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South America: Amazon Basin, northwestern Brazil

The Juruá-Purus moist forest ecoregion lies in the western part of the Brazilian Amazon basin between the Solimões (Amazon) and Purus rivers. This area lies entirely on the low Amazon basin, and the terrain is mostly uniform consisting of flat forested plains dissected by large rivers which are characterized by endless meanders, frequent oxbows, and thousands of tiny streams, all of which flood annually. This ecoregion does not include the abundant riparian flooded várzea, which is described under the Purus várzea ecoregion. This hot and humid aseasonal tropical region receives on average 2500mm of precipitation per year but with some areas to 3500mm. Elevations in this terrain range between 20m and 60m above sea-level.

  • Scientific Code
  • Ecoregion Category
  • Size
    93,700 square miles
  • Status
    Relatively Stable/Intact
  • Habitats

Location and General Description
The Juruá-Purus moist forest ecoregion lies in the western part of the Brazilian Amazon Basin between the Solimões (Amazon) and Purus Rivers. It covers the interfluves between the westermost Amazon River, crossing the basins of the Jutaí, Jurua, Tefé, and Tapauá Rivers east to the Purus River. The southwestern limit is before the lowest elevations of the Carauari Arch, an ancient zone of uplift, excluding the upper portions of the Juruá and Purus Rivers. Thus, the ecoregion lies entirely on the low Amazon Basin whose soft sediments emerged relatively recently during the late Tertiary (2 to 5 million years ago).

The terrain is mostly a uniform, flat plain dissected by large rivers characterized by endless meanders, frequent oxbows, and thousands of tiny streams, all of which flood annually. Because these rivers drain the relatively young (and erodable) lowland basin, they carry a load of suspended solids (both mineral and organic) and are classified as whitewater rivers. The flooded forests along these rivers are described in the Purus Várzea ecoregion and are not discussed here. This hot and humid aseasonal tropical ecoregion receives on average 2,500 mm of precipitation per year. Some areas receive as much as 3,500 mm. The rain averages about 200-300 mm per month. July is the driest month with 100 mm on average. Temperatures over the year average 26° to 27° C with negligible monthly variation. The soils of this ecoregion are variable, alternating between sandy podzols and hydromorphic clay, both of which hold limited nutrients and tend to be acidic. Elevation of this terrain is between 20 and 60 m.

The ecoregion is almost entirely forested with evergreen tropical rain forests with dense high canopy (approximately 30 m in height, with emergent trees as high as 45 m) and some structural heterogeneity. There are small patches of forest with a more open canopy and less dense understory. Most trees are of small diameter (less than 300 mm) with very few stems growing to between 400 and 700 mm diameter. A few giants with the largest trunks (2,400 mm) include Cariniana decandra, Osteophloem platyspermum, Piptadenia suaveolens, Brosimum sp., Eschweilera blanchetiana, and Sclerobium paraense. The most important families in these forests, which are typical of other Amazon Basin forests, are Leguminosae, Sapotaceae, Lecythidaceae, Moraceae, Chrysobalanaceae, Lauraceae, and Myristicaceae. The most important species in terms of density and frequency are Eschweilera alba, E. odora, Pouteria guianensis, Vantanea guianensis, Jessenia bataua, Ragala sanguinolenta, Licania apetala, and Iryanthera ulei. Four palms are common here: Astrocaryum tucuma, Jessenia bataua, Maximilliana regia, and Socratea exorrhiza. The Jurua River Basin seems to be the western limit to the natural occurrence of Bertholletia excelsa, the Brazil nut tree.

Biodiversity Features
The forests of this ecoregion have a super-high floristic diversity with tree richness up to 250 species per hectare near Carauari . The diversity of members of the tree family Sapotaceae is unequaled elsewhere in the Amazon lowlands, with 60 species present . There is a high diversity of timber species (15 commercial species identified in one area), but they occur at low density.

