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Introduced Honeybees (Apis mellifera) Reduce Pollination Success withoutAffecting the Floral Resource Taken by Native PollinatorsAuthor(s): Roselaini Mendes do Carmo , Edivani Villaron Franceschinelli , Fernando Amaral da SilveiraSource: BIOTROPICA, 36(3):371-376. 2004.Published By: The Association for Tropical Biology & ConservationDOI: http://dx.doi.org/10.1646/03094URL: http://www.bioone.org/doi/full/10.1646/03094

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371

BIOTROPICA 36(3): 371–376 2004

Introduced Honeybees (Apis mellifera) Reduce Pollination Successwithout Affecting the Floral Resource Taken by Native Pollinators1

Roselaini Mendes do Carmo, Edivani Villaron Franceschinelli2

Departamento de Botanica, Universidade Federal de Minas Gerais, C.P. 486, 31.270-901, Belo Horizonte,MG, Brazil

and

Fernando Amaral da Silveira

Departamento de Zoologia, Universidade Federal de Minas Gerais, C.P. 486, 31.270-901, Belo Horizonte,MG, Brazil

ABSTRACTDetrimental effects of introduced honeybees (Apis mellifera L.) on native plant pollination have been predicted basedon their observed deterrence or expulsion of native pollinators or their depletion of floral resources. Here, a case isreported in which floral visitation by the introduced honeybee affects male fitness and probably fruit and seed pro-duction of a tropical tree without affecting the resource sought by the native pollinator. Clusia arrudae is a dioeciousspecies and is pollinated by individuals of Eufriesea nigrohirta that visit its flowers to collect resin. Male flowers,however, are also visited by individuals of A. mellifera, which remove ca 99 percent of their pollen grains. When E.nigrohirta leaves flowers previously visited by A. mellifera, they carry on their bodies less than 0.1 percent of the pollengrains carried by bees leaving flowers not visited by the honeybee. This may explain why the frequency of A. melliferaat male flowers is negatively correlated with the number of seeds produced by female flowers. This novel situationmay affect other plants offering alternative rewards for pollinators, such as resins, oils, and aromatic compounds.

RESUMOA polinizacao de especies nativas de plantas pode ser prejudicada em decorrencia das visitas da abelha introduzida Apismellifera. Isto tem sido previsto com base em registros de situacoes em que esta abelha impede o acesso de polinizadoresnativos ou os expulsa das flores e tambem pelo fato dela exaurir os recursos florais. Clusia arrudae e polinizada porEufriesea nigrohirta que busca resina em suas flores. Apis mellifera visita as flores masculinas de Clusia arrudae e removecerca de 99 percento dos graos do polen da flor visitada. Quando E. nigrohirta visita flores ja visitadas por A. mellifera,ela carrega em seu corpo menos de 0.1 percent dos graos de polen carregados por abelhas que visitam flores aindanao visitadas por A. mellifera. Isto pode explicar porque a frequencia de A. mellifera em flores masculinas e negativa-mente correlacionada com a quantidade de sementes produzidas por flores femininas. Esta e uma situacao que aindanao havia sido registrada e que pode afetar outras plantas que oferecem recursos alternativos para o polinizador, comoresinas, oleos, e compostos aromaticos.

Key words: Apis mellifera; Brazil; campo rupestre; Clusia arrudae; Eufriesea nigrohirta; male fitness; seed set.

POLLINATORS OR FLORAL VISITORS MAY INTERFERE

WITH EACH OTHER when visiting the same flowers,inflorescences, or plants. Interference may be di-rect, when they repel or deter each other, or indi-rect, when they deplete floral resources. These in-teractions can affect the reproductive success of theplant. For example, by removing floral nectar, Bom-bus occidentalis (a nectar-robbing bumble bee) maychange the behavior of legitimate pollinators, andconsequently, the pollination and fertilization suc-cess of Ipomopsis aggregata (Irwin & Brody 1998).

