Size of protected areas is the main determinant of species diversity in orchids
Introduction
One of the most pressing issues facing the global conservation community is how to distribute limited resources between regions identified as priorities for biodiversity conservation – the “hotspots” (Possingham and Wilson, 2005, Wilson et al., 2006). However, the efficient allocation of conservation resources will be achieved only if the hotspots are correctly defined. To achieve this we need to pinpoint the main hotspots.
Questions concerning species diversity have attracted ecologists for over a century. 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) is still 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 Currie, 1993a, Wylie and Currie, 1993b, Pelkey et al., 2000, Evans et al., 2005, Storch et al., 2005). Area is clearly the most influential determinant, but the relative importance of the other two factors has been little studied.
In many taxa, especially those confined to natural habitats which have declined in size in recent years, most of the diversity is now concentrated in protected areas. The orchid family, with its enormous species diversity, but extreme susceptibility to disturbance of natural habitats (Kati et al., 2004, Padmawathe et al., 2004, Flores-Palacios and Valencia-Díaz, 2007, Jacquemyn et al., 2007), is one of the best examples. At present, protected areas in the agricultural/industrial landscape can be considered as habitat islands (Begon et al., 1990, Shriver et al., 2004, McDonald et al., 2008), i.e., islands of remnants of natural vegetation surrounded by a landscape that is unsuitable for many species (Forman, 1995). Thus, species richness may be more closely correlated with the size of protected areas rather than with total area.
To test the relative importance of total area, size of protected areas, energy available and latitude, we compiled data on species richness of orchids for various countries worldwide, factored out the influence of area and then correlated the residuals with the mean Normalized Difference Vegetation Index (NDVI, a measure of energy available in these countries – see Section 2) and with latitude. We did this for both the total area of the country and for the size of protected areas available in each country, in order to determine which is the better predictor.
Section snippets
Methods
The counts of orchid species recorded in 67 countries on five continents (Africa, Asia, Europe, North and South America) were extracted from the published orchid floras for each country. The assumption was made that the whole area of each country was surveyed. The list of countries and relevant literature references are included in Appendix A. Tropical and Temperate America were delineated in accordance with the natural boundaries of the tropical/temperate regions, i.e. tropical America is in
Results
When RSS was used as a criterion, protected area predicted the number of species better than total area of a country on three out of five continents and for tropical America the prediction was the same; only for Europe was the total area a slightly better predictor (Table 1). Further, protected area explained more of the variability (the dependence was associated with a larger R2) than the total area, except for Europe, where there was no difference (Table 2). The influence of ln(NDVI) on
Discussion
We aimed to determine which factor best predicts the number of orchid species: total area of the country, size of protected areas of the country, energy available or latitude. Some may assume that protected area is more closely correlated with species richness than is total area. This could be because most countries manage locations with high diversity as reserves or protected areas and the rest of the area consists of an agricultural landscape with a low diversity. This is true especially for
Conservation implications
One important finding of this paper is that the size of protected areas predicts orchid diversity much better than the total area of the country. This strongly emphasizes the need to maximize the size of protected areas in each country in order to preserve most of its biodiversity. The close correlation between the size of protected areas and orchid species diversity shows that many endangered orchid species might be saved from extinction just by increasing the size of protected areas of
Acknowledgements
This research was supported by the Grants LC06073 and MSM6007665801 of the Czech Ministry of Education. D.L.R was funded by the Sarah and Daniel Hrdy Fellowship in Conservation Biology from Harvard University.
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