Multiple plant traits influence community composition of insect herbivores: a comparison of two understorey shrubs

Abstract

Structural and nutritional plant traits influence the ability of insect herbivores to locate, consume and persist on their hosts yet it is uncommon for ecologists to consider how multiple plant traits influence insect community composition. We sampled herbivorous insects on two understorey shrub species common to eucalypt forests of south-eastern Australia, namely Cassinia arcuata (Asteraceae) and Daviesia ulicifolia (Fabaceae). Regression analyses were used to assess the relative influence of plant structure (canopy volume), nutritional quality (macronutrients and total phenolics) and plant productivity (leaf litter) on insect abundance and species richness. Total N content of D. ulicifolia was significantly higher than C. arcuata, while the concentrations of P, K, Ca and Mg were higher in C. arcuata. Total phenolics and leaf litter were significantly lower in D. ulicifolia compared to C. arcuata. Insect composition was similar between the two shrubs but C. arcuata supported greater abundances. Canopy volume and the macronutrients P and Ca were important predictors of insect abundance on C. arcuata, whereas canopy volume alone, but neither plant productivity nor macronutrients, influenced the abundance of insects on D. ulicifolia. Ca was an important predictor of insect species richness on C. arcuata and P was an important predictor on D. ulicifolia. By quantifying a range of plant traits, we have provided an understanding of factors likely to influence the composition of herbivorous insects inhabiting these two shrubs. Traits including leaf architecture, foliar morphology and volatile terpenoids may yet explain the greater number of insects on C. arcuata since they influence the availability of microhabitats and apparency of host plants.