Original article

Nectar foraging by stingless bees in Costa Rica:botanical and climatological influences

on sugar concentration of nectar collected by Melipona

Jacobus C. Biesmeijer* Martin J.A.P. Smeets, Julliette A.P. Richter,Marinus J. Sommeijer

Ethology and Socio-ecology Group, Laboratory of Comparative Physiology, Utrecht University,PO Box 80.086, 3508 TB Utrecht, the Netherlands

(Received 2 March 1998; accepted 23 October 1998)

Abstract - Nectar foraging by two species of stingless bees, Melipona beecheii and Melipona fas-ciata, was studied in Costa Rica. In the humid forest environment M. beecheii collected more con-centrated nectar than M. fasciata. Pollen analysis showed that their nectar sources overlapped con-siderably and that M. beecheii collected richer nectar than M. fasciata even from the same plantspecies. Thus, the botanic origin of the nectar explained only part of the differences. There was no dif-ference in the sugar concentration of the nectar collected by the two species at a dry tropical sitewhere M. fasciata was introduced and M. beecheii occurs naturally. In the dry habitat, both speciescollected more concentrated nectar than at the humid site. Foraging behaviour of the bees is suggestedas the basis for resource partitioning in humid forests. © Inra/DIB/AGIB/Elsevier, Paris

Melipona / nectar foraging / sugar concentration / pollen analysis / Costa Rica

1. INTRODUCTION

The nectar foraging community in trop-ical habitats is very diverse [2, 13] and plantsand animals have evolved many adaptationsconcerning nectar production and collect-

ing [6], e.g. variation in nectar quality andquantity in plants [2, 3] and morphology ofnectar uptake organs of nectar collectors[17]. Flower visitors partition nectarresources based on the amount of nectar in

a flower [13], the sugar concentration [13,

* Correspondence and reprints: Apartado Postal 2270, 3000 Heredia, Costa RicaE-mail: kbiesmei@sol.racsa.co.cr

19, 22], the type of sugar [2], the amino acidcomposition [22] and the lipid compositionof the nectar [2].

In the Neotropical lowlands the mostabundant group of nectar collecting insectsare the stingless bees, subfamily Meliponini[8, 9]. Because stingless bees are eusocialand live in large permanent colonies, theycollect pollen and nectar all the year roundand are polylectic [17]. Several dozenspecies of Meliponini occur sympatricallyand their diets partly overlap in terms ofplant species used [11, 12, 15, 16, 21, 22,24]. Roubik and Buchmann [19] observedthat, in Panama, two species of Melipona,M. compressipes and M. marginata, usedand might prefer higher sugar concentra-tions than the other two species, M. fasci-ata and M. fuliginosa. Another species,M. beecheii, was found to be a high-sugarspecialist in Quintana Roo, Mexico [22].These studies did neither identify the floris-tic origin of the nectar nor related the resultsto climatological variation, but attributedthe differences in sugar concentration to avariation in imbibing rates and to the caloric

rewards that the different species obtainedfrom different sugar solutions [19].

Factors that influence the sugar content ofthe nectar that bees harvest, especially botan-ical origin and local climate, have mainlybeen studied separately (see above citedpapers). Therefore, our main aim was torelate data on bee-collected nectar to botan-ical origin of the nectar and variation inregional climate. These data were used toinvestigate whether partitioning of nectarresources is based on preferences of the beesfor certain plant species or regional climate.We selected two congeneric sympatric stin-gless bees, Melipona beecheii and M. fas-ciata, and analysed the sugar concentrationof the nectar they collected and related it tothe time of day, the seasonal variation, theregional climate and botanical origin of thenectar.

