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1 Niche partitioning by frugivorous bats in the San Luis Valley, Costa Rica Rachel M. Johnson Department of Biology, University of Wisconsin Madison ABSTRACT Niche partitioning is an important form of ecological differentiation that allows two or m ore species to co exist. It has also been shown to be responsible for maintaining the vast diversity of bat species found in the tropics (Aguirre et al. 2002). A community of frugivorous bats in the San Luis Valley, Costa Rica, was sampled to determine t he parameters of niche differentiation used by the bats. The study took place at the San Luis Ecolodge at 1100 meters in elevation in premontane wet secondary forest. Bats were mist netted using two 12 meter mist nets over seven nights in the month of Ap ril, 2005. Time of capture, forearm measurements and weights were recorded and fecal samples were collected. Seeds present in the samples were identified to species to determine if the bats were differentiating niches based on species of plant consumed. There was no niche partitioning due to plant species, but statistical significance was found when time of foraging and the size of the bat compared to the species of fruit eaten were analyzed. It was determined that the species were differentiating niche s based on fruit size and time of foraging. RESUMEN La divisin de los nichos es una forma importante de diferenciacin ecolgica que permite a dos o ms especies coexistir. Se ha demostrado que tambin es responsable de mantener la amplia diversidad d e especies de murcilagos que se encuentran en los trpicos (Aguirre et al. 2002). Una comunidad de murcilagos frugvoros en el Valle de San Luis, Costa Rica fue usada para determinar los parmetros de la diferenciacin del nicho utilizados. El estudio se llev a cabo en el San Luis Ecolodge de San Luis, a 1100 metros de elevacin, en un bosque hmedo premontano secundario. Los murcilagos fueron atrapados, con dos redes de 12 metros de largo cada una, durante siete noches en el mes de abril del 2005. El t iempo de la captura, la longitud del antebrazo y el peso fueron registrados; tambin se recolectaron muestras fecales. Las semillas presentes en las muestras fueron identificadas para determinar si los murcilagos repartan los nichos de acuerdo con las es pecies de plantas que consuman. No hubo una divisin significativa de nichos en las especies de plantas, pero se encontraron diferencias significativas cuando se compar el tiempo de forrajeo y el tamao de los murcilagos con el tamao de la fruta comsum ida. Por lo tanto, se determin que las especies repartan sus nichos de acuerdo con el tamao de la fruta y el tiempo de forrajeo. INTRODUCTION The tropics are famous for the incredible diversity of organisms that they house. According to Fleming (19 73), there are twice as many species of mammals in tropical forests than in temperate forests. Many studies have attempted to explain the abiotic and biotic factors that allow this dense and diverse array of flora and fauna to co exist.
2 Orians (1966) fou nd that tropical forests contain 2 2.5 times as many resident birds than temperate forests and attributed the increase in species to greater stability of food sources in the tropics. In fact, it was determined that 25 50% of the increase in species was du e to the addition of a new food source (Orians 1966). Yet, the Competitive Exclusion Principle states that organisms that compete for the same resources cannot co exist (Hutchinson 1959). In order to coexist, two species must differ in the ecological res ources they utilize by evolving specialized roles or niches within the community (McNab 1971). Many of these parameters of differentiation are associated with the partitioning of food resources (McNab 1971). Most importantly, niche partitioning has been shown to produce and sustain a greater diversity in bat species for the tropics (Aguirre et al. 2002). Niche partitioning can occur for any resource that a species utilizes including food, space, time and microhabitat (Petren 2001). By understanding the niche parameters of an organism, species interactions and community structure can be better understood (Macarthur 1958). Frugivorous bats rely on a large variety of fruiting plants throughout the year because at any one time of the year, only a certain subset of the plants is fruiting. Thus, one would expect that resource partitioning is occurring among these communities. For example, there are13 species of frugivorous bats that utilize 41 species of fruiting plants in the Monteverde region of Costa Ri ca (Dinerstein 1983). The 41 species were located in four life zones; of these only ten fruited in April of 1981 and more specifically, of the eight that were located in zone one at 1300 meters, only four fruited during the month of April (Dinerstein 1983 ). This reduction in variety of resources in one month during the dry season raises the question of how niche partitioning is occurring in this community and along which parameters. The purpose of this study was to determine how a frugivorous bat communi ty in the San Luis Valley, Costa Rica divided resources during this time of lower food availability. MATERIALS AND METHODS Study Site The study was conducted at 1100 meters in premontane wet secondary forest at the San Luis Ecolodge in the San Luis Va lley, Costa Rica. The study site was located on a system of trails directly behind the Ecolodge, some of which were lined with cuadrado patches and casitas, while the rest were lined by secondary undergrowth (Figure 1). Field Work Bats were mist nette d seven nights at the end of April and beginning of May 2005 (see Table 1 for weather data). Two 12 meter nets were used each night, except for one night in which only one net was used, and they were opened at 6:30 PM and closed at 10:00 PM. The nets wer e placed lengthwise on the trails and were never within visual distance of each other. They were moved every two days to prevent avoidance by the bats; however, several sites were used more than once after seven or more days without use.
