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Distribucin de la herpetofauna y la riqueza de especies a lo largo de un gradiente altitudinal
Herpetofauna distribution and species richness along an elevational gradient
Monteverde, Costa Rica has experienced a significant increase in temperature and reduction in mist frequency in the last 20 years. These changes in abiotic conditions have been linked to the extinction 40 percent of local amphibian populations and several reptile species. The purpose of this study was to determine if the herpetofaunal community composition of Monteverde is still transitioning in response to continued changes in abiotic conditions. Reptiles and amphibians were censused along a 191 m elevational gradient to observe zonal distributions, and species richness and abundance. Species distributions were observed for shifts from their expected ranges. Visual and audio data were collected in the study. Visual data include collection of herpetofauna from trail transect day and night hikes and tree buttress plots while audio data consist of nighttime amphibian call documentation. Upslope shifts in distribution were found for three frog species. Species richness and abundance were not significantly correlated with elevation. Distributional shifts to higher elevations as demonstrated by the herpetofauna in this study seem to follow the trend of upslope shifts in cloud and mist distribution of tropical cloud forests. Changes in abiotic conditions including temperature, cloudiness and mist frequency may be exerting increasing pressure on herpetofauna that is resulting in distributional shifts and changing species richness trends in Monteverde.
Monteverde, Costa Rica ha tenido un aumento significativo de temperatura y una reduccin de frecuencia de la neblina en los ltimos 20 aos. Estos cambios en las condiciones abiticas se han relacionado con la extincin del 40% de las poblaciones de anfibios locales y varias especies de reptiles. El propsito de este estudio fue determinar si la composicin de la comunidad de herpetofauna de Monteverde todava esta cambiando en respuesta a cambios continuos en las condiciones abiticas.
Text in English.
Amphibians--Costa Rica--Puntarenas--Monteverde Zone
Anfibios--Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology 2007
Ecologa Tropical 2007
t Monteverde Institute : Tropical Ecology
Herpetofauna Distribution and Species Richness Along An Elevational Gradient Beth Schlimm Department of Biology, Villa Julie College ABSTRACT Monteverde, Costa Rica has experien ced a significant increase in temperature and reduction in mist frequency in the last 20 years. These changes in abiotic conditions have been linked to the extinction 40 percent of local amphibian populations and several reptile species. The purpose of this study was to determine if the herpetofaunal community composition of Monteverde is still transitioning in response to continued changes in abiotic conditions. Reptiles and amphibians were censused along a 191 m elevational gradient to observe zonal distributions, and species richness and abundance. Species distributions were observed for shifts from their expect ed ranges. Visual and audio data we re collected in the study. Visual data include collection of herpetofauna from trail transect day and night hikes and tree buttress plots while audio data consist of nighttime amphibian call documentation. Upslope shifts in distribution were found for three frog species. Species rich ness and abundance were not significa ntly correlated with elevation. Distributional shifts to higher elevations as demonstrated by the herpetofauna in this study seem to follow the trend of upslope shifts in cloud and mist distribution of tropical cloud forests. Changes in abiotic conditions including temperature, cloudiness and mist frequency may be exerting increasing pressure on herpetofauna that is resulting in distributional shifts and changing species richness trends in Monteverde. RESUMEN Monteverde, Costa Rica ha tenido una aumentacin significante de temperatura y una reduccin de frecuencia de neblina en los 20 anos mas reciente s. Esos cambios en condiciones abiticas han sido conectado a la extincin de 40% de las poblaciones locales de anfibios y unas especies de reptiles. El propsito de este estudio fue determinar si la composicin de la comunidad herpetofaunal todava esta cambiando en respuesta a cambios abiticos continuados. Se hizo un censo de reptiles y anfibios en un pendiente de elevacin para observar distribucion es zonales, riqueza de especies y abundancia. Distribuciones de especies fueron observado por notar cambios de sus mbitos anticipados. Data visual y auditorio fueron colectados en es te estudio. Data visual incluyo colecciones de herpetofauna de los senderos de da y de noche y contrafuertes de rboles mientras data auditoria consisto de documentacin de los llames de noche de los anfibios. Cambios de distribucin arriba del pendiente fueron encontrado por tres especies de rana. No hubo correlacin entre ni riqueza de espe cies ni abundancia con elevacin. Cambios de distribucin a elevaciones mas altos como fu e demostrado por el herpetofauna en este estudio estan en concordancia con los cambios de nubes y distribucin de neblina de los bosques tropicales. Cambios de condiciones abioticas incluyendo temper atura, cantidad de nube s y frecuencia de neblina pueden ejercer presura aumentando sobre herpetofauna que resulta en cambios de distribuciones y cambios de tendencias de riqueza de especies en Monteverde. INTRODUCTION Reptiles and amphibians have received much a ttention with the increasing awareness of climate change and its potential impacts on global species distributions. Herpetofaunal species are accurate indicators of environmental health beca use of the tight association between their physiology and ambient humidity and temperature (Gibbons et al. 2000).