1 Received 4 June 2003; revision accepted 24 April 2004.2 Corresponding author; e-mail: edivani@icb.ufmg.br

This effect, however, is not observed in all plantspecies visited by B. occidentalis (Maloof 2001).

Exotic pollinators may cause populations of na-tive pollinators to decline, and as a result may de-crease the pollination success of plants (Kearns etal. 1998). Apis mellifera is the most widely distrib-uted exotic pollinator in the world (Kearns et al.1998). These bees may visit ca 40,000 plant speciesworldwide (Crane 1990), thus having strong im-pacts on plant–pollinator systems. In the WesternHemisphere, where honeybees are introduced, theymay be important competitors of native pollinators(Roubik 1980, 1983; Huryn 1997). Although it isthought that their impact on the reproductive suc-

372 Carmo, Franceschinelli, and Silveira

FIGURE 1. Female flower of Clusia arrudae visited byEufriesea nigrohirta.

cess of plants may be severe (Roubik 1989), studiesdemonstrating this impact are rare (Paton 1993,Aizen & Feinsinger 1994, Gross & Mackay 1998),and detailed research on plant species that span therange of plant reproductive systems is needed (Oso-rio-Beristain et al. 1996). Detrimental effects of A.mellifera on native plant pollination have been pre-dicted from observations of the deterrence or ex-pulsion of native pollinators (Gross & Mackay1998) or depletion of resources shared with nativepollinators (Roubik 1989, Paton 1993). On theother hand, honeybees sometimes have neutral oreven beneficial effects on the pollination of nativeplants (Gross 2001, Dick et al. 2003, Suzuki2003).

We observed A. mellifera visits to flowers of anative Brazilian tree Clusia arrudae. Clusia is aNeotropical genus with 250 species of tree, treelet,climbers, or epiphytes (Bittrich & Amaral 1996,1997); most species are dioecious. Flowers of mostClusia species offer resin as rewards to their oftenspecialized pollinators (Bittrich & Amaral 1997).The resin is produced by a ring of staminodesaround the pistil in female flowers, and in maleflowers, either by the expansion of the anther con-nectives or by a ring of staminodes around the fer-tile stamens.

Worker honeybees visit male flowers of C. arru-dae intensively. They begin to forage before theflowers are completely open and collect largeamounts of pollen. Here, we quantified the effectof these visits on pollen availability to the nativepollinator and on seed production. Specifically, wesought to answer the following questions: (1) Howmany pollen grains do A. mellifera workers collectfrom single flowers of C. arrudae?; (2) Do A. me-llifera visits to male flowers affect the amount ofpollen carried by the native pollinator of C. arru-dae?; (3) Does the frequency of A. mellifera visitsto male flowers of C. arrudae affect the seed set offemale plants?; and (4) Does A. mellifera have ter-ritorial or aggressive behavior against native visitorsof C. arrudae flowers?

METHODS

Data were collected at Serra da Calcada, a smallmountain range in the state of Minas Gerais, Brazil(208059360S, 438599000 W; ca 1480 m elev.). Thepredominant vegetation at the site is the ‘‘camposrupestres,’’ a gradient of field to chaparral-like veg-etation developed on rocky soils above 1000 m el-evation. Experiments and observations began on 10

January 2000 and the last fruits from these exper-iments were collected on 22 July 2000.

