Elevation, habitat structure and Monteverde bird communities Karl Fairchild Department of Fisheries and Wildlife, Oregon State University ABSTRACT One hypothesis for why Â“biodiversity begets biodive rsityÂ” when this pattern is observed in bird communities is that by increasing vegetative comple xity finer niche partitioning is allowed for among bird species. Bird diversity has been shown to correlat e positively with vegetation structure along a successional gradient. However, no such studies ha ve been done in actively managed areas in the tropi cs. I used three 50 m fixed-radius point counts each in t wo pairs of study sites at different elevations to compare bird diversity in structurally simple and structura lly complex pastures at two different elevations in Monteverde, Costa Rica. Within each pair, one site was simple and the other complex. I measured diversity within and across study sites using a var iety of indices. I found that the total number of birds, species diversity, MargalefÂ’s index, and the Shanno n-Weiner index were all highest in the low elevatio n site with high structural complexity. These measur es were lowest at the lower elevation site with low structural complexity, and intermediate at the two higher elevation sites, which showed no significant pattern. Additionally, I found that the two struct urally complex sites were most similar in terms of diversity. This information lead me to conclude th at vegetative structural complexity was a more impo rtant factor in determining bird diversity than altitude. This provides more evidence to support the Â“diver sity begets diversityÂ” hypothesis when one considers tha t structural diversity leads to increased resource diversity and feeding opportunities for bird popula tions. RESUMEN Una hiptesis del porque Â“biodiversidad engendra bi odiversidadÂ” cuando se observa este patrn en comunidades de aves es que al aumentar la complejid ad vegetativa se permite una particin de nicho ms fina entre las especies de aves. La diversidad de aves se ha mostrado correlacionada positivamente co n la estructura de la vegetacin a travs de gradientes sucesionales. Sin embargo, ninguno de estos estudi o se ha realizado en reas activamente manejadas en los trpicos. Yo utilic tres puntos de conteo de 50 m de radio cada uno en dos pares de sitios de estudio a diferentes elevaciones para comparar la diversidad de aves en pastos estructuralmente simples y complejos a dos diferentes elevaciones en Monteverde, Costa Rica. En cada par, un sitio fue simple y el otro c omplejo. Med la diversidad dentro y entre los sit ios de estudio utilizando diferentes ndices. Encontr qu e el nmero total de aves, diversidad de especies, ndice de Margalef y el ndice de Shannon-Weiner fueron ma yores a baja elevacin en el sitio estructuralmente complejo, e intermedio en los dos sitios de mayor e levacin, lo cual muestra un patrn no significativ o. Adems, encontr que los dos sitios estructuralment e complejos son ms similares en trminos de diversidad. Esta informacin me lleva a la conclus in que la complejidad estructural vegetativa es un factor ms importante para determinar la diversidad de aves que la altitud. Esto provee ms evidencia para soportar la hiptesis de que Â“biodiversidad engendr a biodiversidadÂ” cuando uno considera que diversida d estructural ayuda a aumentar la diversidad de recur sos y las oportunidades de alimentarse para las poblaciones de aves. INTRODUCTION
Monteverde, Costa Rica, is one of the most biodiver se communities in the world. It contains rich assemblages of many species, but o ne of the richest is its bird community. Within the Monteverde region, there are over 450 sp ecies, over 200 of which breed in the region (Young and McDonald 2000). The Monteverde a rea also has a very high level of local variation within the region, making this an e xcellent area to study patterns of biodiversity. Two causes of variation in bird communities in the Monteverde area are altitude and vegetative structure. This high variability res ults in many diverse habitat types being available. Altitude is frequently the most common ly attributed factor to variation in bird diversity in the Monteverde area, largely because o f its six Holdridge life zones, each which contains bird communities with diverse assemb lages, and is positively associated with altitude. For example, the middle-elevation P acific Premontane Wet zone contains 6 unique species, while the higher-elevation Lower Mo ntane Rain zone contains 11 (Young and McDonald 2000). Structure has also been examine d as a mechanism for diversity in the Monteverde area. Latham conducted bird point c ounts across a successional gradient with structural complexity that increased from regr owing pasture to secondary forest and primary forest, and found that diversity was strong ly positively correlated with both canopy height and foliage height diversity (Latham 1998). An explanation for this variation of richness in bi rd communities is that their resource base varies correspondingly, which is know n as the hypothesis that Â“diversity begets diversity.