LAUSR

Biotic homogenization (BH), the process by which β-diversity erodes, is suspected to be severe among plant communities due to widespread anthropogenic disturbances. However, few studies have directly linked anthropogenic disturbance with patterns of BH. The US National Wetlands Condition Assessment (NWCA) provides an opportunity to investigate patterns of β-diversity related to anthropogenic disturbances. Our objectives were to compare β-diversity between highly, intermediately and less-disturbed herbaceous emergent wetlands across the temperate region of the conterminous US, and to identify species with greatest influence on β-diversity patterns within disturbance categories. Using species occurrence and abundance data, average distances to group centroids in ordination space were used to assess wetland β-diversity across a disturbance gradient. Species contributions to β-diversity were calculated per disturbance category. Compared to the least and intermediately disturbed wetlands, highly disturbed wetlands had significantly lower β-diversity based on species abundances across the study area. When using only species occurrences, highly disturbed wetlands had significantly lower β-diversity than intermediately disturbed wetlands. No significant differences were found amongst disturbance categories at the within-site scale. The invasive grass Phalaris arundinacea had the greatest influence on β-diversity, especially in the most disturbed wetlands. Our results demonstrate that anthropogenic disturbances have homogenized regional botanical diversity. This homogenization is a function of disturbance intensity, which, after a threshold is crossed creates environmental conditions that lead to the local dominance of a single, widespread invader. This process is repeated across the region, scaling up to homogenize the flora of the entire region.