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Una comparacin en elevacin de las comunidades de colepteros en un bosque de ribera en San Luis, Costa Rica
An elevational comparison of coleopteran communities in a riparian forest in San Luis, Costa Rica
The purpose of this study was to determine whether altitude has a measurable effect on coleopteran community diversity. Data for this analysis were collected along the Rio San Luis in Monteverde, Costa Rica. It was found that the community at lower elevations (710-740 m) had significantly higher diversity (H = 2.45) than the community at higher elevations (1170-1220 m) (H = 2.01). This difference is most likely caused by changes in microhabitat but could also be a result of variation within community sampling.
El propsito de este estudio fue determinar si la altitud tiene un efecto mensurable en la diversidad de la comunidad de colepteros. Los datos para este anlisis se colectaron cerca del ro San Luis en Monteverde, Costa Rica. Se encontr que la comunidad ms baja (710-740 m de altura) tuvo una diversidad ms alta (H' = 2,45) que la comunidad en elevaciones ms altas (1170-1220 m de altura) (H' = 2,01). Esta diferencia es probablemente causada por cambios en el micro hbitat pero podra ser tambin el resultado de la variacin en el muestreo de comunidad.
Text in English.
Costa Rica--Puntarenas--Monteverde Zone--San Luis
Diversidad de especies
Costa Rica--Puntarenas--Zona de Monteverde--San Luis
Tropical Ecology Spring 2005
Ecologa Tropical Primavera 2005
t Monteverde Institute : Tropical Ecology
1 An Elevational Comparison of Coleopteran Communities in a Riparian Forest in San Luis, Costa Rica Emily Sampson Department of Earth and Environment, Mount Holyoke College ABSTRACT The purpose of this study was to determine whether altitude has a measura ble effect on coleopteran community diversity. Data for this analysis were collected along the Rio San Luis in Monteverde, Costa Rica. It was found that the community at lower elevations (710 740 m) had significantly higher diversity community at higher elevations (1170 likely caused by changes in microhabitat but could also be a result of variation within community sampling. RESUMEN El propsito de este estudio fue determinar si la altitud tiene un efecto mensurable en la diversidad de la comunidad de colepteros. Los datos para este anlisis se colectero cerca del ro San Luis en Monteverde, Costa Rica. Se encontr que la comunidad ms bajas (710 740 m de altura) tuvo una diversidad ms alta (H' = 2,45) que la comunidad en elevaciones ms altas (1170 1220 m de altura) (H' = 2,01). Esta diferencia es probablemente causada por cambios en el microhabitat pero podra ser tambin el resultado de la variacin en el muestreo de comunidad. INTRODUCTION Coleopterans, with more than 400,000 described species, comprise approximately 25% of all described organisms on Earth (Nadkarni and Wheelwright 2000). Costa Rica is host to 110 of the 178 families of coleopterans worldwide ( Solis 1999 ). Th ey represent a large body of the worldwide biodiversity, and it is important for us to understand their biology and natural history. Unfortunately, little is known about the factors that most influence the insect diversity in the tropics (Lucky et al 2002 ), and an even smaller amount of information is available on the influences of coleopteran diversity in Costa Rica. Many physical and environmental factors contribute to species diversity. The most important factors in determining basic differences betwee n ecosystems are temperature and rainfall. These factors directly affect plant and animal species composition (Holdridge 1947). Consequently, temperature and rainfall patterns are greatly influenced by latitude and altitude (Holdridge 1967), and animal s pecies composition is either directly or indirectly determined by altitude. Community diversity can change in relation to a number of variables. Number of individuals, number of species, and number of individuals within a species all play a large role i n determining niche partitioning and community diversity. A species with many individuals present can out compete a species with only a few representatives. Another community may have many species that all have similar numbers of individuals; these commu nity dynamics all
2 contribute to diversity. The kind and abundance of species within an area are greatly affected by environmental factors, and affect community composition greatly. Costa Rican insects are affected by many environmental factors that are mainly driven by seasonal changes that influence secondary factors such as temperature, moisture, and food availability (Janzen 1983). If insect diversity can be affected by these factors on a large scale, it is possible that they will also influence ins ect diversity within different microclimates. Changes in altitude affect temperature and moisture, causing changes in microclimate. These variations in microclimates will cause the diversity of insects to change. As coleopterans are such a diverse order, they can be a good representation of overall biodiversity, especially insect biodiversity in a given area. The purpose of this study is to determine the effects of changes in elevation on coleopteran community diversity. Previous studies have shown that some insect groups decrease in diversity with an increase in elevation (Lucky et al 2002). Therefore, the hypothesis of this study is that coleopteran communities at lower altitudes will be more diverse than coleopteran communities at higher altitudes. METHODS Study Area This study was conducted along the Rio San Luis in San Luis, Puntarenas, Costa Rica East. Six study sites were chosen along the river, three in lower San Luis and three in upper San Luis. The six sites were at elevations of 710, 720, 740, 1170, 1190, and 1220 meters. These sites were grouped according to altitude into two communities: the lower river community was made up of the 710, 720, an d 740 m sites, whereas the upper river community was made up of the 1170, 1190, and 1220 m sites. Sites were chosen based on distance from the river, canopy cover, and understory cover. All sites chosen had maximum available canopy cover for that altitud e, minimum understory cover, and were within ten meters of the river. Methods Coleopterans were sampled using flight interception traps (Longino, pers. comm., Southwood 1996) ( see Figure 2 ). A trap was placed at each of one the six sites along the ri ver. Traps were constructed from one piece of clear plastic, 1 m 2 in area, that was tied to bamboo poles or trees on either side and stretched taut. The area below the plastic was cleared of understory vegetation and leaf litter. A trench that was 1m lo ng by 25 cm wide and 10 15 cm deep was dug directly below the sheet of plastic. The trench was lined with plastic and rocks were placed inside to hold the plastic in place. Approximately 5 L of water were added to the trench, along with one tablespoon of powdered laundry detergent and 100 mL of ethanol. The trap worked such that insects would fly into the sheet of plastic and fall directly into the trench. They were then killed by the detergent and preserved by the alcohol until they could be collected. Coleopterans were collected every three days for six days, for a total of two sampling days. All debris was removed from each trap using an aquarium net and placed in a plastic Ziploc bag. All spec imens from each site were sorted in the lab according to morphospecies and later identified
