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La distribucin espacial, la produccin de nctar, y las visitas de los polinizadores en Habenaria monorrhiza (Orchidaceae)
Spatial distribution, nectar production, and pollinator visitation in Habenaria monorrhiza (Orchidaceae)
This study examines the effect of patchy distribution on the production and concentration of nectar of Habenaria monorrhiza at Monteverde, Costa Rica. Sixty patches of H. monorrhiza were identified; patch size ranged from one to six individuals. One group of patches (N=24) was covered to prevent pollination and was examined for the amount and concentration of nectar. Another group of patches (N=36) was left uncovered and was tested to determine the effect of patch size on visitation rates of pollinators. There was no significant effect of patch size on the amount or concentration of nectar produced (F = 1.3039; p = 0.2568; df total = 35). However, increasing patch size significantly increased the percent of pollinia removed on a plant (F = 45.1034; p < 0.0001; df total = 35). Furthermore, many flowering plants depend on the nectar production of their neighbor to attract pollinators, but H. monorrhiza was proven in this study not to regulate the amount or concentration of nectar. It does, however, depend on its large patch size to increase visitation by its moth pollinator.
Este estudio examin el efecto de la distribucin en grupos aislados sobre la produccin y la concentracin de nctar de Habenaria monorrhiza en Monteverde, Costa Rica. Sesenta parches de H. monorrhiza fueron identificados; el tamao de los parches vari entre uno y seis individuos. Un grupo de parches (N=24) fue cubierto para prevenir la polinizacin y fue examinado para determinar la cantidad y la concentracin de nctar. Otro grupo de parches (N = 36) no estuvo cubierto y fue examinado para determinar el efecto del tamao de grupo sobre la frecuencia de visitas por los polinizadores. No se encontraron diferencias significativas en el efecto del tamao del parche sobre la tasa de visitas de los polinizadores (F = 1.3039; p = 0.2568; df total = 35). Sin embargo, el aumento del tamao del parche increment significativamente el porcentaje de polinia removido en una planta (F = 45.1034; p < 0.0001; df total = 35). Muchas plantas con flores dependen de la produccin de nctar de su vecino para atraer al polinizador, pero se demostr que en el caso de H. monorrhiza no hay regulacin de la cantidad o la concentracin de nctar. No obstante, H. monorrhiza depende del gran tamao del parche para aumentar el nmero de visitas de los polinizadores.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone--Monteverde
Monteverde Biological Station (Costa Rica)
Plantas de miel
Costa Rica--Puntarenas--Zona de Monteverde--Monteverde
Estacin Biolgica de Monteverde (Costa Rica)
Tropical Ecology Fall 2005
Nectar producing plants
Biologa Tropical Otoo 2005
Plantas productoras de nctar
t Monteverde Institute : Tropical Ecology
Spatial Distribution, Nectar Production, and Pollinator V isitation in Habenaria monorrhiza (Orchidaceae) M.L. Tsupros College of Arts and Sciences: Department of Biology, Indiana University ABSTRACT This study examines the effect of patchy distribution on the production and concentration of nectar of Habenaria monorrhiza at Monteverde, Costa Rica. Sixty patches of H. m onorrhiza were identified; patch size ranged from one to six individuals . One group of patches (N=24) was covered to prevent pollinatio n and were examined for the amount and concentration of nectar. Another group of patches (N=36) were left uncovered and were tested to determine the effect of patch size on visitation rates of pollinator s . T he re was no significant effect of patch size on the amount or concentration of nectar produced ( F = 1.3039; p = 0.2568 ; df total = 35). However, increasing pa tch size significantly increased the percent of pollinia r emoved on a plant ( F = 45.1034; p < 0.0001 ; df total = 35 ). Furthermore, many floweri ng plants depend on the nectar productio n of their neighbor to attract pollinator s , but H. monorrhiza was proven in this study not to regulate the amount or concentration of nectar . I t does , however, depend on its large patch size to incre ase visitation by its moth pollinator. RESUMEN Este estudio examin Ã³ el efe c to de la distribuciÃ³ n en grupos a islados sobre la producciÃ³n y la concentraciÃ³n de nÃ© ctar de Habenaria monorrhiza en Monteverde, Costa Rica. Sesenta parches de H. monorrhiza fueron identificado s; el ta maÃ± o de los parches variÃ³ entre uno y seis individuos . Un grupo de parches (N=24) fue cubierto para prevenir la polin izaciÃ³n y fue examinado para determin ar la cantidad y la concentraciÃ³n de nÃ© ctar. Otro grupo de parches (N = 36) no e stuvo cubier to y fue examinado para determinar el efecto de l tama Ã± o de grupo s obre la frecuencia de visitas por los poliniza dores. No se encontraron diferencias significativas en el efecto del tamaÃ±o del parche sobre la