The Abundance and Pollen Foraging Behaviour of Bumble Bees in Relation to Population Size of Whortleberry (Vaccinium uliginosum) Carolin Mayer 1 *, Denis Michez 2 , Alban Chyzy 2 , Elise Bre ´ dat 2 , Anne-Laure Jacquemart 1 1 Earth and Life Institute, Catholic University of Louvain, Louvain-la-Neuve, Belgium, 2 Zoology Lab, University of Mons, Mons, Belgium Abstract Habitat fragmentation can have severe effects on plant pollinator interactions, for example changing the foraging behaviour of pollinators. To date, the impact of plant population size on pollen collection by pollinators has not yet been investigated. From 2008 to 2010, we monitored nine bumble bee species (Bombus campestris, Bombus hortorum s.l., Bombus hypnorum, Bombus lapidarius, Bombus pascuorum, Bombus pratorum, Bombus soroensis, Bombus terrestris s.l., Bombus vestalis s.l.) on Vaccinium uliginosum (Ericaceae) in up to nine populations in Belgium ranging in size from 80 m 2 to over 3.1 ha. Bumble bee abundance declined with decreasing plant population size, and especially the proportion of individuals of large bumble bee species diminished in smaller populations. The most remarkable and novel observation was that bumble bees seemed to switch foraging behaviour according to population size: while they collected both pollen and nectar in large populations, they largely neglected pollen collection in small populations. This pattern was due to large bumble bee species, which seem thus to be more likely to suffer from pollen shortages in smaller habitat fragments. Comparing pollen loads of bumble bees we found that fidelity to V. uliginosum pollen did not depend on plant population size but rather on the extent shrub cover and/or openness of the site. Bumble bees collected pollen only from three plant species (V. uliginosum, Sorbus aucuparia and Cytisus scoparius). We also did not discover any pollination limitation of V. uliginosum in small populations. We conclude that habitat fragmentation might not immediately threaten the pollination of V. uliginosum, nevertheless, it provides important nectar and pollen resources for bumble bees and declining populations of this plant could have negative effects for its pollinators. The finding that large bumble bee species abandon pollen collection when plant populations become small is of interest when considering plant and bumble bee conservation. Citation: Mayer C, Michez D, Chyzy A, Bre ´dat E, Jacquemart A-L (2012) The Abundance and Pollen Foraging Behaviour of Bumble Bees in Relation to Population Size of Whortleberry (Vaccinium uliginosum). PLoS ONE 7(11): e50353. doi:10.1371/journal.pone.0050353 Editor: Nigel E. Raine, Royal Holloway University of London, United Kingdom Received December 16, 2011; Accepted October 24, 2012; Published November 27, 2012 Copyright: ß 2012 Mayer et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors thank the Belgian Fund for Scientific Research [FRS-FNRS, www.frs-fnrs.be, contract no. 2.4540.09] for providing funding for this research and C. Mayer. The funder had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: carolin.mayer@uclouvain.be Introduction The destruction and fragmentation of formerly continuous plant communities is considered to be one of the major threats for plant- pollinator interactions [1]. Besides mere reduction of habitat size, fragmentation also implies increased isolation, edge effects, and reduced connectivity among different patches which could further enhance or alter the negative effects on plants and animals [2,3]. Within a pollination network, the loss of some dominant species can lead to concurrent decline or extinction of associated species [4]. Typically, plants may suffer from a reduced abundance and diversity of pollinators resulting in limited pollen transfer [5,6] and reduced reproductive success in fragments [7,8]. Pollinators, such as bees, can be affected by resource limitation and competition for food may increase in small habitat fragments [9,10]. Certain life history traits may render some pollinator species more sensitive to habitat loss: specialized species for example might not be able to shift to alternative host plants [11]. Large bee species require larger amounts of pollen to feed their larvae and would leave sites with low flower abundance first [12– 14]. On the other hand, smaller bee species unable to cover long distances [15,16] might be incapable of re-colonising fragments and could suffer more severely from increasing isolation of habitat fragments [3]. The majority of studies investigating effects of habitat fragmen- tation on plant communities concentrate on single plant species during only one flowering season [17,18] and typically focus on abundance and diversity patterns of flower visitors [19]. In the few cases where pollinator behaviour has been considered, only variables such as number of inflorescences and/or flowers per inflorescence visited, time spent per flower or search duration are analysed [8,20–22]. Other aspects of foraging behaviour related to diet breadth or food resource availability are still poorly investigated, especially for bees [19,23]. In this three-year study, we studied the effects of population fragmentation of Vaccinium uliginosum (whortleberry, Ericaceae) on the abundance, species richness and behaviour of its main pollinators, bumble bees, and how this affected plant reproductive success. Vaccinium uliginosum is a deciduous perennial shrub growing in bogs and is rare and threatened by land use and climatic changes due to this habitat preference [24]. It is present in Belgium only at higher altitudes, such as the Upper Ardenne (.500 m). Here, peat bogs have largely been destroyed to give PLOS ONE | www.plosone.org 1 November 2012 | Volume 7 | Issue 11 | e50353