3 Each night, the nets were each baited with two cuadrados and were checked every 15 to 30 minutes. Upon capture, bats were carefully taken from the nets and if a fecal sample was available at that time, they were processed and released immediately. If a fecal sample coul d not be obtained directly after capture, the bats were placed in cloth holding bags for 30 to 45 minutes to allow defecation. The bats were never held for more than an hour. Costa when identification could not be made in the field, and Richard LaVal was consulte d at a later date for accurate species identification. The reproductive states were assessed after identification and weights and forearm measurements were taken and recorded (Appendix B, Figures 3 and 4, respectively). The forearm was defined as the len gth of the wing when folded. Bats were marked with paint pens prior to release either on the wing or tail membrane to ensure that recaptures were not counted as different individuals. Samples were collected from the holding bag and placed in a marked gla ss vial, and the holding bag was cleaned of all feces and seeds before being reused (Figures 3 and 4). Lab Work Seeds from the fecal samples were cleaned using a tweezers and alcohol in a petri dish and placed in clean alcohol for preservation. They we re examined under a dissecting collection. Individual species found were preserved and labeled for future reference. RESULTS A total of 56 bats were caught, 14 specie s in seven genera, 12 of which were frugivorous (Figure 2, Appendix B). Fecal samples were obtained from 37 bats, 22 of which contained Cecropia obtusofolia 12 samples contained Piper bisasperatum one sample contained Piper auritum one sample contained Piper cuspidispicum and one sample contained both C. obtusofolia and P. bisasperatum seeds (Appendix A). Both the weight and forearm measurements differed significantly between species that yielded samples containing Cecropia seeds versus those measureme nts of the species that yielded samples containing Piper seeds (t test for weights p = 0.018, t test for forearm lengths p = 0.001) (Figures 3 and 4). Statistical significance was also found when the difference in the time of capture was analyzed for all species (Kruskal Wallis p = 0.009), for species that ate Cecropia fruit (Kruskal Wallis p = 0.006) and the difference in time of capture for the large species that ate Cecropia fruit (Kruskal Wallis p = 0.014) (Appendix B). Statistical significance was no t found when the difference in time of capture was analyzed for species that ate Piper (Kruskal Wallis p = 0.176) nor was it significant for the small species that ate Piper (Kruskal Wallis p = 0.156).