As a result of this association, their ranges ar e likely to be affected by climate change on tropical mountains where steep climatic gradients are present. In the tropics, the general trend shows that species richness of herpetofauna decreases as elevation increa ses (Scott 1976). In the list of herpetofauna present in Monteverde, the number of species found at lower elevations is greater that the number of species found at higher elevations (Hayes et al. 1989). According to a study by Stevens (1992), species richness is lower at higher elevations as highland species are more specialized and have a lower tolerance to changes in biotic a nd abiotic conditions. Populations at higher elevati ons are put at high risk fo r extinction because geographic range size decreases as elevation increases (Pounds et al. 2006). Over the last two decades, the climate of Monteverde has experienced warming temperatures and a reduction in mist frequency (Pounds et al. 2006). One study hypothesizes that rising temperatures have resulted in cloud formation shifting further upslope where temperatures are cooler in cl oud forests. This woul d reduce the amount of mist in the lowlands (Pounds et al. 1999). Co rrelations have been discovered between these changes in climate and the local loss of herpetofauna (Pounds et al. 2006). In this time, 40 percent of amphibian populations and se veral reptile species of Monteverde have gone extinct likely in response to changing air temperature, precipitation and cloudiness. The purpose of this study was to determine if herpetofaunal community compositions and distributions are continuing to change in Monteverde. It was hypothesized that abiotic factors of elevational gradients are importa nt determinants of herpetofaunal species distributions. It was predicted that different elevations woul d support different aggregates of reptile and amphibian species. In addition, it was predicted that herpetofaunal populations would experience upslope shifts in response to increased temperatures and drier conditions. MATERIALS AND METHODS This study was performed in the forest in terior on property of the Estacin Biolgica de Monteverde at Monteverde, Co sta Rica, which includes tropical lower montane wet forest and tropical lower montane rain forest (Hayes et al. 1989). Censuses were made along a 191 m elevational gradie nt (1548-1739 m) by collecting herpetofauna and documenting amphibian calls during July. Visual and audio data were collected at night and during the day; the visual portion of the study consisted of day and ni ght collections of he rpetofauna along the Principal Trail in addition to tree buttress pl ot censuses. Daytime trail collections began at 12:00 noon and lasted approximately two hours. Nighttime collections began at 18:30 when the trail was completely dark and la sted about two hours. Two day and two night censuses were performed with the first census moving from highest to lowest elevation while the second moved from lowest to highe st. Leaf-litter along th e trail was disturbed with a stick to make individuals more visi ble and mobile. All reptiles and amphibians found were collected in plastic bags and brought back to the station for identification. The elevation and distributional zone of each individual was documented according to the trail marker directly below its location. All individuals were photographed and identified
to species using The Amphibians and Reptiles of Costa Rica (Savage 2002) and A Guide to Amphibians and Reptiles of Costa Rica (Leenders 2001). In order to perform a census of herpetofauna off of the trail, plots were set up around tree buttresses. Three m by three m plot s were set up around trees with three or more buttresses. To meet the size criteria, at least one buttress extended no less than one m up the base of the tree and one m out from the trunk. Only the corners of each plot were marked with flagging tape so as not to inhibit the movement of herpetofauna in or out of the plot. The location of the plots ranged from 5 m to 20.2 m off of the Principal Trail. A total of nine plots were set up al ong the elevational gradient. Each plot was censused by disturbing the leaf litter and vegetation between buttre sses with a hand rake. Herpetofauna was collected and identified in the same manner as for the trail censuses. Two daytime plot censuses were performed. The first started at the plot with the highest elevation and moved down while the second started at the lowest plot and moved up. Each census started at 12:00 noon and took approximately two hours. Audio data were collected by listening to amphibian calls