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Efectos de la ruta del trfico en la depredacin de huevos por parte de las poblaciones de mamferos en el bosque nuboso de Monteverde
Effects of trail traffic on egg-predation by mammal populations in the Monteverde Cloud Forest Preserve
Digitized by MVI
The effects of human traffic within trails on egg predation and predatory mammal compositions between heavily trafficked trails and secluded trails were studied at the Monteverde Cloud Forest Preserve. This study was able to find differences in egg-predatory mammal compositions between these two trail types by looking at teeth impressions in eggs made of plasticine. The main predator was found to be Nasura narica (common coati) who was involved in the majority of the egg predation events and the vast majority of the nests preyed upon were on the highly trafficked trail. There were only a few predation events from other mammals such as rodents and Cebus capucinus (white-faced monkey). In this case, the abundance of tourists have altered populations of a mesopredator, Nasua narica by providing a new food source for them (garbage and tourists lunches) and by affecting the rarity of their predators (boas, cats and Tayras) whom steer-clear of human populated areas and therefore steer-clear of N. narica. This dualistic relationship acting on the population of N. narica makes for an increase in the carrying capacity and, hence, further predation. This imbalanced egg-predation by N. narica on tourist trails allocates the disruption of their prey populations. Studies of the impacts of tourism have important conservation implications because tourism can have a tremendous disturbance on structure and community function.
Los efectos del trfico humano en la depredacin de huevos en los senderos y en las composiciones de las comunidades de mamferos depredadores entre los senderos con mucho trnsito y los senderos recluidos fueron estudiados en la Reserva del Bosque Nuboso de Monteverde. Este estudio determino las composiciones de las comunidades de mamferos depredadores de huevos en dos tipos de senderos de acuerdo a las impresiones de dientes en los huevos de plasticina. El depredador principal fue Nasua narica (El coati comn), que estuvo implicado en la mayora de los acontecimientos de la depredacin de huevos; la mayor parte de los nidos depredados ocurri en el sendero de mucho trnsito. Hubo muy pocos acontecimientos de depredacin por otros mamferos tales como los roedores y el mono carablanca, Cebus capucinus. En este caso, la abundancia de turistas ha alterado las poblaciones de un medodepredador, N. Narica, proporcionando una nueva fuente de alimento (la basura y los almuerzos de los turistas) y provocando la escasez de los depredadores del pizote (Boas, felinos y tolomucos) que evitan las reas pobladas por humanos. Esta interaccin doble que acta en la poblacin de N. Narica, aumenta la capacidad de carga del ambiente y por lo tanto aumenta la depredacin de huevos. Este equilibrio en la depredacin de huevos por N. narica en los senderos tursticos produce la interrupcin de las poblaciones de la presa. Los estudios de los impactos del turismo tienen implicaciones importantes en la conservacin porque el turismo puede tener un impacto enorme en la estructura y la funcin de la comunidad.
Text in English.
Monteverde Cloud Forest Reserve (Costa Rica)
Costa Rica--Puntarenas--Monteverde Zone--Monteverde
Reserva del Bosque Nuboso de Monteverde (Costa Rica)
Costa Rica--Puntarenas--Zona de Monteverde--Monteverde
Tropical Ecology Fall 2005
Ecologa Tropical Otoo 2005
t Monteverde Institute : Tropical Ecology
Effects of trail traffic on egg predation by mammal populations in the Monteverde Cloud Forest Preserve. Karmen Scott Department of Environmental Science, University of Oregon ABSTRACT The effects of human traffic within trails on egg predation and pr edatory mammal compositions between heavily trafficked trails and secluded trails were studied at the Monteverde Cloud Forest Preserve. This study was able to find differences in egg predatory mammal compositions between these two trail types by looking at teeth impressions in eggs made of plasticine. The main predator was found to be Nasura narica (common coati) who was involved in the majority of the egg predation events and the vast majority of the nests preyed upon were on the highly trafficked trail. T here were only a few predation events from other mammals such as rodents and Cebus capucinus (white faced monkey). In this case, the abundance of tourists have altered populations of a mesopredator, Nasua narica by providing a new food source for them (gar cats and Tayras) whom steer clear of human populated areas and therefore steer clear of N. narica This dualistic relationship acting on the population of N. narica makes for an increase in the carrying capacity and, hence, further predation. This imbalanced egg predation by N. narica on tourist trails allocates the disruption of their prey populations. Studies of the impacts of tourism have important conservation implications because tourism can have a tremendous disturbance on structure and community function. RESUMEN Los efectos del trfico humano en la depredacin de huevos en los senderos y en las composiciones de las comunidades de mamf eros depredadores entre los sen deros con mucho trnsito y los senderos recluidos fueron estudiados en La Reserva del Bosque Nuboso de Monteverde. Este estudio determin las composiciones de las comunidades de mamferos depredadores de huevos en dos tipos de senderos de acuerdo a las imp resiones de dientes en huevos de plasticina. El depredador principal fue Nasura narica (El coat comn), que estuvo implicado en la mayora de los acontecimientos de la depredacin de huevos; la mayor parte de los nidos depredados ocurri en el sendero de mucho trnsito. Hubo muy pocos