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19How uniform is species diversity in tropical forests?

Pavel Kindlmann1 and Carlos A. Vergara Cassas2

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Biodiversity Research Centre, Institute of Systems Biology and Ecology AS CR, Na Sádkách 7, CZ-370 05 České Budějovice, and Institute for Environmental Studies, Faculty of Science, Charles University, Benátská 2, CZ-128 01 Prague 2, Czech Republic

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Universidad CatólicaBoliviana, Unidad Académica Campesina de Carmen Pampa, Coroico - Nor Yungas, La Paz, Bolivia

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Abstract

Questions concerning species diversity have attracted ecologists for over a century. One of these factors is the sampling effort - the “botanist effect,” which can be, for example, the number of orchidologists in the region. The sampling effort can also account for the fact that most endemicorchid species are found close to roads, which indicates that orchid diversity may decline from the forest edge towards its interior. Here we tested this hypothesis, using data on orchid species from Bolivia. We found opposite trends in terrestrial and epiphytic species. Both species diversity and number of individuals of terrestrial species declined towards the forest interior. However, inepiphytic species both species diversity and number of individuals of terrestrial species increased towards the forest interior. Only because of the prevalence of the terrestrial species did the total number of species and the total number of individuals decline towards the forest interior. Thus, when making conclusions about the trends in orchid diversity towards the interior of the forest, their lifemode should be taken into account. The reasons for the trends observed are quite straightforward. Towards the forest interior, density of the trees increases, and therefore the amount of light available on the ground declines. Hence, habitats close to the openings (roads, meadows, fields, etc.) are more suitable for terrestrial species, whereas those deep in the forest interior are more suitablefor epiphytic species because of the availability of host trees.

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Questions concerning species diversity have attracted ecologists for over a century (Schödelbauerová et al.. 2009). Increase in species richness from the poles to the tropics (Pianka, 1966; Rohde, 1992; Willig et al., 2003; Hillebrand, 2004) and with area (Arrhenius, 1921; Gleason, 1922; Williamson, 1988; Rosenzweig, 1995) isstill one of the main topics in contemporary ecology. More recently, the amount of energy available (i.e., that which can be converted into biomass) for net primary productivity has been revealed to be an important determinant of species richness (Wright, 1983; Wylie and Curie, 1993a, 1993b; Pelkey et al., 2000; Evans et al., 2005; Storch et al., 2005). Area is clearly the most influentialdeterminant, but other factors may also be important. One of these factors is the sampling effort - the “botanist effect” -- which is related to, for example, the number of orchidologists in the region. The “botanist effect” is thought to be the reason for higher plant species richness in areas where botanists are disproportionately present as an artefactual consequence of a more thorough sampling(Pautasso and McKinney, 2007). For orchids this is illustrated in Figure 1 using the examples of Ecuador, Costa Rica, and Bolivia, which are similar to each other in many respects: they are tropical, mountainous countries with similar diversity of habitats and similar climatic conditions. Ecuador and Costa Rica were well studied and are above the regression line. There is much less known about orchidspecies in Bolivia, which is below the line. The sampling effort can also cause the effect observed by Endara et al. (2007) – most endemic orchid species were found close to the roads. Thus it seems that the orchid diversity declines from the forest edge towards its interior. Here we test this hypothesis, using data on orchid species from Bolivia.

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