Â” One such study involved antwrens in Amazonian rainforests. It found that up to 10 species coexisted in the same patch o f rainforest via niche partitioning, and then compared the niches they used with the number of niches used by a comparable temperate foraging guild. It further found that se veral of these feeding niches were unique to the tropics because the structures they r elied on were either only present in the tropics or were only available seasonally in temper ate zones, preventing specialization. For example, one species foraged exclusively on dea d leaves caught in branches, a niche found exclusively in the wet tropics because decidu ous forests lack a continuous supply of dead leaves to be caught. Likewise, another ant wren that foraged by gleaning lianas also enjoyed a niche found exclusively in the tropi cs, because temperate forests have very few such vines (Terborgh 1992). It is likely that structurally rich communities can support more diversity because it allows for more diverse resource types to be presen t. In addition to providing more foraging niches that do not overlap and allow for m ore specialization, it allows many more opportunities for other behaviors such as nest ing and predator avoidance as well. According to this pattern, it seems that greater st ructural diversity should positively relate to greater resource availability (in the form of fo raging opportunities, predator avoidance, and nesting resources). Despite being renowned for its diversity, Monteverd e has experienced fairly high levels of human alteration, primarily in the form o f clearing forest for cattle grazing, which has led to reductions in structural and resou rce diversity. Structural reduction varies across a gradient, with extremes present on both ends. Studying the diversity in these extremes allows one to examine the role of mo derate reduction in structural diversity as compared to greatly reduced structural diversity as it relates to biodiversity and the diversity begets diversity gradient. Follo wing the studies of Terborgh, any
degree of structural simplification will reduce the available resources and number of niches present. This was exactly what was found in a pastureland study in Sweden (Snderstrm 2001). However, no such study has bee n done in tropical regions, which have greater structurally diversity and greater spe cies richness. Perhaps examining the relationship between structure and biodiversity her e could lead to more evidence that Â“diversity begets diversity.Â” An additional reason to examine pasturelands around Monteverde is that most previous studies of bird diversity in this area hav e largely examined forest or successional habitats that are not being actively used by humans However, pastures are a common habitat type in the area around Monteverde and repr esent no less than 50 of available habitat (personal observation). Since they are so dominant, it is important to understand their effects on local ecology and biodiversity. In addition to increasing knowledge about the patte rns of biodiversity in Monteverde, examining bird species diversity in Mon teverde pastures could also have conservation implications. While many farmers inte rested in creating or expanding pastures do not prioritize planning for any conserv ation, evidence of a strong correlation between retained structure and species diversity wo uld still be important information to conservation planners from the perspective of range improvement. Windbreaks and the planting of native grass species have successfully improved productivity of many pastures in the Monteverde area, thanks largely to organizations such as the Monteverde Conservation League. The same could be applied to tree structure retention if it is shown to positively correlate with bird abundance. This study examined the effects of vegetative struc tural complexity and altitude on bird diversity in four study sites in the Montev erde area. It used two pairs of sites, one at higher elevation and one at lower elevation. Wi thin each pair, one site was structurally simple while the other one was complex. I predict that bird communities in Monteverde pastures will positively correlate with vegetative structural complexity. However, I make no predictions as to whether altitude or vegetative structure will have the greater effect, because I have found no studies addressing this. MATERIALS AND METHODS Study Sites I selected two pairs of study sites; one pair was l ower in elevation and was located in the Caitas area, while the other was higher elevation and located near the Monteverde Cloud Forest Preserve. Within each pair of sites, one si te was in a completely open pasture, while the other pasture had a retained overstory of large canopy trees. Table 1 summarizes exact location data. TABLE 1. Description of each study site. All location measu rements were taken with a handheld Magellan SporTrak Pro GPS u nit.
