Global Diversity of Amphibians (Amphibia) in Freshwater
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Hydrobiologia (2008) 595:569–580 DOI 10.1007/s10750-007-9032-2 FRESHWATER ANIMAL DIVERSITY ASSESSMENT Global diversity of amphibians (Amphibia) in freshwater Miguel Vences Æ Jo¨rn Ko¨hler Ó Springer Science+Business Media B.V. 2007 Abstract This article present a review of species amphibians is very high, with only six out of 348 numbers, biogeographic patterns and evolutionary aquatic genera occurring in more than one of the major trends of amphibians in freshwater. Although most biogeographic divisions used herein. Global declines amphibians live in freshwater in at least their larval threatening amphibians are known to be triggered by phase, many species have evolved different degrees of an emerging infectious fungal disease and possibly by independence from water including direct terrestrial climate change, emphasizing the need of concerted development and viviparity. Of a total of 5,828 conservation efforts, and of more research, focused on amphibian species considered here, 4,117 are aquatic both their terrestrial and aquatic stages. in that they live in the water during at least one life- history stage, and a further 177 species are water- Keywords Amphibia Á Anura Á Urodela Á dependent. These numbers are tentative and provide a Gymnophiona Á Species diversity Á Evolutionary conservative estimate, because (1) the biology of many trends Á Aquatic species Á Biogeography Á Threats species is unknown, (2) more direct-developing spe- cies e.g. in the Brachycephalidae, probably depend directly on moisture near water bodies and (3) the Introduction accelerating rate of species discoveries and descrip- tions in amphibians indicates the existence of many Amphibians are a textbook example of organisms more, yet undescribed species, most of which are living at the interface between terrestrial and aquatic likely to have aquatic larvae. Regional endemism in habitats. They fulfil this role both in an ecological context, with typically a strictly aquatic larval and Guest editors: E. V. Balian, C. Le´veˆque, H. Segers & K. Martens largely terrestrial adult phase, and in an evolutionary Freshwater Animal Diversity Assessment context, representing the intermediate bauplan level between aquatic and fully terrestrial vertebrates M. Vences (&) (‘‘fishes’’ vs. amniotes). Most amphibians are strictly Division of Evolutionary Biology, Zoological Institute, dependent from water for their larval development, Technical University of Braunschweig, Spielmannstr. 8, Braunschweig 38106, Germany and water for this group of animals is synonym to e-mail: [email protected] freshwater. Although a few amphibians are able to tolerate high-salinity levels (Balinsky, 1981), there J. Ko¨hler are no marine representatives of this class. Department of Zoology, Hessisches Landesmuseum Darmstadt, Friedensplatz 1, Darmstadt 64283, Germany Although existence of an aquatic larval phase e-mail: [email protected] is probably the ancestral condition for recent 123 570 Hydrobiologia (2008) 595:569–580 amphibians, there are only few amphibian taxa with The (aquatic) larvae of caecilians and salamanders also fully aquatic adult phases. In contrast, multiple are morphologically largely similar to their adults, evolutionary trends towards more terrestrial repro- except for the presence of external gills which usually duction have led to a plethora of reproductive modes are reduced in the adults. In contrasts, the larval stage (Duellman & Trueb, 1986) which make it difficult, in of frogs, the tadpole, is a larval phase radically some instances, to decide if and to what degree a different from the adults (Altig & McDiarmid, 1999). particular species is indeed strictly dependent on Especially the oral and digestive system of tadpoles is freshwater. composed of numerous features which are not Recent amphibians are often named Lissamphibia. homologous to the corresponding structures in the They are divided in three orders: frogs (Anura), adult, such as a horny beak, oral papillae and salamanders (Urodela), and caecilians (Gymnophi- keratinous labial teeth, mainly due to the fact that ona). Dubois (2004) recommended abandoning sev- typically tadpoles are omnivorous suspension feeders, eral other higher taxa names based on arguments of ingesting a high degree of vegetable matter, while nomenclatural priority. Although these priority rules adult frogs are strictly carnivorous (with only a single do not strictly apply to names above the family level, known exception, Xenohyla truncata, a species that we here follow Dubois (2004) in not using the names also eats fruits). Apoda (for caecilians), Caudata (for salamanders), Several excellent resources on amphibians were Salientia (for Recent frogs), and Archaeobatrachia