The two species are sympatric only in asmall zone in Costa Rica (transition betweendry forest and rain forest) and allopatric inthe remaining part of their distribution (fig-ure 1). M. beecheii is a Central American

species and is found from Mexico (YucatanPeninsula and Quintana Roo) to Costa Rica,mainly in the dry forest zones ([1]; Bies-meijer, unpublished data). M. fasciata is amember of a species complex that is widelydistributed in Central and South America

[7, 18, 23], mainly in the humid lowlands.Our bees probably belong to the samespecies as M. panamica in Roubik et al.[22]. We will refer to it as M. fasciata fol-lowing Hanson and Gauld [7].

Studying these two species allowed usto assess: 1) whether they show similar high-and low-sugar specialisation as reported inallopatry [19, 22]; 2) the variation and botan-ical origin of the nectar; and 3) the influ-ence of regional climate on the sugar contentof the collected nectar.

2. MATERIALS AND METHODS

2.1. Study sites and bee colonies

Most observations were carried out in El Surde Turrubares, San José Province, Costa Rica(9°45’N; 84°35’W; alt. 180 m; figure I). El Surwas chosen because it is situated in a zone wherethe distribution of the two bee species overlaps.

Nine colonies were used: five of M. beecheii

(hereafter referred to as B1, B2, B3, B4, B5) andfour of M. fasciata (hereafter referred to as F1,F2, F3, F4). The colonies were housed either intree-trunks (B2, B3, B4, B5, F1, F2, F3) or inone-compartment hives (B1, F4) and wereinstalled at one of two locations that were 50 m

apart: ARBOFILIA centro de educación y inves-tigación (B 1, B5, F F2, F3) or at a private house(B2, B3, B4, F4). Five colonies had been in situfor at least 1 year (B1, F1, F2, F3, F4). Colony B5was transferred from the neighbouring villageof Bijagual (10 km away) to El Sur in January1996, and B2, B3 and B4 were transferred fromSanta Cruz, Guanacaste Province (about 150 kmto NW in dry forest zone) to El Sur in mid-December 1995. The colonies consisted of anestimated 2 000-3 000 individuals; B3 and B4were exceptions in that they were estimated tocontain only 1 000-1 500 individuals.

To study the climatological influence on thesugar concentration of the collected nectar addi-tional observations were performed in Pozo Azul

de Abangares, Guanacaste Province, Costa Rica(10°10’N; 85°00’ W; alt. 200 m; figure 1). PozoAzul is situated in the tropical dry forest zone.Only M. beecheii is native to the Pozo Azul area(nb.: this Pozo Azul is different from Finca PozoAzul regularly cited as a collection site forM. fasciata by Wille [25], which is located inrain forest environment near the Central Pacificcoast, 9°39’N; 84°18’W). Colony B2 was movedfrom El Sur to Pozo Azul on 12 January 1996.Colony F4 was moved to Pozo Azul on 28 Febru-ary after a regular sampling session in El Suron 27 February.

2.2. Nectar sampling

From 17 November 1995 (week 1) till 23 Fe-bruary 1996 (week 15) nectar samples were takenin El Sur once a week from five to ten return-

ing foragers per colony, five times a day(07:00-08:00; 09:00-10:00; 11:00-12:00;13:00-14:00 and 15:00-16:00). Nectar foragerscarried pollen of different colours and on differ-ent parts of their bodies (but did not carry pollenloads in their corbiculae). By sampling selec-tively at least one forager from each visually dis-tinguishable dusting and colour type, we obtaineda wide variety of nectar samples. This may havecaused a slight overestimation of the diet diver-sity index. Four colonies were sampled fromweeks 1-15 (B5, F1, F2, F3), B2 in weeks I Iand 13, B3 from weeks 6-11, B4 from weeks6-13 and B5 from weeks 11-15. Temperature,relative humidity and cloudiness were recordedduring each sampling session. The relativehumidity (RH) was calculated from a non-aspi-rated psychrometer. Cloudiness was recorded ona 5-step scale: I = clear sky, 2 = scattered clouds,3 = half cloudy, 4 = heavy clouds, 5 = rain.