Site Name Region Structure Lat (N) Lon (W) Alt (m) ALB Caitas Simple 10.32466 84.84502 ~1350 VJT Caitas Complex 10.32121 84.83540 ~1275 BUL Cloud Forest Complex 10.29640 84.80131 ~1500 MAR Cloud Forest Simple 10.29705 84.80423 ~1475 The pair of study sites in the Caitas was located in the Pacific Premontane Wet life zone, as classified by the Holdridge system. Before the 20th century, much of this area was forested, but much of it has now been clea red for dairy farming, coffee production, and rural housing. The higher elevatio n study sites, by contrast, were located in the area immediately north and west of the Monte verde Cloud Forest Preserve. This area is classified as Lower Montane Wet, according to the Holdridge system. Much of the area to the east is primary and secondary fores t that is protected in the Monteverde Cloud Forest Preserve and the ChildrenÂ’s Eternal Ra inforest (Fogden 1993). In the Caitas area, one site was located on the fa rm of the Victor Torres family. This site was characterized by a moderate elevation al gradient, an uneven distribution of retained trees, occasional windbreaks and a few sma ll sun-grown coffee patches (<2 ha each. The canopy structure here was sufficient to facilitate many foraging opportunities and the vegetation considered complex. The other s ite in the Caitas area was located on the southeast corner of Alberto CastroÂ’s farm. Whi le the Castro farm is very large (>50ha) and highly varied, this particular section was characterized by a steep, eastfacing pasture with rocky soil and mostly devoid of vegetation at higher elevations. At lower elevations, several species of trees and shru bs grew in a somewhat stunted form to approximately 8m tall. Many trees exhibited varyin g degrees of wind-restricted growth, presumably due to a nearly continuous east to north east breeze. The general lack of vegetation above 1 m tall (with the exception of th e few trees mentioned) led this site to be considered structurally simple. The structurally complex study site near the Montev erde Cloud Forest Preserve was a currently inactive pasture with fairly dense, tall trees in a fairly uniform distribution. There was virtually no vegetation of intermediate heightÂ—only a dense herb layer and fairly uniformly aged trees, most approxi mately 20m tall. The pasture area was fairly small (<5ha) and surrounded on all sides by a dense and structurally complex forest. The structurally simple site was also loca ted in a relatively small pasture (<10 ha) and was characterized by active cattle grazing and fairly widely distributed and nearly leafless trees that supported rich epiphyte communi ties. Like the closed site, it was surrounded fairly closely by closed forest, but was bordered on one side by the road running between Monteverde and San Luis. Methodology Within each of the four study sites, three 50 m, fi xed-radius point counts (Ralph et al., 1991) were established, for a total of 12 survey po ints. Each point was at least 100m from the next nearest point and ideally at least 50 m from the edge of the selected habitat type. Each point was visited 5 times from November 2 to November 12, 2008 during the hours of 0600-1100 and 1400-1700. At each point, I counted birds for 10 minutes,
recording all stationary birds detected visually or aurally. Birds flying over were noted as well, but were recorded separately. Statistical Analysis To analyze my findings, I combined all observations from each site into one Excel spreadsheet and tallied the number of individuals f ound. For each study site, I calculated species richness, number of individuals observed, e venness, MargalefÂ’s index and the Shannon-Weiner diversity index. I then graphed eac h measure across all four study sites to visually compare the results. In addition, I co mpared the Shannon-Weiner diversity index of each site with all others using StudentÂ’s paired t-test. Finally, I used EstimateS 8.0 (Cowell 2006) to compare species presence and s pecies abundance distribution across all study sites (calculated with SorensonÂ’s Qualita tive and Morisita-Horn indices, respectively). RESULTS Diversity Information I observed a total of 49 bird species during my sur veys, with 13 additional species seen flying over (see Appendix 1 for complete observatio ns). I found that the lower elevation open site had the lowest number of individuals, whi le the lower elevation closed had the highest (Fig. 1). Likewise, the lower elevation op en site showed the lowest species diversity, MargalefÂ’s index, and Shannon-Weiner div ersity index, while the lower elevation closed had the highest (Figs 2 and 3, res pectively). The higher elevation open had the highest evenness, while the lower elevation open had the lowest (Fig. 4). FIGURES 1A AND 1B. Number of species detected at each of four study si tes (left), along with species richness and MargalefÂ’s index fo r each of the four study sites (right). Study sites are lower elevation open, lower elevati on closed, higher elevation closed, higher elevation open, left to right. Monteverde, Costa Rica, November 2008. 0 5 10 15 20 25 30 35 ALBVJTBULMARSpecies Richness S Smarg 0 40 80 120 160 ALBVJTBULMARNumber of Individuals
