Testing the utility of stable isotope analysis for analyzing bee foraging patterns across habitats

Abstract

Concern about declining bee populations has stimulated interest in promoting habitat for bees and other pollinators, but a better understanding of habitat requirements of diverse bee species is needed. Traditional methods of studying bee foraging are labor intensive and limited spatially, temporally, and to certain bee genera. I tested the utility of stable isotope analysis, a new method for tracking the habitat origin of bee nutritional resources by comparing stable isotopes present in bee tissue to those in distinct foraging habitats. In a preliminary study, I showed that stable isotope analysis could distinguish bee habitat use over small spatial scales. I analyzed 21 native bee species collected in forests and old fields to determine if isotopic ratios of body tissue reflected that of flowers in their foraging habitat. Discriminate analysis assigned the bees to habitats with respect to δ15N and δ13C with 86% accuracy. Some of the variation in isotopic signature may come from species differences and the integration of larval and adult diet across years. In a second study, therefore, I investigated isotopic signatures of a single taxon, bumble bee workers, caught during two distinct seasons: spring and the mid-late summer, to determine what role time of capture and diet play in influencing isotopic signature. Early summer bees fed on early spring resources (mainly in forests), while mid-summer bees fed on early summer resources (mainly in fields). Therefore, isotopic signatures should differ based on time of capture due to spatial separation of floral resources available at different times in the season. I collected 35 bumble bees and 75 flowers in 2016. Samples were identified, dried, pulverized, before being analyzed by an isotope ratio mass spectrometer. The results should help illuminate stable isotope effectiveness in tracking bee habitat use and as a tool in conservation ecology.