tasa de visitas de los polinizadores (F = 1.303 9; p = 0.2568; df total = 35 ). Sin embargo , el aumento del tamaÃ±o del parche incrementÃ³ significativamente el porcentaje de polinia removido en una planta (F = 45.1034; p < 0.0001; df total = 35 ). Muchas plantas con flores dep ende n de la producciÃ³n de n Ã©ctar de su vecino para atraer a l polin iza dor , pero se demostrÃ³ que en el caso de H. monorrhiza no hay regulaciÃ³n de la cantidad o la concentraciÃ³n de nÃ© ctar. No obstante, H. monorrhiza dep ende del gran tamaÃ±o del parche para aumentar el nÃº mero de visita s de los polin iz adores. INTRODUCTION Habenaria monorrhiza is part of the largest genus of orchids in the neotropics w ith over 500 known species (Gentry 1993) . This is a primitive group in which little is known. This lack of knowledge includes, but is not limited to the pollination of Habenaria , although the phenotype s of mos t species in this genus indicate that they are moth pollinated (Zomlefer 1994; Dressler 1990). Through observation, it is known that Habenaria monorrhiza share many of these tr a its, therefore is also presumed to be moth pollinated (Walter 1983). The m ost noticeable such qualities ar e the large ra ceme inflorescences that feature 2 15 sp irally arranged flowers that bloom synchronously (Hammel et al . 2003). They have a patchy dist ribution , meaning they grow in clumps, and are found in disturbed , open areas along r oad sides and in grassy habitats (Mora and Atwood 1992) . These open areas make flowers easy for a moth to find when foraging. The lip of the flower is deeply dissected i n to 3 linear lobes , which create a good landing
pad for the pollinator (Mora and Atwood 1992). Habenaria monorrhiza ha s a large, specialized spur (up to 25 mm) , which contains nectar (Fig. 1a ) (Zomlefer 1994; Gentry 1993). The l ong proboscis of a m oth matc hes this morphology well . Both sectile pollinia are supported on a slender caudicle , which is about 5 mm long (Mora and Atwood 1992). When a moth stops for nectar , the rostellum that surrounds the pollinia (Fig. 1b) gets stuck to the wings or the face of the mot h . If the pollinator continues to search for nectar, pollen from the pollinia will be deposited on the stigma of another flower, thus beginning the repr oduction process (Mora and Atwood 1992; Zomlefer 1994) . The nectar production of Habenaria mo norrhiza has also been understudied. However, o ther wild plant popul ations have been found to produce flowers with little nectar if a neighbor is already producing large amounts of nectar (Jones and Little 1983). Also , the concentration of nectar for som e flower species can be highly variable b etween individuals depending on their distribution and location (Howe and Westley 1988). It is possible that a similar relationship between spatial distribution and nectar production occurs in H. monorrhiza . In th eory , larger patches of H. monorrhiza will offer more ne ctar to a pollinator than a single inflorescence . N ectar production , which is presumabl y energ etically expensive , s hould be avoided if possible, so long as pollinator visitati on is not negatively aff ected. If moths are attracted to larger patches containing more flowers and hence more nectar, it may be possible for H. monorrhiza to reduce nectar production without sacrificing pollinator visitation. Therefore, to increase fitness by saving energy , it is predicted that flowers within larger patches of H. monorrhiza will produce less nectar at lower sugar concentration s . Additional ly , if the s e larger patches are more desirable to pollinators , the n , once said pollinators are present , they will tend to s tay within that spatial range, and visit nearby inflorescences (Howe and Westley 1988). Both nectar production and spatial distribution are proposed method s to increase out crossing, which is the favored pollinati on mechanism employed by orchids. (Walter 1983). This study investigates the effect of s patial distribution on nectar production and concentration, as well as poll inator visitation. Plants cannot control how they grow with respect to members of the same or other species . Therefore , w hen attract ing a pollinator, an individual may have to compensate for being in a bad location . Likewise, an individual may be offered the opportunity to take advantage of being in a good location. One solution is to according ly alter nectar volume and concentration . The prediction is that patches with more plan ts will produce, on average, a lower amount and concentration of nectar. This is contrary to single plant patches , in which nectar is predicted to be produced in higher amounts and concentra t ions to attract p ollinator visitation . Lastly, a higher percentage of pollinia are predicted to be missing from H. monorrhiza flowers that occur on inflorescence s in larger patches than on flowers on an inflorescence in a single plant patch. This is predicted because larg er patches shou ld attract more pollinators, and , because they contain larger amounts o f nectar, m oths should tend to remain in them . M ETHODS Study site and determination of patchiness This study was conducted in Monteverde, Puntarenas Province, Costa Ric a from October 22 to November 15, 2005 along the roadside between the EstaciÃ³n BiolÃ³ gica and Hotel Belmar. The first step in this experimen t was to define patch es withi n the study site. The d istance between each plant and its nearest neighbor was estimate d with a tape m easure
