Grazing and an invasive grass confound spatial pattern of exotic and native grassland plant species richness

Previous work has shown exotic and native plant species richness are negatively correlated at fine spatial scales and positively correlated at broad spatial scales. Grazing and invasive plant species can influence plant species richness, but the effects of these disturbances across spatial scales remain untested. We collected species richness data for both native and exotic plants from five spatial scales (0.5–3000 m²) in a nested, modified Whittaker plot design from severely grazed and ungrazed North American tallgrass prairie. We also recorded the abundance of an abundant invasive grass, tall fescue (Schedonorus phoenix (Scop.) Holub), at the 0.5-m² scale. We used linear mixed-effect regression to test relationships between plant species richness, tall fescue abundance, and grazing history at five spatial scales. At no scale was exotic and native species richness linearly related, but exotic species richness at all scales was greater in grazed tracts than ungrazed tracts. Native species richness declined with increasing tall fescue abundance at all five spatial scales, but exotic species richness increased with tall fescue abundance at all but the broadest spatial scales. Severe grazing did not reduce native species richness at any spatial scale. We posit that invasion of tall fescue in this working landscape of originally native grassland plants modifies species richness-spatial scale relationships observed in less disturbed systems. Tall fescue invasion constitutes a unique biotic effect on plant species richness at broad spatial scales.