Demographic benefits of early season resources for bumble bee (B. vosnesenskii) colonies
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The temporal distribution of resources is an important aspect of habitat quality that can substantially impact population success. Although it is widely accepted that floral resources directly influence wild bee population sizes, we lack experimental data evaluating how resource availability affects colony growth via demographic mechanisms. To achieve this, we tracked marked individuals in bumble bee (Bombus vosnesenskii) colonies to evaluate whether worker survival and reproduction responded to experimentally elevated forage early in colony development. Specifically, we assessed the effect of early resource environment on worker and sexual offspring production, and the survival and body size of individual workers. We also assessed whether responses of colonies differed when exposed to higher or lower resource environments at a relatively smaller (~ 10 workers) or larger (~ 20 workers) size. Resource supplementation always resulted in greater total offspring and male production; however, the influence of supplementation on worker production and quality depended on colony size at the start of supplementation. Among colonies that were initially smaller, colonies that were supplemented produced fewer but larger bodied and longer lived workers compared to control counterparts. Among colonies that were initially larger, colonies that were supplemented produced more workers than corresponding controls, but without changes to worker quality. Collectively, these results provide clear experimental evidence that greater resource availability early in colony development increases overall productivity, and indicate that colonies may pursue different allocation strategies in response to the resource environment, investing in more or better workers.
KeywordsBumble bee Demography Floral resources Life history Population dynamics
This project was supported by the National Science Foundation (DEB1354224 to EC and DEB1354022 to NW). We thank Jim Jackson and the UC Davis Plant Sciences Division for technical support in preparing and maintaining our study site, Hedgerow Farms (Winters, CA) for donated native plant seed, and a multitude of people, including technicians and students, who provided various forms of assistance in the field and laboratory: S. Glasser, N. Dorian, C. Fagan, J. Mola, J. Drost, A. Buderi, T. Zapalac, L. Cech, M. Epperly, K. Bolte, N.Kerr, C. Nye, K. Ward. We also thank two anonymous reviewers whose comments helped to improve this manuscript.
Author contribution statement
NW, EC, and RM collaborated on experimental design and methodology. RM and NW collected the data. RM and EC analyzed the data. RM led the writing of the manuscript with input from NW and EC. All authors contributed critically to the drafts and gave final approval for publication.
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