FIGURES 2A AND 2B. Shannon-Weiner diversity index for each of the four study sites (left) and evenness index (right). Study sites are lower elevation open, lower elevation closed, higher elevation closed, higher elevation o pen, left to right. Monteverde, Costa Rica, November 2008. When a paired t-test was performed to compare the S hannon-Weiner diversity index of each site to the others, the lower elevati on open site showed significantly less diversity than its partner closed site, and also le ss diversity than either of the two other sites (p<0.5 for all three comparisons). None of t he other pairwise comparisons showed significant differences. TABLE 2. Modified t-test of H'. Bold text indicates signifi cance (critical value of t=2.00) First Sample ALB ALB ALB VJT VJT BUL Second Sample VJT BUL MAR BUL MAR MAR t -55.17 -5.56 -6.61 -1.78 -0.43 -1.59 df 97.37 48.30 47.44 60.13 63.91 167.88 Finally, I used EstimateS to calculate the Sorenson qualitative index and MorisitaHorn index. I found that the lower elevation open site differed strongly from the closed sites and was more similar to the high elevation si te. The closed sites were most similar in terms of diversity, while the lower elevation cl osed was most similar to the high elevation open in terms of composition (Fig. 5). 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 ALBVJTBULMARDiversity Index Values 0.70 0.75 0.80 0.85 0.90 0.95 ALBVJTBULMAREvenness Index
0 0.2 0.4 0.6 0.8 1 ALB/VJTALB/BULALB/MARVJT/BULVJT/MARBUL/MARIndex Values Sorensen (Qualitative) Morisita-Horn (Quantitative) FIGURE 3. Sorenson qualitative index and Morisita-Horn index comparing bird community composition in each of four study sites w ith all others, Monteverde, Costa Rica, 2008. Species Composition Information I noted the most common species in each study site and also which species was present in which habitats. I found that Great-tailed Grackles only appeared at the lower elevation sites and Common Bush-Tanagers only appeared at the higher elevation sites. I also found that WilsonÂ’s Warblers only appeared in close d habitat types. One species of bird made up almost half of all observations (15 of 31) in the lower elevation open habitat (Yellow-faced Grassquit) and was never observed mor e than three times in any other site. DISCUSSION My prediction that vegetative structural complexity is correlated with bird diversity was supported. I conclude this in two different ways. First, I found a positive relationship between bird diversity and vegetation structure. S econd, the composition of specialist species in each habitat type led me to conclude tha t these habitat types differed from a birdÂ’s perspective (in terms of resources such as f ood and nesting opportunity) as well as structurally. Diversity Analysis I used several reasons to conclude that structural diversity correlates strongly with bird diversity. First, the lower elevation site with si mple structure had significantly lower bird species diversity than all other habitats, especial ly the lower elevation complex site both from a visual comparison and from pairwise t-tests. It was also the least structurally diverse of any site. None of the other sites diffe red significantly according to the t-tests. I also conclude that the diversity of habitats crea ted via greater structural diversity is more important in determining bird diversity that i s altitude. If altitude were important, I would expect there to be other significant differen ces. Additionally, community comparisons showed that the two closed sites were t he most similar in terms of diversity. One reason that the lower elevation closed site may have had the highest diversity was because it had the greatest structure, both hor izontally and vertically. While much of
the study area was active pasture, it was mixed wit h small patches of coffee and sugarcane. One of my survey points was near a wind break, and trees were frequently distributed in clumps and varied in age in several instances. Perhaps this indicates that horizontal structural heterogeneity plays a role in regulating diversity in bird communities around Monteverde by providing opportun ities for species that require several structures for different purposes. While th ese species may forage in the open habitats, they may still utilize grown-over areas s uch as sugarcane or windbreaks for roosting and predator avoidance. Additionally, the increased heterogeneity may provide more food sources and enable greater niche partitio ning. Indeed, Terborgh (1979) found a positive relationship between horizontal structur al diversity and bird communities. While the difference in bird diversity between the lower elevation open and closed sites was large, this difference was much le ss at the higher elevation sites. In fact, the structurally simpler site had higher measures o f diversity than the complex site. There are several possible reasons for this. First the higher elevation simple site was appreciably more complex than the lower elevation s imple one. At this site, there were a fairly high number of trees within the pasture (eve n though nowhere near enough to form a closed canopy), and while at first glance they ap peared to be dead or dying, many of them actually supported some live leaves. Perhaps even more significantly, many of these trees had healthy and diverse epiphyte commun ities that I observed birds utilizing for foraging, especially hummingbirds. Based on th ese observations, it appears the moderate level of structural complexity here and