Plant diversity and composition compensate for negativeeffects of urbanization on foraging bumble bees

Marietta HÜLSMANN1, Henrik von WEHRDEN

1,2,3, Alexandra-Maria KLEIN

1,4,

Sara Diana LEONHARDT5

1Institute of Ecology, Faculty of Sustainability, Leuphana University Lüneburg, Scharnhorststr. 1, 21335, Lüneburg,Germany

2Centre of Methods, Leuphana University Lüneburg, Scharnhorststr. 1, 21335, Lüneburg, Germany3Research Institute of Wildlife Ecology, Savoyen Strasse 1, Vienna 1160, Austria

4Chair of Nature Conservation and Landscape Ecology, Institute of Earth and Environmental Sciences, University ofFreiburg, 79106, Freiburg, Germany

5Department of Animal Ecology and Tropical Biology, University of Würzburg, 97074, Würzburg, Germany

Received 6 October 2014 – Revised 21 February 2015 – Accepted 18 March 2015

Abstract – Bumble bees play an important role as pollinators of many crop plants and wild flowers. As in manywild bees, their abundance and diversity have declined in recent years, whichmay threaten the stability of pollinationservices. The observed decline is often linked with the loss or alteration of natural habitat, e.g., through urbanization,the conversion of natural habitat into largely sealed areas (concrete) inhabited by humans. The effects of urbanizationon bumble bees remain as yet controversial with both positive and negative effects reported. We investigated howhabitat isolation through increasing areas of concrete, as well as the diversity, abundance, and communitycomposition of floral resources, determine bumble bee abundance and diversity in cities. We found plant speciesdiversity and abundance to be more important than the amount of concrete in driving the abundance and speciesrichness of common bumble bees in a German city. Moreover, plant species composition, i.e., the presence ofspecific plant species and families (e.g., Fabaceae), played a prominent role. In particular, flower-rich parks andgardens can offer a continuous food supply for bumble bees and attract bumble bee foragers even to isolated patchesin the city center.

bee decline / habitat fragmentation / Hymenoptera / pollination / urban landscape

1. INTRODUCTION

Globally, the decline of pollinators and po-tential loss of pollination services was estimat-ed to result in an annual economic deficit of153 billion EUR of the agricultural sector due

to failure of fruit production in animal-pollinator-dependent crop plants (Gallai et al.2009). For Europe alone, the annual economicbenefit of bee pollination exceeds 14 billionEUR (Leonhardt et al. 2013). Primitively euso-cial bumble bees (Apidae: Bombus Latreille)are particularly important wild bee pollinatorsof crops and wild flowers in most temperatecountries (Fussell and Corbet 1992) becausesingle colonies comprise relatively high foragernumbers and foragers show high individualflower constancy (Heinrich 1976; Heinrichet al. 1977; Kleijn and Raemakers 2008), startforaging early, and forage for most of theflowering season (Goulson 2010).

Electronic supplementary material The online version ofthis article (doi:10.1007/s13592-015-0366-x) containssupplementary material, which is available to authorizedusers.

Corresponding author: S. Leonhardt,sara.leonhardt@uni-wuerzburg.deD. Leonhardt,sara.leonhardt@uni-wuerzburg.deManuscript editor: James Nieh

Apidologie (2015) 46:760–770 Original article* INRA, DIB and Springer-Verlag France, 2015DOI: 10.1007/s13592-015-0366-x

Although several bumble bee species are stillcommon (but see Lye et al. 2012), the occurrenceof a general bumble bee decline has been debatedfor 60 years (Fitzpatrick et al. 2007; Goulson et al.2008b; Connop et al. 2010). One frequentlydiscussed cause for the observed decline is habitatalteration and loss, especially through intensifiedfarming (Fischer and Lindenmayer 2007;Goulson et al. 2008b; Bommarco et al. 2010;Pellissier et al. 2013) and increasing urbanization(Ahrné et al. 2009; Kearns and Oliveras 2009;Bates et al. 2011; Geslin et al. 2013). In manyEuropean countries, up to 80 % of the populationlives in cities with urbanization further increasing(Antrop 2004). Increasing urbanization is gener-ally associated with an increase in concrete and adecrease in animal and plant diversity (Geslinet al. 2013; Martins et al. 2013; Williams andWinfree 2013).

