Inferring trophic positions of generalist predators and their linkage to the detrital food web in agroecosystems: a stable isotope analysis

We investigated the feasibility of using stable isotope analysis of carbon and nitrogen to infer the trophic level of generalist arthropod predators and the relative strengths of their linkages to detrital and grazing food webs in agroecosystems. Generalist predators are potential biocontrol agents because they prey on herbivores in the grazing food web. Many of these predators also feed on detritivores and fungivores in the soil food web; thus, knowledge of this detrital trophic link may be instrumental to the effective manipulation of generalist predators to enhance their effectiveness in biological control. We analyzed patterns of isotopic concentrations of delta C-13 and delta N-15 in several groups of arthropod predators and their potential prey in replicated cucurbit gardens to which a detrital supplement had been added. Similarity in delta C-13 values between spiders and Collembola suggests that detritivores in this crop system may represent a key prey resource for small spiders, including sheet-web weavers (Linyphiidae) and juvenile wolf spiders (Lycosidae). Isotopic values of delta N-15 place spiders more than one trophic level above Collembola, perhaps dire to substantial intraguild predation and cannibalism. Patterns of delta C-13 and delta N-15 for carabid beetles reveal large interspecific variation in the extent of omnivory, in agreement with documented broad ranges of carabid feeding habits. We conclude that stable isotope analysis is a promising tool for investigating trophic connections in arthropod-dominated food webs in agroecosystems.