Returning nectar foragers were caught witha suction tube. Nectar was collected from nec-tar foragers by means of 20 and 50 μL capillarytubes (minicaps, Hirschmann) that were placedbetween the mandibles, while slightly pressingthe bee’s abdomen manually. The sugar con-centration of the nectar was measured by a hand-held refractometer corrected for ambient tem-

perature (Atago 500, National Co., Japan).Refractometer values are in % BRIX which isthe weight of total dissolved solids per totalweight of solution.

Nectar samples were taken from colonies F4and B2 in Pozo Azul, on 29 February and5 March, according to the methodology describedabove.

2.3. Pollen analysis

To identify the nectar sources used by thebees we analysed pollen from the bodies of thenectar foragers. We did not analyse the pollenin the nectar, as has been done regularly(reviewed by [16]), because preliminary workshowed that many nectar samples did not con-tain any pollen, whereas most bees carrying nec-tar had pollen grains on their bodies. Only ifanthers are close to the nectar or if pollen hasfallen into it, bees will imbibe pollen grains withthe nectar. On other occasions pollen grains willadhere to the bees’ body, but not necessarilyenter the nectar. Pollen grains adhering to thebody of the bee were collected by tapping thebee with a small piece of glycerine jelly mountedon a pin. The colour and the amount of the pollenwere recorded as well as the part of the bodyfrom which it was collected. The glycerine jellywas melted and mounted on a microscopic slidefollowing the addition of a drop of fuchsin. Thevast majority of the slides contained only onetype of pollen indicating that bees tended to col-lect nectar from one plant species only. Pollenslides that contained more than one type werenot included in our analysis. Some bees did notcarry pollen grains in adequate numbers on theirbodies. The origin of the nectar harvested bysuch bees remains unknown, because in none ofthese cases did the nectar contain an identifiableamount of pollen grains. The different pollentypes were coded, described, drawn and identifiedby making use of our reference collection (seebelow) and the pollen literature from Panama[20] and Yucatan [14].

2.4. Pollen reference collection

To facilitate identification of the plant speciesused as nectar sources by the bees, we prepareda pollen reference collection for the floweringplants of the area. During the 15 week observa-tion period 3 days per week were used for thecollection of flowering plants in an area of about1 km around the bee colonies. Standard routeswere covered, all flowering recorded and all flow-ering plant species (230 in total) collected, driedand kept as a reference. Vouchers were depositedat Utrecht University, the Netherlands andCINAT, Universidad Nacional, Costa Rica. Apollen slide was made of each plant collectionto serve as reference for identification. Plantswere identified by the authors with considerablehelp from José Gonzalez and Barry Hammell

(Lauraceac, Convolvulaceae) of INBIO and LuisPoveda (Compositae) of the UniversidadNacional.

3. RESULTS

3.1. Sugar concentrationof nectar collected by foragers

Both species collected nectars with awide range of sugar concentrations (M. bee-cheii: 7.1-72.4 %; M fasciata: 6.6-65.4 %).Occasionally bees carried loads of less than5 % sugar concentration. These loads prob-ably consist mainly of water [19] and willnot be considered further. M. beecheii col-lected nectar with higher sugar concentra-tions than M. fasciata overall (figure 2) andduring each of the five daily sampling peri-ods (figure 3). Sugar concentration increasedfrom 7:00 to 11:00 and stabilised after 11:00.

A MANOVA was performed on all data(2 028 cases) to find the factors associatedwith sugar concentration of nectar. Theresults indicate that the factors species(P < 0.001) and colony within species(P < 0.001) as well as the covariants tem-perature (P < 0.001), week (P < 0.001) andcloudiness (P < 0.01) significantly explainedpart of the variance in sugar concentration(model explained 41.4 % of the variation insugar concentration). Using the mean sumsand t-values as a measure of relative con-

tribution, factors and covariants can be putin order of descending importance: beespecies, week, temperature, cloudiness andcolony.