(cm) . These measurements were then atches were defined as a group of two or more plants that were as close or closer to each other than the mean distance (150 cm). Nectar volume and concentration To be included, an in dividual had to have at least three open, unpollinated flower s on its i nflorescence. Each infl orescence on a chosen plant was covered using a mesh polli nation bag, so that flowers were not visited during the course of the experimen t. T he height of nectar in the spur of each flower was measured; t his entailed removing the three uppermost flowers from each plant and hold ing a flashl ig ht behind the spur. S ubtle line s were exposed on the spur , which marked the height of the nectar within; this was then measu red using calipers (mm) . Next , each spur was cut open, exposing the ne ctar. The nectar was tr ansferred to a refractometer to determine sugar concentration. The c oncentrat ion and volume of nectar were compared between pl ants in different patch sizes . A regression analysis was used to compare these relationship s . The total number of patches of bagged individuals equaled 24 and the total number of individuals equaled 66. Rate and Visitation A second set of patches was found using the same criteria above. The only difference was that , to be eligible for this set, the inflorescences only needed to have one open flower, and it could be poll inated. These plants were used to study visitation by pollinators , instead of for nectar observations . Therefore, pla nts in this set of patches were not bagged. Using a 10X hand lens, the presence or absence of pollinia was noted for each flower on each inflorescence. Patch size was recorded , and the relationship with the percentage of flowers with missing pollinia was analyzed with a regression analysis. If one or two polli nia were absent, the flowers were considered to have been pollinated , because the moth pollinator frequently only removes one pollinium per visit (Singer 2001). A regr ession graph was used to compa re the relationship between the average percent of pollinia missing within a whole patch and patch size ; the average pe rcent of pollinia missing for each flower on an individual plant within a patch and patch size was also analyzed . Ninety three total inf lorescences, comprising 3 6 total patches, were observed. RESULTS The h eight of nectar in the spur, the percent concentrati on of the nectar sampled , and the patch size we re regressed. There was no significant relationship fou nd between these parameters ( F = 1.3093; p = 0.2568 ; df total = 65 ) (Fig. 2 ). The t otal average percent of pollinia removed within a patch and patch size showed a significant correlation ( F = 45.1034 ; p < 0.0001 ; df total = 35 ) (3a.) . The relationship between average percent pollinia removed per inflorescence and patch size yielded was also found to be significant (F= 36.3948 ; p < 0.0001 ; df total = 92 ) (3b.) . DISCUSSION The results proved the first hypothesis ( larger patches would have less nectar amounts and concentrations ) to be i nsignificant (Fig. 2 ). This means that , since growth patterns are random, single plant patches must find other ways to compensate and attract a pollinator . However , the latter hypothesis, which stated that patch size affects the rate of
pollinator visita tion , was supported . Percent pollinia removed , w hen compared to patch size, was signif icant (Fig. 3 ). This suggests that pollinator visits increase along with patch size . Once again, we know that nectar production is presumed to be energetically costly to a plant. For example changing the different types of speciali zed sugars, depending on the particular pollinator tha t a plant is trying to attract , this can change seasonally in some species (Howe and Westley 1988) . Since there was no trend in the nect a r production for different patch sizes , it must be concluded that H. monorrhiza does not use nectar regulation in order to save energy. Even though this trend has been seen in other plant species, nectar production for H. monorrhiza was s o highly variabl e that it was apparently indepe ndent of patch size . One reason that H. monorrhiza does not put e nergy into nectar regulation could be that it can self pollinate; this means that no pollinator is necessary in order for the plant to reproduce (Tannourji 200 0). Most orchids are self compatible , but their structures tend to favor out crossing (Dressler 1992). Since this act of selfing is possible , H. monorrhiza does not need to depend on nectar regulation to compensate if it does not grow in a patch . Repro duction is still possible without pollination ; there have been many ca ses of solitary orchids that self pollinate in the absence of other means of reproduction (Dressler 1992). In addition to alternative forms of reproduction, H. monorrhiza has other met hods of attracting pollinators. Since missing pollinia imply that a flower has been visited by a pollinator , the observed results suggest that larger patches have more success with pollination (F ig. 3 ). Not only were a higher percentage of po llinia missi ng as patch size increased a s a whole, but individuals within larger patch es also showed a sign ificantly higher rate of visitation . T his could be due to the foragi ng habits of pollinating moths . It has been found that once a pollinator finds a plant , the pollinator is more likely to visit the next closest plant than a further one (Howe and Westley 1988) Thus , a patchy distribution may be an effective way for H. monorrhiza to increase visitation by a pollinator to an individual inflorescence . O nce the po llinator has been drawn to the patch by nectar cues, all individuals that are lucky enough to be in that patch are likely to be visited. In conclusion, self p ollination could be a reason that nectar production is not regulated . Habenaria monorrhiza doe s not expend energy to change the amount or concentration of nec tar it produces in accordance with its spat ial distribution , because it has alternative forms of reproduction in the worst case scenario (not being pollinated by an external source ) . Instead , H. monorrhiza depends on larger patches to maximize pollinator visitation. Further studies could review the rate of selfing as compared to out crossing , in accordance with patch size. Further more , since the inflorescences of H. monorrhiza are so variabl e, the size of inflorescences and their effect on foraging behavior could be explored . ACKNOWLEDG E MENTS I would like to take this opportunity to thank Karen Master s for always telling me how well I am doing. Also my beloved Alan Masters for always bein g so hip and for tons of hugs ! Jointly I would like to thank Karen and Alan for providing all those delicious snacks to us rascals. Thanks to Ollie and Marie for answering all my really SMART! questions and for supporting me in all my spinning endeavors. forget Javi, for making my neck Last but not least E staciÃ³n BiolÃ³ gic a and Costa Rica for revealing all its beauty and magic. P.S . Bottton Feeders stink !!!
LITERATURE CITED Dressler, R.L. 1990. The Orchids: Natural History and Classification. Harvard University Press, Cambridge, Mass. Gentry, A. H. 1993. Woody Plants of Northwest South America. Conservation Intenationa l, Washinton DC. Howe, H.F. and L. C. Westley. 1988. Ecological Relationships of Plants and Animals. Oxford University Press, New York, New York. Hammel, B.E., M.H. Grayum, C. Herrera, and N. Zamora. 2003. Manual de Plantas de Costa Rica, Volumen III. M issouri Botanical Garden Press. St. Louis, Missouri. Jones, C. E. and R.J. Little. 1983. Handbook of Experimental Pollination Biology. Van Nostrand Reinhold. New York, New York. Mora, D.E. and J.T. Atwood. 1992. Habenaria monorrhiza. Icones Plantarum Tro picarum Sarasota, Florida. Singer, R. B. 2001. Pollination Biology of Habenaria parviflora (Orchidaceae: Habenariinae) in Southeastern Brazil. Darwiniana 39 (3 4): 201 207. Tannourji, D. 2000. Reproductive Strategies and Modes of Pollination found in the Primitive Orchid Habenaria monorrhiza (Orchidaceae). UCEAP, Monteverde Tropical Biology, Fall 2000. Walter, K.S. 1983. Orchidaceae (Orquideas, orchids). In: Costa Rican Natural History , D.H. Janzen, ed. The University of Chicago Press, Chicago, Illi nois, pp. 282 291. Zomlefer, W.B. 1994. Guide to Flowering Plant Families. The University of North Carolina Press, Chapel Hill, North Carolina, pp. 293 296.
Figure 1. Vegetative and reproductive structures of Habenaria monorrhiza (a) specialized spur (25mm) containing nectar of plant (b) pollinia ( from Mora and Atwood 1992).
a. b. Figure 2. See text for details on experiment setup. Patch size equals the number of individual inflorescence s that are 150cm or closer from each other (a ) Mean height of nectar (mm) between flowers for one plant within a patch vs. the size of patch in which it was present (b ) mean percent concentration of nectar between flowers for one plant within a patch vs. the size of patch in which it was pre sent. a. b. Figure 3 . See text for details on experimental setup. Patch size equals the number of individual inflorescences that are 150cm or closer from each other (a) percent of flowers with missing pollinia per patch, with data from total number of flowers on all plants in patch averaged, vs. the size of patch (b) percent of flowers with missing pollinia per plant, with data from all flowers on a plant averaged vs. the siz e of patch plant was present in.