COS 273-5 - Plant community assembly in a semi-arid grassland is jointly influenced by herbivory and environmental factors.

Abstract: The assembly of plant species in a plant community is guided by stochastic and deterministic processes. It is likely that these processes act jointly in a plant community, with the importance of either process changing due to ecological factors. Herbivory has been shown to affect plant community assembly (i.e., the process of structuring of plant communities), but the strength of the effects of herbivory may differ due to the length of grazing, and environmental factors. Using paired plots within and outside exclosures, we sought to understand the role of grazing and environmental factors as drivers of plant community assembly. We selected 25 sites from a semi-arid grassland ecosystem across the interior of British Columbia, Canada, covering a broad range of environmental conditions. The selected sites had fenced exclosures, which excluded grazing by ungulates. The exclosures were established between 21 – 84 years ago, covering a wide chronosequence of grazing exclusion treatment. We conducted plant surveys along parallel 10m transects inside and outside the exclosure. We placed 0.25 × 0.25 m2 quadrats at 1 m intervals of 1 m along the transects. We used the "Hellinger" transformed count data from the plant survey to calculate the modified Raup-Crick index (RC index). A positive RC index implies that the community assembly is more dissimilar than expected, implying a more divergent community assembly. A negative RC index signifies a more similar community than expected, representing a converging community. We investigated the community assembly process by examining the impact of the exclosure, exclosure age, mean annual temperature (MAT), and mean annual precipitation (MAP) on the RC index of plants.
Plant communities outside exclosures were more divergently assembled than expected, while those inside exclosures were more convergently assembled than expected compared to the null expectation. This divergence and convergence within and outside exclosures, respectively, intensified with grazing length. From low to high elevation and precipitation, plant communities are transitioned from convergence to divergence regardless of the existence of herbivory. Higher resource presence at higher elevation communities may allow for the co-existence of a variety of species, which can be partitioned into various niches, reducing biotic filtering. Our results show that grazing exclusion may play a role in homogenizing plant communities, while increasing environmental heterogeneity may reduce convergence and increase divergence in community assembly.