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La eficacia del tordo de montaa (Turdus plebejus) como un dispersor de semillas en la zona de Monteverde
The effectiveness of the mountain robin (Turdus plebejus) as a seed disperser in the Monteverde area
Quantitative and qualitative factors are important in determining the effectiveness with which bird species disperse seeds. This study examined several factors that may influence the quantitative component of
dispersal in the mountain robin. Regurgitation times of three common fruits in the area were examined. Additionally, the size at which seeds transition from being regurgitated to defecated was studied. Results show that regurgitation times between these three fruits do not differ significantly. However, there was a
significant difference in the size of regurgitated (X = 5.97 mm 2.17 mm) and defecated seeds (X = 4.25 mm 2.55 mm) (t = -17.192, P < 0. 0001). This information may be important in determining the way in which seeds of different sizes are dispersed. Furthermore, if combined with future field studies documenting number of visits made to a fruiting tree and length of each visit, insight will be gained regarding the effectiveness of the mountain robin as a seed disperser.
Los factores cuantitativos y cualitativos son importantes para determinar la eficacia con la cual las especies de aves dispersan las semillas. Este estudio examin varios factores que pueden influenciar el componente cuantitativo de dispersin del tordo de montaas. Se examin los tiempos para regurgitar tres frutos comunes en el rea. Adicionalmente, se estudi el tamao en el cual las semillas se pasan de ser regurgitada a ser defecadas. Resultados muestran que los tiempos de regurgitacin entre estas tres fructosa no difieran significativamente. No obstante, hubo una diferencia significativa (t = -17.192, P < 0. 0001) en el tamao de las semillas regurgitadas (X = 5.97 mm 2.17 mm) y defecadas. Esta informacin puede ser importante para determinar la forma en que las semillas de diferentes tamaos son dispersadas. Adems, si se combina con futuros estudios de campo que documenten el nmero de visitas hechas a un rbol con frutos y la duracin de cada visita, se ganara informacin con respeto a la eficacia del tordo de montaa como un dispersor de semillas.
Text in English.
Monteverde Biological Station (Costa Rica)
Estacin Biolgica de Monteverde (Costa Rica)
Tropical Ecology Summer 2004
Ecologa Tropical Verano 2004
t Monteverde Institute : Tropical Ecology
The Effectiveness of the Mountain Robin ( Turdus plebejus ) as a Seed Disperser in the Monteverde Area Sean Brady Lundquist College of Business, University of Oregon ABSTRACT Quantitative and qualitative factors are important in determining the effecti veness with which bird species disperse seeds. This study examined several factors that may influence the quantitative component of dispersal in the Mountain Robin. Regurgitation times of three common fruits in the area were examined. Additionally, the siz e at which seeds transition from being regurgitated to defecated was studied. Results show that regurgitation times between these three fruits do not differ significantly. However, there was a significant difference in the size of regurgitated (X = 5.97 mm 2.17 mm) and defecated seeds (X = 4.25 mm 2.55 mm) (t = 17.192, P < 0. 0001). This information may be important in determining the way in which seeds of different sizes are dispersed. Furthermore, if combined with future field studies documenting num ber of visits made to a fruiting tree and length of each visit, insight will be gained regarding the effectiveness of the Mountain Robin as a seed disperser. RESUMEN Los faactores cuantitativos y cualitativos son importantes para determinar la eficacia c on la cual las especias de auesa disperson las semillas. Este estadio examin varias factores que pueden influenciar el componente cuantitativo de dispercin del tordo de montaas. Se examin los tiempos para regurgitar tres frutos communes en el area. Adi cionalmente, se estudi el tamao en el cual las semillas se pasan de ser regurgiterada a ser deficadas. Resultitados muestran que los tiempos de regurgitacin entre estos tres frutosa no difieran significativamente. No obstante, hubo una diferencia signif icativa (t = 17.192, P < 0. 0001) en el tamao de las semillas regurgiitas (X = 5.97 mm 2.17 mm) y defecatadas. Este informacin puede ser importante para determinar la forma en que las semillas de diferentes tamaos son dispersadas. Ademas, si se combi na con futuros estudios de campo qe documenten el nmero de visitas hechas a un rbol con frutos y la duracin de cada visita, se ganara informacin con respeto a la eficacia del tordo de montaa como un dispersor de semillas. INTRODUCTION In the cloud f orest of Montverde, Costa Rica more than 171 tree species rely at least partially on birds for seed dispersal (Wheelwright et al. 1984). Correspondingly, there are over 70 species of fruit eating birds in the area. These species form a complex web of mutua lisms in which birds depend on plants for food and plants rely on the birds to disperse their seeds. However, not all birds are equal in their effectiveness as seed dispersers. Effectiveness is measured using both quantitative and qualitative components (S chupp 1993). This means many factors must be studied in order to determine the dispersal effectiveness of any particular species. An important seed disperser in the Monteverde area is the Mountain Robin, ( Turdus plebejus Turdidae). This species is com mon in montane forests from S. Mexico
