Diet selectivity in a terrestrial forest invertebrate, the Auckland tree wētā, across three habitat zones.

Abstract

Insects are important but overlooked components of forest ecosystems of New Zealand. For many invertebrate species, information on foraging patterns and trophic relationships is lacking. We examined diet composition and selectivity in a large bodied invertebrate, the Auckland tree wētā, in three habitat zones in a lowland New Zealand forest. We asked if Hemideina thoracica selectively forage from available plant food sources, and if these choices were lipid-rich compared to non-preferred available plants. We also identified the proportion of invertebrates in their frass as a proxy for omnivory. From reconnaissance plot sampling, together with faecal fragment analysis, we report that more than 93% of individual wētā had eaten other invertebrates before capture. Wētā in the highest elevation hillslope habitat consumed significantly fewer plants on average than wētā on either the low elevation terrace or mid-slope habitat. Hillslope wētā also had the highest average number of invertebrate fragments in their frass, significantly more than wētā in the low elevation terrace habitat. Wētā showed high variability in the consumption of fruit and seeds across all habitats. Generally, we did not observe diet differences between the sexes (although it appears that male wētā in the mid hillslope habitat ate fruits and seeds more voraciously than females), suggesting that the sexes have similar niche breadths and display similar degrees of omnivorous behaviour. Extraction of leaf lipids demonstrated a range of lipid content values in available plants, and Ivlev’s electivity index indicated that plant species which demonstrated high electivity, tended to have higher concentrations of lipids in their leaves. Our findings indicate that H. thoracica forage omnivorously and selectively, and hence play multiple roles in native ecosystems and food webs.