclusive dietary constituents for many species. Scale insects (Hemiptera) produce sugar-rich honeydew, which can be a keystone sugar source in honeydew ecosystems worldwide. In New Zealand, most previous research in honeydew ecosystems has been conducted in areas where herpetofauna are heavily suppressed by introduced predators. Consequently, little is known about potential trophic interactions between endemic lizards and scale insects. Korapuki Island is one of the few remaining locations in New Zealand where endemic scale insects and lizards survive in densities likely to be representative of prehuman conditions. We examined the relative importance of different sugar resources on Korapuki Island to Duvaucel's geckos (Hoplodactylus duvaucelii) and common geckos (Woodworthia maculatus). We recorded the abundance and morphometrics of geckos attending five sugar-producing plant species (two of which host honeydew-producing scale insects) three times daily along a fixed transect. Large numbers of Duvaucel's and common geckos were recorded nocturnally feeding on honeydew produced by the scale insect Coelostomidia zealandica (Coelostomidiidae). Duvaucel's geckos of all sizes and genders fed extensively on honeydew throughout the year, favouring ngaio (Myoporum laetum) trees with high scale insect infestations, but were seldom recorded at other sugar resources. In contrast, juvenile common geckos were infrequently recorded on honeydew-producing trees. Common geckos fed on a variety of other sugar resources, with all sizes and sexes abundant on nectar and sap of flax (Phormium tenax) and seasonally exploiting nectar of pohutukawa (Metrosideros excelsa). The strength of interactions between scale insects and geckos, particularly for the Duvaucel's gecko population on Korapuki Island, indicates the importance of honeydew in addition to more ephemeral sugar resources such as nectar. Accordingly, the re-establishment of honeydew-producing Hemiptera populations should be considered in future conservation and restoration plans.