Weed seed availability, carabid intraspecific interference and their interaction drive weed seed consumption by carabid beetles

Abstract

Abstract Carabid beetles may play a role in the regulation of arable weeds, when seed predation is directly density‐dependent and consumption increases more than proportionally with seed density. We quantified the effect of seed and carabid density on consumption by two carabid species (Poecilus cupreus and Harpalus affinis) of the weed species Taraxacum officinale and Viola arvensis. For each pairing of carabid and weed species, we quantified per capita seed mortality (PCSM) over 72 h under combinations of low and high densities of carabids and seeds, aiming to assess whether per capita consumption increased more or less than proportionally with seed density. We tested three hypotheses: (i) PCSM is constant as seed availability increases; (ii) for a fixed seed density, PCSM decreases with increasing numbers of carabid conspecifics; and (iii) the response of PCSM changes with the interaction between seed and carabid density. Without intraspecific competition, we detected a seed density‐independent predation effect, which became inverse density‐dependent over time. At similar seed densities, the presence of conspecific carabids did not affect PCSM, although it led to a faster shift towards inverse density dependence in T. officinale. The findings of the present study suggest that a sufficiently large carabid numerical response might lead to direct predation density‐dependence.