Eating at altitude: Using stable isotopes to unravel the feeding ecology of Protodendrophagus antipodes (Coleoptera: Silvanidae)

Abstract

Protodendrophagus antipodes is an endemic, silvanid beetle known from ranges along much of the length of the New Zealand Southern Alps. The beetle is unique among the tribe Brontini, for which there are 12 globally-distributed genera, in adopting an alpine existence. All other Brontini genera live in forest habitat underneath loose bark, where they consume fungi. Since P. antipodes lives well above the treeline, this tree-based, fungal diet is not possible. Based on abundance of lichens in typical P. antipodes habitat and, in some instances, the absence of any obvious alternative food source, we hypothesised that P. antipodes had made a dietary shift to lichen feeding. Lichen consumption was analysed initially by microscopic examination of gut contents. The only identifiable items were scattered ascospores of the crustose lichens, which provided some support for lichen feeding, however, the gut contents predominantly consisted of unrecognisable amorphous material. We then used an indirect method, the measurement of carbon and nitrogen stable isotope ratios. Beetle samples and associated soil, plant and invertebrate material were collected from rock outcrops at around 1800 m altitude on Mt Hutt ski field at the eastern edge of the Southern Alps. Isotopic analysis results were consistent with crustose and fruticose lichens being the food source for P. antipodes adults and larvae. Our results are one of the few unequivocal examples of lichen feeding in beetle larvae, and highlight the value of stable isotope analysis in assessing arthropod food sources.