Bees and Butterflies in Burned and Unburned Alvar Woodland: Evidence for the Importance of Postfire Succession to Insect Pollinator Diversity in an Imperiled Ecosystem

Abstract

The apparent importance of successional habitat to pollinating insects, specifically bees (Hymenoptera) and butterflies (Lepidoptera) was quantified in an alvar landscape in the Ottawa valley through a comparison of burned and unburned alvar woodland. The two adjacent habitats on the same successional gradient were sampled by sweeping with additional data from pitfall traps for bees and by direct observation with close focus binoculars and occasional verification through capture with a net for butterflies. The sampling was done during 11 visits in 2008 beginning 16 May and ending 13 September. Both bee and butterfly diversity were higher in the post-fire burned alvar woodland compared to the adjacent unburned woodland based on species richness, number of individuals and Brillouin’s Biodiversity Index which takes evenness and heterogeneity into account. No bees were captured in the unburned area, but 34 species and 201 individuals were captured in the burned site. The most abundant bee species was Augochlora aurata. Lepidoptera were represented in the burned site by 35 species and 408 individuals compared to 15 species and 21 individuals in the unburned woodland. The most common butterfly species in the burned woodland was Callophrys polios. The higher diversity of pollinators in the burned site was correlated with both higher vascular plant diversity and much higher cover and frequency values for insect-pollinated plants providing nectar and pollen including flowering shrubs such as Amelanchier alnifolia var. compacta, Arctostaphylos uva-ursi and Prunus virginiana. The burned site also provided more cover of larval food plants for butterflies and apparently more nesting sites for bees. We suggest that a decrease in fire frequency and in the availability of open successional habitats are contributing factors in the decline of pollinators, and that endangered ecosystems where fire has been a natural phenomenon may require fire or fire-simulated management to sustain their biodiversity.