3 the coleopteran reference collection at the Monteverde Biological Station. For each site, as well as for both lower river and upper river communities, S S marg Weiner y were applied to all six sites to determine if the sites within each community were statistically similar to each other in terms of species richness. RESULTS A total of 1131 individuals were collected for this study, representing 66 species and 20 f amilies of coleopterans. A full list of all individuals collected is provided in Appendix 1 The 5 most common families found were Scarabaeidae (14 species), Chrysomelidae (10 species), Carabidae (8 species), Elateridae (6 species), and Staphilinidae (6 species). The lower river community had a total of 590 individuals, while the upper river community had a total of 541 individuals (Figure 3). The values for alpha, beta, and gamma diversity for the lower river community were 20.67, 0.69, and 43 respectiv ely; the values for alpha, beta, and gamma diversity for shown in Table 2 Figure 4 ) (regression, y= 0.0007x + 2.6579, r 2 = 0.4 and upper river communities and were 2.45 and 2.01, respectively (Table 2). These values were found to be significantly different (t test, t value = 4.78, t critical = 1.96, 1012.73 degrees of freedom). Also, species evenness for all six sites and for the lower and upper river communities was calculated (Table 1). The upper and lower communities had evenness values of 0.65 and 0.53, respectively. normalize for sample size for each site. These values are shown in Table 1 upper river communities. The S marg value for the lower river community was 6.58, while the S marg value for the upper river c ommunity was 6.99 ( Table 2 ). There proved to be a positive correlation between species richness and elevation (regression, y= 0.0075x + 15.35, r 2 = 0.0599) (Figure 5). Table 3 ). The lower river community was not significantly dissimilar with an average value of 0.16 (one sample t test, t value = 2.14, p value = 0.065), and the upper river community was significantly similar with an average value of 0.58 (one sample t test, t value = 7.33, p value < 0.0001). DISCUSSION The results supported the original hypothesis that coleopteran communities at lower altitudes were more diverse than coleopteran communities at higher altitudes. The community at lower elevations did indeed have higher diversity than the community at higher elevations. Although the higher river community had more overall species richness, the lower river community was higher in evenness and therefore species
4 diversity. It can be concluded that no one species or group of species was necessarily dominating the whole lower river community. However, it is interesting to note that the three sites that comprised the lower river community were extremely different from each ilarity, the upper river community was much more of a meta community than the lower river community, as it had more species in common. The upper river community, therefore, can be described as having more species homogeneity, whereas the lower river commu nity has more species heterogeneity. The question that arises here is: why would three sites so close together at lower elevations have such different community dynamics than those sites at higher elevations? I have, so far, been only able to attribute t his difference to differences in site microhabitat. It is true that the two communities were set up to have differences in microhabitat; however the three sites within each community were selected because they appeared to be similar. Therefore, the fact that the three lower river sites have such a low within community similarity may be due to unknown differences. Possible explanations for this variation include sampling error. Each trap was made on site, and it could be that they were not made to be o f equal size. The plastic sheet could have been at different levels from the ground, and, while they appeared to be beetles. The location of the traps could have also been a factor. When choosing locations for the traps, I looked for areas with more open understory and could be a natural flight path of beetles. However, it could have been that some trap locations were more of a beetle highway than others, and the refore more individuals were flying through the area per day. Finally, the variation between sites in the lower river community could have to do with some factor that I could not see, such as the presence different species of host plant or a rotting plant or animal that was attracting certain groups of beetles in large numbers. In this way one group or groups of organisms (such as the staphylinid beetles in site 720 m) could be throwing off the balance within the community. Still, the factors which mos t influence coleopteran diversity are yet unknown. Similar to the study by Lucky et al (2002) of the effects of altitude on leaf mining insects, there is no definite trend as to which factors most greatly influence insect diversity. Although some groups may exhibit trends, it is hard to quantify the exact correlation between changes in environmental factors and changes in biodiversity. Especially here in the tropics, where diversity is high and niches are small, there could be many variables that infl uence speciation and movement of organisms. The trends found are significant, yet there is enough variation in the original data to suggest that further research is necessary. In future studies, it would be more useful to set up a complete transect withi n an elevational range, such that there would be an altitudinal gradient with sites evenly spaced between the two end elevations. This would more clearly show how coleopteran diversity changed as a function of increasing altitude. Also, it would be usefu l to have more within site replicate traps, to help normalize the potential variation such as that which was found at site 720 m. More sites and more traps, as well as potentially more sampling days, would provide much more information about exactly how c oleopteran community structure changed with elevation.