However, cities can also provide new(foraging) habitat for animals (Paker et al. 2014),and some studies revealed gardens, parks, andmeadows as islands for insects (Gaston et al.2005a; Matteson et al. 2008; Ahrné et al. 2009;Garbuzov et al. 2013).

Nevertheless, in Stockholm (Sweden), bumblebee abundance decreased with increasing housingdensity (Ahrné et al. 2009). Cities comprise dif-ferent types of (urban) green areas, e.g., parks,gardens, flower strips, patches, etc., which candifferentially affect the abundance and speciesrichness of bumble bees (Pellissier et al. 2013).Particularly, gardens provide a variety offlowering plant species that are frequently visitedby bumble bees (Eremeeva and Sushchev 2005;Nielsen et al. 2008; Sikora and Kelm 2012) andmay compensate for the negative effect of con-crete. How these two factors interact has, howev-er, not yet been determined. Moreover, the quality(i.e., composition) of floral resources at specificurban green areas, such as gardens, varies andplays an important role in determining bee abun-dance and community composition (Tommasiet al. 2004), but this has also not been investigatedin detail for bumble bees visiting flowering plantsin urban areas. It is also unclear whether theirflower visitation patterns, e.g., the degree of spe-cialization of bumble bee–plant networks, in ur-ban areas differ from patterns observed for (semi-

)natural areas. One of the few studies that hasanalyzed the degree of specialization in flower-visiting bumble bees found the network to berather generalized in semi-natural areas (mediannetwork specialization index H 2

′=0.33) and thatthe degree of specialization did not change withthe diversity of flowering plant species (Fründet al. 2010).

All things considered, we still do not fullyunderstand how different aspects of urbanizationaffect abundance, diversity, and flower visitationpatterns or the degree of network specialization ofbumble bees. Therefore, we investigated the dis-tribution of bumble bees in the city of Lüneburg(Germany) in relation to urbanization (which wedefine as the amount of concrete that increasestowards the city center and correlates withflowering plant abundance), plant diversity, andplant community composition. We hypothesizethat the abundance and species richness of bumblebees is (1) negatively correlated with increasingurbanization, but (2) positively correlated with theabundance and diversity of floral resources in thecity. We further hypothesize that bumble beeabundance and richness respond more to plantcomposition than to urbanization per se and thatthe degree of specialization of the urban bumblebee–plant network is similar to networks recordedfor (semi-)natural areas.

2. MATERIALS AND METHODS

2.1. Study sites

The study was conducted in Lüneburg from begin-ning of April to the beginning of October 2013.Lüneburg is a small provincial town close to themetropole region of Hamburg in the north of Germany(Lower Saxony) with about 70,000 inhabitants. Beyondthe city boundary, there are several crop fields dominat-ed by arable fields, pastures, and a high density of smallvillages.

Fifteen study plots (comprising an area of 50 m indiameter) were chosen within the city boundary witharound 1 km in between two adjacent study plots: fivein the suburbs, five in the city center, and five betweenthem (here defined as urban area). The main criteria forour site selection were the distance from the centralpoint, the distribution over the city area (including

Distribution of bumble bees in urban areas 761

central urban, suburban, and periphery areas), and theabundance and richness of flowering plant species at asite.

The BHauptschule Stadtmitte^ was chosen as thecentral point of Lüneburg (as indicated by GoogleEarth: latitude 53.246407° N/longitude 10.411525°N). For each site, the distance from the central pointwas measured with the freely accessible ruler functionin Google Earth (Figure S1, Table SI).

Additionally, a 500 m radius (785,398.16 m2) wasdrawn around the central point of each 50 m study siteusing Google Earth and the Keyhole Markup Language(KML) Circle Generator for Google Earth to assess thedegree of urbanization around the study site. The circledarea was then divided into 16 fragments of same size(49,087 m2), and the area of trees, meadows, gardens,concrete, farmland, and water (see Table SIII for adetailed description of coverage types) within eachfragment was visually assessed at a magnitude of10 % steps in Google Earth. The percentage amountsof each coverage type were summed up for each circle(Table SI).

