Regional species pools interact with local community assembly mechanisms to shape plant and pollinator beta-diversity across wildfire-disturbance gradients
Community assembly is hypothesized to reflect the interplay of deterministic processes such as niche-based environmental filtering, stochastic processes leading to ecological drift, and dispersal from the regional species pool. To test the relative importance of these processes, ecologists often examine how environmental filters such as disturbance alter variation in community composition (beta-diversity). However, because disturbance can alter beta-diversity by changing the relative importance of community assembly mechanisms, local numbers of individuals (community size), or the composition of the regional species pool, most studies have been unable to unambiguously determine the mechanisms by which disturbance influences biodiversity, especially among biogeographic regions where species pools vary. Here, we examined whether disturbance systematically alters the relative importance of community assembly mechanisms in co-occurring plant and pollinator communities across a large-scale biogeographic gradient in Montana, USA. In each of three regions that varied in the species-pool diversity of plants, we compared patterns of plant and pollinator beta-diversity among communities within unburned landscapes and landscapes disturbed by wildfires. To test whether disturbance alters stochastic community assembly, we used a null-model approach to compare how disturbance influenced non-random patterns of beta-diversity after controlling for differences in community size and species pools within and among regions.
Wildfire disturbance altered non-random patterns of beta-diversity in both plant and pollinator communities. However, the direction and magnitude of this effect varied between trophic levels and biogeographic regions. In pollinator communities, disturbance increased non-random assembly in all regions. In plant communities, in contrast, disturbance had no systematic influence on community assembly in the highest-diversity region, but decreased non-random assembly in the lowest-diversity region. Our results suggest that regional variation in disturbance-diversity relationships can be partly explained by biogeographic differences in species pools and stochastic effects of disturbance on local community size. In addition, the contrasting responses of plants and pollinators to wildfire may reflect shifts in the relative importance of local-scale dispersal limitation and environmental filtering across trophic levels. We highlight implications of these results for understanding how patterns of beta-diversity within trophic levels may scale-up to influence the beta-diversity of trophic interactions at local and biogeographic scales.