acontecimientos de depredacin por otros mamferos tales como roedores y el mono carablanca, Cebus capucinus En este caso, la abundancia de turistas ha alterado las poblaciones de un mesodepredator, N. Narica, proporcionan do una nueva fuente de alimento (la basura y los almuerzos de los turistas) y provocando la escasez de los depredadores del pizote (Boas, felinos y tolomucos) que evitan las reas pobladas por humanos. Esta interaccin doble que act a en la poblacin de N. Narica aumenta la capacidad de carga del ambiente y, por lo tanto, aumenta la depredacin de huevos. Este dequilibrio en la depredacin de huevos por N. narica en los senderos tursticos produce la interrupcin de las poblaciones de la presa. Los estudio s de los impactos del turismo tienen implicaciones importantes en la conservacin porque el turismo puede tener un enorme impacto en la estructura y la funcin de la comunidad. INTRODUCTION Ecotourism is a major part of the Monteverde, Costa Rica communi ty. It is a growing market that has been increasingly bringing in tourists, money, local jobs, biodiversity conservation, and environmental education opportunities for local and visiting populations. This growth in ecotourism is shown in the Monteverde Cl oud Forest Preserve (MCFP), which has seen a dramatic increase from 25 thousand visitors in 1990 to over 73 thousand tourists in 2004 (Rojas 2004). The MCFP also protects 121 species of mammals and 265 species of birds (Nadkarni 2000, Fogden 1993). For th at reason, it is important to study the implications of this increase in tourists on the biology of the forest. Scholer, in 2005, examined the abundance of avifaunal populations between popular tourist trails and isolated trails in MCFP (Scholer 2005). T he results found that bird populations were greater on the trails with more human traffic. This was attributed to a new surplus of food resources provided by the Preserve and/or a decrease in predation on such trails. Though never tested, the decrease in predation was most likely associated with a greater abundance of people on trails scaring away possible egg predators. These expected mammalian
egg predators were opossum ( Diedelphis marsupialis ), tayra ( Eira Barbara ), grey fox ( Urocyon cineroargenteus ), o lingo ( Bassaricyon gabbi ), coyote ( Canis latrans ), coati ( Nasua narica ), and raccoon ( Procyon lotor) (Conwell 1997). In the food web, terrestrial mammals are highly omnivorous, consuming prey from all quinary trophic levels. Thus, it is important to note that this study only looks into the predation of these mammals on bird eggs play important functional roles in communities by affecting many pat hways of energy flow and nutrient cycling (Reagan, 1996). Therefore, if tourism has negatively influenced these populations than there could be damaging consequences to the entire function of the community. Sustainable tourism requires an awareness of po tential threats to the ecosystem and needs efforts to minimize the impact visitors have on the environment (Scholer, 2005). This study seeks to quantify the effects of ecotourism on composition of mammalian egg predators in Monteverde, Costa Rica. So, bas ed on the prier study by Scholer (2005) it is expected that there was lower amounts of predatory mammals. Hence, It was predicted that there would be greater egg predation events. METHODS Data were collected on the trails of MCFP, at elevations between 1500 to 1600, during13 days from November 18 to November 18, 2005. Human traffic on each trail was determined by one of the founding members of the MCFP, Wilford Guindon. Three individual experimental sites were used for each trail type (isolated and heavi ly trafficked) to avoid a high proportion of returning predators. The three experimental sites were on the sendero bosque nuboso (cloud forest trail) (Fig. 1) because W. Guindon said it was the most heavily trafficked trail to date. The third experimental site was very close to the highly trafficked park entrance opening. The two trails used for secluded sites were trails not drawn on the trail map given at the reserve, yet one is shown on the internet version of the MCFP trail map. The first two secluded experimental sites were on the sendero George Powell which was roped off to public and had no human traffic. The third site was not a part of the network of trails. It was an abandonded trail close to the MCFP laboratory. The Laboratory was never used duri ng the study. The individual experimental sites got progressively closer to the main entrance. Nests were made by weaving small sticks into a circular shape about the size of an outstretched hand (Fig. 2). Four nests were used in total, two for each trail type. The nests were placed either directly on the ground or on a decaying log (no more than 25 cm off the ground). The two nests were placed within ten meters of each other at each site. One quail egg (Length three cm, circumference eight cm) and one h and made plasticine egg (mimicking quail egg) were placed in each nest. The quail eggs were used to draw in predators and to make the situation more realistic. The plasticine eggs were used to identify the different predators by teeth imprints left in the plasticine. In addition, to make the nests seem more realistic to the predators, chicken feathers and small samples of chicken feces were put on the nests. The eggs were left for periods of 24 hours. Predation was quantified and the nests were refilled w ith one quail and one plasticine egg. A nest was described as preyed upon if one or both eggs were attacked thus, each site could only have a maximum of two predation events each day. Daily qualitative weather patterns were noted as well. Teeth impri nts were quantified by using dental formulas from Reid (1997) and referring to drawn teeth impressions from a similar egg predation study (Benson 2004).