4 DISCUSSION Since each of the twelve species of frug ivorous bats ate either C. obtusofolia or P. bisasperatum with little or no overlap, niche partitioning was not occurring according to the species of plant consumed (Appendix A). However, niche partitioning was occurring according to the size of the bat a nd the size of the fruit (Figures 3 and 4). The species that ate C. obtusofolia were significantly larger than those that ate P. bisasperatum and although the dry mass of these fruits was never measured, according to Richard LaVal and personal observatio ns, C. obtusofolia is the larger of the two fruits (LaVal pers. comm.). The bats were also partitioning according to the time of foraging, as proven by the statistical significance found when the times of capture were analyzed (Appendix B). More specifi cally, temporal niche partitioning was shown in the Cecropia eating subset of bats with significance found not only when the time of capture for all species was compared, but also when the time of capture for the large species of bats were analyzed. Howev er, this was not the case when the times of capture were analyzed for all Piper eating bats or for the small species found eating Piper This could be due to the fewer number of individuals that ate Piper compared to the number that ate Cecropia (Figures 3 and 4). It could also be that there is greater competition between the larger bats for the Cecropia fruits in order to minimize foraging efforts and maximize nutrient intake. The Optimal Foraging Theory states that the fitness of a foraging animal is a function of the efficiency of foraging as assessed in terms of some currency, usually energy (Pyke et al. 1977). Of the many factors that influence energy expenditure in mammals, the most important one is body size (McNab 1979). Mass specific measuremen ts show decreasing energy use with increasing body mass meaning that larger bats would have a lower rate of energy use when proportionally compared to smaller bats (Masters K. and A. Masters 2005). Even though small bats have higher energy requirements th an large bats, it is still beneficial to the fitness of both to be as efficient as possible when foraging. Because of this, it would be more efficient for both sizes to minimize activity spent foraging by eating large fruits. However, there is a larger b ase of fruits that small bats can efficiently use if both Cecropia and Piper are considered to be available resources. Conversely, if by the Optimal Foraging Theory, Cecropia is considered the only fruit present in the samples that large bats could effici ently exploit, then there is a smaller base of fruits available to them. In order for the species of large bats to coexist with these restricted parameters and increased competition, finer niche partitioning is occurs. This is one of the possible explana tions for the more specialized time niche partitioning seen not only in the guild of Cecropia eating bats, but in the large bat species that ate Cecropia as compared to the Piper eating guild which had no significance regarding time of foraging. In summary the community of bats studied differentiated along fruit size and temporal parameters and did not differentiate based on species of fruiting plants exploited. Unfortunately, a list of species and their fruiting peaks and patterns in the San Luis Valley does not exist; therefore, it is not possible to know if these four species were the only fruiting plants available at that time, or if there were others fruiting which were neglected by the bats. However, it is clear that the individual species in this c ommunity exploited resources during this dry season month through niche partitioning based on temporal and fruit size parameters.
5 Future studies could add to the knowledge of this topic by assessing the fruiting peaks and patterns of the San Luis Valley a nd comparing these trends to the manner of niche partitioning utilized by the bat populations during that time of year. In addition, a comprehensive list of bat species for the San Luis Valley does not exist and would be very helpful for future studies co nducted there. ACKNOWLEDGEMENTS I would like to thank my advisor, Javier Mndez for supporting my batty ideas with enthusiasm, Richard Laval for inspiring my interest in bats, the use of his seed collection and for his stories (bats really are cute, w hours spent helping me in the field and in the classroom and all the laughter in between, Alan and Karen Masters for their endless dedication, patience and compassion, Arturo Obando for his help with identification, Christina Wong for all of her help setting up the nets and intense study/contemplation sessions, the wonderful people at the San Luis Valley Ecolodge for allowing me to use their beautiful trails and lab, mi familia tica para muchas bolsas de azcar, rico gallo pinto and y todas las risas, to all the students who lived in San Luis, to Emily Whitman for donating her head lamp to my project I wish you could have done bats with me! and lastly than k you to all the bats that involuntarily endured my learning curve and the nets. LITERATURE CITED Aguirre L. F., Herrel A., van Damme R., Matthysen E. 2002. Ecomorphological analysis of trophic niche partitioning in a tropical savannah bat community Biological Sciences 269(1497): 1271 1278. Dinerstein, E. 1983. Reproductive ecology of fruit bats and seasonality of fruit production in a Costa Rican cloud forest. Ph.D. dissertation. State University of Washington, Washington. Fleming, T. H. 1973. Th e number of mammal species in several North and Central American forest communities. Ecology, 54:555 563 Hutchinson, G. E. 1959. Homage to Santa Rosalia or Why are there so many kinds of animals? The American Naturalist 93: 145 159. Laval, R. K. and B. Ro drguez H. 2002. Murcilagos de Costa Rica Instituto Nacional de Biodiversidad, Costa Rica. LaVal, R. K. 2005. Personal communication. Macarthur, R. 1958. Population ecology of some warblers of northeastern coniferous forests, In Real and Brown (Eds.) Foundations of Ecology, pp. 686 706. Univeristy of Chicago Press, Chicago, Illinois. Masters, K. and A. Masters. 2005. Tropical diversity laboratory: diveristy day handouts pg. 90 McNab, B. K. 1979. Food habits, energetics, and the population biology of mammals. The American Naturalist 116(1): 106 124. Orians, G. H. 1969. The number of bird species in some tropical forests. Ecology 49: 565 566. Pyke G. H., H. R. Pulliam, and E. L. Charnov. 1977. Optimal Foraging: A selective review of theory and tests. The Quarterly Review of Biology, 52(2): 137 154. Timm, R. M. and R. K. LaVal. 1998. A field key to the bats of Costa Rica. Occasional Publication Series from The University of Kansas 22: 1 30.