beginning at 18:30, at nine elevations along the tra il, on the night of July 29th, 2007. All amphibian call types heard in a five minute period were documented and described for each elevation. The call types were compared to known amphibian calls and potential species identifications were proposed for four of the five total call types. A comparison was made between the expect ed zonal distributions according to Hayes et al. (1989) and the observed distribut ions for all herpetofauna collected. The distributional zones are defined by elevation and topography and correspond loosely to the Holdridge life zones. Zone Three exte nds from 1470 m to 1600 m and is considered tropical lower montane wet forest. Zone Four, tropical lower montane rain forest, covers the study site above 1600 m on the Pacifi c slope to 1450 m on the Caribbean slope (Hayes et al. 1989). In addition, Spearman ra nk correlation analyses were conducted to test the effects of elevation on abundance and species ri chness. The species richness values were based on visual data a nd the number of amphibian call types. RESULTS In a comparison of expected herpet ofaunal zonal distributions with the distributions recorded in th e study, several amphibian species were found out of their expected zones (Hayes et al. 1989). Six herpetofaunal species we re collected in total from the trail and plot censuses. Three Eleuthroda ctylus species collected during trail censuses were found outside of their expected zonal distribution (Table 1). The same comparison was made with the frog call data (Table 2). Fi ve distinct calls were recorded. The four species compared in this table are prop osed species identific ations, as visual identification was not possible. There was no statistical significance in the correlation of visual results for the number of individuals versus elevati on (Spearman rank corr elation, rho = 0.3274, p = 0.2336, n = 15, Figure 1). The corr elation of species richness versus elevation for the visual data also had no statistical signifi cance (Spearman rank correlation, rho = 0.1563, p = 0.5780, n = 15, Figure 2). The number of am phibian call types were significantly correlated with elevation (Spearman rank correlation, rho = 0.6172, p = 0.1398, n = 7, Figure 3).
DISCUSSION In the recent past, herpetofaunal commun ities of Monteverde have experienced changes in species composition and shifts in distribution due to changes in temperature, cloudiness and mist frequenc y on mountains. The hypothesis of this study stated that abiotic factors of elevational gradients are important factors in determining herpetofaunal species distributions. Predictions were also made stating that herpetofaunal community compositions would change with differences in elevation an d that populations would shift upslope in response to changes in abiotic factors. The hypothesis was supported by zonal distribution results as they showed an upslope shift in distribution. The correlation analyses had no statistical significance and could not be used to substantiate the hypothesis. The comparisons of expected and study distributions showed several disparities in the zonal distributions of herpetofauna. Of si x total species found duri ng trail transect and plot census collections, three were found outside of their expected zonal distribution (Table 1). Eleuthrodactylus bransfordii and E. fitzingeri were both in Zone Four while their expected distribution was Zone Three. Eleuthrodactylus altae was also found in Zone Four. The study distribution of this species is especially interesting as the expected distribution was Zones Five and Six which are located on the Caribbean slope. Climate change induced range shift is distinct possibility for these species. The results of the ca ll data were similar to those of the visual data. Although calling individuals could not be positively identified (frogs were not directly observed), they corroborated the results from the visual data. The distribution z ones for E. fitzingeri and E. podiciferus were the same for the visu al and audio data. In addition, the call of Bufo coccifer was identified in Zones Three and Four while the expected distribution was only in Zone Three showing an upslope shift in distribution. Species found outside of their expected di stributions experienced a shift to a zone at a higher elevation with the exception of E. altae which shifted to the Pacific Slope. This upward zone shift seems to mirror the upwar d shift of the cloud ba se and mist in the tropics (Pounds et al. 1999). It is logical th at the moisture dependent life cycles and biology of herpetofauna