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The effects of edge and pollution on lichen richness, abundance, and distribution in Cañitas, Costa Rica

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Title:
The effects of edge and pollution on lichen richness, abundance, and distribution in Cañitas, Costa Rica
Translated Title:
Los efectos del borde y la contaminación en la riqueza, abundancia, y distribución de líquenes en Cañitas, Costa Rica ( )
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Bedell-Stiles, Jenny
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Lichens--Ecology   ( lcsh )
Air--Pollution   ( lcsh )
Costa Rica--Guanacaste--Cañitas   ( lcsh )
Líquenes--Ecología
Aire--Contaminación
Costa Rica--Guanacaste--Cañitas
Tropical Ecology Fall 2004
Ecología Tropical Otoño 2004
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Reports   ( lcsh )
Reports

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Abstract:
Lichens are commonly known to be biological indicators of pollution and edge effects. Their use as indicators in temperate regions is well documented, however, less is known about their function as a bioindicator in the tropics. There is also debate over the consequences of edge effects, such as increased temperature, and less moisture and wind, on lichen richness and abundance. In this study lichen communities on tree trunks in three habitat sites (primary forest, pasture, and roadside) were examined. In total 88 trees were censused in order to determine lichen richness, abundance, and similarity. Overall 68 species of lichen were observed. The pasture site demonstrated the highest total number of species (42), average number of species (5.97), and average abundance (28% coverage). In contrast, the road showed the lowest total number of species (16), average number of species (2.39), and average abundance (.37%). It was speculated that these trends were due to the strong effects of edge and air pollution, however, these factors were not directly measured. The moist primary forest site appeared to have moss competing with lichen for area on the tree trunk, possibly limiting the number of species and abundance of lichen in that site. The highest similarity between sites was observed between the pasture and roadside, most likely due to the similarity of these two environments. Both were subject to edge effects, however the road had the additional pressures of air pollution and dust.
Abstract:
Los líquenes son indicadores biológicos de contaminación y efectos de orilla. Su uso en regiones templadas está bien establecido, sin embargo, sabemos menos de su función como indicador biológico en el trópico. Además, hay desacuerdos acerca de las consecuencias del efecto de borde, como lo son el aumento de temperatura, la disminución de la humedad, y el viento sobre la riqueza y la abundancia de líquenes. En este estudio se examinaron tres hábitats (Bosque primario, potreros y borde del camino), 88 árboles fueron considerados y 68 especies de líquenes fueron considerados. Yo analicé comunidades de líquenes sobre los troncos de árboles en términos de la riqueza de especies, abundancia y similitud. El potrero mostró el número de especies (42), número promedio de especies (5.97) y abundancia promedio más altos (28%). Por otro lado, el camino mostró el número de especies (16), el numero promedio de especies (2.39), y abundancia promedio más bajos (.37%). Yo especulé que esto se debe a los efectos fuertes del borde y a la contaminación. El bosque primario húmedo pareció tener musgo compitiendo con liquen por el área en el tronco; por lo tanto posiblemente limita el número de especies y abundancia de líquenes en ese hábitat. La similitud más alta entre los sitios se observó entre el potrero y el camino, probablemente debido a la similitud de los dos terrenos. Los dos están sujetos a fuertes efectos del borde; sin embargo, el camino también exhibe las presiones adicionales de contaminación del aire y el polvo.
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1 The Effects of Edge and Pollution on Lichen Richness, Abundance, and Distribution in Caitas, Costa Rica Jenny Bedell Stiles Department of Environmental Studies, University of Oregon ABSTRACT Lichens are commonly known to be biological indicators of pol lution and edge effects. Their use as indicators in temperate regions is well documented, however, less is known about their function as a bioindicator in the tropics. There is also debate over the consequences of edge effects, such as increased temperat ure, less moisture and wind, on lichen richness and abundance. In this study lichen communities on tree trunks in three habitat sites (primary forest, pasture, and roadside) were examined. In total 88 trees were censused in order to determine lichen rich ness, abundance, and similarity. Overall 68 species of lichen were observed. The pasture site demonstrated the highest total number of species (42), average number of species (5.97), and average abundance (28% coverage). In contrast, the road showed the lowest total number of species (16), average number of species (2.39), and average abundance (.37%). It was speculated that these trends were due to the strong effects of edge and air pollution, however, these factors were not directly measured. The mois t primary forest site appeared to have moss competing with lichen for area on the tree trunk, possibly limiting the number of species and abundance of lichen in that site. The highest similarity between sites was observed between the pasture and roadside, most likely due to the similarity of these two environments. Both were subject to edge effects, however the road had the additional pressures of air pollution and dust. RESUMEN Los lquenes son indicadores biolgicos de contaminacin y efectos de oril la. Su uso en regiones templadas est bien establecido, sin embargo, sabemos menos de su funcin como indicador biolgico en el trpico. Adems, hay desacuerdos acerca de las consecuencias del efecto de borde, como lo son el aumento de temperatura, la di sminucin de la humedad, y el viento sobre la riqueza y la abundancia de lquenes. En este estudio se examinaron tres hbitats (Bosque primario, potreros y borde del camino), 88 rboles fueron considerados y 68 especies de liquen fueron considerados. Yo analic comunidades de lquenes sobre los troncos de rboles en trminos de la riqueza de especies, abundancia y similitud. El potrero mostr el nmero de especies (42), nmero promedio de especies (5.97) y abundancia promedio ms altos (28%). Por otro l ado, el camino mostr el nmero de especies (16), el numero promedio de especies (2.39), y abundancia promedio ms bajos (.37%). Yo especul que esto se debe a los efectos fuertes del borde y a la contaminacin. El bosque primario hmedo pareci tener mu sgo compitiendo con lquen por el rea en el tronco; por lo tanto posiblemente limita el nmero de especies y abundancia de lquenes en ese hbitat. La similitud ms alta entre sitios se observ entre el potrero y el camino, probablemente debido a la simi litud de los dos terrenos. Los dos estn sujetos a fuertes efectos del borde; sin embargo, el camino tambin exhibe las presiones adicionales de contaminaction del aire y polvo. INTRODUCTION Lichens, although commonly thought of as a single entity, ar e in fact a symbiosis between a fungus and a photosynthesizing organism. Lichen fungi, many (over 40%) belonging to the Division Ascomycota, associate with green algae or cyanobacteria (Purvis 2000). The fungal partner, or mycobiont, provides the physica l structure for the photosynthesizing component, or photobiont, and absorbs minerals and moisture from the environment (Brodo et al. 2001). The photobiont photosynthesizes and provides carbon