6 TABLE 1. Weather data during April and May of 2005 for t he nights when the mist nets were set up. The full moon occurred on April 24, 2005. FIGURE 1. Diagram of the San Luis Ecolodge in the San Luis Valley, Costa Rica. All of the mist net sites used are marked by a net on the trail. Ne ts were always placed lengthwise along the trails and were only left in the same place for two days but may have been used again with a week in between.
7 FIGURE 2. Frequency distributions of the bat species caught at the San Lu is Ecolodge in the San Luis Valley, Costa Rica. There was an uneven distribution of individuals caught for each species. Also see Appendix B. FIGURE 3. Forearm (mm) and weight (g) measurements for all individuals, by species that ate C. obtusofolia
8 FIGURE 4. Forearm (mm) and weight (g) measurements for all individuals, be spe cies that ate P. auritum, P. bisasperatum, and P. cuspidispicum FIGURE 5. Time Niche Partitioning within all species of frugivorous bats caught in the San Luis Valley, Costa Rica. Time scale is every half hour from 6:30 PM to 10:00 PM. Also see Appendix B.
9 APPENDICES Appendix A: List of individuals, by species, caught in the San Luis Valley Costa Rica and what was found in their fecal sample.
10 Appendix B: List of all individuals, by species, ca ught in the San Luis Valley of Costa Rica, the time of capture of each individual, and the reproductive state of the individual. weight measurement. Species Time of Capture Reproductive State Artibeus intermedius 19:00 Lactating Female Artibeus intermedius 21:3 0 Reproductive Male Artibeus jamaicensis 20:00 Reproductive Male Artibeus jamaicensis 19:20 Lactating Female Artibeus jamaicensis 20:45 Male Artibeus jamaicensis 20:45 Male Artibeus jamaicensis 20:50 Artibeus lituratus 20:00 Reproductive Male Arti beus lituratus 20:30 Reproductive Male Artibeus toltecus 20:15 Lactating Female Artibeus toltecus 19:30 Male Artibeus toltecus 19:45 Lactating Female Carollia brevicauda 18:30 Lactating Female Carollia brevicauda 18:30 Term Pregnancy Carollia brevica uda 18:30 Term Pregnancy Carollia brevicauda 18:45 Male Carollia brevicauda 19:30 Term Pregnancy Carollia brevicauda 20:00 Reproductive Male Carollia brevicauda 18:40 Male Carollia brevicauda 19:20 Reproductive Male Carollia brevicauda 19:50 Reproduc tive Male Carollia brevicauda 19:11 Term Pregnancy Carollia brevicauda 18:30 Male Carollia brevicauda 21:10 Reproductive Male Carollia brevicauda 21:10 Lactating Female Carollia perspicallata 20:15 Male Glossophaga commissarisi 19:30 Reproductive Mal e Glossophaga commissarisi 19:30 Male Glossophaga commissarisi 19:45 Reproductive Male Glossophaga commissarisi 19:45 Reproductive Male Glossophaga commissarisi 19:45 Reproductive Male Glossophaga commissarisi 19:10 Male Myotis keaysi 21:00 Male Pl atyrrhinus helleri 19:00 Lactating Female Platyrrhinus helleri 19:05 Male Platyrrhinus vittatus 19:00 Reproductive Male Platyrrhinus vittatus 19:00 Reproductive Male Platyrrhinus vittatus 20:15 Reproductive Male Platyrrhinus vittatus 20:15 Male Platy rrhinus vittatus 19:30 Reproductive Male