3.2. Diversity and overlapof nectar sources

During the 15 weeks of observation thebees collected nectar from 55 different plantspecies, 21 of which were shared by the twospecies. M. fasciata used 47 plant species(26 exclusively) and M. beecheii used29 plant species (eight exclusively). M. fas-

ciata also collected nectar from a wider

range of plant species per colony per weekthan M. beecheii (MWU-test: Z = 5.70,P < 0.001 ; Nfasciala = 45, Nbeecheii = 35).Colony B5 collected nectar from more plantspecies per week than the other colonies ofM. beecheii (table I; MWU-test: Z = 2.74,P < 0.01; NB5 = 5, Nother B = 30).

The Shannon index of diversity, H’, washigher for M. fasciata than for M. beecheii(table I; MWU-test: Z = 5.84, P = 0.0001;Nfasciata = 45, Nbeechcii = 34). Again colony

B5 had a higher diversity than the otherM. beecheii colonies (MWU-test: Z = 2.62,P = 0.01; NB5 = 4, Nother B = 30).

Intraspecific overlap, measured as Horn’sindex, was higher for M. fasciata than forM. beecheii (table II). Interspecific overlapbetween M. beecheii colony 1 and the

M. fasciata colonies was high and interme-diate between the intraspecific overlap ofthe two species. This indicates that a con-siderable part of the important nectar sourceswas shared between the species.

3.3. Botanical origin of the nectar

By means of palynological analysis halfof the nectar sources could be identified at

generic or species level. A nectar sourcewas referred to as a major source if it con-tributed at least 30 % of the samples of onecolony during one observation day. All othersources are minor sources including the

major sources outside their ’major’ period.The presence of the major sources over theobservation period is depicted in table III.Four of the 11 most important sources wereCompositae (seven species in total; all herbs,vines and shrubs), two were Sapindaceae(one tree and one vine). Mimosaceae (fourspecies total; three trees and one liana), Tili-aceae (tree), Simaroubaceae (tree) and Labi-

atae (herb) accounted for one major sourceeach. Additional nectar sources that are notmentioned in table III or IV include Crotonschiedens (Euphorbiaceae; tree), Protiumsp. (Burseraceae; tree) and Cordia spinescens(Boraginaceae; liana).

3.4. Nectar concentration harvestedin relation to botanic origin

From half the plant species they shared,M. fasciata collected significantly morediluted nectar than did M. beecheii. A sim-ilar trend was found for three other nectarsources (0.1 > P > 0.05; table IV). No dif-ference was found for the remaining fourshared nectar sources. M. fasciata collecteda significantly higher sugar concentration

from the plant species it shared withM. beecheii than from sources that it used

exclusively (t-test: t = 4.27; P < 0.001;shared sources: average = 35.3 ± 5.6; n = 15;exclusive sources: average = 23.6 ± 7.6;n = 9; table IV).

3.5. Sugar concentrationof bee-collected nectarand micro-climate

To investigate whether the interspecificdifferences in nectar sugar concentrationwere influenced by regional climatologicalconditions, we compared nectar foraging ofM. beecheii and M. fasciata in El Sur (humidtropical forest zone) and Pozo Azul (drytropical forest zone). Both species collected

significantly richer nectar in Pozo Azul thanin El Sur and M. beecheii collected moreconcentrated nectar than M. fasciata in ElSur (table V; 3-way ANOVA: sample site:P < 0.001; bee species: P < 0.01). The sig-nificant interaction between bees and sam-

ple sites (P < 0.05) reflects the fact that thenectar sugar concentration collected by thetwo species was quite different in El Sur(figure 3) but similar in Pozo Azul (table V).Most of the nectar at both sample sites wascollected from Vernonia patens (Composi-tae; shrub) and Bravaisia integerrima (Acan-

thaceae; tree). The sugar concentration ofthe nectar of these plant species showedsimilar variation over the sample sites andbee species (table V).