RESULTS A higher egg predation on the trails was expected with less human traffic, b ut the opposite results were found. This contradicts the original hypothesis. In fact, predation on the popular tourist trails was greater by a factor of nine (Fig. 3; out of 26 possible predation events per trail type (13 experimental days, two nests per site), the heavily trafficked trails had 18 nests preyed upon and the non trafficked trails had only two nests that were preyed upon. These data illustrate that ecotourism was having an effect on egg predation and predatory mammal populations. The chi squ ared value showed that egg predation between both sites was significantly different (x 2 = 12.8, df=1, p< 0.05), but this obvious difference in egg predation can be seen in Figure 4. Daily composition of predation showed a different frequency of nests pre yed upon as the individual sites moved toward the park entrance (Fig. 4). The closer the nests got to the entrance the more egg predation occurred (Fig. 5). The first sites had an average of 1.25 nests preyed upon per day (np/d) for the tourist trail (Fig. 5a) and zero nd/d at the secluded site (Fig. 5b). The second sites had an average of 1 np/d for the tourist trail (Fig. 5a) and 0 np/d for the secluded trail (Fig. 5b). Finally, the third sites (closest to the entrance) had an average of 1.8 np/d for the tourist trail (Fig. 5a) and 0.5 np/d on the secluded trail (Fig. 5b). The mammals preying upon the eggs and naively biting into the plasticine eggs varied between the trail types. Nine plasticine eggs were found within 5 10 meters of the original nest on the heavily trafficked experimental sites. Eight were preyed upon by Nasua narica and one was preyed upon by Cebus capucinus (Fig. 7a). There were only two predation events on the secluded trails and the teeth impressions belonged to a small rodent (squi rrel, mouse, or rat) from the Order Rodentia, and a Cebus capucinus (Fig. 7b). In both of these predation events the plasticine eggs were found within a meter from the nest.
Figure 3. Number of nests preyed upon between the heav ily trafficked and secluded trails within 13 experimental days during November 3 18, 2005 at the MCFP. Eighteen nests were preyed upon on the tourist trails while only two were preyed upon in the secluded trails. A maximum of two nests could be preyed upon at each trail type, each day (total maximum predation being 26 per trail type). The daily average predation for the tourist trail was 1.38 nests per day, while the daily average predation was 0.15 nests per day. Figure 4. Dail y composition of egg predation on each trail type at the MCFP during November 3 18, 2005 There was a greater abundance of egg predation, within both sites, in the last four days. These four experimental days were in sites closest to the main entrance wh ere there was the most human traffic.
Figure 5. The daily average of egg predation at each site. (a) Tourist trail predation events: site one, furthest away from the main entrance, had an average of 1.25 np/d. Site two had an average of 1 np/d. Site 3, closest to the main entrance, had an average of 1.8 np/d. (b) Secluded trail predation events: both site 1, furthest from the main entrance, and site 2 had no egg predation. Site 3, closest to the main entrance, had an av erage of 0.5 np/d. Both (a) and (b) showed an increase in egg predation toward the main entrance. Study site at the MCFP during November 3 18, 2005
Figure 6. Typical bite marks in plasticine eggs by the three egg predators found in this study. The ani mals inside the parenthesis are the most probable egg predators (at MCFP) in that family. The most common bite marks (8 out of 11) were from the family Procyonidae ( Nasua narica) Procyonids have dental formulas of i1/1 and c1/1 and the plasticine eggs at tacked by them were always much more mangled and torn up. One bite mark was found from an animal in the Order Rodentia and would most likely from a mouse, squirrel or rat. Rodentia animals have a dental formula of i1/1 and c0/0. Two plasticine eggs were found with bite marks from a Cebus capucinus in the Order Primate. Primates have dental formulas of i2/2 and c1/1. Dental formulas from Reid (1997).