5 ACKNOWLEDGMENTS Thanks to Javier Mndez, my advisor; Alan and Karen Masters for helping Javier advise me; Matt Gasner and Ollie Hyman for their help with the acquisition of non potable alc ohol, glass vials, and the like; the San Luis Ecolodge staff and the University of Georgia at Atlanta for allowing us to use their space; Miguel Leitn for the use of his trails; the Ramrez Brenes family for housing me during my project; John Longino for his entomological methods; and finally Andrs Vaughan and the San Luisers for their support. LITERATURE CITED Fernandes, G. W., F. M. Caldeira Castro, M. L. Garia, E. S. A. Marques, and M. K. Barcelos Greco. 2004. Effects of hygrothermal stress, plant r ichness, and architecture on mining insect diversity. Biotropica 2004; 36(2): 240 247. Janzen, D. H., editor. 1983. Costa Rican natural history. Chicago: University of Chicago Press. 816 p. Longino, John. 2005. Personal communication. Monteverde, Co sta Rica. Lucky, A., T. L. Erwin, and J. D. Witman. 2002. Temporal and spatial diversity and distribution of arboreal Carabidae (Coleoptera) in a Western Amazonian rain forest. Biotropica 2002; 34(3): 376 386. Nadkarni, N. M., and N. R. Wheelwright, ed itors. 2000. Monteverde: Ecology and conservation of a tropical cloud forest. New York: Oxford University Press. 573 p. Solis, A. 1999. Costa Rica beetles: the most common families. Heredia: INBio/NORAD. 110 p. Southwood, T. R. E. 1996. Ecologi cal methods with particular reference to the study of insect populations. 2 nd ed. New York: Chapman and Hall. 524 p. White, R. E. 1983. A field guide to the beetles of North America. Boston: Houghton Mifflin Company. 368 p.
6 FIGU RES FIGURE 1. (See attached). A map of the San Luis area and the rio San Luis where the study was conducted. Xs mark each sampling site. FIGURE 2. Diagram of a flight interception trap as described by Longino (2005) and Southwood (1996). Sheet of plastic stretched taut between two poles with a trench dug underneath. Coleopterans fly into the sheet of plastic (number 1) and drop (number 2) into a trench containing water, detergent, and alcohol where they are preserved until they can be collecte d. FIGURE 3. The total number of coleopteran individuals collected at each site.
7 FIGURE 4. The values for Shannon altitude. The equation for the line is y = 0.0007x + 2.6579, and the r 2 value = 0.4635. Coleopteran diversity decreases with altitude. FIGURE 5. A regressional comparison between number of species (species richness) of coleopterans at each site and increasing atlitude. There is a slight positive correlation b etween increasing altitude and species richness. The equation for the line is y = 0.0075x + 15.35, and the r 2 value = 0.0599.
8 TABLES Weiner Diversity Index), E (ev enness) and S marg presented in meters. marg for the lower river and upper river communities. Altitude is presented in meters. TABLE 3. Sor average value of 0.16, and the upper river community sites had an average value of 0.58. The upper river sites were found to be significantly similar, while th e lower community sites were found to be marginally significantly dissimilar. Altitude is presented in meters.
9 APPENDICES APPENDIX 1. A complete list of all individuals collected throughout the study period at each site. They are or ganized by family and morphospecies. A total of 1131 individuals were collected and identified, representing 66 species and 20 families.