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Niche overlap and diet breadth in bumblebees; are rare species more specialized in their choice of flowers? Article (Published Version) Goulson, Dave and Darvill, Ben (2004) Niche overlap and diet breadth in bumblebees; are rare species more specialized in their choice of flowers? Apidologie, 35 (1). pp. 55-63. ISSN 0044- 8435 This version is available from Sussex Research Online: http://sro.sussex.ac.uk/id/eprint/51244/ This document is made available in accordance with publisher policies and may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher’s version. Please see the URL above for details on accessing the published version. Copyright and reuse: Sussex Research Online is a digital repository of the research output of the University. Copyright and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable, the material made available in SRO has been checked for eligibility before being made available. Copies of full text items generally can be reproduced, displayed or performed and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. http://sro.sussex.ac.uk Apidologie 35 (2004) 55–63 © INRA/DIB-AGIB/ EDP Sciences, 2004 55 DOI: 10.1051/apido:2003062 Original article Niche overlap and diet breadth in bumblebees; are rare species more specialized in their choice of flowers? Dave GOULSON*, Ben DARVILL Division of Biodiversity and Ecology, School of Biological Sciences, Biomedical Sciences Building, Bassett Crescent East, Southampton SO16 7PX, UK (Received 4 November 2002; revised 23 May 2003; accepted 10 July 2003) Abstract – The ecology of all bumblebees (Bombus spp.) is similar, yet some species have declined greatly while others remain abundant. We examine whether abundance is related to diet breadth. The floral visits of bumblebees were examined on Salisbury Plain, UK. All of the species examined gathered pollen mostly from Fabaceae. All species gathered nectar from a broader range of flowers than they did pollen, and longer- tongued bees had a narrower diet breadth when collecting nectar. B. hortorum (the species with the longest tongue) specialized on Trifolium pratense. As predicted, abundant species had a broader diet than rare species. Species with similar-length tongues visiting similar flowers. However, interspecific competition did not appear to be important since species with similar tongue lengths and high niche overlap co-occurred at high abundance. We suggest that the rare species may be those with short colony cycles, in which dependence on high quality food to rear larvae quickly forces specialization. Hymenoptera / Bombus / rarity / tongue length / pollen / competition 1. INTRODUCTION In general, it seems that those species with restricted geographic ranges also tend to be Many bumblebee (Bombus) species have less abundant where they do occur (Williams, declined dramatically in recent decades, both 1986). At present we have no indication as to in Europe and in North America (Peters, 1972; why bumblebee species differ so widely in Williams, 1982, 1986; Rasmont, 1995; Kosior, their range, abundance, and susceptibility to 1995; Buchmann and Nabhan, 1996; Westrich, environmental change. Morphologically, all 1996; Westrich et al., 1998). Of the 25 species species are remarkably similar, the only obvi- known from the UK, three species are extinct ous differences being variation in size and and several more are now confined to a handful tongue length. They have broadly similar of sites and have uncertain futures. The con- annual life cycles (although a few species are sensus is that declines in numbers of bumble- partially bivoltine), and all depend exclusively bees are linked to the intensification of farming on nectar and pollen for food. Most species do practices (Williams, 1986; Osborne and Corbet, not have precise habitat requirements, so far as 1994). However, a small subset of species (6 in is known (Williams, 1986). the UK) have proved to be at least partially The plight of our bumblebee fauna deserves immune to these changes; their ranges have not particular attention because loss of bee species contracted, and they remain abundant and will have negative consequences for other ubiquitous. wildlife. A large number of wild plants are * Corresponding author: [email protected] 56 D. Goulson, B. Darvill pollinated predominantly or exclusively by Studies were carried out between 19 July and 13 bumblebees, sometimes by particular species August 2002. This is late for some bumblebee of bumblebee (Corbet et al., 1991; Osborne et al., species, but facilitated access since all live firing 1991). Thus it seems probable that reductions ceases on SPTA for two weeks in early August. Thirty five sites were selected to cover the