Figure 7. Composition of predatory mammals invo lved in eating and/or attempting the eat the eggs (in the case of the plasticine eggs). The mammals were identified by using dental formulas from Reid (1997). These figures do not represent every predatory event; sometimes the plasticine eggs were found bitten to pieces (most likely behavior of Nasua narica ) or just not found. (a) The identified mammalian predators on the tourist trails consist mainly of the Nasua narica with only one predation event from Cebus capucinus. (b) The mammals identified on th e secluded trails, showed one event from an animal in the Rodentia Order and one predation event from Cebus capucinus DISCUSSION The results of this study contrasts with the study by M. Scholer (2005) which concluded that the higher bird populations on secluded trails were due to populations of egg predators in the area. However, there were a number
of studies done that compliment the results of this study; for example, studies by Andrn and Angelstam (1998) confirmed increased egg predation in areas of disturbance. Willis (1970) studied quail egg predation in Barro Colorado Island and found 15 times more predation on the island than on the (forest) mainland of close tend to have more edges and negative influences from neighboring open spaces while mainland forest tend to have a more coherent forest with nts at MCFP and are effected by the edges and high human traffic, while the secluded trails are, in some way, like the It was obvious that more h eavily trafficked trails, with higher influences from tourism, had higher rates of egg predation and the main predatory mammal was N. narica. Comparing mammal composition and the amount of egg predation events between the two trails, I concluded there is an invasive population of N. narica. The next important question would be: what is allowing their population to be disproportionately higher in areas closer to tourists? Two major components that keep populations in check are abundance of food and levels of predation. It was a common observation, at the MFCP, to see N. narica jump on picnic tables and aggressively grab sacs with food as well as to see tourists feeding N. narica completely new a nd additional food source. Additionally, the predators of N. narica, which are mainly boas, cats, and Tayras, tend to steer away from human trafficked areas. Also, in the case of the family Felidae, populations of top predators are declining, endangered, and rare due to deforestation and over hunting (Janzen 1983). This new food source and absence of predators are acting on populations of N. narica by raising their carrying capacity. There was a study taken place at El Verde, Puerto Rico on the consequenc es of the omnivorous predator, Rattus rattus (black rat) whose populations increased and had negative effects on the forest biodiversity and community structure (Reagan 1996). The introduction of R. rattus had a disruptive effect on the structure of the fo od web. These black rats are similar to N. narica in that they are mobile arboreal and terrestrial omnivores who forage on many types of fruits, invertebrates and vertebrates. Six vertebrate species are considered to be endangered partly because of R. r attus and their invasive population and opportunistic behavior (Raffaele et al 1973). The endangered birds construct ground or exposed nests which are vulnerable to predation by terrestrial mammals. The long term consequences of such disruptions on the d ynamics of the food web at El Verde are unclear, but have already caused extinctions. Due to simple predator prey dynamics, the raise in the carrying capacity of N. narica will have implications on their prey. Predation can contribute to the stabilizatio n of a population if the predation acts in a density dependent manner to increase the death rate as density increases (Whittaker 1975). Referring to the 12 species of birds in Monteverde in 1994 that were listed by BirdLife International as being vulnerabl e or nearly threatened with extinction due to their restricted ranges in the cloud forest (Nadkarni et al. 2000) it is obvious bird populations are not following the same population trends as N. narica Therefore, bird populations are at risk to over preda tion by N. narica To conclude, bird populations are not the only organisms at risk to the invasive population of N. narica Their opportunistic, flexible and omnivorous feeding habits include small lizards, young caviomorphs (agouti and paca spp.), forest litter invertebrates (insects, spiders, millipedes and snails to name a few), fruit, other vertebrates (like mice), and snakes (Finch 1973, Smyth 1978, Kaufmann 