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 m2 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.
Highlights
The destruction and fragmentation of formerly continuous plant communities is considered to be one of the major threats for plantpollinator interactions [1]
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 on the abundance, species richness and behaviour of its main pollinators, bumble bees, and how this affected plant reproductive success
The finding of changes in pollen collecting behaviour by bumble bees according to plant population size is novel and should receive further attention
Summary
The destruction and fragmentation of formerly continuous plant communities is considered to be one of the major threats for plantpollinator interactions [1]. 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]
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