Allometric constraints on carabid diets: interspecific differences in carabid-to-seed mass ratios impact seed choice

Abstract

Seed choice in seed-feeding omnivorous carabid beetles (Coleoptera: Carabidae) is influenced by numerous ecological factors, including the chemical and physical properties of seed species. Previous work has shown that seed chemistry can drive seed selection decisions by carabids only when the size and mass of seed species are within certain limits. In a model system composed of eight carabid species and seeds of three brassicaceous weeds, we explored predator-to-seed mass relationships and their impact on seed choice by carabids. We show that carabid-to-seed mass ratio scaling relationships are likely to drive seed choice when seed species of different mass are presented to carabid species varying in body mass. Smaller seed species in the experiments were more preferably chosen by smaller carabid species, and mass of the preferable seed species increased as a function of the body mass of carabid species. The taxon-specific mass of carabid predators in relation to the species-averaged mass of available seeds was the main driver of seed choice decisions in the model system under study. These mass-driven changes in seed preferences suggest that feeding interactions between carabid and seed species in agricultural fields are likely driven by mass-structured dynamics. Given this, the intensity of predation pressure imposed by carabids on seed species in the field may potentially be determined by the match/mismatch between the distribution of seed mass in the weed community and the structure of functional body mass in the carabid community.