Fire‐induced change in floral abundance, density, and phenology benefits bumble bee foragers

Abstract

AbstractFire is a dominant, and well‐studied, structuring force in many temperate and semi‐arid communities; yet, few studies have investigated the effects of fire on multi‐trophic interactions. Here, we ask how fire‐induced changes in flowering affect the abundance of bumble bee foragers (Bombus vosnesenskii) and whether differences in floral resource availability are due to changes in plant species composition or lengthened bloom of a consistent set of species within burned and unburned grasslands. Following fire, burned and unburned sites had similar early spring bee and floral abundances. However, after the early bloom, forager activity remained high only in burned sites, where floral abundance persisted for longer. Importantly, the increased floral abundance following fire was due to a lengthening of within‐species flowering phenology, as burned areas later in the season retained floral abundance and composition similar to that of unburned areas early in the season. Furthermore, density of flower patches chosen by bumble bee foragers was significantly higher at burned sites, suggesting an increase in patch quality for foragers in post‐fire communities. Our results suggest positive effects of fire for bumble bee foragers and forb communities in California grassland ecosystems in the year following disturbance, namely through differences in plant phenology and floral density. We conclude that fire‐induced changes in flowering phenology can alter interspecific interactions and benefit pollinators.