Local and landscape determinants of species diversity and composition of beneficial insects in agricultural environments

Background/Question/Methods

Natural and semi-natural habitats are important to the diversity of pollinating and predatory insects in intensively managed agricultural landscapes because they provide resources, such as nectar, alternative prey, and nesting habitats.  We studied the species richness and composition of bees (Hymenoptera: Apoidea) and predatory beetles (Coleoptera) in 10 warm-season grasslands that were established as part of the USDA Conservation Reserve Program and other wildlife enhancement incentives.  The grassland patches ranged in size from 1.2 to 17.8 ha and age since planting from 1 to 12 yrs.  Pitfall and combination pan/flight intercept traps were established along a single transect at each site, with the number of traps scaled to log area of the patch.  Vegetative cover and flowering stems were recorded in two 10-m² circular plots at each trap location along each transect.  Land cover of the surrounding landscape was digitized from orthodigital photos (3-m resolution) and mapped using GIS at different radial distances from the focal patch.  We tested the hypothesis that species richness and composition of bees and predatory beetles were determined by patch size, age, and vegetative composition.  We also tested the hypothesis that the surrounding land use/cover types would influence insect species richness and composition.

Results/Conclusions

Poisson regression models showed that bee species richness was positively related to patch area (Deviance = 4.14, df = 1,8, p = 0.0418) and to the proportion of semi-natural land cover within 525m of the patch (Deviance = 4.16, df = 1,8, p = 0.041).  Species richness of predatory beetles was unrelated to any local patch measures, but was negatively related to the proportion of intensive agriculture within 130m of the patch (Deviance = 4.03, df = 1,8, p = 0.044).  Ordinations using distance-based redundancy analysis with patch variables showed that patch area explained 22% of the variance of bee species composition, while 19% of variation in beetle species composition was explained by forb cover.  Ordinations using surrounding landscape variables revealed that intensive agriculture within 525m of the patch explained 23% of the variation in bee species composition, while intensive agriculture within 740m of the patch explained 17% of the variation of beetle species composition.  We conclude that larger semi-natural grasslands with more abundant forb cover can support diverse assemblages of beneficial insects in agricultural landscapes, especially if grassland patches are in landscapes with other semi-natural and natural habitats.