1962). To add, they are widespread and common in all regions of Central America and are terre strial and arboreal (Reid 1997). Many species could be threatened because of the high population of N. narica and this could have devastating outcomes for the tropical forest at MCFP. Ecotourism has provided a lot of money to conservation and protection projects (such as the MCFP). It is a great way to promote and publicize the importance of biodiversity and to bring in money to a local and regional community. However, with all things good comes the possible bad. In the tropics, there needs to be an emph asis on scientists working with land managers and the local community to find a balance between protecting biodiversity and having a sustainable economy. Tourism has been increasing in the Costa Rica and, as this study shows, could have negative future imp lications on tropical environments. I would like to see a longer future study involving more data to work with. I also qualitatively weather patterns and noticed that there was more predation on sunnier and drier days. This could have an even profounder
ef fect on mesopredator populations because there are more tourists in sunnier and drier seasons, meaning more food for these populations to grow. Figure 8. Pictures taken at MCFP during November 3 18, 2005. (a) A N. narica eating a bag of lunch stolen f rom a tourist. It was looking through the bag under a bench where two tourists were sitting (note their shoes). (b) A tourist feeding the N. narica a banana This was a common sight. ACKNOWLEDGMENTS I would like to give thanks to my advisor Javier Mnde z for helping me get my project organized, going to San Jos to get my quail eggs and for all his time working with me. Federico Chinchilla was a great help for finding article resources and experimental advice. Much thanks to the MCFP, especially Wilford Guindo for not only giving me the ins and outs of what trails to use, but for also giving me elevation maps and other answering specific questions about the reserve. Thanks to Carlos Hernndez for giving me permissi on to use the MCFP as my experimental sit e. Recognition and thanks must also go to Yanni Francisco SanMiguel Borrero for spending hours with me constructing wooden nests : you are a nest making master. LITERATURE CITED in Habitat Islands: 547 p. Benson, H. 2004. Edge effects on the terrestrial mammal community of a tropical premontane wet forest. UCEAP IMV Spring 2004. Collar, N., M. Crosby, and A. Strattersfield. 1994. Birds t o watch 2: The world list of threatened birds. In: Monteverde: Ecology and Conservation of a Tropical Cloud Forest, N. Nadkarni et al. Oxford, New York, USA. 202 p Conwell, E. 1997. Seed and egg predation of forest fragments and intact forest. UCEAP IMV Spring 1997. Finch, H. 1973. Field study of Costa Rican lizards. In: Costa Rican Natural History, D. Janzen, ed. The University Chicago Press, Chicago, IL, 478 480 p. Fogden, M. 1995. An annotated checklist of the birds of Monteverde and Peas Blancas. I n: Monteverde: Ecology and Conservation of a Tropical Cloud Forest, N. Nadkarni et al. Oxford, New York, USA. 182 p. Nadkarni, N., N. Wheelwright. 2000. Monteverde: Ecology and Conservation of a Tropical Cloud Forest. Oxford, NY, USA
Kaufmann, J. 1962. Ecology and social behavior of coati, Nasua narica, on Barro Colorado Island. In: Costa Rican Natural History, D. Janzen, ed. The University Chicago Press, Chicago, IL, 478 480 p. Raffaele, H., M. Velez, R. Cotte, J. Whelan, E. Keil, &W. Cupiano. 1973. R are and endangered animals of Peurto Rico: A committee report. In: The Food Web of a Tropical Rain Forest. Reagan, D., R. Waide. The University of Chicago Press. Chicago, IL. Reagan, D., R. Waide. 1996. The Food Web of a Tropical Rain Forest. The Univers ity of Chicago Press. Chicago, IL. Reid, F. 1997. A Field guide to the Mammals of Central America and Southest Mexico. Oxford university Press, Oxford, NY, USA. Rojas, J. 2004. Economa se Dispara Debido al Ingreso de Turismo por Aeropuertos. La Nacin. July 26. 2 p. Scholer, M. 2005. Impact of tourism on bird diversity, abundance and community ecology. CIEE Summer 1999. 69 72 p. Smyth, N. 1978. The natural history of Central American agouti. In: Costa Rican Natural History, D. Janzen, ed. The Univers ity Chicago Press, Chicago, IL, 478 480 p. Terborgh, J. 1992. Diversity and the Tropical Rainforest. New York, NY: Scientific America Library. 207 211 p. Whittaker, R. 1975. Communities & Ecosystems. 2 nd ed. Macmillan Publishing Co.. New York, NY. Young, B., D. McDonald. 2000. Birds. In: Monteverde: Ecology and Conservation of a Tropical Cloud Forest. Oxford, NY, USA.