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2 and nitrogen to the mycobiont. Lichens occur in diverse habita ts worldwide, from the tropics to polar regions, and to the most arid and harsh environments. About 8% of 14,000 species of lichens are known, exhibiting diverse col or, form and size (Brodo et al. 2001). For over 140 years, lichens have been recognized as being extremely sensitive to air pollution (Brodo et al. 2001). Having no roots, they efficiently absorb water and nutrients from air and rainwater across their su rface. This adaptation makes them vulnerable and sensitive to chemicals in their environment. The symbiosis between the mycobiont and photobiont further contributes to this sensitivity; if one component becomes damaged by the pollutant then the partnersh ip will not function and the lichen will die (Brodo et al. 2001). For these reasons lichens are commonly used as biological indicators of pollution. Air quality, specifically concentrations of sulfur dioxide, fluoride, and ammonia, has been monitored usi ng lichens. Although lichens are well studied bioindicators in temperate regions, less is known about their usefulness as pollution indicators in the tropics partly due to their higher species diversity (Wolseley and Aguirre Hudson 1997). Studies to de termine the pollution tolerance of lichen in the tropics are becoming more common. A popular technique is to transplant lichens from a habitat without pollution to an area with pollution, and vise versa to monitor what is absorbed and the rate of absorpti on. One such study in Venezuelan cloud forests transplanted lichens from an urban and presumed polluted habitat to an area without pollution, and from the pristine habitat to the urban area (Gordon et al. 1995). Fog frequencies in both habitats were very high (up to 326 days a year) and previous studies showed that the fog from the urban area was characterized by high concentrations of Pb, Zn, and Mn. These metals were thought to come from human sources; including heavily leaded gasoline and Zn from tire dust. Elevated concentrations of these metals were found in the transplanted lichen at the urban site after only a few months. Conversely, the lichen transplanted from the polluted site to the forest lost significant amounts of Pb after six to ten month s. The findings of a tropical lichen study in San Jose, Costa Rica suggested that the Pb and other chemicals in gasoline had an adverse effect on lichen communities (Monge Najera et al. 2002). Lichen cover change was recorded from 1976 1997 within a 10 x 10 cm template positioned 1.5 m above ground on the south, east, north, and west sides of ten trees per station in 11 study stations. Mean lichen cover was 23% in 1976, 12% in 1986, 9% in 1990, and 22% in 1997. Most stations reflected a reduction in me an coverage after 1976, but increased after 1990, possibly due to improved traffic regulations and elimination of lead from gasoline, which was banned in Costa Rica in 1995 (Muoz, 11/23/04, www.tierramerica.net/2002/1006/iacento3.shtml). Lichens are sen sitive not only to pollution, but to edge effects as well, although there is debate as to how these affect lichen abundance and species richness. Forest fragmented by a road, pasture or other natural or human caused disturbances, have decreased moisture, higher temperature, and more wind along forest edges. A 1998 study collecting data on the fruticose lichen, Alectoria sarementosa showed that a significantly lower mass of lichen grew along the forest edge than deeper within the forest (Esseen and Renhor n 1998). Other studies found that edge effects may increase lichen species richness and abundance. In one study of two managed forests, the percent of a plot

