COS 13-5 - CANCELLED - Low levels of cattle grazing result in the highest arthropod abundance in mixed-grass prairie rangeland

Background/Question/Methods

Herbivory plays a vital role in shaping the plant communities of nearly all grasslands worldwide, 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 arthropods. The Trophic Level Hypothesis states that the effects of herbivorous mammals on the plant community indirectly alter the arthropod community composition because arthropods are highly reliant on the food and shelter that plant hosts provide; large grazers typically reduce arthropod diversity. However, how cattle grazing alters the arthropod community has seldom been studied in the mixed-grass prairie ecosystem of Montana and in the context of changing cattle grazing regimes. 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 for two years 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

Cattle grazing significantly affected the average weight of arthropods in both sweep net and modified leaf blower samples. Specifically, using both methods of collection, low cattle grazing levels resulted in a higher average weight of arthropods compared to no and high cattle grazing. This pattern is seen for total weight (summing across all arthropods) and was mainly driven by orthoptera, where low cattle grazing resulted in a higher average weight of orthoptera compared to no grazing and high grazing. Finally, when comparing communities using Bray Curtis dissimilarity values, grazing treatment did not alter arthropod community composition (PERMANOVA not significant) or variability within the community (PERMDISP not significant) for either sampling method. Overall, the low cattle grazing treatment resulted in the highest abundance of arthropods in 2020, a particularly wet year in Montana. This suggests that low levels of cattle grazing increases orthoptera abundance when competition is potentially lower because of increased precipitation. The results here will assist in determining changing herbivory patterns in mixed-grass prairie rangelands which will have cascading consequences for the plant community.