Size matters: trap size primarily determines prey spectra differences among sympatric species of carnivorous sundews

Abstract

Even though carnivorous plants (CPs) are a popular focus of ecological research, surprisingly few studies have investigated their prey spectra (the number and composition of captured prey). This knowledge gap has important implications for our understanding of sympatric speciation processes in CPs and may potentially hinder effective conservation and ecological restoration efforts. We applied a novel photography-based analysis method to characterize the in situ prey spectra of eight species from Drosera sect. Arachnopus, including five species that were studied across multiple populations in northern Australia. The prey spectra of all studied species predominantly comprised of flying insects, with small Nematocera (Diptera) being the most common prey group across all species. While the prey spectra of most species varied significantly among locations, differences in prey spectra among sympatric species were most strongly determined by trap size. The number of prey captured per plant and per centimeter of trapping leaf was strongly associated with increasing leaf length, and species with larger trapping leaves also captured comparatively greater numbers of large prey items than species producing smaller trapping leaves. Although niche segregation in prey spectra was not observed at any of the study sites, at one location D. fragrans (a species producing a strong, honey-like scent from trapping leaves) was found to capture significantly more winged Hymenoptera than the unscented sympatric D. aquatica, potentially indicating selective prey attraction in D. fragrans. Small species (such as D. nana) captured a disproportionally low amount of prey, despite being relatively widespread over large areas of northern Australia. Results indicate that carnivory may not have been a primary driver of diversification in D. sect. Arachnopus.