to W. Panama (Stiles and Skutch 1989). Though the Mountain Robin possesses a relatively small gape (12 mm) it has been known to consume more than 44 species of fruits in the Monteverde area (Wheelwright et al. 1984). S till, the effectiveness with which the Mountain Robin disperses the seeds of these fruits is not known. Moreover, it is not known if effectiveness changes with fruit type or seed size. Quantitative factors that may influence the abilities of the Mountain Robin as a seed disperser include number of visits to trees, the number of seeds dispersed per visit and the total number of seeds dispersed (Schupp 1993). Since this particular species of ting the fruit in any way) and visit length is not known, gut processing time may be an important trait in determining seed dispersal effectiveness (Schupp 1993). Furthermore, given that passage time is almost always longer in defecation than regurgitation (Johnson et al. 1985), the threshold at which this change occurs may be important in predicting gut processing time. The purpose of this study was to determine the gut processing time of the Mountain Robin for Viburnum costaricanum (Caprifoliaceae) Cham aedorea pinnatifrons and Chamaedorea costaricana (Arecaceae), three common fruit species in the cloud forest. Additionally, this study used the varying size of V. costaricanum fruits and seeds to determine the size range at which seeds go from being defec ated to being regurgitated. It was hypothesized that the small size of the V. costaricanum seeds would result in it having the fastest regurgitation time of the three fruits. Also, it was predicted that rather than having a clearly defined size boundary, t here would be a gradient in which seeds move from being defecated to regurgitated. MATERIALS AND METHODS Site Description In this study four Mountain Robins were captured near the Estacin Biolgica in Monteverde, Costa Rica. All four birds were capture d at an elevation of 1500 m in an area where many individuals were observed feeding and/or roosting. Data collection on captive birds was done in the lower lab of the Estacin Biolgica. V. costaricanum C. pinnatifrons and C. costaricana fruits were col lected from seven different sites in forest and pastures adjacent to the Estacin Biolgica. Sampling Mountain Robins were captured using mist nets. Nets were opened at 5:00 am and closed at 7:30 am. They were reopened at 4:30 pm and closed at 6:00 pm. W eather permitting, mist netting occurred each day from July 20 th to July 31 st During the netting process a variety of birds were captured; all except for the Mountain Robin were released. Once in captivity, birds were held in 1 m x 1 m x 1 m enclosures c rafted from mosquito nets. The enclosures were made dark by covering them with sheets and black plastic bags to calm the birds. The gape of each bird was measured to the nearest millimeter using calipers and weight was calculated to the nearest gram using a pissola scale. Each Mountain Robin was held for no more than three days to avoid
undernourishment. At the end of this time period individuals were released outside of the Estacin Biolgica. To ensure fresh food for the birds, ripe fruits of V. costaric anum C. pinnatifrons and C. costaricana were gathered no more than an hour prior to each feeding. Vibernum costaricanum fruits were chosen non randomly to assure variance in fruit and seed size. All fruits were cut from their respective trees using a ma chete and stored in plastic bags until feeding. Feeding sessions lasted approximately one hour and were done an average of two times per day. At each feeding all three fruit types were simultaneously offered to the Mountain Robin. Time of consumption and fruit type were recorded each time a fruit was eaten along with time of defecation or regurgitation of seeds. At the end of each session, expelled seeds were collected and stored separately depending on the way in which they were processed (regurgitated o r defecated). Each night, birds were left with all three fruit types. The following morning, processed seeds were collected and separated depending on if they were defecated or regurgitated. The difference was easy to determine as defecated seeds were foun d in feces while regurgitated seeds lay cleanly at the bottom of the enclosure. Calipers were used to measure seed size to the nearest 0.1 mm at the widest point. Analysis An ANOVA test was run to determine the effects of fruit type on regurgitation time The difference in regurgitation time compared to defecation time and regurgitated seed size compared to defecated seed size in V. costaricanum were compared using unpaired t tests. Box plots were created and compared for regurgitated and defecated seeds to examine the size threshold at which seeds change from being defecated to regurgitated. RESULTS No significant difference was found between the regurgitation times of seeds from V. costaricanum, C. costaricana and C. pinnatifrons (Fig. 1) Still, C. pi nnatifrons was regurgitated most quickly and C. costaricana took the longest amount of time (Fig. 1). When examining regurgitation time without regard to fruit type, the mean gut processing time for regurgitated seeds was 11.4 minutes 20.4 minutes (N = 2 01). The size of defecated V. costaricanum seeds (N = 175) was found to be significantly smaller than regurgitated seeds, (N = 187) t = 17.192, p < 0.0001. Defecated seed size ranged from 2.5 mm to 6.8 mm and regurgitated seed size ranged from 3.8 mm to 8.0 mm (Fig. 2). Defecated seeds took significantly longer to process (X = 27.6 min. 9.4 min., N = 11) than regurgitated seeds (X = 11.2 min 13.8 min., N = 133) t = 12.545, p < 0.0001. DISCUSSION Seed size is a good indicator of whether seeds will b e defecated or regurgitated by the Mountain Robin. Though there is not a fixed boundary, the box plot (Fig. 2) shows that seeds over 6.8 mm in size will always be regurgitated and seeds under 3.8 mm in size