The fauna is also very diverse with a high occurrence of endemism. Of the 171 mammals recorded for this ecoregion, nearly 120 have been recorded at a single site at the headwaters of the Urucu River (Peres 1999). Vagrant mammals that travel between flooded and non-flooded forests include squirrel monkeys (Saimiri sciureus), brown pale-fronted capuchins (Cebus albifrons), woolly monkeys (Lagothrix lagothricha), and collared peccaries (Tayassu tajacu). Other species do not make the jump across the rivers, such as at the Purus and Tapaua Rivers, which separate subspecies of some primates and insects. Other large mammals found in the ecoregion include anteaters (Cyclopes didactylus, Tamandua tetradactyla, Myrmecophaga tridactyla), sloths (Bradypus variegatus), cats such as jaguars (Panthera onca) and pumas (Puma concolor), deer (Mazama americana and M. gouazoubira), and tapir (Tapirus terrestris).

The avifauna boasts 554 species with seasonal migrant birds such as toucans (Ramphastos cuvieri), large parrots (Amazona spp.), and macaws (Ara spp.), and locals including tanagers (Tangara spp., Tachyphonus spp.), woodcreepers (Xyphorhynchus spp.), pavonine quetzals (Pharomachrus pavoninus), curasows (Crax globulosa, Nothocrax urumutum, Mitu tuberosa), and tinamous (Crypturellus spp., Tinamus spp.).

Current Status
Because no roads traverse this ecoregion, lack of access prevents over-hunting and extensive habitat disturbance. The Brazilian company Petrobras has been prospecting for oil and natural gas in this ecoregion for years, regularly deforesting patches throughout the area. Near Tefé, a large patch of forest was removed for an experimental agriculture project, but it has returned to secondary forest. Aside from these, the forests of the interior remain largely intact. Along the rivers, a few large urban centers (Carauari, Tefé, Coari, Jutaí) and many small settlements of farmers have an impact on the forest environment through clearing for urbanization, agriculture, and cattle ranching.

Types and Severity of Threats
There are plans to complete the Transamazon Highway through this ecoregion from Lábrea on the Purus River to Tabatinga on the Peru-Brazil-Colombia border. This would result in extensive areas of land conversion, burning, and water pollution. Presently, hunting and extractive logging put pressure on mammals such as tapir, peccaries, deer, and some primates, and several species of trees sought for timber. The expansion of small-scale cattle ranching also threatens the forest environment.

Justification of Ecoregion Delineation
The delineation’s for this ecoregion follow major waterways, which act as formidable barrier to the distributions of many species. The northern boundary traces the Rio Solimões (Amazon), and the southern follows the Rio Purus to the confluence with the Solimões, forming the eastern corner of the ecoregion. The western delineation follows the IBGE (1993) classification of "dense lowland ombrophilous Amazonian forest". The Juruá River in contained within, and characterizes this ecoregion. The rivers which bind this river also have isolated it and many species are restricted here, many of which are endemic. Further justification is offered by da Silva (1998).

Ducke, A., and G. A. Black. 1953. Phytogeographical notes on the Brazilian Amazon. Anais da Academia Brasileira de Ciências 25: 1-46.

Fundação Instituto Brasilero de Geografia Estatástica-IBGE. 1993. Mapa de vegetação do

Brasil. Map 1:5,000,000. Rio de Janeiro, Brazil.

Peres, C. A. 1999. The structure of nonvolant mammal communities in different Amazonian forest types. Pages 564-581 in J. F. Eisenberg and K. H. Redford, eds. Mammals of the Neotropics: the Central Neotropics. Chicago: University of Chicago Press.

Silva, J.M. C. 1998. Um método para o estabelecimento de áreas prioritárias para a conservação na Amazônia Legal. Report prepared for WWF-Brazil. 17 pp.

Silva, A. L. L. de, P. L. B. Lisboa, and U. N. Maciel. 1992. Diversidade florística e estructura em floresta densa da bacia do Rio Juruá-AM. Boletim do Museu Paraense Emílio Goeldi, Botânica 8: 203-258.

Prepared by: Robin Sears
Reviewed by: In process


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