would force distributio ns to follow the change in cloud and mist distribution as is suggested in this study. The study distribution Eleuthrodactylus alt ae was an interesti ng exception to the upslope migration hypothesis. While its study distribution was upslope of the expected distribution, the distri bution also shifted from the Cari bbean Slope to the Pacific Slope. One hypothesis for this distribut ional shift is that local he rpetofaunal extinctions and distribution migrations have modified community compositions such that fewer competitor and predator species are present. As a result, E. altae is able to expand its zonal distribution and niche space. Eleuthrodactylus melanostictus is a Zone Four species that has declined in the past and shares similar behaviors with E. altae. Both are primarily nocturnal species that are active on low vegetation. During the day, these species spend time hiding in leaf litter or bromeliads. It is pos sible that E. altae is filling the niche of E. melanostictus. This hypothesis is one suggestion for further study. Fewer species and individuals were expect ed to be found at the higher elevations of Zone Four thus decreasing species richness and number of individuals. However, there was a lack of significance and trend between sp ecies richness and elevation. This is likely a result of limited sampling. Visual results of the study show that several species from
Zone Three have moved up in elevation and were only found in Zone Four. This should increase the species richness at the higher, Zone Four elevations. Further research is needed to test this prediction. Though this study suggests that herpetofauna are migra ting upslope to escape the effects of climate change, shifts in distributio n do not secure the survival of every reptile and amphibian species affected by climate ch ange. Mountain slopes act as filters against upward migrations of populations (Scott 1976 ). Discontinuities in vegetation and water sources, differences in microhabitats and co mpetition with native species prevent some migrant species from surviving at higher elevations (Hofer et al. 2000). As a result, small shifts in distribution can lead to extinctions at the local level and for endemic species, extinction at the global level (Blaustein et al. 1994). Future studies of changes in species distributions are very important and sorely needed as they may help in predicting loss of species and biodiversity in the future. This study could be improved upon by using an elevational grad ient with even coverage of Zones Three and Four. A larger pr oportion of the elevational gradient in this study had a Zone Four distribution. This may account for th e fact that several species were missing from their expected distributional zones but it cannot e xplain their presence in a higher zone. Additional collections of visual and audio data could also be performed at time intervals throughout the day and night. This might incorporate a greater diversity of herpetofauna as different spec ies are active at different times. Some suggestions for further research include studying a larger elevational gradient that covers more than two herpetofa una distributional zones. An increase in the number of zones in the study may shed light on specific characteristics of shifting species and whether or not upward shifts are occurri ng evenly along the elevational gradient. The Caribbean Slope might be incorporated in the study to see if upslope shifts are occurring with the same number of species and at the same rate. It would also be interesting to conduct a similar study on zone shifts during th e dry season when individuals are more directly affected by a reduction in mist freque ncy and an increase in temperature. Lastly, herpetofauna could be collected and amphibian calls could be documented from different habitats the forest interior in addition to th e trail and tree buttre ss habitats. This would potentially incorporate a great er number of species into the study. It may also help answer the question of whether or not zonal sh ifts occur more easily in forest clearings than in the forest interior with more dense vegetation. ACKNOWLEDGEMENTS I would like to thank Karen Masters fo r her guidance and encouragement in pursuing my interest in herpetology. Thank you to Pablo for his assistance in herpetofauna collection and to Cam for all of her excellent advice. I would also like to thank Jess for her help and much appreciated company on night hikes. Many thanks also to the Estacin Biolgica de Monteverde fo r allowing me to conduc t this study along the beautiful Principal Trail.
