Location and General Description
The Chaco Humedo ecoregion is located in northeastern Argentina, the center of Paraguay, and small areas in southwestern Brazil. The region slopes gently towards the east and overlooks areas full of depressions. The soils are generally sedimentary, originating from river flows and composed of fine materials. Sometimes there are impermeable soil layers that cause the formation of bogs (Cabrera, 1976). The average annual temperature drops from north to south, from 23° C along the Paraguay border, to around 18° C in the centre of the province of Santa Fe in Argentina. Precipitation varies between 1,300 mm annually in the east and 750 mm in the west. A mosaic of strips of high, well-drained land dominates the countryside in this ecoregion, with forests along the river courses alternating with interfluves at the base of the creeks, and gorges feeding the rivers. These strips feature natural pastures, savannas and areas of tall, coarse grass. (Burkart et al. 1999).
The vegetation consists of xenophile forests mixed with palm savannas. The forests are composed of quebracho colorado (Schinopsis balansae) and quebracho blanco (Apidosperma quebracho-blanco). Other important arboreal species in this community are: guayacán (Caesalpinia paraguariensis), espina corona (Gleditsia amorphoides), urunday (Astronium balansae), viraró (Ruprechtia laxiflora), palo piedra (Diplokeleba floribunda), guayaibí (Patagonula amaricana), zapallo caspini (Pisonia zapallo), lapacho negro (Tabebuis ipe), itin (Prosopis kuntzei), palo borracho del flor rosada (Chorisia speciosa) and others (Cabrera 1976).
In lower areas that are easily subject to flooding, there are bogs of algarrobo negro (Prosopis nigra), churqui espinillo (Acacia cavens), tala (Celtis tala), and cina-cina (Parkinsonia aculeata) (Burkart et al. 1999); while the slightly alkaline low country contains predominately algarrobo blanco (Prosopis alba) and caranday (Copernicia australis), a palm 10-12 m-high (Cabrera 1976).
The grasslands of this region are quite varied depending upon the soils. Elionurus muticus is found in soils similar to that of the forests but at slightly lower elevations. Sorghastrum agrostoides is found in soils that are flooded for a short time, and Panicum prionites in the bottom of the ravines and depressions without permanent water (Burkart et al. 1999).
The fauna is very diversified resulting due to the heterogeneity of the habitat. Among the large mammals we find: maned wolf (Chrysocyon brachyurus), black howler monkey (Alouatta caraya), Azara’s night monkey (Aotus azarae), pecaries (Tayssu pecari and T. tajacu), giant anteater (Mymercophaga tridactyla), capybara (Hydrochaerys hydrochaeris), deer (Mazama sp), marsh deer (Blastocerus dichotomus), pampas deer (Ozotoceros bezoarticus), puma (Felis concolor), and jaguar (Panthera onca). Some of the birds include the ñandú (Rhea americana), Crypturellus undulatus, Celeus lugubris, Heterospizias meridionalis, and others.
Restricted-range avifauna found in the southern part of the Humid Chaco and other ecoregions in the area include the endangered marsh seedeater (Sporphila palustris), the threatened Entre Ríos seedater (Sporophila zelichi) and the chesnut seedeater (Sporophila cinnamomea). These birds are found in seasonally wet grassland, marshes and riparian thickets (Stattersfield et al. 1998).
A large part of the aquatic inhabitants are reptiles like the yacaré negro (Caiman crocodylus yacare), the yacaré ñato (Caiman latirostris), the tortuga cavaleta (Acantochelys spixii) and the boa acuática curiyu (Eunecte notaeus) (Burkart et al 1999). Some of the areas where these reptiles are found include Estero Patiño, Laguna Blanca (RAMSAR site) and Bajos (Canevari et al. 1998).
Due to the extensive land use for cattle ranching in the ecoregion, there are various endangered animals including birds such as Rhea americana; mammals -Aotus azarae, Hydrochaerys hydrochaeris, Panthera onca, Blastocerus dichotomus, Ozotoceros bezoarticus, and Chrysocyon brachyurus; and reptiles- Acantochelys spixii and Eunecte notaeus (Chebez, 1988; García Fernández et al. 1997).
Dinerstein et al. (1995) have classified this ecoregion as vulnerable, and of importance on a local and a national scale. Cattle raising and forest exploitation have profoundly changed the composition of the vegetative communities either by destroying the herbaceous layer, eliminating the hardiest forest species or by burning all of the forest for agriculture. The quebracho colorado has been very exploited for its hard, almost imperishable wood, which has been used to make railroad ties and posts. Collection of the quebracho blanco has also been a problem; it is often used to make carbon, and its bark is used for medicine.
In this region there are numerous natural reserves with different degrees of protection and use. Among them we could mention: Parque Nacional Pilcomayo, Parque Nacional Chaco, Reserva Provincial Iberá, Reserva Provincial Granja Yatay, Reserva Provincial Pampa del Indio, Reserva Provincial Laguna La Loca, Reserva Provincial Vire-Pitá, Reserva Provincial del Medio- Los caballos, Reserva Provincial Cayastá, provincial reserve Lote 7B Los Quebrachos, among others.
Types and Severity of Threats
Commercial exploitation of plant species has led to a decline in their abundance, and its continued practice threatens to permanently change the vegetative structure of the ecoregion. This change in vegetation in turn threatens the species that rely on its abundance. Desertification in the ecoregion is likely to occur due to the clearing of trees combined with grazing of livestock. Commercial hunting and poaching also threaten the fauna chaqueña. Illegal shipments of leather and furs violating the established quotas and/or containing threatened, and thus prohibited, species are found at ports.
Justification of Ecoregion Delineation
This chacoan ecoregion is distinguishable from others by the species associations brought about by increased precipitation and reduced seasonality. The western border abut to the chaco ecoregion, which we classify as a dry forest habitat type as opposed to this shrubland. Linework follows national vegetation assessments. In Argentina we follow Daniele and Navarro (1994) who classify this region as "forests and wetlands of the humid chaco". Additional reference was made to Cabrera (1976) and Morellos (1968. In mapping the small extensions of this ecoregion into Brazil we followed IBGE (1993) classifications by lumping "open savanna steppe" and "savanna steppe – seasonal forest transition" vegetation types at the confluence of the Apa and Paraguay Rivers. We distinguish this ecoregion from the flooded grasslands along the Paraná River, which we classify as Paraná flooded savannas.
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Prepared by: Claudia Dellafiore
Reviewed by: Dr. Dan Brooks