Clusia arrudae Planchon & Triana is a dioe-cious tree that occurs in the mountains of south-eastern Brazil (V. Bittrich, pers. comm.). A detailedstudy of flowers and pollination biology has beenpublished by Carmo and Franceschinelli (2002).According to these authors, blooming of C. arrudaepeaks between December and mid-February. Maleand female individuals produce flowers daily; how-ever, a synchronized anthesis peak occurs everythree days. The sex ratio in the population is 1:1,although male plants produce three times moreflowers than female plants. The flowers are largeand have eight white, thick petals. Male flowershave many stamens that are densely distributedaround nonfunctional stigmas and release largeamounts of pollen continuously for three days. Fe-male flowers have five to eight fused carpels withlarge stigmas surrounded by staminodes. Each car-pel sets a maximum of seven to eight seeds. Neitherfemale nor male flowers produce nectar. Rather, aring of staminodes close to the corolla of flowersof both sexes secretes resin that is collected by theorchid bee Eufriesea nigrohirta (Apidae, Euglossi-na), which is the primary pollinator of C. arrudae(Fig. 1; Carmo & Franceschinelli 2002). Thesebees probably employ resin in nest construction(Kimsey 1982, Armbruster 1984). Eufriesea nigro-hirta is a solitary bee endemic to open environ-ments at elevations above 1000 m in eastern Brazil.Although these bees occur naturally in low abun-dance, they seem to be efficient pollinators of C.arruda because they touch anthers and stigmasproperly when collecting the resin from male andfemale flowers, respectively (Carmo & Franceschi-nelli 2002). No asexual reproduction has been re-ported for C. arrudae. Apparently, reproduction of

Impact of Honeybee on a Native Plant 373

TABLE 1. Types of flower reward collected by each species of visitor and frequencies of visits to male and female flowersof Clusia arrudae. For the less frequent species, the values in parenthesis are the total number of bees observedvisiting flowers during the entire period of study (from Carmo & Franceschinelli, 2002).

Bee visitors

Rewards

Pollen Resin

Frequency of visits (bees/minute/flower)

Male Female

Apis melliferaEufriesea nigrohirta

yesno

noyes

30.001 (8)

0 (2)a

0.01 (11)Melipona quadrifasciataNeocorynura sp.

noyes

yesno

0.001 (4)0.002 (5)

0 (1)0

Trigona spinipes yes yes 1.7 0.03 (13)

a Very short visits without contact with the stigma.

C. arrudae is completely dependent on seeds pro-duced by the fertilization of ovules by spermaticnuclei.

Other bees observed visiting the male flowersto collect pollen were A. mellifera, Trigona spinipes,and Neocorynura sp. Melipona quadrifasciata wasseen visiting male and female flowers of C. arrudaefor resin, but they were very rare visitors (Table 1)and do not touch the stigma of female flowersproperly. Apis mellifera was by far the most com-mon visitor at male flowers (Table 1).

To estimate the amount of pollen taken by A.mellifera, a total of eight male buds on six plantsof C. arrudae were marked just prior to anther de-hiscence. Half of those buds were bagged to pre-vent contact with visitors and the others were leftopen to visitation by A. mellifera. Although otherbees were seen on marked flowers, their visits werebrief and rare. For this reason, it was assumed thatA. mellifera was responsible for almost all pollencollected from the four flowers open to visitation.After the three days of anthesis, all pollen releasedby the anthers of each flower was washed in 70percent ethanol and stored. The amount of pollenextracted from each flower was estimated using ahemacytometer (Kearns & Inouye 1993). Thus,the amount of pollen taken by A. mellifera wasestimated using the following formula: p 5100(t2r)/t, where p is the percentage of pollen col-lected by A. mellifera relative to the total amountof pollen produced by the flowers; t is the averageamount of pollen released per flower during thethree days, measured on the flowers protected fromvisitation; and r is the average amount of pollenremaining on the open flowers after three days ofvisitation.

To evaluate the impact of A. mellifera visitationon the amount of pollen carried on the bodies ofE. nigrohirta, three individuals of this bee were cap-tured when departing from flowers already visited