The proportion of concrete within the 500 mradius decreased significantly with increasing dis-tance from the city central point (Pearson’s corre-lation, r =−0.88, P <0.001). Distance from the citycentral point was therefore used as a measure fordecreasing urbanization.

Before we started the monthly monitoring, allflowering plants at each study site were identified tospecies level (or genus level if the species could not beclearly identified) using taxonomic keys (Jäger 2011;Schmeil and Fitschen 2011). Identification of floweringplants was performed directly at the study plots beforethe bumble bee monitoring started.

2.2. Monitoring

Every study site was surveyed once per month be-tween 10AM and 7PM on nonrainy days with tempera-tures above 15 °C and an average wind speed below2.5 m/s. Site visitation was repeated in intervals of28 days, and visitation order was randomized to ensurethat every study site was surveyed in the morning,afternoon, and early evening. Overall, we performed105 surveys.

Following Braun-Blanquet (1994), we counted thenumber of flowering trees, bushes, and herbal plants(within the 25 m radius). To classify all flowering plant

species according to their flowering area provided intotal per site (rendering single flowering plants and largetree species comparable), a blooming product (B p) wasdefined for each plant species:

Bp ¼ Np � ba � Nb

where b a represents the area of a single blossom[measured in diameter (cm)],N b the average num-ber of blossoms per plant species, and N p thenumber of individuals per plant species per site.Data on blossom area and estimated number ofblossoms per plant were obtained from Schmeiland Fitschen (2011), Rothmaler (Jäger 2011), andBIllustrierte Flora von Mitteleuropa^ (Hegi et al.1995). Thus, the blooming product represents ameasure for the flowering area, which likely at-tracts foragers from a distance and potentially, butnot necessarily, correlates with the amount of flo-ral resources.

The entire study site (for a detailed description ofeach site, see SI and SII) was examined for 30 min forthe presence of bumble bees, which was sufficient toobserve small flowering herbal plant species for approx-imately 5 s and trees for up to 3 min. We always startedat the same point at a site and randomly crossedmeadows and lawns. This way of monitoring was per-formed for all sites except for gardens with beds wherewe walked from flowering plant to flowering plant. Allbumble bees observed on flowers or in flight werecounted and classified into one of six species groupsaccording to the identification keys of Prŷs-Jones andCorbet ( 2011), Hagen et al. (2003), and Edwards andJenner (2009) and listed under the name of the mostcommon species in this group (see Table SIV). If thespecies group could not be clearly identified on thespot, bumble bees were caught with a plastic tube oran insect net and identified using a magnifier. Weused the species group categories because some rarespecies (e.g., Bombus magnus Vogt and Bombuscryptarum Fabricius) and several common species(e.g., Bombus terrestris Linnaeus) could not bedistinguished in the field using a visual taxonomicapproach (Lye et al. 2012). Queens and workerswere distinguished by their size and the season ofobservation. Drones were identified based on theirclypeus setae (Edwards and Jenner 2009).

On large trees that can grow up to 30 m (e.g., Tiliaplatyphyllos Scop. (Malvaceae) (Jäger 2011)), only

762 M. Hülsmann et al.

those bumble bees where counted that could be seenfrom the ground.

Note that, we may have counted the same individualmore than once, as we did not capture bees.

2.3. Statistical analysis

We analyzed species richness and abundance usinggeneralized linear mixed effect models (GLMM), with aPoisson error distribution and sites and time included asrandom effects. Individual species were modeled with abinomial error distribution. We used an informationtheoretic approach for model selection to identifymodels that best explained bumble bee species richness,abundance, and presence of individual species and todifferentiate between the effects of variables related tourbanization and plants. We constructed six alternativecandidate models arising from specific hypothesis de-rived combinations of the four groups of fixed variables(plant diversity=P, blooming product=B , vegetationcommunity patterns=V, habitat ordination=H ) and thenull model. Plant diversity (P ) was entered as speciesrichness. Vegetation community patterns (V ) includedthe distribution of all plant species and were analyzedwith a detrended correspondence analysis (DCA, seeFigure S2) using loadings on the first axis as a predictorfor the main vegetation community dynamics based onall vegetation surveys. Habitat data (H ) comprised thepercentage amounts of each vegetation type (summedup for each circle), which was transformed via a princi-pal component analysis (PCA, see Figure S3) and cor-related negatively with distance from the city centralpoint (GLMM: model estimate±SD: −0.64±0.31,P=0.04). We used the loadings on the first axis as apredictor for the most important habitat dynamics.