as sociated resources were somewhat intermediate between the low elevation simple site and the complex sites. Species Composition Also important in concluding that structure played an important role was the observation that several specialist species occurred only in ce rtain habitats or elevations. WilsonÂ’s Warblers, for instance, were the most abundant spec ies in the low elevation complex site and the second-most abundant in the high elevation complex site, but were entirely absent in both simple sites. It is perhaps surprising to observe how common they were, considering they are associated with well-developed understory habitats, which appeared largely absent in any of the pasture habitats I sur veyed. Perhaps the fairly complex canopy communities with dense vegetation, lianas, a nd epiphytes provided sufficient substrate for the insects they feed on. Additional ly, none of the points where they were common were more than 100 m from an area with a den ser understory. It is possible that they utilize more open areas for feeding but never venture too far from denser understory habitats that offer them a safe retreat when necess ary. I also noticed a similar pattern of abundance with regard to an open-area specialist, the Yellow-faced Grassquit, which was e xtremely abundant at the lower elevation open site and rare other places. The stru ctural complexity of this site was very low both horizontally and vertically with only a fe w small, stunted trees in a pasture of uniformly distributed grass clumps that appeared to be of a single species. Some studies show that open habitat specialists like grassquits (Stiles and Skutch 1989) actually show a negative correlation between abundance and structur al diversity (Laiolo 2004). This appeared to be the case here, and could be due to t he fact that grassquits get all their
necessary resources from open fields. They feed on grass seeds, nest in dense grass, and use it for predator avoidance as well (Stiles and S kutch 1989, pers. obs.). Even with evidence to conclude that there is a rela tionship between vegetation structure and bird diversity, altitude and/or metac ommunity appears to have some effects as well, especially on species composition. For ex ample, Common Bush-Tanagers were the most common species in the high elevation close d site and were also present in the open site, but were completely absent from both low er sites. They feed primarily on berry-producing epiphytes, which were far more comm on at high elevations, and might explain the birdsÂ’ distribution. Likewise, Great-t ailed Grackles, which are associated with human activity, were present at both lower sit es, but absent from both upper sites. Additional Results/ Future Study One explanation for the higher diversity in the hig her elevation open site as compared to the closed site is that the forest surrounding the pasture was influencing the open site. The high elevation closed site was also a fairly sm all habitat patch, and it may have been experiencing these effects as well (I did indeed de tect several closed forest specialist species here, Stiles and Skutch 1989). However, th is contrast was much greater in the open habitat, possibly leading to a more pronounced effect. One final explanation for the higher diversity in the higher elevation open habit at was that it was simply difficult to observe birds in the closed habitat. Many birds pa ssed through unidentified, flying from closed forest on one side of the pasture to closed forest on the other and only pausing briefly in between. In considering improvements for future studies, a q uantitative investigation would provide stronger evidence of the connection between habitat diversity and bird diversity. A second reason for a quantitative study is that it proved very difficult to find study sites that fit the description of Â“completely openÂ” or Â“c ompletely closed canopy.Â” Hence, it would probably be more useful to select a gradient of study sites at each altitude and attempt to numerically correlate vegetative structu ral complexity with bird diversity. Finding that bird diversity in pasture habitats is positively correlated with vegetation structure suggests a relationship betwee n structural diversity, resources that birds use (e.g. food resources), and the birds them selves. It also provides more evidence for the hypothesis that diversity begets diversity across many different habitat types. In addition, it provides an argument for conservation of vegetative complexity in pastures and also may provide a new route for farmers seekin g to improve habitat quality in their pastures. ACKNOWLEDGEMENTS I would like to thank my adviser, Karen, for invalu able advice and support throughout. I would also l ike to thank Alan, Pablo, Moncho, and Tania for their advi ce and support and making my time in Costa Rica unforgettable. I would also like to extend a huge thank you to Nery Rojas and Vctor Torres for hosti ng me during the project and letting me use their farm as a study site, and to the Campbell, Guindon, Tandy, Santamara and Castro families who granted me acces s to their lands for the project. Finally, I would like to thank the Estacin Biological Monteverde for the ir facilities and hospitality, and to the rest of t he students in the program, who made life interesting and amiably tolerated a birding freak in their mids t.