extent of in the abundance and species richness of bum- SPTA, with each site at least 1.5 km from any other. blebees may lead to widespread changes in All sites lay between 51o 06’ and 51o 20’ N and 1o plant communities (Corbet et al., 1991). And 40’ and 2o 00’ W, and were between 150 and 200 m of course these changes will have further altitude. Each site consisted of a circle of knock-on effects for associated herbivores and approximately 100 m radius, and was searched for other animals dependent on plant resources. one hour. All searches were conducted between 0900 h and 1700 h, and during warm dry weather For practical reasons, most studies of bum- favourable to bee activity. All Bombus species were blebee ecology and behaviour focus on the recorded. No attempt was made to distinguish common species, notably B. terrestris, B. luco- workers of B. terrestris and B. lucorum (to do so rum, B. lapidarius, B. pratorum, B. pascuorum would have been very time-consuming and and B. hortorum in Europe. For most of the unreliable in the field). B. humilis and B. muscorum approximately 40 remaining European bum- are indistinguishable on the wing, so all specimens blebee species, we have very little information of these species were captured and examined with a on foraging preferences, or indeed on any other hand lens. Any other individuals that could not be aspect of their ecology. We do not know readily identified were also captured, notably Psithyrus spp. and B. ruderarius. The area was whether the behaviour and foraging prefer- searched systemically, to avoid recording the same ences of the common species are representa- bees more than once, but this probably occasionally tive of all bumblebees. The reverse may be occurred. Most bees were visiting flowers. The true; the common species are probably common flower species visited was recorded, and the bee because they are atypical generalists. If this is was briefly observed to determine whether it was so, then our knowledge of common species collecting pollen or nectar. Bees were classified as may be of little help in conserving the rare nectar collectors (those not collecting pollen) or ones. Ecological studies of rare and declining pollen collectors (those observed actively grooming pollen into their corbiculae), but it must be noted species are urgently needed if appropriate con- that most pollen collectors were also collecting servation measures are to be deployed. Here nectar. The total number of inflorescences within we examine whether rarity in bumblebees is the search area (the circle of 100 m radius) was related to specialization in flowers utilized. We estimated by eye for each forage species present. test the prediction that rare species tend to be To compare the diet breadth of the species more specialized in terms of the flowers that recorded, a Simpson’s index was calculated for the they visit. We also examine whether diet diversity of flowers visited (Simpson, 1949): breadth and niche overlap between species s ()n ()n – 1 ---------------------------i i - relates to tongue-length. Ds = ∑ i = 1 ()NN()– 1 where ni is the number of flowers of the ith species that were visited, N is the total number of flowers 2. METHODS visited, and s is the total number of flower species visited. As is usual, results are presented as 1/D, so The study was carried out on Salisbury Plain that larger values indicate higher diversity. This Training Area (SPTA), southern UK, the largest index is insensitive to sample size (Magurran, 1988) area of unimproved chalk grassland that remains in which is important because samples are inevitably north-west Europe. Its status as a military training larger for the more common species. Only workers area since 1897 has protected it from most of the were included in the calculation, and only species farming changes that have occurred elsewhere, and for which there were at least 10 records of both much of its 38 000 ha consists of lightly grazed pollen and nectar collection. It must be noted that flower-rich grassland and scrub. As a result, it the combined index for B. terrestris and B. lucorum supports a diverse bumblebee assemblage, with at is likely to overestimate the true value for these two least 16 species present, perhaps the most that occur species, if the two species have different preferences. at any UK site (Carvell, 2002). It thus provides an Spearman’s rank correlations were calculated to opportunity to compare the forage used by both examine relationships between total abundance common and rare species when foraging in the same (workers, queens and males combined) and diversity habitat. indices. Niche overlap and diet breadth in bumblebees 57 Figure 1.

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