available over the world wide web at the time of and Neobatrachia (for basal and modern frogs). preparation of the present article. The Amphibian However, we decided to here continue using Liss- Species of the World database (Frost, 2004), hosted amphibia for the clade containing all three recent by the American Museum of Natural History, con- amphibian orders, and we use ‘‘Archaeobatrachia’’ tinues previous efforts (Frost, 1985; Duellman, 1993) and ‘‘Neobatrachia’’ in quotation marks since these to document from a taxonomic point of view all established terms make discussion of anuran rela- amphibian taxa. Amphibiaweb (2005, www.amphib- tionships easier. iaweb.org), hosted by the University of California at Due to the large diversity of extinct Paleozoic Berkeley, provides a full species list of amphibians, amphibians, the phylogenetic relationships of lissam- too, but aims at providing also additional information phibians relative to amniotes has in the past been such as distribution, photographs, and biological data. questioned. Current evidence converges on their The Global Amphibian Assessment (www.globalam- monophyly, based on morphological characters such phibians.org) has compiled, during 2002–2004, the as, for instance, their pedicellate teeth, special visual expertise of regional and taxonomic experts world- cells (green rods) in the retina, or the ear structure wide and provides an estimate of threat status (Duellman & Trueb, 1986), and on molecular char- (according to IUCN criteria) and distribution for all acters (e.g. Meyer & Zardoya, 2003; San Mauro amphibian species. et al., 2005). The paucity of fossils, especially from In this article, we summarize species diversity and the Mesozoic, makes it difficult to trace the early distribution, and zoogeography, of extant amphibians, evolution of lissamphibians, but they appear to be a based on a species list and distributional information very old group according to molecular clocks which compiled from these three online data sources as date the separation among the three orders back into accessed in December 2005. Furthermore, we cate- Paleozoic times before Pangaean break-up (San gorize all amphibian species according to their water Mauro et al., 2005; Roelants & Bossuyt, 2005). dependence on a regional and taxonomical level. For Furthermore, deep divergences are also typical for taxonomy we generally follow Frost et al. (2006). In amphibians at the species level. As already noted by the following sections, we use the definitions of the Wilson et al. (1974), amphibian species have much freshwater diversity assessment project in defining (1) larger molecular divergences (and, consequently, aquatic species as those with at least part of their life ages) than other vertebrates e.g. mammals and birds, cycle in or on the water, (2) water dependent species and also the large divergences among populations as those which do not live directly in the water but considered to be conspecific are typical for amphib- closely depend on it e.g. for habitat or food, (3) water ians (e.g. Vences et al., 2005a, b). related species as aquatic plus water dependent 123 Hydrobiologia (2008) 595:569–580 571 species, and (4) nor water related species as those more difficult to decide which of the non-aquatic which are neither aquatic nor water dependent. species may be water dependent, i.e. with close/ specific dependence on aquatic habitats (see Figs. 1 and 2). In our categorization, all amphibians charac- Species diversity terized by direct development, viviparity with terres- trial birth, or tadpole development in terrestrial jelly A striking aspect of amphibian taxonomy is the or foam nests are considered to be non-aquatic, while increasing rate of new species discoveries (Glaw & species with tadpoles in water-filled leaf axils of Ko¨hler, 1998;Ko¨hler et al., 2005). As of December plants or tree holes were included in the aquatic 2005, a total of 5,828 amphibian species (aquatic+ category (Figs. 2 and 3). To be able to categorize all non-aquatic) were known, but still at the end of 1992, species, we have here assumed that species share the there were only 4,533 species (Duellman, 1993). The reproductive mode of their closest relatives (usually absolute number of newly described amphibian spe- congeneric taxa). Although certainly not fully pre- cies per decade (not only the cumulative number of cise, this estimate should be a relatively reliable valid and described species) has been steadily approximation to the real situation. In our analysis, increasing since the decade of the 1960s, with species were categorized as ‘‘unknown’’ with respect especially steep increases since the 1990s (Glaw & to their dependence from freshwater only when no Ko¨hler, 1998;Ko¨hler et al., 2005). The new species data at all were available to us concerning