4. DISCUSSION

4.1. Nectar harvesting by Melipona

According to ecological theory, speciesthat share a common environment need touse their environment in a slightly different

way, e.g. show resource partitioning. Essen-tial resources for bees include food sourcesand nest sites. We studied the partitioning ofnectar resources by M. beecheii and M. fas-ciala. These stingless bees occur sympatri-cally in the Central Pacific zone of CostaRica, but are allopatric in most of their dis-tributions. M. beecheii collected richer nec-tars than M. fasciata in humid tropical CostaRica. A similar differentiation has been

reported for four Melipona species normallyin overlapping disributions in Panama [ 19].In Panama M. compressipes collected richernectar than M. fasciata and in MexicoM. beecheii collected highly concentratednectar [22]. This agrees with the idea thatM. beecheii and M. compressipes are sisterspecies with mutually exclusive distribu-tions: M. compressipes occurs in Panamabut not in Costa Rica, whereas M. beecheiioccurs in Costa Rica and not in Panama [5].The results suggest that M. fasciata andM. beecheii are consistent in their respec-tive low- and high-sugar specialisation indifferent parts of their distribution.

4.2. Sugar concentrationof nectar harvested by Melipona

Many different factors may influence thesugar concentration of nectar harvested byMelipona. With the present results we can

disprove the influence of some factors andsuggest which are the main factors for the

interspecific differences.

The first set of factors are those related tothe botanical origin of the nectar and flowerchoice in the field. It is clear that preferencefor different plant species or different flowertypes has at the most only limited impact.The two species collected nectar from plantspecies in the same families and with simi-lar morphology (mostly open and sometubular flowers) and shared many of theirmajor nectar sources (tables III and IV). Thefact that M. beecheii collected more con-centrated nectar from the same plant speciesthan M. fasciata indicated that the twospecies use different foraging strategies thatleads them to visit different patches (or sub-patches) within the same habitat.

Climatological factors, e.g. temperatureand humidity, are known to influence thesugar concentration of floral nectar. At the

dry climate site (Pozo Azul) both speciescollected richer nectar than at the humid cli-mate site owing to the fact that both majorsources (V. patens and B. integerrima) pro-duced richer nectar in Pozo Azul than in ElSur. The two Melipona species collectednectars of similar richness in the dry cli-mate. This clearly indicates that local cli-mate highly influenced the sugar concen-tration presented by the plants and harvested

by the bees. In dry forest habitat almost nohumid shady places are present in the dryseason; however, in the rain forest habitatof El Sur both sunlit and shaded patches areabundant in the dry season, especially beforenoon. Therefore, it is hypothesised that inthe dry season the bees partition nectarsources based on patch climate in the wetforest habitat, but not in the dry forest habi-tat.

We are aware that differences in sugarconcentration of bee-collected nectar canbe (partly) due to other factors, such as beemorphology (e.g. tongue characteristics,body size and colour), specific needs ofsocial bees (e.g. colony thermo-regulationand humidity regulation) or interspecificcompetition. The influence of these ’bee-related’ factors on partitioning of nectarresources by the two species of Meliponaare dealt with in another paper [4].

4.3. Behavioural regulationof partitioning of nectar resources

This study does not address the questionin what aspect of their foraging strategiesM. beecheii and M. fasciata differ. How-ever, it was shown that even at the sametime of day the differences are considerable,so that we can rule out temporal differencesin foraging activity. Moreover, the habitatwas not different further away from thehives than close by, so that even if thespecies foraged at different distances fromthe colony it would not have led to differ-ences in harvested nectar (additional obser-vations demonstrated that neither of the

species changed the nectar sugar concen-tration between collecting and returninghome - unpubl. data). Interspecific differ-ences in communication system could nothave lead to such results, because we stan-dardised the number of samples per colonyper hour and it resulted that different nectar

quality was collected from the same plantspecies.