by A. mellifera. The pollen on their bodies was re-moved by rubbing the bees with small pieces ofglycerin jelly. These pieces were mounted on mi-croscope slides and pollen was counted under amicroscope. This process was repeated until no pol-len was found for three consecutive trials on eachbee. Three individuals of E. nigrohirta were alsocollected when departing from flowers not previ-ously visited by A. mellifera. These flowers werebagged before anthesis and then unbagged and ob-served after anthesis. Arriving A. mellifera werechased away before landing and native bees wereallowed to forage freely. Due to the large amountsof pollen present on bodies of E. nigrohirta col-lected on these flowers, direct counting was notpossible. Thus, these bees were shaken in 70 per-cent alcohol and the amount of pollen grains inthe suspension obtained was estimated using a he-macytometer, as above. A t-test was performed totest for differences between the amounts of pollenpresent on the bodies of E. nigrohirta followingvisits to flowers that had and had not been visitedby A. mellifera. Small sample sizes for this experi-ment were due to the difficulty we encountered inprotecting open flowers from visits by A. melliferawhile permitting visits by E. nigrohirta to theseflowers.

The influence of A. mellifera on the reproduc-tive success of female plants was estimated by cor-relating the frequency of visits by A. mellifera tomale flowers with the number of seeds producedper locule by female flowers open on the same days.For this, 9–20 buds of five female plants weremarked on the day prior to each of five synchro-nized anthesis peaks; these peaks began on 15 Jan-uary, 18 January, 21 February, 26 February, and 4March 2000. Subsequently, their development tofruits was checked and number of seeds per fruitwas counted. During those same anthesis peaks, thefrequency of A. mellifera visits to 15 flowers of

374 Carmo, Franceschinelli, and Silveira

FIGURE 2. Correlation between the average numberof Apis mellifera worker visits to male flowers of Clusiaarrudae and the number of seeds produced per locule byfemale flowers (rs 520.9; P , 0.05). Dates are the firstdays of data collection in each of the five synchronizedanthesis peaks. See text for full explanation.

three male plants (5 flowers/plant) was estimatedfor their three days of anthesis. Five flowers perplant were observed simultaneously for three ten-minute intervals beginning at 0900, 1000, and1100 h (the highest frequency of flower visits in-cluding those by A. mellifera occurred during thelate morning) on each of the three days of anthesis.Thus, each male flower was observed for 90 min-utes (30 min/d 3 3 d of anthesis) and each maleplant was observed for a total of 450 minutes (30min/d 3 3 d of anthesis 3 5 periods of synchro-nized blooming). Interactions among A. melliferaand native bees on flowers of C. arrudae were ob-served and territorial and aggressive behaviors wererecorded.

After six months, fruits of female plants werecollected and seeds counted. The relationship be-tween the mean number of A. mellifera visits permale flower during each three-day synchronizedanthesis peak and mean seed number per locule inthe fruits produced by female flowers in anthesis atthe same time was evaluated with Spearman’s rankcorrelation test.

RESULTS

Apis mellifera collected ca 99 percent of the pollengrains produced by flowers of C. arrudae. Meannumbers of pollen grains remaining after the thirdday of anthesis in flowers protected from visitationand visited by A. mellifera were 11,587,667 (SD 51,342,278) and 68,500 (SD 5 8172), respectively(N 5 4 flowers for each treatment). As a result,females of E. nigrohirta leaving flowers of C. arru-dae already visited by A. mellifera carried on average177.3 (SD 5 64.0) pollen grains on their bodiesor fewer than 0.1 percent of the pollen grains car-ried by females leaving flowers not yet visited (x 5236,667; SD 5 46,458; N 5 3; t 5 8.817; df 54; P , 0.01).

The frequency of A. mellifera at male flowerswas negatively correlated to seed production on fe-male flowers open during the same interval on fe-male plants (rs 5 20.9; P , 0.05; Fig. 2), whichmay indicate that when visits of A. mellifera arevery frequent, female fitness may decline.

No aggressive behavior of A. mellifera was ob-served toward other visitors. Apis mellifera workersdid not expel other bees from the flowers they werevisiting. In fact, they seemed to avoid flowers beingvisited by other bees and were observed being ex-pelled from flowers by workers of T. spinipes, a no-toriously aggressive bee (Almeida & Laroca 1988).We could not determine, however, if the presence

of A. mellifera on flowers inhibited native pollina-tors from visiting those flowers.

