Bumblebee Communities Research Articles

Effects of flowering plant's patch size on species composition of pollinator communities, foraging strategies, and resource partitioning in bumblebees (Hymenoptera: Apidae).

In 4 common Middle-European mainly bumblebee-pollinated plant species (Impatiens glandulifera, Echium vulgare, Aconitum napellus, Symphytum officinale) the influence of patch size on species composition of the pollinator community was studied. Short-tongued species were most dominant in large patches, while small patches were frequented by middle- and long-tongued bumblebees. This phenomenon was extremely obvious in Symphytum officinale and Aconitum napellus, where short-tongued species had bitten a hole in nearly every flower of large patches. Long-tongued species were forced to small patches, where nectarrobbing occurred only exceptionally. In small patches visitationrate (Number of visits per flower per hour) was not lower but either equal or even higher then in large patches. Nectar measurements in Echium vulgare showed, that not only the mean quantity of nectar but also the variance was lower in small patches. As a result, the possible gain can be predicted much more precisely in a small patch than in a large one, and bumblebees have less difficulties in making the right foraging decisions. According to this, foraging strategies depend on patch size. This was confirmed by a computer simulation. The conclusion can be drawn, that many bumblebee species are able to share the same resource by using different patch sizes. Since large flower patches occur mainly in man-made habitats, the dominance of short-tongued species in many bumblebee communities studied by other authors may be unnatural.

Local Geographic Distributions of Bumblebees Near Crested Butte, Colorado: Competition and Community Structure

It was hypothesized that the local geographic distributions of bumblebees near Crested Butte, Colorado and the community patterns exhibited by these bumblebees are the products of competition for plants. To evaluate this hypothesis several transects were established and at regular intervals throughout a summer growing season, data were collected along each transect on how many bumblebees of each species and caste were visiting the various plant species. The distributions and abundances of the plant species involved were recorded qualitatively. Seven species of bumblebees accounted for 97% of all bumblebees observed and in this paper attention is restricted to these species. Five other species were observed in very low numbers. Each bumblebee species had a different distributional pattern. The seven bumblebee species form four groups in terms of both their proboscis lengths and the corolla lengths of the plants they preferentially visit. Long—, medium—, and short—tongued groups were most often observed foraging at flowers with long, medium, and short corollas, respectively. Furthermore proboscis lengths of bumblebees tended to be very similar within each group but quite dissimilar between groups. The fourth group consisted of a single short—tongued species which has well—developed mandibles which enable it to rob nectar from many plants with long corollas. It also feeds legitimately on short—corolla flowers. Some anomalies in the above groupings are mentioned and discussed. When the data on bumblebees and plant distributions are combined with data on flower preferences, a pattern consistent with the competition hypothesis emerges. Within each proboscis—length group, bumblebee species tend to replace one another altitudinally in a manner consistent with the hypothesis. The nectar—robbing species is most abundant in areas where a plant that is usually visited by hummingbirds is most common. Other species of bumblebees are unable to gain access to the nectar of this plant. In any uniform well—isolated area, similar floristically to the present study area, only three or four species of bumblebees appear to be able to coexist. Furthermore, a bumblebee community in such an area will apparently consist of a short—, a medium—, and a long—tongued species and in some cases a short—tongued nectar—robber. These patterns are also consistent with the competition hypothesis, as similarity in proboscis length reflects similarity in diet and the intensity of competition should, for these bumblebees, be closely related to diet similarity. It is possible that the observed distributional patterns could also be explained on the basis of different distributions of suitable nest sites for each bumblebee species or different responses to local variations in climatic conditions. Neither alternative seems able, however, to explain the observed patterns.

Species Structure of North European Bumblebee Communities

The structure of North European bumblebee communities was studied on regional and local scales. As the bumblebee proboscis has a potentially important role in resource partitioning, the structure of bumblebee communities was analysed in terms of proboscis length relationships. The regional numbers of bumblebee species in 100-km squares in NW Europe were frequently between 11 and 15. No obvious south to north gradient in species richness was found. The slightly lower regional numbers of species in Finland are suggested to be due to the lack of alpine habitats with associated species. England and Ireland have fewer species than equally sized areas in Scandinavia. Most of the bumblebee species (25/29) in Northern Europe have proboscises shorter than 12 mm. Short-tongued species tend to be more widely distributed than those with long tongues. B. hortorum, a long-tongued species, deviates clearly from this trend by being more widely distributed than the other species with a long proboscis, all of which are either habitat (alpine) or food specialists. Local communities consist of a number of coexisting species, most of which differ only slightly in terms of proboscis length. Only a few species (1-3 per community) have long proboscises and these species are seldom abundant. When species in the realized communities were compared in terms of proboscis length distances, no statistically significant differences were revealed from distances expected on the basis of random sampling from the local species pool. The same was the case also when only the abundant species were taken into account. Thus, propositions advocating proboscis length differences and food competition as the single assembly rule of bumblebee communities should be supported by good evidence. Other conceivable factors include competition for nesting sites, nest predation, and other mechanisms that shape the present-day local communities and regional species pools.

