Biotic and abiotic drivers of plant–pollinator community assembly across wildfire gradients

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Abstract Understanding how abiotic disturbance and biotic interactions determine pollinator and flowering‐plant diversity is critically important given global climate change and widespread pollinator declines. To predict responses of pollinators and flowering‐plant communities to changes in wildfire disturbance, a mechanistic understanding of how these two trophic levels respond to wildfire severity is needed. We compared site‐to‐site variation in community composition (β‐diversity), species richness and abundances of pollinators and flowering plants among landscapes with no recent wildfire (unburned), mixed‐severity wildfire and high‐severity wildfire in three sites across the Northern Rockies Ecoregion, USA. We used variation partitioning to assess the relative contributions of wildfire, other abiotic variables (climate, soils and topography) and biotic associations among plant and pollinator composition to community assembly of both trophic levels. Wildfire disturbance generally increased species richness and total abundance, but decreased β‐diversity, of both pollinators and flowering plants. However, reductions in β‐diversity from wildfire appeared to result from increased abundances following fires, resulting in higher local species richness of pollinators and flowers in burned than unburned landscapes. After accounting for differences in abundance, standardized effect sizes of β‐diversity were higher in burned than unburned landscapes, suggesting that wildfire enhances non‐random assortment of pollinator and flowering‐plant species among local communities. Wildfire disturbance mediated the relative importance of mutualistic associations to β‐diversity of pollinators and flowering plants. The influence of pollinator β‐diversity on flowering‐plant β‐diversity increased with wildfire severity, whereas the influence of flowering‐plant β‐diversity on pollinator β‐diversity was greater in mixed‐severity than high‐severity wildfire or unburned landscapes. Moreover, biotic associations among pollinator and plant species explained substantial variation in β‐diversity of both trophic levels beyond what could be explained by wildfire and all other abiotic and spatial factors combined. Synthesis. Wildfire disturbance and plant–pollinator interactions both strongly influenced the assembly of pollinator and flowering‐plant communities at local and regional scales. However, biotic interactions were generally more important drivers of community assembly in disturbed than undisturbed landscapes. As wildfire regimes continue to change globally, predicting its effects on biodiversity will require a deeper understanding of the ecological processes that mediate biotic interactions among linked trophic levels.