Conserving bee habitat requires detailed information about habitat use, which may require multiple habitats to meet their nesting and nutritional requirements. Therefore, a landscape-scale perspective is needed when making conservation management decisions. Traditional methods of bee monitoring are time-intensive and are highly sensitive to local flower availability and weather conditions. The small amounts of naturally-occurring heavy isotopes of C and N can be used in ecological studies to track spatial and temporal changes in animal diets. Stable isotopes have the benefit of integrating diet information over the individuals’ life, rather than providing an instantaneous assessment. If bees’ isotope signature can be used to predict habitat use at the landscape scale, it will introduce a new tool for tracking bee foraging. We examined C and N isotope ratios of bees and flowers collected from early spring to late summer in two locations with different environments (arboretum and reclaimed mine). Based on differential isotope fractionation of plants grown under conditions of variable water- and nutrient stress, we hypothesize that flower tissue will differ in ∂13C and ∂15N by habitat and sample month. Bee tissue ∂13C and ∂15N will reflect spatial and temporal differences in flower isotope fractionation.
The isotopic ratios of flowers collected in late summer were significantly enriched in 13C from those early in the spring, but 15N did not track predicted seasonal nitrogen availability. Flowers from the reclaimed mine were significantly enriched in 13C, but not in 15N. ∂13C of bees differed across seasons, while ∂15N were inconsistent across the season. The bees collected early in the spring on the arboretum site had significantly different ∂13C than those collected later in the season, but ∂15N did not vary consistently across the season. This pattern was true for both solitary bees and bumble bees on arboretum land, but only for bumble bees on reclaimed mine land. The largest differences in isotopic ratios that we found, however, were among locations. The bees from reclaimed mine sites showed significantly enriched ∂13C and significantly depleted ∂15N relative to the arboretum bees, regardless of habitat. Bee and flower isotopic ratios showed congruence between the two locations, but not across seasons. This result suggests that C and N stable isotopes may be of more use at large spatial scales than at spatial scales at which bees forage.