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3 occupied by gaps was strongly correlated with lichen species richness (Neitlich and McCune 1997) Larger gaps supported higher species richness, suggesting that edge effect, which increase with the size of a gap, may contribute to higher species richness and diversity. Another study conducted by Wolseley et al. (1991) suggested that high lichen spe cies richness may be found along forest edges or roads because less sensitive species succeed in disturbed areas. Some species of lichen are more sensitive than others to specific types of pollution and disturbance, and therefore many types of lichen may still thrive in mildly polluted areas (Brodo et al. 2001). An edge effect specific to unpaved roads is increased dust and airborne particulates. Monteverde, Costa Rica has two distinct seasons: wet and dry/misty. The wet season ranges from April to Nove mber and the dry/misty season for the remainder of the year. The dry/misty season is defined by more wind and increased intervals without measurable precipitation. Lacking moisture at this time of year, the roads become dustier and vegetation alongside t hem is covered in a film of sand and dust. Physical effects of dust and sand on plants include cell destruction and blocked stomata (Spellerberg 1998). A 1993 study of the effects of dust on plants and plant communities found that epiphytic lichens, spha gnum, and some mosses were the most sensitive of the taxa studied (Farmer 1993). Therefore, in Monteverde dust must be included as an edge effect of roadside disturbances. Monteverde offers many environments to study the consequences of pollution/dust an d edge effects on lichen. Unpaved roads provide areas to study the effects of pollution and dust as well as strong edge effects such as lower moisture, increased temperature, and wind. Pasture areas are relatively unpolluted and free of dust, but still a re subject to common edge effects. And primary inner forest environment shows very little edge and pollution/dust effects. In this study I expect to find that both pollution/dust and edge effects have a negative effect on lichen species richness and ab undance. Therefore, the forest interior lichen should demonstrate the highest species richness and abundance. Without the effects of pollution and dust, pasture lichen should show the second highest richness and diversity. Due to pollution/dust and edge effects, roadside lichen will have the lowest species richness and abundance. I expect the roadside and pasture sites to have the most similar species compositions because they are both subject to edge effects. My pasture site borders the forest, so per haps the forest and pasture sites will have the next highest species composition, leaving the forest and roadside sites to have the lowest similarity. MATERIALS AND METHODS This study was conducted at three sites on or near the property of Jim Wolfe and Martha Salazar in Caitas, Costa Rica, from October 25 to November 15, 2004. The sites included primary forest (1405 m), pasture (1400 m), and roadside (1305 m), all near or in forest classified as Holdridge Lower Montane Wet Forest. The primary fore st was a fragment between two dairy farm pastures. Thirty trees from this habitat were censused from the interior no closer than 8 m from the forest edge. Pasture data were collected from 30 trees in one of these dairy farm pastures. Data for the roadsi de were gathered from 28 trees directly bordering the main road, Calle Principal, to the West below the property of Jim and Martha.

