Professor of Biology and Environmental Science Scripps and Pitzer Colleges Claremont, California
Climate change is increasing fire frequency and intensity in many ecosystems. Recent research shows fire can benefit bees by enhancing floral resources. Yet where non-native Apis mellifera compete with other pollinators, fire disturbance could favor the more generalist introduced species. I measured densities of A. mellifera, native bumble bees (Bombus spp.), and their floral resources for 21 years before and the first year after a 51,820 hectare fire on the California coast. I quantified post-fire changes in the most visisted flowering species and both bee taxa. In the first summer after fire (2021), Bombus reached the highest forager densities since monitoring began in 1999. At the same time, A. mellifera dropped to the lowest densities recorded since 2011. Flower numbers for the shrub most visited by A. mellifera (Eriophyllum staechadifolium) fell to near zero in burned areas. Conversely, floral abundances of herbaceous species preferred by Bombus (Phacelia malvifolia, Scrophularia californica, Stachys bullata) increased significantly. Even when controlling for variation in floral abundances and annual weather, A. mellifera densities declined steeply in 2021. Bombus densities in 2021 did not differ from pre-fire years after accounting for floral resources and weather effects. These results suggest that responses to fire can differ dramatically among bee species, reflecting both direct effects such as mortality and indirect effects mediated by changes in preferred floral resources. Such differential impacts of fire could alter interspecific competition between A. mellifera and native bees, although for how long remains uncertain.
