Boundaries

This ecoregion encompasses much of the northern (Chaco Boreal) and central (Chaco Central) Chaco region that includes the drainages of the western Paraguay basin across Bolivia, Paraguay, and Argentina. Its boundaries extend from the southern slopes of the Sierra Sunsas in Bolivia to the Río Bermejo drainage in northern Argentina.  It is limited to west by the Central Andes and to the east by the main stem of the Río Paraguay. The Michicola Arch, which forms the ecoregion’s northwestern boundary, is the drainage divide between the Amazon and Paraná systems (Lundberg et al. 1998).

Topography

Despite the flat topography of the Chaco, elevations in this ecoregion range from less than 50 m along the Rio Paraguay to peaks exceeding 5000 m asl in the Andes (Hijmins 2004). The upper Pilcomayo drains the Central Andes in an area characterized by steep slopes, deep canyons, and narrow ridges. To the southeast lies the low-lying Chaco, a vast sedimentary alluvial plain that slopes gently (.004 degree gradient) toward the Río Paraguay. Elevations here do not exceed 300 m asl and there is little relief. The nutrient-rich sedimentary soils include luvisols, cambisols, and regosols (Schulenberg & Awbrey 1997).  

Freshwater habitats

The Chaco contains numerous intermittent streams, rivers, and swamps, as well as several permanent rivers and large swamps (Chernoff et al. 2001).  The Bermejo and Pilcomayo rivers are the two largest tributaries flowing into the Paraguay, which is the main river draining the Gran Chaco (Lundberg et al. 1998). They have courses comprised of sloughs, oxbow lakes, braided channels, sandbars, seasonally flooded grasslands, permanent marshes, and extensive swamps, which is typical of streams of this ecoregion (Resende 2003). Most of the flow, however, does not reach the Paraguay due to the flat topography and losses from flooding, seepage, and evaporation. The lower Chaco is so poorly drained that the shallow channels flood extensively into seasonal marshes and small ponds during the rainy summers (Resende 2003). Many of these areas become desiccated and deoxygenated during the dry season (Lowe-McConnell 1987).

Terrestrial habitats

Savannas and thorn forests are predominant in the Chaco with species like vinal (Prosopis ruscifolia) and cacti (e.g. Opuntia sp.). The undergrowth is composed of spiny shrubs and grasses (Chernoff et al. 2001). The easternmost part of the ecoregion has higher rainfall and poor drainage, resulting in a swampy plain of open savannas, quebracho woodlands (e.g. quebracho colorado, Schinopsis balansae; quebracho blanco, Apidosperma quebracho-blanco), and palm forests (e.g. Caranda’y palm, Copernicia australis) (WWF 2001). The westernmost part of the ecoregion spans yungas, dry forests, and puna as elevations increase.

Description of endemic fishes

The ecoregion contains two strict endemic genera: Ixinandria (I. montebelloi, I. steinbachi) and the monotypic genus, Nans (N. indefessus), which is restricted to the Río Bermejo basin. Papiliolebias (P. bitteri) is a near-endemic monotypic genus that also occurs in the Paraguay ecoregion [343]. In total there are 16 endemic species presently recorded, including four species of Astyanax, three Trichomycterus, and two Ixinandria.

Other noteworthy fishes

Some species have adapted to the deoxygenated and desiccated seasonal swamps of the Chaco. For example, the marbled swamp eel (Synbranchus marmoratus) is a facultative air breather that aestivates in burrows during the dry season (Lowe-McConnell 1987).

Ecological phenomena

There are a number of fishes that undergo spawning migrations in the ecoregion. For example, fishes such as the boga (Leporinus obtusidens) and spotted sorubim (Pseudoplatystoma corruscans) undergo 400 km migrations from the lower Chaco, up the Pilcomayo, and into the Andean foothills to spawn. Most return during the large flood period, between November and December (Lowe-McConnell 1987).

Justification for delineation

This ecoregion lies entirely within the Parano-Platense ichthyographic province defined by Ringuelet (1975) and Paranean region defined by Géry (1969). The Chaco is recognized internationally as unique both from a freshwater and terrestrial standpoint.

Level of taxonomic exploration

Fair to poor