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Using stable isotopes to show that an herbivorous beetle from alpine rock outcrops relies on ammonia-absorbing lichens

Date
2018
Type
Conference Contribution - published
Keywords
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Abstract
The endemic beetle Protodendrophagus antipodes (Coleoptera: Silvanidae) is unique among 12 globally distributed genera of the tribe Brontini in exchanging the forest for an alpine existence. Larvae from other Brontini genera consume fungi found underneath loose bark, but since P. antipodes lives well above the treeline, this tree-based fungal diet is clearly impossible. Because lichens are composite organisms comprising fungi and either algae or cyanobacteria, we proposed that lichen consumption could be the dietary link between P. antipodes and the remaining fungivorous Brontini. Our samples came from rock outcrops at 1800 m on Mt Hutt ski field at the eastern edge of the Southern Alps. Primary producer δ15N values fell into two clusters; crustose and fruticose lichens at very low δ15N, and mosses, vascular plants and foliose lichens close to 0‰. Adult and larval P. antipodes δ15N values were 1-3‰ higher than crustose and fruticose lichens and so consistent with these lichens as their food source. Lichens depend for their N on either atmospheric deposition or atmospheric N2 fixation. The remarkably negative δ15N values of the crustose and fruticose lichens at our site best matched gaseous ammonia as their N source. Ammonia sources to New Zealand alpine environments may have changed, as ammonia from increasingly intensive agriculture has replaced that from formerly widespread surface-nesting river bird colonies. If P. antipodes is a significant prey animal for alpine lizards, birds and predatory arthropods, this unusual beetle would be an important conduit for lichens and atmospheric ammonia in supporting alpine biodiversity.
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