Tested hypotheses were B +V , B +H , V +H ,D+B+H , B+V+H , andD+B+V+H (i.e., models witheach of these variable combinations were tested againsteach other). Predictors within the GLMM were testedfor collinearity, but no redundant predictors were de-tected within our predictor combinations. We used the Rpackage BAICmodavg^ to rank the candidate models,based on Akaike’s information criterion corrected(AICc) values to account for small-sample bias.

The quantitative network-level specialization indexH 2

′ (Blüthgen et al. 2006) was applied to characterizethe degree of floral specialization across bumble beespecies groups with regard to plant species visited. H 2

′

ranges between 0 (bumble bees of all species groups

visit similar plant species) and 1 (each species groupvisits a specific set of plant species).

3. RESULTS

In total, 699 bumble bees were counted overthe sampling period, belonging to the speciesg roups of Bombus pascuorum (247 ) ,B. terrestris (208), Bombus lapidarius (128),Bombus hypnorum (44), Bombus hortorum(37), and Bombus pratorum (35) (Tables SII,SV, and SVI). Overall, 381 (54.5 %) bumble beeswere recorded in the urban area, 208 (29.8 %) inthe suburban area, and 110 (15.7 %) in the citycenter. The study sites with fewest bumble bees(fewer than 27) were all located in the city center(see Tables SII and SVI).

Most bumble bees and bumble bee specieswere recorded in August (210 bees in total).Most plant species were recorded in July (208)and June (171) (Tables SII, SV, and Figure 1).These plants were found at the BKurpark^ (146),Leuphana botanical garden (101), BPferdeteich^(90), BHasenburger Weg^ (59), and the town hallgarden (59) (Tables SII and SVI). All of thesestudy plots were cultivated continuously withflowering plants by private or professionalgardeners.

We recorded overall 255 different plant speciesover the study period (Table SI). Of these, 86 werevisited by bumble bees (Figure 2). If ranked byblooming product (i.e., flowering area), the plantwith largest flowering area was Malus domesticaBorkh,, which was visited by only two bumblebee queens in April (Table SII), while the mostabundant flower in terms of numbers was Bellisperennis L. (Asteraceae Bercht and J. Presl),which was not visited by any bumble bees.Trifolium repens L. (Fabaceae Lindl.) was theplant species visited most in this study(Figure 2). Fabaceae (190 bumble bees),Lamiaceae L. (121), Rosaceae Juss. (84),Asteraceae (76), and Malvaceae Juss. (41) werethe most frequently visited plant families (see alsoTable SII). Overall, plant diversity increased withincreasing distance from the city central point(Spearman rank correlation: r=0.35, P<0.001).

Both bumble bee abundance and species rich-ness were best described by models including all

Distribution of bumble bees in urban areas 763

parameters [i.e., plant diversity (D ), bloomingproduct (B ), vegetation community patterns (V ),and habitat parameters (H )] as was the abundanceof all bumble bee species groups, except forB. pascuorum (Table I). However, the bloomingproduct generally had the least effect on the totaland species group abundance or number of bum-ble bee groups, while overall abundance and spe-cies richness both increased strongly with increas-ing plant diversity (Table II, Figure 3) and de-creased slightly towards the city center (see neg-ative correlation with habitat PCI coordinates(Table II), which increase towards the city centralpoint (see BMaterials and methods^)).

All bumble bee species groups visited a broadspectrum of plant species and plant species over-lapped between bumble bee species (Table SII,Figure 2). All bumble bee groups thus showed ageneralized foraging behavior (H 2

′=0.42,Table SII, Figure 2). The highest number of plantspecies was visited by B. pascuorum (47 differentplant species) and B. terrestris/lucorum (51 dif-ferent plant species) (Table SII, Figure 2). Theabundance of the latter was most strongly deter-mined by plant diversity, while plant communitycomposition had the strongest effect on the abun-dance of all other species groups (Table II).