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4 Trees with a diameter at breast height (DBH) of more than 28 cm were randomly chosen for study in all three habitats. Fo r each tree, an area of 56 x 225 cm was censused for lichen area covered and for the presence of morphospecies. A clear laminate grid was placed 1 m from the ground over the bark of the tree to delineate the study area. The lichen was censused by countin g the total number of squares filled by lichen in the 56 x 225 cm area. Morphospecies found within this area were recorded as well as tree DBH. Photos of each species were taken with a Sony Cyber shot DSC S85 digital camera and later used to accurately i dentify them throughout the study. A Simple Linear Regression Test was used to determine whether the number of species or percent coverage was related to the average tree DBH. A one way ANOVA was used to test whether the mean number of species and percen t coverage differed similarities in morphospecies composition between the three sites. RESULTS A total of 68 species were found at the three different sites: 28 in the prima ry forest, 42 in the pasture, and 16 along the road (Figure 1). The pasture had the highest total species richness, followed by the primary forest and roadside sites. The average species richness and average coverage were both highest at the pasture site (Figure 2). Primary and roadside sites had very similar average species values (2.7 and 2.4 respectively) but dissimilar average proportion coverage values (23.9% and 3.7% respectively). One way ANOVA analysis showed tha t average species richness for both the primary forest/pasture and pasture/road sites was significant. The primary/road sites were not significantly different. The difference in average lichen abundance was significant for the primary forest/road and pas ture/road sites. However, the lichen abundance for the primary forest/pasture sites was found to be not significant. Figure 1 Total lichen morphospecies observed at three sites in Caitas, Costa Rica.

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5 A B A B 3) showed that there was highest similarity between the pasture/road sites, followed by Mean Diff Crit. Diff. P Value Primary/Pastu re 0.048 0.086 0.275 Primary/Roadside 0.202 0.087 <.0001 Pasture/Roadside 0.249 0.088 <.0001 Mean Diff. Crit. Diff P Value Primary/Pasture 3.267 0.911 <.0001 Primary/Roadside 0.307 0.928 0.512 Pasture/Roadside 3.574 0.928 <.0001 Figure 2. Showing variation in average number of lichen species (A) and average lichen coverage (B) at three sites in Caitas, Costa Rica A F = 36.791, p<.0001; n = 30, 30, 28 for primary forest, pasture, and roadside respectively. B F = 17.873, p <.0001; n = 30, 30, 28 for primary forest, pasture, and roadside, respectively. Table 1. Post Hoc Analysis of average lichen species richness (A) and average proportion lichen coverage (B) in the primary forest, pasture, and along the roadside.

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6 the road/primary forest sites, and lastly, the primary forest/pasture sites (Quan titative values = 0.464, 0.148, and 0.1 respectively). three sites in Caitas, Costa Rica showing the highest similarity of lichen species between the pasture/road sites, followed by the road/prim ary forest and finally the primary forest/pasture.

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7 There were no significant relationships found between the DBH of trees at the three sites and their species richness or abundance in single linear regre ssions (Figure 4). Species richness and abundance showed a slight negative trend with DBH in the primary forest. For the pasture site species richness showed no correlation with DBH and abundance demonstrated a very slight positive trend. Both species r ichness and abundance demonstrated a slight positive trend with DBH along the roadside. A B C D E F DISCUSSION The greatest total species were observed at the pasture site, followed by the primary forest, and the road sites (Figure 1). Contrary to my hypothesis, that the primary forest would have the highest species richness, in fact the pas ture was found to have the most species of lichen. One observation that may account for this is the interaction between lichen and moss. Moss survival is dependent upon constant moisture. Of the three sites, moss appeared to flourish most in the primary forest, seemingly competing with the Figure 4. Relationships between tree DBH, lichen species and lichen coverage for each tree at each site in Caitas Costa Rica. Simple linear regressions showed no significance for any o f the relationships.

