Reproduction and energy reserves of a predatory carabid beetle relative to agroecosystem complexity

During their lifetimes, mobile species are likely to experience a mosaic of habitat types offering a variety of food resources and shelter. The lifetime fitness of such species may depend on the quantity and quality of resources encountered in the different habitat types as the organisms move across the landscape. Because alternative farming practices can lead to contrasting degrees of resource availability among habitats, agroecosystems provide opportunities to address such questions experimentally. Here, I examine how landscape structure and pesticide use influence feeding rate, measured with the Energy Reserve Index (ERI) based on body mass and length, and fecundity of a generalist arthropod predator Pterostichus cupreus L. (Coleoptera, Carabidae) in an agroecosystem setting. Landscape structure was measured at the scale of the lifetime range of P. cupreus. Landscape was characterized by the number of fields within range of P. cupreus, the size distribution and perimeter-to-area ratios of arable fields, and the proportion of annual and perennial crops. Feeding rate, fecundity, and adult body size were compared among five farms, two conventional and three organic. For this generalist predator, fecundity and body size were correlated with the degree of landscape heterogeneity within its range of mobility. Localities with small fields, high perimeter-to-area ratios, and a high percentage of perennial crops harbored beetles that were larger and had almost three times higher fecundity than beetles from localities with low spatial complexity. Adults from all localities were food limited, but to different extents, with as much as 42% difference between localities. Effects of pesticides could not be separated, but this study indicates large effects of landscape composition on success of adults and larvae.