Species abundance typically decreased towardsthe city center, except for B. pratorum (Table II).

4. DISCUSSION

Along an urbanization gradient in the Germancity of Lüneburg, plant species diversity and com-munity composition strongly affected the speciesrichness and abundance of foragers of six bumblebee groups, while the distance from the city centerand hence an increasing amount of concrete (i.e.,urbanization) had a comparatively weak effect. Asplant species diversity decreased with decreasingdistance from the city’s central point, our resultsconfirm a negative effect of urbanization on plantspecies diversity and thus bumble bees, whichagrees with previous studies (Ahrné et al. 2009;Bates et al. 2011; Banaszak-Cibicka andŻmihorski 2012; Jha and Kremen 2013b;Aronson et al. 2014), but indicates that floralrichness and composition, not urbanization perse, determine the presence of bumble bee foragersin small cities, such as Lüneburg.

Interestingly, most bumble bees were found atsites with intermediate urbanization (urban area),such as the Kurpark, the Leuphana campus, andthe Pferdeteich. All of these sites were all highly

Figure 1. Changes in bumble bee abundance (a ), species richness (b ), and plant species richness (c ) across months.

764 M. Hülsmann et al.

Figure 2. Bumble bee–flowering plant visitation networks. Level of shading of matrix entries in top networkpositively correlates with number of observations. Below Lines between a bumble bee and plant species show theplant species visited by that species group; line width correlates with the number of visits recorded for this particularplant species; bar width correlates with the number of individuals recorded per species group in relation to allindividuals recorded (top ), or with the proportion of visits to each plant species in relation to the overall number ofvisits (bottom ), respectively.

Table I. Model results for abundance, species richness, and individual species.

Species Model Log (L ) K AICc Wi AICc null model

Abundance D+B+V+H −1610 8 7842 1 3349

Species richness D+B+V+H −3913 8 3630 1 753

B. terrestis D+B+V+H −296 8 608 1 37

B. lapidarius D+B+V+H −385 8 787 1 196

B. hortorum D+B+V+H −243 8 502 1 361

B. pratorum D+B+V+H −115 8 246 1 646

B. pascuorum V+H −358 6 729 1 37

Model summary: log (L) the maximized log-likelihood, K number of estimated parameters, AICc Akaike’s information criterioncorrected for small sample bias, Wi Akaike weights, AICc null model difference in AICc compared with the nullmodel, D plant diversity, B blooming product, V vegetation community pattern, H habitat parameters

Distribution of bumble bees in urban areas 765

cultivated by (semi-)professional gardeners andoffered a comparatively high plant species rich-ness over the entire survey period (see Tables SIand SII). Especially the Kurpark and Pferdeteichwere further surrounded by additional gardens(see Table SI), which are known to attract andsupport insects (Samnegård et al. 2011). Anotherreason for the high abundance of bumble bees atthese sites could be the comparatively high abun-dance of Fabaceae and Lamiaceae (see Tables SI

and SII), which are known to be preferentiallyvisited by bumble bees (Goulson et al. 2008a;Sikora and Kelm 2012).

We also found high numbers of bumble beeson study plots in the city center of Lüneburg,e.g., 15 B. terrestris and 12 B. pascuorum inthe Rathaus garden (with 65 % of concrete inthe surrounding) at 1 day in August, demon-strating that bumble bees are able to forageeven at very isolated patches in the city center,

Table II.Model coefficients (and standard errors) of the environmental variables included in the best ranked modelsin Poisson/binomial GLMMs.

Species Model Intercept Plantdiversity

Bloomingproduct

Plant community Habitat

Abundance D+B+V+H 0.36±0.08 0.29±0.03 0.02±0.02 −0.83±0.06 −0.13±0.07Diversity D+B+V+H 1.21±0.21 0.15±0.01 0.05±0.01 −1.74±0.04 −0.17±0.20B. terrestis D+B+V+H 4.29±1.48 2.01±0.43 −0.10±0.11 −1.44±0.37 −1.05±1.48B. lapidarius D+B+V+H 0.36±0.56 1.38±0.18 0.13±0.10 −3.11±0.36 −0.71±0.52B. hortorum D+B+V+H −13.41±2.74 2.95±0.40 0.41±0.14 −9.17±0.95 −2.57±2.74B. pratorum D+B+V+H −1.75±4.82 4.56±0.85 −0.09±0.26 −18.8±1.90 12.7±5.35

