The effects of grazing and site productivity on Carabid Beetles (Coleoptera: carabidea) in a semi-arid grassland

The grasslands of British Columbia (BC), Canada are an important resource for the ranching industry, even though they occupy less than one percent of BC‟s land area. Cattle grazing can alter the structure and composition of the plant community, which may indirectly affect insect communities. Insects are an integral component of grassland ecosystems; for example, carabid beetles (Coleoptera: Carabidae) prey on lower trophic levels while providing food resources for upper trophic levels. In the semi-arid grasslands of southern BC, soil-water availability increases with elevation, and plant community composition differs along the elevation (productivity) gradient. I investigated the effects of cattle grazing and productivity on ground beetle abundance, dried weight (biomass), species richness and diversity. Over three sessions of pitfall trapping in 2008 in Lac Du Bois BC Provincial Park, 600 individuals of six carabid species were captured. To test for main and interacting effects of elevation, grazing, and month of capture, carabid beetles were quantified by trap for abundance, dried weight (biomass), species richness and Shannon‟s diversity. I found that elevation was the most important predictor of carabid abundance, biomass, species richness and diversity. Lower elevation had an average abundance of 1.04 carabids, dried weight of 25.50 mg, species richness of 0.46 and Shannon diversity of 0.06, compared to upper elevation with an average abundance of 2.88 carabids, dried weight of 113.71 mg, species richness of 1.00 and Shannon diversity of 0.15. Grazing had a significant effect on carabid diversity with higher diversity in upper elevations. There were significant effects of month of capture on carabid beetle biomass, species richness, and Shannon diversity. To examine energy differences, calorimetry experiments were performed on the seeds of four dominant grasses (Poa sandbergii, Pseudoroegenaria spicata, Festuca campestris, and Poa pratensis) and on four carabid beetles (Carabus taedatus, Calosoma moniliatum, Amara obesa, and Cymindis borealis). Average calories per gram were significantly different between two dominant species of carabid, Cymindis borealis (3114.741 cal/g), and Carabus taedatus (5321.862 cal/g). Grass species did not differ in calories per gram of seed. Caloric value (calories per gram x gram) of carabids and seeds were higher in the upper grasslands compared to the lower grasslands. As one of three Provincial parks that protect natural grasslands, the management of Lac Du Bois benefits by considering biodiversity of all biota, including invertebrates.