will always be defecated. It can be inferred from Fig. 2 that within this range larger seeds are more likely to be regurgitated and smaller ones are more likely to be defecated. This is consistent the prediction made by Johnson et al. (1985) that regurgitation is more likely for large seeds and small dis persers. For regurgitated seeds such as C. pinnatifrons C. costaricana, and larger seeds of V. costaricanum (over 6.8 mm), there is no apparent correlation between plant species or seed size and gut processing time. Within each plants species some seeds were regurgitated in less than five minutes and others took more than 20 minutes. Still, as predicted by Johnson et al. (1985), mean regurgitation time was much less than mean defecation time. Knowing the time it takes a Mountain Robin to regurgitate see ds and the threshold at which they begin to do so has important implications in the effectiveness of this species as a disperser. Though visitation lengths are unknown for the Mountain Robin in these plant species, calculating regurgitation time is the fir st step in predicting the probability that seeds will be dispersed away from the parent tree. Similarly, the size at which seeds begin to be regurgitated and/or defecated is important in determining where fruits of various sizes will be dispersed by this s pecies. According to Schupp (1993), a disperser that regurgitates may be less likely to disperse seeds away from the parent tree than one that defecates. For this reason, knowing the size at which seeds start to be regurgitated is important in determinin g which fruit species are dispersed most effectively by the Mountain Robin. If the hypothesis proposed by Schupp (1993) is true, it can be stated with confidence that the Mountain Robin is a more effective disperser for seeds that are less than 2.5 mm at t heir widest point than for seeds that measure greater than 6.8 mm at their widest point. Due to its relative abundance and the fact that it consumes more than 44 species of fruits (Wheelwright et al. 1984), the Mountain Robin is clearly an important dispe rser in the Monteverde area. This study is the first step in determining how effective the Mountain Robin is at dispersing fruits of various sizes. However, to know the full extent of dispersal capabilities, a more complete analysis regarding time from see d consumption till defecation should be done using a larger sample of birds and a wider variety of fruit. If coupled with field studies documenting number of visits made and length of each visit, most of the quantitative factors outlined in Schupp (1993) w ould be known, providing further insight into the level of effectiveness of the Mountain Robin as a seed disperser. ACKNOWLEDGEMENTS hero! I would als o like to thank Oliver Hyman and Maria Jost for getting up every day at 5:00 am with me, for editing this paper and for being incredibly cool (Schmergens!). It would have been impossible to do this l forever remember our time together in the lower lab. Thanks to Mike for helping me come up with the idea of this project and for exploring the woods with me. Jimmers, thanks for ensuring the structural integrity of my birdcages! Rachael, it was a lot of fun learning how to mist net with you and taking an awesome break together on the final day! Thanks to Vanessa for fixing all my typos and cleaning my paper up! Jess, thank you for cheering me up when I was down about this project and for being my light wh to my entire family for sending me on this trip. Thanks to Stupendous and Snoop Dogg for being the soundtrack to this project. I also have to give a big shoutout to the boys back home, you know who you are. Go Ducks!
LITERATURE CITED Johnson, R. A., M. F. Willson, J. N. Thompson and R. I. Bertin. 1985. Nutritional values of wild fruits and consumption by migrant frugivorous birds. Ecology 66: 819 827. Schupp, E. W. 1993. Quantity, quality and the effectiveness of seed dispersal by animals. Vegetatio 107/108: 15 29. Stiles, G. and A. F. Skutch. 1989. A guide to the birds of Costa Rica. Cornell University Press, Ithaca, New York. pp. 362 365 Wheelwright, N. T., W. Haber, G. Murray, and C. Guindon. 1984. Tropical fruit eating birds and their food plants: a survey of a Costa Rican lower montane forest. Biotropica 16: 173 192.
Figure 1 Comparison of regurgitation times of seeds for C. costaricana (N = 54, X = 11.94 20.6 minutes), C. pinnatifrons (N = 14, X = 10.21 4.79 minutes), and V. costaricanum (N = 133, X = 11.15 13.85) by the Mountain Robin. Differences were not significant indicating that these three fruits have similar regurgitation times. Figure 2 Box plot for seed size of defecated and regurgitated seeds of V. costaricanum Defecated seeds ranged in size from 2.5mm to 6.8mm with a mean of 4.25 mm (N = 175). Regurgitated seeds varied in diameter from 3.8mm to 8.0mm with a mean of 5.97 mm (N = 187 ). The Difference between the two methods of processing was significant (t = 17.192, P < 0.0001) indicating that larger seeds are more likely to be regurgitated and smaller ones more likely to be defecated. Regurgitation Time (Minutes)