Boundaries

Encompassing the extreme southern part of Madagascar, the Southern Basins ecoregion is characterized by its aridity. Many streams and rivers are dry from May to October (Aldegheri 1972; Benstead et al. 2000) and much of the ecoregion is essentially devoid of rivers or surface water (Aldegheri 1972). As a result, the ecoregion has a depauperate aquatic fauna.

Freshwater habitats

Karst formations characterized by fissures, subterranean streams, sinkholes, and caverns produced by erosion occur within the western portion of the ecoregion. Groundwater discharge that draws its water from a perched water table feeds Lake Tsimanampetsotsa. The original natural stream course in the lake’s catchment has disappeared and only traces remain of an ancient network of river valleys that once existed under a more humid climate (Wetlands International 2002). Rivers throughout the ecoregion are wide, shallow, and subject to extreme seasonality in flow. 

Terrestrial habitats

Vegetation is referred to as “spiny desert,” and members of the endemic family Didiereaceae dominate the landscape (Lowry et al. 1997).

Description of endemic fishes

Distinctive habitats within this ecoregion include systems of limestone caves, located north and south of the Onilahy River as well as in the southwestern part of the Western Basins ecoregion [868]. These caves are home to a small radiation of endemic blind cave eleotrids (Typhleotris spp.). Among these is the vulnerable blind fish, Typhleotris madagascariensis, found in the underground rivers and caves of the Mahafaly Plateau (IUCN 2002).

Justification for delineation

This ecoregion is defined by the arid southern portion of Madagascar that receives less than 50 mm rain/month and characterized by a depauperate freshwater fauna.

Level of taxonomic exploration

Poor. Little data are available for basins in this ecoregion or for the fishes that inhabit this remote part of the island. Southern Madagascar has not been the focus of ichthyofaunal collecting expeditions, given the dry desert-like climate, homogeneity of habitat, and few native fish species that would be expected to inhabit such areas. Similar to Madagascar\'s Western Basins [579], to date there has been no comprehensive ichthyological survey of the drainages in southern Madagascar. Ichthyologists have historically focused on other regions of Madagascar due to the fact that few species would be expected to occur in xeric habitats, and, therefore, data quality remains poor.