Among the behavioural traits that aremost likely to explain the observed differ-ences in nectar quality are competitive abil-ity of the species [10], innate preference forcertain sugar concentration or type, optimalsugar intake rates (but see [19, 22]) and tol-erance to direct insolation [26]. Differencesin sunlight regime contribute to the distri-bution of bees over flower patches ([26];Biesmeijer, unpubl. data). In El Sur light-coloured stingless bees seem to be moreabundant on the sunlit patches than in theshade, whereas the dark-coloured bees aremore abundant in the shade. Consequently,body colour may be an additional factor inthe partitioning of nectar resources betweenthe light-coloured M. beecheii and the dark-coloured M. fasciata, but other morpholog-ical and physiological factors as well asinterspecific competition cannot beexcluded. This study clearly shows that thesugar content of bee-collected nectar isbased partly on food plant choice and localclimate.

4.4. Why is M. fasciata absent fromthe dry forest in NW Costa Rica?

The foraging activity of M. fasciatacolony F4 was very high during its 3 weekstay in Pozo Azul, indicating that lack offood is not the main reason for its naturalabsence in the tropical dry forest zone inCosta Rica. In the longer term, regulationof the colony climate may be a problem forM. fasciata in the tropical dry forest, becauseit did not collect water during the observa-tions to cool down the colony (0 of 99 loadscontained less than 5 % sugar in Pozo Azul)but, in general, harvests watery nectar (fig-ure 1). In the tropical dry forest no waterynectar was available during the observationsin the middle of the dry season. M. fasci-ata, however, did not switch to water col-lecting which may in the long run lead tooverheating of the colony and damage tobrood or nest structures. M. beecheii doescollect water during dry hot weather (14 of

108 loads contained less than 5 % sugar inPozo Azul) and is capable of regulating itscolony climate under such circumstances.The behavioural difference may be the keyto the largely allopatric distribution ofM. beecheii and M. fasciata in Costa Ricaand most of its range.

ACKNOWLEDGEMENTS

Miguel Soto and the other members ofARBOFILIA allowed us to use their bees,arranged our stay in El Sur and were very help-ful in many ways. Our colleagues from CINATand PRAM let us use their facilities and assistedon many occasions. Fernando Ramirez helpedto obtain some bee colonies. The people of ElSur and Pozo Azul are thanked for their friend-

ship and help. Plants were identified with thehelp of Luis Poveda (Universidad Nacional,Hcredia) and José Gonzalez and Barry Hammell(INBIO, Santo Domingo). Hayo Velthuis, DickKocdam, Judith Slaa and two anonymous refer-ees made valuable suggestions that improved themanuscript and Sheila McNab made linguisticimprovements. This study was financed by theNetherlands foundation for research in the trop-ics (WOTRO), the Lucy Burgers Foundation,Uyttenboogaart-Eliasen Foundation and STIR(Utrecht University).

Résumé - Récolte de nectar par lesabeilles sans dard du Costa Rica :influence de l’origine botanique et du cli-mat sur la concentration en sucres dunectar récolté par Melipona. La récolte denectar par deux mélipones indigènes, Meli-pona beecheii et Melipona fasciata (Api-dae, Meliponinae), a été étudiée au CostaRica. Les deux espèces ont une aire de répar-tition qui se recouvre dans une petite partiedu Costa Rica (figure 1). M. beecheii se ren-contre principalement dans la forêt sèchedu NW du pays, tandis que M. fasciata estprésente dans tous les habitats de la forêtpluviale. Les mesures de la concentrationen sucres du nectar récolté par les butineusesmontrent que dans le milieu de la forêt tro-

picale humide M. beecheii récolte unifor-mément un nectar plus riche que M. fasciata

