The importance of wild bees as pollinators of crops and wild flora is widely appreciated (Corbet et al., 1991; Williams, 1996; Holzschuh et al., 2007) and yet the contribution and dynamics of the many different species towards pollination is much less frequently reported (Westrich, 1996; Kremen et al., 2002) and have tended to focus on larger bee species (Osborne et al., 1999; Cresswell et al., 2000; Walther-Hellwig & Frankl, 2000; Osborne et al., 2008). There is some justification for this bias as the larger bees have the capacity to forage and transfer pollen over great distances (Gathmann et al., 1994). Some species of solitary bee, e.g. Megachilinae spp., have been more extensively studied because of their importance for the spring pollination of fruit trees (Torchio, 1976; Bosch et al., 2006; Krunić & Stanisavljević, 2006; Guedot et al., 2009).
Smaller solitary and social bee species have received much less attention, even though their species diversity is greater than larger bees, and are often in greater abundance (Westphal et al., 2008).
With the exception of the cleptoparasitic species, all female bees prepare a nest in which each cell is stocked with pollen before laying an egg within. Consequently at the time of nest building, as a central place forager, the provisioning bee must forage strategically in the landscape for pollen, and make a number of journeys depending upon the total volume of pollen required and the bee’s carrying capacity in flight (Westrich, 1996; Franzén & Larsson, 2007). Foraging strategy is complex (Goulson, 2010) but the energy required for each journey will at least depend upon whether the pollen demand can be readily satisfied by a general source of locally abundant flora or whether a particular species or family of flower is required. In the latter case, greater flight capacity is required where the specific flora is either sparsely distributed or some distance away in sufficient abundance. In some cases the nesting and foraging habitats may be close to one another; however in many cases the nesting requirement, such as a sandy soil, sunlit slope or vacated beetle hole, may be some distance from the pollen source. Therefore, in conserving for wild bees or managing for their pollination potential, it is important to recognise that each species may require two or more partial habitats for nesting and foraging (Westrich, 1996) and the distances between habitats will be a critical factor for species viability (Westrich, 1996; Williams & Kremen, 2007;
Goulson et al., 2010; Jha & Kreman, 2012).
On any scale, as connectivity between habitats becomes weakened by fragmentation of the natural landscape, the viability of central place foragers becomes threatened as the fragmentation approaches the limits of their capacity to travel. In the case of foraging bees it is the physical size of the bee that limits the effective distance that a species is able to travel for pollen; the larger the bee, the greater will be the distance it may forage from its nesting site (Gathmann et al., 1994; van Nieuwstadt & Ruano Iraheta, 1996;
Gathmann & Tscharntke, 2002). Therefore as partial habitats become increasingly isolated and distant from each other, notwithstanding their quality, it will be the smaller bees that are most disadvantaged (Araújo et al., 2004).
In previous work, the different methods of trying to estimate the maximum foraging ranges of different species of bee demonstrates both the difficulty and the aspiration to evaluate this critical statistic. An overview and objective comparison of methods is given by Greenleaf et al. (2007) who suggest that most methods are likely to indicate a shorter foraging range when compared to their optimised model of flight distances, and by an amount which may be associated with the particular method. The underlying problem is that most methods introduce an unnatural circumstance upon a natural landscape to allow for an obserEur. J. Entomol. 112(2): 303–310, 2015 doi: 10.14411/eje.2015.028
ISSN 1210-5759 (print), 1802-8829 (online)
Evidence of forage distance limitations for small bees (Hymenoptera: Apidae)
Ivan R. WRIGHT 1, Stuart P.M. ROBERTS 2 and BonnIe E. COLLINS 1 1 Shotover Wildlife, 15, Blenheim Way, Horspath, Oxford, OX33 1SB, UK; e-mail: email@example.com 2 1, Waterloo Road, Salisbury, SP1 2JR, UK; e-mail: firstname.lastname@example.org
Key words. Hymenoptera, Apidae, ground-nesting bees, forage, flight distance, partial habitats, habitat fragmentation
Abstract. The distribution of ground-nesting bees was investigated using transects of water traps in a mosaic of nesting and forage habitats at Shotover Hill in Oxfordshire, UK. The site includes a large area of ground-nesting bee activity and is adjoined on three sides by floristic hay meadows. This study showed that the females of small bee species (< 1.5 mm intertegular span) that were foraging in the hay meadows demonstrated a functional limitation to their homing range. The abundance of small bees declined rapidly with increasing distance from areas of high density nesting; declining more rapidly than might be expected from uniform dispersal into the surrounding landscape. By modelling the occurrence of bees along each transect it was found that the probability of observing a small bee in the hay meadows was reduced to 10% at a distance of 250–370 m from the nesting habitat. The result emphasises the scale on which habitat fragmentation will begin to impact upon bee diversity, and the relative contribution of managed “pollen and nectar” strips to areas of nesting habitat. 304
The sward of this mildly acidic field is somewhat unproductive for hay, and the flora is not especially abundant. Some bare soil is created by the burrowing activities of Talpa europaea Linnaeus (Talpidae). To the east of the SSSI is a tenanted “set-aside” field which is on the same free draining sand as the north field but with a negligible gradient. This field has not been cropped for about 30 years, but still retains a residual fertility with a strong growth of tall grasses each year. Consequently, compared to the other two fields this east field has a greater abundance of tall Asteraceae, such as Crepis biennis Linnaeus. There is some bare soil, mostly around the edges of the field. The southern field slopes gently to the south and is on a glacial deposit of clay-loam with impeded drainage in places. There is a little bare soil around the edges.