In many regions, biodiversity is concentrated in specific "hot spots" of high species richness. For example, riparian areas and riverine wetlands typically maintain a much higher biodiversity than the proportion of the landscape that they occupy (National Research Council 2002). Large, ancient ecosystems such as Lake Baikal, the Amazon River, and the Pantanal wetland are other examples of especially diverse biotic communities. The seasonally flooded forests in the Amazon basin contain about 20 percent of the 4,000-5,000 estimated Amazonian tree species (Junk et al. 1989), and fish diversity in the Amazon Basin is exceptionally rich. The Pantanal contains more than 400 species of fish and many species of benthic invertebrates (see Giller et al., Chapter 6) in addition to providing habitat to endangered species such as the giant river otter (Pteronura braziliensis).
In the Santa Monica Mountains of Southern California, less than 1 percent of the total land area is comprised of wetlands but approximately 20 percent of the native vascular plant species have their primary habitat there (Rundel & Sturmer 1998). In Sweden, 13 percent (>260 spp) of the country's entire vascular plant flora occur along the Vindel River (Nilsson 1992). In France, 30 percent of the country's 1,386 vascular plant species occur along the Adour (Planty-Tabacchi et al. 1996). Approximately 28 percent of the threatened or endangered plants, 58 percent of threatened or endangered vertebrates and mussels, and 38 percent of threatened or endangered insects in the United States occur in wetlands (Niering 1988). Nevertheless, half of the world's wetlands are estimated to have been lost during the 20th century (Dahl 1990). More than half of this loss has been in the United States, and most resulted from conversion to agriculture and other land uses (Dahl et al. 1991).
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