B. pascuorum V+H0.23±0.58 −1.98

±0.32−0.44±0.49

D plant diversity, B blooming product, V vegetation community pattern, H habitat parameters

Figure 3. Correlation between bumble bee and plant species Shannon diversity in relation to the distance from thecity central point. Different symbols indicate different urbanization categories: squares mark study plots in the citycenter, circles plots in the urban area and triangles plots in the suburban area. Each symbol represents one survey;we refrained from labeling individual months because month had low explanatory power if included as a randomintercept in a mixed effect model (data not shown).

766 M. Hülsmann et al.

provided these patches comprise sufficient flo-ral resources. This primarily applies to perma-nently cultivated gardens, which may explainwhy several studies consider gardens in citiesBharbors or islands^ for bumble bees and otherinsects (Ahrné et al. 2009; Samnegård et al.2011; Goulson et al. 2012; Pereira-Peixotoet al. 2014).

Consequently, the availability and diversity ofattractive plant species represents a prerequisitefor the presence of bumble bee foragers in thecity center. Our finding agrees with Jha andKremen (2013a) who also found that foragingranges in the North American native bumble beeBombus vosnesenskii Radoszkowski were pri-marily driven by floral resource diversity in aheterogeneous landscape. Likewise, multiplestudies found that the abundance of several bum-ble bee species was positively correlated with highflowering plant diversity as well as with the pres-ence of particular plants families (Williams 1986;Mänd et al. 2002; Goulson and Hanley 2004;Hines and Hendrix 2005), further stressing theimportance of plant diversity and communitycomposition. Interestingly, however, close to30% of the bumble bees foraged on only six plantspecies: T. repens (Fabaceae), Lotus corniculatusL. (Fabaceae), Rubus fruticosus (Rosaceae),M e n t h a s p i c a t a L . ( L a m i a c e a e ) ,Rhododendron×hybrida (Ericaceae), and Salvianemorosa L. (Lamiaceae). Five of these plantsbelonged to the most visited plant families:Fabaceae, Lamiaceae, and Rosaceae. Most bum-ble bees (27.2 %) were recorded on Fabaceae,whereas other plant fami l ies , such asHydrangeaceae Dumort. (five recorded visits) orIridaceae Juss. (two), were hardly visited by anybumble bees. This differential visitation patterncannot be explained by availability alone, as indi-cated by the comparatively weak effect of theblooming product, which is correlated with plantabundance at our sites. Moreover, even the veryabundant plant family Asteraceae, with more than250 individual plant recordings in this study, wasonly visited by 76 (11 %) bumble bees. In con-trast, we only recorded 130 Fabaceae plants.

The strong preference of bumble bees forFabaceae also agrees with results from previousstudies (Goulson and Darvill 2004; Goulson et al.

2005, 2008a; Connop et al. 2010; Redpath et al.2010). Fabaceae seem thus to be highly importantfor bumble bees, especially for longer-tonguedspecies, such as B. hortorum (19 visits toFabaceae in this study, 47.5 %), B. lapidarius(68 visits, 52.3 %), and B. pascuorum (76 visits,32.2 %). A potential explanation for their impor-tance may be that Fabaceae pollen is richer inprotein compared to pollen of other plant families,as suggested by Goulson et al. (2005) and Hanleyet al. (2008). The differential importance ofdifferent plant families/species can thus likelybe explained by differences in the plants’nectar or pollen quality, as bumble bees ap-pear to require pollen of high protein content(Leonhardt and Blüthgen 2012), which wasfound to increase their survival and immuno-competence (Genissel et al. 2002; Tasei andAupinel 2008; Brunner et al . 2014) .Interestingly, most of the plants visited bybumble bees in this study were native plants,such as T. repens , R. frut icosus , orL. corniculatus (Table SII). This findingagrees with recent studies that suggest nativeplant species to promote bee abundance inurban gardens (Pawelek et al. 2009; Pardeeand Philpott 2014).