11 Platyrrhinus vittatus 19:10 Reproductive Male Platyrrhinus vittatus 20:00 Reproductive Male Platyrrhinus vittatus 20:20 Reproductive Male Platyrrhinus vittatus 19:20 Reproductive Male Platyrrhinus vittatus 19:5 0 Reproductive Male Phyllostomus discolor 20:25 Male Phyllostomus discolor 20:50 Reproductive Male Phyllostomus discolor 21:00 Male Phyllostomus discolor 21:30 Male Phyllostomus discolor 21:40 Male Phyllostomus discolor 21:40 Male Sturnira lilium 19 :50 Reproductive Male Sturnira lilium 20:20 Lactating Female Sturnira ludovici 20:15 Term Pregnancy Sturnira ludovici 21:30 Term Pregnancy Sturnira ludovici 20:00 Term Pregnancy
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Johnson, Rachel, M.
Particin de nichos por los murcilagos frugivorous en el valle de San Luis, Costa Rica
Niche partitioning by frugivorous bats in the San Luis Valley, Costa Rica
Niche partitioning is an important form of ecological differentiation that allows two or more species to coexist. It has also been shown to be responsible for maintaining the vast diversity of bat species found in the
tropics (Aguirre et al. 2002). A community of frugivorous bats in the San Luis Valley, Costa Rica, was sampled to determine the parameters of niche differentiation used by the bats. The study took place at the
San Luis Ecolodge at 1100 meters in elevation in premontane wet secondary forest. Bats were mist netted using two 12-meter mist nets over seven nights in the month of April, 2005. Time of capture, forearm measurements and weights were recorded and fecal samples were collected. Seeds present in the samples
were identified to species to determine if the bats were differentiating niches based on species of plant consumed. There was no niche partitioning due to plant species, but statistical significance was found when time of foraging and the size of the bat compared to the species of fruit eaten were analyzed. It was determined that the species were differentiating niches based on fruit size and time of foraging.
La divisin de los nichos es una forma importante de diferenciacin ecolgica que permite a dos o ms especies coexistir. Se ha demostrado que tambin es responsable de mantener la amplia diversidad de especies de murcilagos que se encuentran en los trpicos (Aguirre et al. 2002). Una comunidad de murcilagos frugvoros en el Valle de San Luis, Costa Rica fue usada para determinar los parmetros de la diferenciacin de los nichos utilizados. El estudio se llev a cabo en el Ecolodge de San Luis, a 1100 metros de elevacin, en un bosque hmedo premontano secundario. Los murcilagos fueron atrapados, con dos redes de 12 metros de largo cada una, durante siete noches en el mes de abril del 2005. El tiempo de la captura, la longitud del antebrazo y el peso fueron registrados; tambin se recolectaron muestras fecales. Las semillas presentes en las muestras fueron identificadas para determinar si los murcilagos repartan los nichos de acuerdo con las especies de plantas que consuman. No hubo una divisin significativa de nichos en las especies de plantas, pero se encontraron diferencias significativas cuando se compar el tiempo de forrajeo y el tamao de los murcilagos con el tamao de la fruta consumida. Por lo tanto, se determin que las especies repartan sus nichos de acuerdo con el tamao de la fruta y el tiempo de forrajeo.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone--San Luis
Costa Rica--Puntarenas--Zona de Monteverde--San Luis
Tropical Ecology Spring 2005
Ecologa Tropical Primavera 2005
t Monteverde Institute : Tropical Ecology