Urbanization effects on wild bee carbon and nitrogen stable isotope ratios in the Paris region

Abstract

Urban environments have been shown to have profound, yet still poorly understood effects on key ecosystem processes such as carbon (C) and nitrogen (N) cycling, as well as for key aspects of organism ecology, such as community assembly and network interactions. The study of natural abundances of C and N stable isotopes, 13 C and 15 N, can help infer mechanistic hypotheses on ecosystem processes at play in urban environments. No existing study has investigated whether a visible urban effect on soil and plant C and N biogeochemistry is being transferred to the animal compartment of urban ecosystems through trophic interactions. Here, we report 13 C and 15 N natural abundance values measured on three pollinating species of wild bees (Lasioglossum laticeps, Lasioglossum morio, and Lasioglossum nitidulum) sampled in 12 locations along an urbanization gradient in the Ilede-France region (Paris area, France). Our objective was to test whether already recorded isotopic patterns in urban soils and plants in the Paris area would also be visible in pollinators, and to reflect on whether this would be indicative of an urban isotopic signal being transferred to pollinators. Results indicate a significant effect of urbanization on the δ 13 C and δ 15 N for all three bee species, with enrichment for both 13 C and 15 N linked to increased urbanization. This could be linked to an urban imprint on the diverse plants on which pollinating insects forage on in the Paris area, or to other factors linked to the physiology and foraging strategies of insects. A better understanding of the drivers influencing pollinator δ 13 C and δ 15 N could help better understand urban biogeochemistry and trophic chains, as well as elucidate the origin of specimens and the migratory capacities of bee species, an important yet still elusive question considering the increase of habitat fragmentation.