The effects of cattle grazing intensity on the arthropod community in a mixed-grass prairie rangeland

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Kathryn J. Bloodworth, Biology, University of North Carolina at Greensboro, Greensboro, NC, Kurt O. Reinhart, USDA-ARS, Miles City, MT and Sally E. Koerner, Department of Biology, University of North Carolina Greensboro, Greensboro, NC

Background/Question/Methods
Herbivory plays a vital role in shaping plant communities–altering plant growth rates and selecting for certain plant species or traits. Within an ecosystem, there are multiple herbivore types competing for resources, resulting in interactions between herbivores, such as those between mammals and insects. The ‘Trophic Level’ hypothesis states that the effects of herbivorous mammals on the plant community indirectly alter the insect community composition because insects are highly reliant on the food and shelter that plant hosts provide; large grazers in particular typically reduce insect abundances. However, the question of how cattle grazing intensity alters the arthropod community has seldom been studied in the mixed-grass prairie ecosystem of Montana. Therefore, here I address the question of how different cattle grazing intensities alter the arthropod community composition and biomass. Arthropod collection occurred in an established grazing experiment in Eastern Montana during peak growing season using two collection methods: a modified leaf-blower and transect-sweep net collections in three different cattle grazing intensities (none, low, high). Following collection, all arthropods were immediately stored in a freezer until identification. Following identification, arthropods were dried and weighed.
Results/Conclusions
Within sweep net samples taken across three cattle grazing treatments, six orders within the class Insecta and one order within the class Arachnida were found. Moreover, there were 1,128 total arthropod individuals, with more arthropods in no-grazing (420 individuals) compared to low (336 individuals) and high (342 individuals) grazing plots. When considering grasshopper species alone, there were 317 total grasshopper individuals with 82, 119, and 116 grasshopper individuals in no, low, and high-grazing plots, respectively, showing the opposite trend as when all arthropod individuals were assessed. However, while the abundance of grasshoppers was lower in no grazing plots, there were more unique species of grasshoppers in no-grazing (13 species) compared to low- and high--grazing plots (10 species each). This suggests that while grazing reduces overall arthropod abundance and grasshopper richness, it unexpectedly increases grasshopper abundance. This therefore supports the ‘Trophic Level’ hypothesis, with cattle grazing potentially altering the plant hosts that arthropods rely on, resulting in decreased richness and abundance of arthropods in cattle-grazed plots, however grasshopper abundance remains unexplained. Currently, we are seeing a global decline in arthropod abundance and the results here may begin to explain these declines through the lens of grazing treatments.