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

1. 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. 2. 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, topography), and biotic
associations among plant and pollinator composition to community assembly
of both trophic levels. 3. 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. 4. 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. 5. Synthesis: Wildfire disturbance and plant-pollinator
interactions both strongly influence 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.