Most bumble bees species observed in thisstudy are ubiquitous (von Hagen et al. 2003) andgeneralized foragers. For example, the secondmost common species group B. terrestris/lucorum (208 individuals recorded) was observedon 19 different plant families. The most abundantbumble bee species, B. pascuorum , also foragedon 14 different plant families. Despite their gen-eralized foraging behavior, the degree of special-ization of the urban bumble bee–plant networkwas slightly higher than in (semi-)natural areas(Fründ et al. 2010). This finding is even moresurprising as we may have grouped several spe-cies, which may visit different plant species, intoone group, thereby increasing the number offlowering plant species per group. Grouping spe-cies should generate a more generalized network(i.e., lower H 2

′ value) than would be observed forthe actual species. The higher degree of speciali-zation as found for our bee group–plant networkindicates that bumble bees tend to more stronglypartition floral resources among each other in

Distribution of bumble bees in urban areas 767

urban compared to (semi-)natural landscapes.This behavior might be explained by a generallyhigher plant diversity in cities compared to theseminatural, often human-influenced landscapesof industrialized countries (Thompson et al.2003; Gaston et al. 2005b; Loram et al. 2008),which may allow different bumble bee species tomore easily distribute themselves across differentplant species (see also Miller-Struttmann andGalen 2014) and, in doing so, avoid competition.However, more comparative network analyses areneeded in order to confirm this hypothesis.

Note that all bumble bees observed in ourstudy belong to species commonly found inand outside of cities (Goulson et al. 2005,2008a; Banaszak-Cibicka and Żmihorski2012), while typically rare species, such asB. sylvarum L. or B. humilis Illiger, werenot observed in our study. Consequently,there are most likely Bwinners and losers^of urbanization (Banaszak-Cibicka andŻmihorski 2012). With regard to bumblebees, winners comprise, e.g., the short-tongued B. terrestis and the medium long-tongued B. pascuorum (Banaszak-Cibickaand Żmihorski 2012). This finding agreeswith Goulson et al. (2005) who suggestedthat mostly short-tongued species persist inaltered areas due to their generally less spe-cialized diet. However, species richness mayhave been underestimated in this study, asindividuals were categorized into groups, po-tentially overlooking rare, similar lookingspecies.

To conclude, plant species diversity andcomposition are the most important factorsdetermining bumble bee abundance and di-versity in (and outside of) small cities, andspecific plant species and families (e.g.,Fabaceae) play a disproportionally importantrole. In particular flower-rich parks and gar-dens can offer a continuous food supplyattracting bumble bees even to isolatedpatches in the city center provided they com-prise a specific composition (e.g., Fabaceaeand Lamiaceae) of plant species. Althoughour findings do not necessarily apply to larg-er cities, such floral resource patches canconsequently compensate the negative effect

of urbanization for this group of pollinators,at least in small cities.

ACKNOWLEDGMENTS

We thank Peter Zurheide, head of „Grünplanung,Friedhöfe und ForstenB, Susanne Schröder (Dipl.-Ing.(FH) Landespflege) and Oliver-Martin Freese, gardener(Gärtnermeister) from the city of Lüneburg and theKleingärtner Bezirksverband Lüneburg for permissionto survey bumble bees in parks, gardens, and onmeadows, and Johann Neumayer as well as Rolf Wittfor help with the identification of some bumble beespecimens. We also thank Dr. Wilhelm Hülsmann fortechnical support and two anonymous reviewers fortheir helpful comments on a previous version of themanuscript. Funding for SDL was provided by theDeutsche Forschungs-Gemeinschaft (DFG project: LE2750/1-1).

La diversité et la composition des plantes compensentl e s e f f e t s néga t i f s de l 'urban i sa t i on pourl'approvisionnement des bourdons

déclin des abeilles / fragmentation de l'habitat / Hyme-noptera / pollinisation / paysage urbain

D i v e r s i t ä t u n d Z u s amm e n s e t z u n g v o nPflanzengemeinschaften kompensieren negativeAuswirkungen der Urbanisierung für sammelndeHummeln

Bienenrückgang / Habitatfragmentierung / Hymenop-tera / Bestäubung / urbaner Lebensraum

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