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# of Endemic Species
579: Western Madagascar
Major Habitat Type:
xeric freshwaters and endorheic (closed) basins
John S. Sparks, Department of Ichthyology, Division of Vertebrate Zoology, American Museum of Natural History, New York, NY, USA
This ecoregion includes the basins of the Manambolo, Tsiribihina, Mangoky, and Onilahy rivers in western Madagascar.
Drainages flowing into:
In contrast to the extremely wet eastern forests, western Madagascar is considerably drier, with an average annual rainfall of only about 20-25 % that of the eastern slopes (Donque 1972).
Western rivers are long, generally slow flowing, and subject to seasonal fluctuations in water level and flow. Tidal influence is significant in the lower reaches of these watercourses (Kiener & Richard-Vindard 1972). Many of western Madagascar’s smaller river basins are dry from April to November (Aldegheri 1972).
Distinctive freshwater habitats within this region include tsingy or karst formations characterized by fissures, subterranean streams, sinkholes, and caverns produced by erosion, located along the western coast and isolated larger rivers in the central highlands with distinct assemblages of fish species. Lac Itasy, located to the east of the capital Antananarivo in the western central highlands, once supported a diverse assemblage of native fishes, although exotics have almost entirely replaced the native species.
Deciduous forest is present in the wetter parts of this ecoregion, but drought-tolerant vegetation, such as Didiereaceae and Euphorbia thickets, dominates the landscape (Lowry et al. 1997).
About fifty freshwater fish, including some endemics (three described), are known from this ecoregion. The larger and relatively undisturbed rivers in western Madagascar, (e.g., those adjacent to the Parc National de Isalo) historically supported extremely rare and localized freshwater fish assemblages, including members of the endemic genera Ptychochromoides, Ptychochromis, and Ancharius (undescribed species). Although isolated populations of Ptychochromoides and Ancharius survive in remote sections of this ecoregion where disturbance is limited, members of Ptychochromis have not been collected in western Madagascar for decades. Ichthyofaunal communities of the Isalo region (southern-central Madagascar) are remarkably similar to those encountered in the southeastern highlands (e.g., upper and middle reaches of the Mananara basin). For example, closely related members of the catfish genus Ancharius and cichlid genus Ptychochromoides are present in both hydrological regions, which suggests that southwestern drainages of the Western Basins ecoregion and southeastern drainages of the Madagascar Eastern Highlands  were recently in contact (Sparks, unpub. data).
Fish communities contain numerous intrusive marine species, many of which migrate far inland (Kiener 1963). Brenon (Brenon 1972) (Brenon 1972)hypothesized that these incursions are largely the result of an expansive continental shelf and the presence of numerous coral reefs containing substantial fish populations. Marine taxa have likely colonized these streams subsequent to drought-induced extinctions of strictly freshwater taxa. The pattern of seasonal desiccation, then, may account for the depauperate assemblages of freshwater fishes inhabiting western basins and the relative dominance of marine species.
Other noteworthy aquatic biotic elements:
In addition to freshwater fishes, the larger wetlands and lakes within this ecoregion provide habitat for the rare and highly endangered Madagascar fish eagle (Haliaeetus vociferoides) (Langrand 1990). The endemic big-headed turtle, Erymnochelys madagascariensis, the only member of the subfamily Podocneminae to occur in the Old World, is widespread but uncommon (due to exploitation as a food source) in the larger lakes and rivers of this ecoregion and in the Madagascar Northwestern Basins  ecoregion (Kuchling 1988; Glaw & Vences 1994).
Justification for delineation:
This ecoregion is defined by those basins in western Madagascar with an annual average of less than 1400 mm of rain (Food and Agriculture Organization & International Institute for Applied Systems Analysis 2000). Compared to the wetter northwestern basins, the dry western basins are substantially more influenced by vast seasonal fluctuations in flow rate that could easily wipe out existing fish populations during periods of complete desiccation.
Level of taxonomic exploration:
Poor. To date a comprehensive ichthyological survey of the western basins has not been completed and thus comprehensive data are lacking. Many of the western basins are extremely difficult to access, as roads are few and in poor condition.
Aldegheri, M. (1972)"Rivers and streams on Madagascar" In Battistini, R.;Richard-Vindard, G. (Ed.). Biogeography and ecology in Madagascar. (pp. 261-310) The Hague, The Netherlands: Dr. W. Junk.
Brenon, P. (1972)"The geology of Madagascar" In Battistini, R.;Richard-Vindard, G. (Ed.). Biogeography and ecology in Madagascar. (pp. 27-86) The Hague, The Netherlands: Dr. W. Junk.
Donque, G. (1972)"The climatology of Madagascar" In Battistini, R.;Richard-Vindard, G. (Ed.). Biogeography and ecology in Madagascar. (pp. 87-144) The Hague, The Netherlands: Dr. W. Junk.
Food and Agriculture, Organization,International Institute for Applied Systems, Analysis (2000) "Global Agro-Ecological Zones - 2000" (2003)
Glaw, F.,Vences, M. (1994). "A field guide to the amphibians and reptiles of Madagascar" Leverkusen, Germany: Moos-Druck.
Kiener, A. (1963). "Poissons, peche et pisciculture a Madagascar" Publ. Centre Technique Forestier Tropical 24(1-224)
Kiener, A.,Richard-Vindard, G. (1972)"Fishes of the continental waters of Madagascar" In Battistini, R.;Richard-Vindard, G. (Ed.). Biogeography and ecology in Madagascar. (pp. 477-499) The Hague, The Netherlands: Dr. W. Junk.
Kuchling, G. (1988). "Population structure, reproductive potential, and increasing exploitation of the freshwater turtle Erymnochelys madagascariensis" Biological Conservation 43 107-113.
Langrand, O. (1990). "Guide to the birds of Madagascar" New Haven, Connecticut, USA: Yale University Press.
Lowry, P. P., II, Schatz, G. E., et al. (1997)"The classification of natural and anthropogenic vegetation in Madagascar" In Goodman, S.M.;Patterson, B.D. (Ed.). Natural change and human impact in Madagascar. (pp. 92-123) Washington, D.C., USA: Smithsonian Institution Press.