LITERATURE CITED Blaustein, A. R., Wake, D. B., & Sousa, W. P. (1994). Amphibian Declines: Judging Stability, Persistence, Susceptibility of Populations to Local and Global Extinctions. Conservation Biology, 8), 60-71. Gibbons, J. W., Scott, D. E., Ryan, T. J., Buhlmann, K. A., Tubervill e, T. D., Metts, B. S., Leiden, Y., Poppy, S., & Winne, C. T. (2000). The Global Decline of Reptiles; DejaVu Amphibians. BioScience, 50, 653-666. Hayes, M. P., Pounds, J. A., & Timmerman, W. W. (1989). An Annotated List and Guide to the Amphibians and Reptiles of Monteverde Costa Rica. Miami University, Oxford, Ohio: D. H. Taylor. Hofer, U., Bersier, L., & Borcard, D. (2000). Ecotones and Gradient as Determinants of Herpetofaunal Community in the Prim ary Forest of Mount Kupe, Cameroon. Journal of Tropical Ecology, 16, 517-533. Leenders, T. (2001). A Guide to Amphibians and Reptiles of Costa Rica. Miami, FL: Distribuidores Zona Tropical. Pounds, J. A., Fogden, M. P., & Campbell, J. H. (1999). Biological Response to Climate Change on a Tropical M ountain. Nature, 398, 611-615. Pounds, J. A., Bustamante, M. R., Coloma, L. A., Consuegra, J. A., Fogden, M. P., Foster, P. N., La Marca, E., Masters, K. L., Merino-Viteri, A., Puschendorf, R., Ron, S. R., Sanchez-Azofeifa, G. A ., still, C.J., & Young, B. E. (2006). Widespread Amphibian Extinctions from Epidemic Disease Driven by Global Warming. Nature, 439, 161-167. Savage, J. M. (2002). The Amphibians and Rep tiles of Costa Rica. London: University of Chicago Press. Scott, N. J., Jr. (1976). The Abundance and Di versity of the Herpetofaunas of Tropical Forest Litter. Biotropica, 8, 41-58. Stevens, G. (1992). Elevational Gradient in Altitudinal Range: An Extension of Rapoportâ€™s Latitudinal Rule to Altitude. The American Naturalist, 140, 60-71.
TABLE 1. A comparison of zonal distributions from visual data. The species were found during day and night trail transe ct censuses. The expected di stribution zone was based on the findings of Hayes et al . (1989). The study distribution zones are based on the elevations where the species was found in th is study. Zone 3: 1470 m to 1600 m ; Zone 4: 1600 m (Pacific Slope) over the ridge to 1450 m (Caribbean Slope); Zone 5: 1150 m to 1450 m (Caribbean Slope); Zone 6: 600 m to 1150 m (Caribbean Slope). Class Species Expected Distribution Zone Study Distribution Zone Amphibia E. altae 5,6 4 E. bransfordii 3 4 E. fitzingeri 3 4 E. podiciferus 3,4 3,4 Reptilia A. humilis 3,4 4 A. tropidolepis 3,4 4 TABLE 2. A comparison of zonal distributions of amphibian calls. The species listed are the proposed call identifications for four of five total call ty pes. The expected distribution zone was based on the findings of Hayes et al. (1989). The study di stribution zones are based on the elevations wher e the species was found in th is study. Zone 3: 1470 m to 1600 m (Pacific Slope); Zone 4: 1600 m (Pacific Slope) over the ridge to 1450 m (Caribbean Slope). Species Expected Distribution Study Distribution B. coccifer 3 3,4 E. diastema 3,4 4 E. fitzingeri 3 4 E. podiciferus 3,4 4
0 0.5 1 1.5 2 2.5 3 3.5 1500 1550 1600 1650 1700 1750 Elevation (m) Figure 1. Number of individuals co llected from leaf-litter along trail transect elevational censuses. All herpetofauna observed were collected from 1548 m to 1739 m on the Principal Trail of Monteverde, Costa Rica. 0 0.5 1 1.5 2 2.5 3 3.5 1500 1550 1600 1650 1700 1750 Elevation (m) Figure 2. Herpetofauna species richness along trail transect elevational census. The species include all herpetofauna collected from 1548 m to 1739 m on the Princi pal Trail of Monteverde, Costa Rica.
0 0.5 1 1.5 2 2.5 3 3.5 1560158016001620164016601680170017201740 Elevation (m)Number of Call Types Figure 3. Number of amphibian call types recognized along an elevational gradient on the Prin cipal Trail of Monteverde, Costa Rica. The number of distinct amphibian calls were documented for nine locations. The number of ca ll types at each location corresponds to the number of species present.