(figure 2). La concentration en sucres aug-mente durant la matinée et se stabilise après11 h (figure 3).En comparant le pollen présent sur le corpsdes butineuses et le pollen des fleurs pré-sentes dans la région, on s’aperçoit que lessources de nectar utilisées par les deux

espèces se recouvrent considérablement(tableau II). Le tableau III donne les prin-cipales sources de nectar durant la périoded’observation. M. beecheii récolte du nectar

plus riche en sucres que M. fasciata, y com-pris lorsqu’elles butinent toutes deux lesmêmes espèces. En moyenne M. fasciatarécolte des nectars à concentration en sucres

plus faible sur les espèces qu’elle utiliseseule que sur les espèces qu’elle partageavec M. beecheii (tableau IV). L’origine bota-nique du nectar explique donc en partie,mais pas en totalité, la différence dans laconcentration en sucres des nectars récol-tés par chacune des deux espèces.La différence observée sur le site de la

région tropicale humide ne se retrouve passur le site à climat tropical sec, où M. bee-cheii est présente naturellement et où unecolonie de M. fasciata a été introduite. Dansl’habitat sec, les deux espèces récoltent dunectar plus concentré que dans la régionhumide, ceci même pour des plantes de lamême espèce (tableau V). Les résultats sug-gèrent que le comportement de butinage desabeilles est à la base du partage des res-sources dans les forêts humides. &copy; Inra/

DIB/AGIB/Elsevier, Paris

Melipona / récolte nectar / teneur ensucres / analyse pollinique / plante nec-tarifère / partage ressources / Costa Rica

Zusammenfassung - Nektarsammeln derstachellosen Bienen in Costa Rica: bota-nische und klimatische Einflüsse auf dieZuckerkonzentration im von Meliponeneingetragenen Nektar. Das Sammeln vonNektar durch zwei heimische stachelloseBienenarten, Melipona beecheii und Meli-

ponafasciata, wurde in Costa Rica unter-sucht. Das Vorkommen beider Arten über-

lappt dort in einer schmalen Zone (Abb. 1).M. beecheii lebt hauptsächlich im Trocken-wald in Nordwest Costa Rica, während sichder Lebensraum von M. fasciata über denganzen Regenwald erstreckt. Messungender Zuckerkonzentration des von Sammle-rinnen eingetragenen Nektars zeigte, dassM. beecheii im tropischen Regenwaldes beihoher Luftfeuchte immer konzentrierterenNektar sammelte als M. fasciata (Abb. 2).Die Zuckerkonzentration zeigte jeden Mor-gen einen Anstieg, stabilisierte sich jedochnach 11:00 Uhr (Abb. 3).Ein Vergleich der Pollen am Körper derNektarsammlerinnen mit Pollen von Pflan-zen zeigte, da&szlig; sich die Nektarquellen derbeiden Arten beträchtlich überlappten(Tabelle II). M. beecheii sammelte Nektarmit einem höheren Zuckergehalt wie M. fas-ciata, selbst wenn er von der gleichenPflanze stammte. Im Durchschnitt sammelteM. fasciata geringere Zuckerkonzentrationenvon Pflanzenarten, die ausschliesslich vonihr besammelt wurde, als von solchen, dieauch von M. beecheii beflogen wurde(Tabelle IV). Demnach lassen sich die Unter-schiede in den Zuckerkonzentrationen zwi-schen den beiden Arten nur zum Teil durchdie botanische Herkunft des Nektars erklären.Obwohl sich ein Unterschied in der Zucker-konzentration des eingebrachten Nektars infeuchten, tropischen Gebieten ergab, wardieser Unterschied im trockenen, tropischenBereich nicht nachzuweisen. Für den Ver-such wurden Völker von M. fasciata in dentrockenen Bereich angesiedelt, währendM. beecheii dort natürlicherweise vorkommt.Im trockenen Lebensraum sammelten beideArten von denselben Pflanzenarten kon-zentrierteren Nektar als in dem feuchtenGebiet (Tabelle V). Es wird angenommen,dass das Sammelverhalten der Bienen die

Grundlage für die Aufteilung der Nah-rungsressourcen in feuchten Wäldern bil-det. &copy; Inra/DIB/AGIB/Elsevier, Paris

Nektareintrag / Zuckerkonzentration /Melipona / Pollenanalyse / Costa Rica

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