LITERATURE CITED Cowell, R.K. 2006. EstimateS 8.0. Department of Ecology and Evolutionary Biology, University of Connecticut. . Downloaded 21 November 2008. Fogden, M. 1993. An Annotated Checklist of the Bi rds of Monteverde and Peas Blancas. Michael Fogden, Monteverde, Costa Rica. Garrigues, R. and R. Dean. 2007. The Birds of Cos ta Rica: A Field Guide. Cornell University Press, Ithaca, New York. Laiolo, P., F. Dondero, E. Ciliento, and A. Rolando 2004. Consequences of pastoral abandonment for the structure and diversity of the alpine avifauna. Jo urnal of Applied Ecology 41: 294-304. Latham, A.C. 1998. Bird species diversity along a successional gradient. CIEE Fall 1998: 41-53. Ralph, C.J., G.R. Geupel, P. Pyle, T.E. Martin and D.F. Desante. 1993. Handbook of Field Methods for Monitoring Landbirds. U.S. Forest Service Pacific Southwest Research Station General Technical Report, PSW-GTR-144-www. Stiles, F.G. and A.F. Skutch. 1989. A Guide to th e Birds of Costa Rica. Cornell University Press, I thaca, New York. Sderstrm, B., B Svensson, K. Vessby, and A. Glims kr. 2001. Plants, insects and birds in semi-natu ral pastures in relation to local habitat and landscape factors. Biodiversity and Conservation 10: 1839-1863. Terborgh, J.W. 1979. Migrant birds in the Neotrop ics: Ecology, behavior, distribution, and conservat ion. Smithsonian Institution Press, Washington, D.C., p5 76. IN Latham, A.C. 1998. Bird species diversity along a successional gradient. CIEE Fall 1998: 41-53. Terborgh, J.W. 1992. Diversity and the Tropical R ain Forest. W.H. Freeman and Co., New York, pp6771. Young, B.E. and D.B. McDonald. 2000. Â“Birds.Â” In: Monteverde: Ecology and Conservation of a Tropical Cloud Forest (N.M. Nadkarni and N.T. Wheelwright, e ds.) Oxford University Press, New York, pp179-183
APPENDIX 1. Summary of point count observations. Â“FÂ” denotes a flyover bird; flyovers were not used in the analysis. Numbers in cells refer to the number of non-flying birds observed in each habitat. Site codes were as follows: ALB=lower elevation with simple structure, VJT=lower elevation complex, BUL= higher elevation complex, MAR=higher elevation simple. Species ALB VJT BUL MAR Total Black Vulture F 1 F F 1 Turkey Vulture F F F F Broad-winged Hawk 2 2 Short-tailed Hawk F F Rock Pigeon F F Red-billed Pigeon 2 2 Ruddy Pigeon F F Crimson-fronted Parakeet F F F Parakeet sp. F F Brown-hooded Parrot 15 15 White-fronted Parrot F 2 7 9 Vaux's Swift F F Green Violet-Ear 4 4 Stripe-tailed Hummingbird 1 1 Hummingbird sp. F 1 3 2 6 Coppery-headed Emerald 1 1 Magenta-throated Woodstar 1 F Emerald Toucanet F F Hoffman's Woodpecker 1 1 Woodpecker sp. 1 1 2 Red-faced Spinetail 1 1 Streak-headed Woodcreeper 1 1 Eastern Wood-Pewee 1 4 5 2 12 Dusky-capped Flycatcher 1 1 Boat-billed Flycatcher 2 2 4 Great Kiskadee 1 1 Social Flycatcher 1 2 5 F Tropical Kingbird 2 3 5 Flycatcher sp. F F Masked Tityra 2 F 2 Yellow-throated Vireo 1 1 Philadelphia Vireo 1 1 Brown Jay 5 1 1 7 14 Blue-and-white Swallow F F F F Barn Swallow F F Swallow sp. F F Plain Wren 2 1 1 1 House Wren 3 2 9 6 2F Gray-breasted Wood Wren 3 3 Slaty-backed Nightingale Thrush 1 1 Clay-colored Robin 1 1