DISCUSSION

Because A. mellifera occurs in such high numbersin male flowers of C. arrudae and starts collectingpollen even before anthesis, chances are small thatE. nigrohirta will visit flowers not previously visitedby A. mellifera. Because honeybees extract almostall the pollen from the flowers they visit, it seemsprobable that they actually reduce male reproduc-tive success of C. arrudae. It is noteworthy that theeffect of A. mellifera on C. arrudae fertility was notdue to expulsion of native pollinators or to deple-tion of resources actively sought by them, as hasbeen suggested by Paton (1993), Roubik (1980),and Vischer and Seeley (1982) for other plant spe-cies.

Trigona spinipes and Neocorynura sp. also col-lected pollen from C. arrudae; however, individualsof these species collect small amounts of pollen pervisit because they are much smaller than A. melli-fera and they also visit flowers at lower frequenciesthan does A. mellifera (Table 1). Notably, Neocory-nura sp. collects pollen leftovers on the petals suchthat these bees probably do not affect the repro-ductive success of C. arrudae. This is supported bythe large difference between the amounts of pollenon the bodies of E. nigrohirta departing from flow-ers previously visited versus not visited by A. me-llifera. In both situations, foraging by other nativeflower visitors was allowed.

When A. mellifera removes large amounts ofpollen from flowers of the same species, acting only

Impact of Honeybee on a Native Plant 375

as pollen robbers, impacts on female reproductivesuccess of such species may be highly significant(Huryn 1997). Carmo and Franceschinelli (2002)noted that seed production by C. arrudae at Serrada Calcada was only 68 percent of that obtainedfrom hand-pollination, suggesting that C. arrudaefertility at the site may be pollen-limited. The datapresented here indicate that at least part of this lowrate of reproductive success may be due to the for-aging activity of A. mellifera. This is supported bythe negative correlation found between the fre-quency of A. mellifera at male flowers and seedproduction by contemporaneous female flowers.

Although frequencies of A. mellifera had dif-fered greatly during the weeks over which the studywas conducted, the average number of seeds perlocule increased only from 3.9 to 5.1 (Fig. 2). Atthe peak of the flowering period (January), seedproduction should have been the highest becausepollen availability was high and pollinators werefrequent. In contrast, the population of C. arrudaeshowed lower seed production in January than atthe end of February and March, when flower pro-duction was 80–90 percent lower than in January(Carmo & Franceschinelli 2002). It seems probablethat without the interference of honeybees, seedproduction from flowers produced during the peakin blooming would be higher than the 5.1 observedat the end of the flowering period.

Plants with specialized pollination systems maybe more negatively affected by efficient pollen rob-bers such as A. mellifera than are species with gen-eralist pollinators (Huryn 1997, Gross & Mackay1998). In the case of C. arrudae, the impact of thisintroduced bee is magnified by specific traits relatedto its reproductive biology. Clusia arrudae relies ona pollinator that seeks a specific resource (resin) notcollected by most other flower visitors. Moreover,because C. arrudae is dioecious and the resourcesought by robbing A. mellifera is found only atmale flowers, the result is a very interesting situa-tion (apparently reported here for the first time) inwhich the introduced pollinator is detrimental tothe plant reproduction without reducing the plant’sattractiveness or affecting resource availability to itsnative pollinator. Exotic bees, as well as highly ef-ficient native pollen robbers, may similarly affectother plants that offer alternative rewards for pol-linators such as resins, oils, and aromatic com-pounds (Simpson & Neff 1981, Armbruster 1984,Buchmann 1987, Vogel 1988).

ACKNOWLEDGMENTS

We wish to thank D. Resende for her help with dataanalysis and Drs. L. McDade, M. McIntosh, J. H. Cane,and S. Menon for valuable suggestions on the manuscript.R. M. Carmo was supported by a fellowship granted byCAPES.

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