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8 lichen for area on tree trunks. Whereas moss was mainly absent in the pasture habitat, which is subject to edge effects such as lower moisture and higher temperature. This suggests that moss prefers habitats without edge effects, and lichen, perhaps a poor competitor with moss, can thrive in disturbed areas. Sulyma and Coxon (2001) found in British Columbia that lichen coverage was overtaken by feather moss mats in mid to late successional forests, with moss mats of ten burying lichen that previously had occupied microsites. Canopy leaf area was found to have a positive effect on the area of moss mats in forest microsites. This finding agrees with the data of this study because highest average proportion coverage wa s found in the pasture (Figure 2B) where there is least canopy leaf area. The lowest richness of lichens was found along the road, and little moss was observed there as well, perhaps due to the limited moisture. The road is subject to the same edge effect s as the pasture, but has the additional pressures of pollution and dust. Fewer total species along the road suggests that the added edge effects of pollution and dust along the road negatively impact many lichen species and those that remain are more tol erant. In addition to total species observed at each site, the pasture had the highest average species richness per tree being greater than both the primary forest and roadside trees (Figure 2A). This further suggests that lichens thrive in areas of di sturbance where there is lower moisture and higher temperatures. Both of these sites have lower average lichen species richness, perhaps because the primary forest lichens must compete with moss, and lichens along the road are limited by pollutants and dus t. However, not all lichens may be classified together by their preferred environments; some lichen need moisture, other need dry habitats, and others such as Xanthoria sp even thrive on nitrogen rich agricultural dust (Stone 2004). The difference in av erage lichen coverage was found to be significant for the primary forest/road and pasture/road sites (Figure 2B, Table 1). Because the road had such an extremely low average percent coverage, both other sites were significantly different when compared wit h it. The pasture showed the highest average coverage of lichen and the road showed the lowest. Both sites were subject to edge effects, while the road also probably experienced pollution and dust. This is again suggestive that pollution and dust contri bute to low roadside abundance. Both the primary forest/pasture sites had relatively high average abundances and were found to have no significant differences in abundance. The high similarity between the pasture and the road sites (Figure 3) could be e xplained by the physical similarity between these two habitats. Both are subject to heavy edge effects, but differ in that the road is subject to pollution and dust as well. Therefore, the species that overlap between these two sites are probably those t hat can best tolerate pollution and dust, and would thus be poor bioindicators because they are least sensitive. Those species that do not overlap and are found only in the primary forest and or pasture might serve as good bioindicators. An example of su ch species is Usnea a genus found only in the pasture site, is commonly used as a bioindicator in Europe to map pollution levels (Brodo et al. 2001). Lichen communities of the road and primary forest demonstrate the second highest similarity followed by the primary forest and the pasture with the lowest similarity (Figure 3). The primary forest is included in the two

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9 lowest similarity comparisons, perhaps because the lichen species that colonize a forest specialize within that habitat and need a moist, l ower light environment. Temperate epiphytic lichen diversity tends to increase with the age of the substrate, therefore, indicators are often associated with trees +200 years (Rose, 1976). However, in a study conducted in tropical forests in Thailand the re was little evidence for an increase in diversity with host tree size (Wolseley and Aguirre Hudson 1997). My study supports the results of Wolseley and Aguirre Hudson (1997). There were no significant relationships found between the DBH of trees at the three sites and their species richness or abundance (Figure 4). Furthermore, in the primary forest, where the oldest and largest trees are expected to be found, both species richness and abundance revealed a slight negative correlation with DBH. Increas ing habitat fragmentation with the building of roads and other human activities in the tropics intensifies the need to understand how these activities affect tropical richness and biodiversity. As more habitat edge is created, lichen community structure w ill be altered, as evidenced in this study. Both of the disturbed areas in my study, pasture and roadside, showed significantly different lichen communities than the relatively undisturbed primary forest. If the findings of the Sulyma and Coxon (2001) ex periment are also applicable in the tropics, that higher leaf area index values correlate with higher amounts of moss, then manipulation of stand structure in managed forests may delay successional changes and promote continued lichen growth where moss mig ht otherwise have dominated. But it is not only lichen and moss communities that are affected by disturbances. The status of lichens as bioindicators is similar to the idea of the ies to assess how changes in lichen communities correlate with changes in other floral and faunal communities. For future studies of lichen in the Monteverde area, it would be interesting to Wilson and Robert fragment of forest is surrounded by pasture, how likely are the forest lichen species to distribute their genes outside of the pasture area, and vise versa. In addition, a closer analysis of the competition between moss and lichen and how it affects the abundance of each in forest environments would be helpful in und erstanding the dynamics of lichen communities. ACKNOWLEDGMENTS I would like to extend my sincere gratitude to Karen Masters for helping me design my project and guiding me through my exploration of lichen In times of complete stress you and Alan someh ow have enough patience to go around. The both of you have broadened by horizons as to how a socially active and conservation conscious biologist can act. A big thank you also goes out to Jim Wolfe and Martha Salazar for allowing me to tramp through thei r farm and use their property in my study. I also want to thank Matt Gasner and Ollie Hyman for their statistics expertise and keeping me grinning by being goofballs. I am grateful to the Estacion Biologica for the use of its resources. And finally, to my family, without whose support I would not have been able to be a part of this program.

