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 a nd the relative strengths of their linkages to detrital and grazing food we bs in agroecosystems. Generalist predators are potential biocontrol agents because they prey on herbivores in the grazing food web. Many of these pred ators also feed on detritivores and fungivores in the soil food web; thus, knowledge of this detrital trophic link may be instrumental to the effectiv e manipulation of generalist predators to enhance their effectiveness in bi ological control. We analyzed patterns of isotopic concentrations of delta C-13 and delta N-15 in several groups of arthropod predators and their pote ntial prey in replicated cucurbit gardens to which a detrital supplement ha d been added. Similarity in delta C-13 values between spiders and Collembol a suggests that detritivores in this crop system may represent a key prey r esource for small spiders, including sheet-web weavers (Linyphiidae) and ju venile wolf spiders (Lycosidae). Isotopic values of delta N-15 place spider s more than one trophic level above Collembola, perhaps dire to substantial intraguild predation and cannibalism. Patterns of delta C-13 and delta N-1 5 for carabid beetles reveal large interspecific variation in the extent of omnivory, in agreement with documented broad ranges of carabid feeding hab its. We conclude that stable isotope analysis is a promising tool for inves tigating trophic connections in arthropod-dominated food webs in agroecosys tems.