Introduction biodiversity of beneficial insects in semi-natural grassland patches within agricultural landscapes

Background/Question/Methods

Natural and semi-natural habitats are important to the diversity of insect predators in intensively managed agricultural landscapes because they provide resources, such as alternative prey sources and natural nesting habitats.  Semi-natural grasslands, such as those established as part of the Conservation Reserve Program (CRP), are potentially important to the conservation of natural enemies because they may complement or enhance resources in natural habitats.  We sampled the diversity and abundance of predatory insects (beetles and true bugs) and ants in 10 warm-season grasslands that were planted using a mix of warm season grasses and forbs.  The patches ranged in size from 1.2 to 17.8 ha and age since planting from 1 to 12 years.  We tested the hypothesis that insect species richness and composition are determined by patch size, age, and vegetative composition.  We predicted that larger, older, and more heterogeneous patches will lead to higher levels of insect species richness.  Pitfall traps 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 plots at each trap location along transect. 

Results/Conclusions

Ant species richness was positively related with patch age (p = 0.057, F = 4.89, df = 1, 7) and patch area (p = 0.053, F = 5.11, df = 1, 7), but neither the beetle nor bug richness was predicted by area or age. Ordination axes from non-metric multidimensional scaling showed that ant species composition varied along gradients of patch age (axis 1, r = -0.56), and grass cover (axis 3, r=0.60) vs. flowering forb density (axis 3, r = -0.85).  Species composition of true bugs was strongly influenced by a gradient of flowering forb density (axis 1, r = 0.57), whereas beetle species composition was primarily influenced by a gradient of patch area (axis 3, r = 0.50).  Our results suggest that patch area and age of semi-natural grasslands are the most important determinants of richness and composition of insect predators and ants, but taxa differed in their response to vegetative composition and floral resources, suggesting complementarily in insect predator composition among heterogeneous semi-natural habitats.