Mountain Robin 3 3 Tennessee Warbler 1 1 Black-throated Green Warbler 8 3 3 14 Black-and-white Warbler 1 1 Wilson's Warbler 21 11 32 Gray-crowned Yellowthroat 1 1 2 Warbler sp. 5 1 1 7 Common Bush-Tanager 14 3 17 Hepatic Tanager 4 4 Blue-gray Tanager 4 5 9 Tanager sp. 1 1 Yellow-faced Grassquit 15 1 3 19 Rufous-collared Sparrow 3 4 12 19 Sparrow sp. 1 1 Eastern Meadowlark 3 3 Great-tailed Grackle F 11 2 Baltimore Oriole 2 2 Golden-browed Chlorophonia 3 3 White-vented Euphonia 4 4 Unknown sp. 1 8 9 3 21 Total 31 99 80 89 276
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La elevacin, la estructura del hbitat y las comunidades de aves de Monteverde
Elevation, habitat structure and Monteverde bird communities
One hypothesis for why biodiversity begets biodiversity when this pattern is observed in bird communities is that by increasing vegetative complexity finer niche partitioning is allowed for among bird species. Bird diversity has been shown to correlate positively with vegetation structure along a successional gradient. However, no such studies have been done in actively managed areas in the tropics. I used three 50 m fixed-radius point counts each in two pairs of study sites at different elevations to compare bird diversity in structurally simple and structurally complex pastures at two different elevations in Monteverde, Costa Rica. Within each pair, one site was simple and the other complex. I measured diversity within and across study sites using a variety of indices. I found that the total number of birds, species diversity, Margalefs index, and the Shannon-Weiner index were all highest in the low elevation site with high structural complexity. These measures were lowest at the lower elevation site with low structural complexity, and intermediate at the two higher elevation sites, which showed no significant pattern. Additionally, I found that the two structurally complex sites were most similar in terms of diversity. This information lead me to conclude that vegetative structural complexity was a more important factor in determining bird diversity than altitude. This provides more evidence to support the diversity begets diversity hypothesis when one considers that structural diversity leads to increased resource diversity and feeding opportunities for bird populations.
Una hiptesis del porque la biodiversidad engendra biodiversidad cuando se observa este patrn en las comunidades de aves es que al aumentar la complejidad vegetativa se permite una particin de nichos mas fina entre las especies de aves. La diversidad de aves se ha mostrado positivamente correlacionada con la estructura de la vegetacin a travs de un gradiente successional. Sin embargo, ninguno de estos estudios se ha realizado en reas activamente manejadas en los trpicos. Yo utilice tres puntos de conteo de 50 m de radio cada uno en dos pares de sitios de estudio a diferentes elevaciones para comparar la diversidad de las aves en los potreros estructuralmente simples y complejos a dos diferentes elevaciones en Monteverde, Costa Rica.
Text in English.
Birds--Costa Rica--Puntarenas--Monteverde Zone
Birds--Variation--Costa Rica--Puntarenas--Monteverde Zone
Plant species diversity--Costa Rica--Puntarenas--Monteverde Zone
Aves--Costa Rica--Puntarenas--Zona de Monteverde
Aves--variacin--Costa Rica--Puntarenas--Zona de Monteverde
Diversidad de especies de plantas--Costa Rica--Puntarenas--Zona de Monteverde
Tropical Ecology 2008
Ecologa Tropical 2008
t Monteverde Institute : Tropical Ecology