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10 LITERATURE CITED Brodo, I. M., S.D. Sharnoff, S. Sharnoff. 2001. Lichens of North America New Haven: Yale University Press. Esseen, P.A., and K.A. Renhorn. 1 998. Edge effects on an epiphytic lichen in fragmented forests. Conservation Biology 12(6):1307 1316. Farmer, A.M. 1993. The effects of dust on vegetation a review. Environmental Pollution 79:63 75. Gordon C.A., R.T. Herrera, C. Hurchinson. 1995. The use of a common epiphytic lichen as a bioindicator of atmospheric inputs to two Venezuelan cloud forests. Journal of Tropical Ecology 11:1 26. Monge Najera J, M.I. Gonzalez, M.R. Rossi, V.H. Mendez Estrada. 2002. Twenty years of lichen cover change in a tropical habitat (Costa Rica) and its relation with air pollution. Revista de Biologia Tropical 50(1):309 319 [cited 2004 Nov. 23]; http://www.tierramerica.net/2002/1006/iacento3.shtml Neitlich, P.N. and B. M cCune. 1997. Hotspots of epiphytic lichen diversity in two young managed forests. Conservation Biology 11(1):172 182. Purvis, W. 2000. Lichens Washington DC: Smithsonian Institution Press. Rose, F. 1976. Lichenological indicators of age and environment al continuity in woodlands. Lichenology Progress and Problems 279 307. Spellerberg, I.F. 1998. Ecological effects of roads and traffic: a literature review. Global Ecology and Biogeography Letters 7(5):317 333. Stone, Daphne. 2004. PhD University of Or egon, Eugene Oregon. Personal Communication. Sulyma, R. and D. Coxon. 2001. Microsite displacement of terrestrial lichens by feather moss mats in late seral pine lichen woodlands of north central Britich Columbia. Bryologist 104(4):505 516. Wolseley, P. A., B. Aguirre Hudson. 1997. The ecology and distribution of lichens in tropical deciduous and evergreen forests of northern Thailand. The Journal of Biogeography 24:327 343. Wolseley, P.A., C. Moncrieff, B. Aguirre Hudson. 1991. Lichens as indicators of environmental stability and the change in the tropical forests of Thailand. Global Ecology and Biogeography Letters 1:170 175.


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speculated that these trends were due to the strong effects of edge and air pollution, however, these factors were not directly measured. The moist primary forest site appeared to have moss competing with lichen for area on the tree trunk, possibly limiting the number of species and abundance of lichen in that site. The highest similarity between sites was observed between the pasture and roadside, most likely due to the similarity of these two environments. Both were subject to edge effects, however the road had the
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Los lquenes son indicadores biolgicos de contaminacin y efectos de orilla. Su uso en regiones templadas est bien establecido, sin embargo, sabemos menos de su funcin como indicador biolgico en el trpico. Adems, hay desacuerdos acerca de las consecuencias del efecto de borde, como lo son el aumento de temperatura, la disminucin de la humedad, y el viento sobre la riqueza y la abundancia de lquenes. En este estudio se examinaron tres hbitats (Bosque primario, potreros y borde del camino), 88 rboles fueron considerados y 68 especies de lquenes fueron considerados. Yo analic comunidades de lquenes sobre los troncos de rboles en trminos de la riqueza de especies, abundancia y similitud. El potrero mostr el nmero de especies (42), nmero promedio de especies (5.97) y abundancia promedio ms altos (28%). Por otro lado, el camino mostr el nmero de especies (16), el numero promedio de especies (2.39), y abundancia promedio ms bajos (.37%). Yo especul que esto se debe a los efectos fuertes del borde y a la contaminacin. El bosque primario hmedo pareci tener musgo compitiendo con liquen por el rea en el tronco; por lo tanto posiblemente limita el nmero de especies y abundancia de lquenes en ese hbitat. La similitud ms alta entre los sitios se observ entre el potrero y el camino, probablemente debido a la similitud de los dos terrenos. Los dos estn sujetos a fuertes efectos del borde; sin embargo, el camino tambin exhibe las presiones adicionales de contaminacin del aire y el polvo.
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