Structure in seven bumblebee communities in eastern Finland in relation to resource availability

The structure of seven local bumblebee communities was studied in eastern Finland. The number of species in the study sites varied from 5 to 10, 11 being the number in the species pool. Most of the species (queens) had proboscis lengths of 8–11 mm. When floral resources were analysed on the basis of corolla tube depths, a group was found between 4 and 8 mm. In terms of flower utilization, species with similar proboscises had higher pairwise niche overlaps than species differing by their proboscis lengths.Spacing between neighbouring species along the proboscis length dimension was not more uniform in the realized communities than what was expected on the basis of random draws from the local species pool. The results were the same for all species observed in the communities and for the abundant species only (frequency >5%), although competition among species in the latter group is expected to be more intense.When the structure of bumblebee communities and of floral resources was expressed in terms of diversity of proboscis lengths and corolla tube depths, respectively, a positive correlation was found. Data suggest that species with similar proboscis tend to co‐occur on fields with availability of appropriate corolla tubes.

Why Are There So Many Species? Spatio-Temporal Heterogeneity and Northern Bumblebee Communities

It is proposed that (1) the different body sizes and proboscis lengths of bumblebee species lead to different optimal foraging patterns in different species. The morphological differences affect niche relations, depending on the pattern of the resource environment. Tongue morphology allows only a basic set of three to four species per community, differing by proboscis length and frequency in indulging in robbery. To explain considerably higher community species numbers (frequently between six and eleven species), (2) the effect of superabundant resources was examined. Data suggest that species-rich bumblebee communities exist locally by the Schlaraffenland effect. As few of the bumblebee communities are located at fields of superabundant resources, or in non-competitive environments, other coexistence mechanisms were sought for. (3) Intensified intraspecific competition relative to interspecific competition may theoretically allow coexistence, but supporting evidence is still lacking. (4) Spatio-temporal heterogeneity was advocated as the main coexistence mechanism: the energy available for colony growth changes seasonally and depends to a large extent on the location of the nest relative to dynamically changing resources. This may lead to temporal reversions in the competitive relations between the colonies and species. All three tests available suggested that spatio-temporal heterogeneity suffices in explaining the high species numbers in northern bumblebee communities. However, specific features of bumblebee foraging behaviour call for measurements of withinand between-colony competition.

Patterns of Resource Utilization in Two Fennoscandian Bumblebee Communities

We studied seasonal changes in food niche relations among species in two Fennoscandian bumblebee communities. A community in southern Finland composed of 8 species utilized altogether 59 flower species during the whole summer. Corresponding figures for a community in northern Sweden were 9 bumblebee species and 50 flower species. Three bumblebee species were common to both communities. The flowering periods of the resource plants were longer in the southern community than in the northern one. In the southern community, but not in the northern one, the observed weekly niche overlaps were smaller than those to be expected assuming random foraging. During the whole season most species utilized more than 15-20 flower species in both areas, concentrating, however, most of their visits to a few of them. Niche overlaps (whole season combined) and differences between the frequency distributions of proboscis length and utilized corolla tube depths, respectively, were smaller in the southern community than in the northern one. These discrepancies may be due to a more stable resource production in the southern community contributing to a more intensive competition between species.

Habitat and Food Utilization in a Subarctic Bumblebee Community

We studied the seasonal dynamics of habitat and food utilization among bumblebee species in a community on the slope of a subarctic mountain (340-1050 m a.s.l.). During the summer all species at times utilized the whole altitudinal range. During spring all species foraged in the basal parts of the mountain, and during autumn in the intermediate zone. The altitudinal utilization of the mountain slope was the most diverse in the middle of the season. The observed weekly habitat niche and the food niche overlaps did not deviate from the overlaps calculated on the assumption of random habitat or food utilization, respectively. The segregation with respect to food was more complete than with respect to habitat. When the overlaps were calculated on the pooled, whole-season observations, a positive correlation was found between the pairwise habitat and food niche overlaps. The bumblebees separated into subalpine species (Bombus pratorum (L.), B. jonellus (K.), B. pascuorum Scop. and B. lucorum (L.)) and into alpine species (B. hyperboreus Schon., B. polaris Curt. and B. alpinus (L.)). The species, B. lapponicus (Fabr.) and B. balteatus Dahlb., utilized the whole altitudinal range. Probable factors affecting the altitudinal distribution patterns are discussed.