Location and General Description
The Chaco ecoregion as defined herein (Olson 2000) is generally restricted to the northwestern two-thirds of western Paraguay, and east of the Andes in southeastern Bolivia and northwestern Argentina. The northern boundary in general falls just west of the center of the continent. The northern, southern, western and eastern boundaries of this ecoregion terminate approximately at the 170 and 310 south latitudes and 650 and 560 west longitudes, respectively (Anon 1980). Mean annual temperature in the central Paraguayan Chaco during 1989-1990 was 260C with monthly means ranging 18.60 - 33.70C, and annual rainfall was 865 mm (mean = 72 mm/month) with monthly means ranging 10 – 164 mm (Brooks 1993, unpubl. data). Most scientists agree that the Chaco formed during Pleistocene postglacial fluctuations, from an arid to humid to semiarid environment, as initially proposed by Lüders (1961).
The Chaco is comprised of several habitats, although savanna’s, thorn forests, or a transition of these two predominant. Savannah and grassland habitats are characterized by a high abundance of grasses. Quebracho woodland (Short 1975) is more open than thorn forest, and is characterized by thorny bushes (e.g., Prosopis sp.), shrubs, and cacti (e.g., Opuntia sp.), with scattered trees (e.g., Aspidosperma quebracho, Bulnesia sarmientii and Schinopsis sp.) up to 13 m high. Dominant species include Prosopis ruscifolia, a thorny legume, and Opuntia sp. cactus (Lopez et al. 1987). Isolated tracts of thick, impenetrable primary thorn forest are sometimes left when land is cleared for agrarian purposes. The understory of primary thorn forest is punctuated with spiny terrestrial plants such as bayonet bromeliads (Bromelia serra) and star cactus (Cleistocactus baumanii) (Stabler 1985). The Chaco is punctuated "tajamars" (ponds made by ranchers for cattle) that support some aquatic life (Brooks 1993).
The Chaco represents a region that was inadequately explored until recently, with new species of large vertebrates such as the Chacoan Peccary (Catagonus wagneri) being discovered as recently as the 1970’s (Wetzel et al. 1975). Moreover, new records of known species (e.g., Wetzel and Lovett 1974, Brooks 2000b) are increasingly documented as the international scientific community realizes more fieldwork-hours.
The Chacoan Peccary (Catagonus wagneri), discovered in the 1970’s (Wetzel et al. 1975), is undoubtedly the most famous Chacoan (if not continental) endemic (Brooks 1992). Armadillos reach their peak diversity in the Chaco, with at least eight and ten species in the Paraguayan (Redford and Eisenberg 1992, Brooks 1995), and Argentinean (Zuleta and Bolkovic 1994) Chaco, respectively.
Other important species include the following: lesser mara (Pediolagus salinicola), giant tuco-tuco (Ctenomys conoveri) (Wetzel et al. 1975, Brooks 1993); greater rhea (Rhea americana), brushland tinamou (Nothoprocta cinerascens), chaco chachalaca (Ortalis canicollis), black-legged serieman (Chunga burmeisteri), chaco blue-fronted amazon (Amazona aestiva), picui Ground Dove (Columbina picui), Guira Cuckoo (Guira guira), Little Thornbird (Phacellodomus sibilatrix), many-colored chaco finch (Saltaitricula multicolor) (Capurro and Bucher 1988, Brooks 1997, 1998, Casenave et al. 1998); paraguayan caiman (Caiman yacare), southern boa (Boa constrictor occidentalis),false water cobra (Hydronastes gigas), horned frog (Ceratophrys sp.), argentine walking frog (Phyllomedusa sauvageii) (Brooks pers. obs.).
Due to its central location in South America, the Chaco harbors migrant birds from both southern (Austral) and northern (Neotropical) regions of South America, as well as migrants from even further north in North America (Brooks 1997, 1998).
The relatively newly established Parque Nacional (PN) Kaa-Iya, in Bolivia, sits abreast the Paraguayan border, relatively close to PN Defensores del Chaco. National Parks exist in the northern (PN Defensores del Chaco) and western sections of the Paraguayan Chaco, as well as several private reserves, including one each in the central (Estancia Boquerón'í) and northwestern (Estancia de South American Ltd.) Paraguayan Chaco (Fundacion Moises Bertoni unpubl., Brooks pers. obs.). Argentina also has several reserves in the Chaco, primarily in the northern Argentine Chaco (RN Formosa, PNs Pilcomayo, Baritú, Callilegua, El Rey, and RPs Agua Dulce, Potreros de Yala, El Bagual), with a couple in the central (RP Los Palmares and RP Copo) and southern (RP Chaco) regions (Caziani et al. 1997).
Reserves in the eastern Paraguayan Chaco and western Bolivian Chaco are noticably lacking, and reserves in the central and southern Argentinean Chaco are scant. A series of corridors connecting existing reserves would be ideal. Additionally, it is important to insure that these reserves are all properly staff, with the personnel well trained in law enforcement and habitat/wildlife management. Perhaps the main threat to the few pristine regions of the Chaco is increased development, which should be dampened wherever possible.
Types and Severity of Threats
Much of the Chaco is in various stages of alteration due to grazing of cattle (Benischke et al. 1989) and goats (J. Pinazzi pers. comm.), the latter especially in the southern Chaco. This development is perhaps least severe around the border of the Paraguayan and Bolivian Chaco, and most extensive in the Argentinean Chaco. Paved road development projects provide easy access to remote sites to hunt game and alter pristine wilderness for agrarian development. A good example of this is the Trans-Chaco highway that connects Paraguay and Bolivia (completed in late 1990s).
Justification of Ecoregion Delineation
Within Argentina, delineation’s for the chaco were derived from Daniele and Natenzon (1994), and linework follows their Bosques y Arbustales del Chaco Semiárido region. Other resources consulted include Cabrera (1976) and Morello (1968). Within Bolivia, we followed Ribera et al. (1994). Our linework encompasses their broad classification of the "chaco plains", with the following components included: "chaco dry forests", "chaco matorral and xeric scrub", "sandy lowlands", and the "Izozog wetlands". Finally, in Paraguay we referred to the UNESCO map (1980) for representation, and linework follows their classification of Drought Deciduous Lowlands (and submontane) Woodland – specifically in the province of Chaco, Paraguay, between the plains of the Paraguay and the Parana Rivers. The chaco region is recognized internationally as unique and needs no justification. We have divided the greater "chaco" into mainly climatic components, to include this Chaco ecoregion and also the Humid Chaco and Arid Chaco ecoregions.
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Prepared by: Dr. Daniel Brooks
Reviewed by: In process