Southwest Amazon moist forests
This ecoregion located in the Upper Amazon Basin, is characterized by a relatively flat landscape with alluvial plains dissected by undulating hills or high terraces.
The biota of the southwest Amazon moist forest is very rich because of these dramatic edaphic and topographical variations at both the local and regional levels.
This ecoregion has the highest number of both mammals and birds recorded for the Amazonian biogeographic realm: 257 with 11 endemics for mammals and 782 and 17 endemics for birds. The inaccessibility of this region, along with few roads, has kept most of the habitat intact. Also, there are a number of protected areas, which preserve this extremely biologically rich ecoregion.
Location and General Description
The southwest Amazon moist forest region covers an extensive area of the Upper Amazon Basin comprising four sub-basins: (1) both the Pastaza-Marañon and (2) Ucayali sub-basins drain into the Upper Amazon River in Peru; (3) the Acre and (4) Madre de Dios-Beni sub-basins drain to the east into the Juruá, Purus and Madeira Rivers; which, in turn, feed into the Amazon River lower down in Brazil. The region is bisected north to south between Peru and Brazil by the small mountain range Serra do Divisor. It extends east to the edge of the Purus Arch, or ancient zone of uplift, in the southwestern area of the Brazilian State of Amazonas. It then extends southeast into northern Bolivia and in a narrow band south along the base of the Andes.
Elevations range from 300 meters (m) in the west to 100 m on the eastern edge of the region. Because the ecoregion covers such a vast area, there are climatic, edaphic and floristic differences within it. Generally, the wetter and less seasonal northern forests (3,000 millimeters (mm) of rain annually) share only 44 percent of the tree species with forests in the slightly drier, more seasonal southern region. This region receives from 1,500 to 2,100 mm of rain annually, in different parts. Temperatures over the year range from 22 to 27° C.
Landforms present in this region include the upland terra firme (non-flooded) mostly on nutrient-poor lateritic soils, ancient alluvial plains (mostly non-flooded) on nutrient-rich soils, and present alluvial plains (várzea, seasonally flooded) of super-rich sediments renewed with each annual flood. Floristically, distinct lowland humid forest types occur on each of these landforms with the terra firme mature forests and late successional, seasonally flooded forest being the two major types. Permanent swamp forests are common on the alluvial plains. Pockets of nutrient-poor white sand soils are found here that host forests of lower height, a more open canopy, and lower alpha diversity, but with many endemics. The forests are mostly dense tropical rain forest, but some patches of open forest exist.
At first glance, large areas may appear to be homogeneous dense forests with a canopy 30 to 40 m high with some emergent trees to 50 m towering above the canopy. Structurally, this may be the case; however, the species composition reflects much the opposite: tree species variability reaches upwards to 300 species in a single hectare. There are a few exceptions to this high diversity, mainly where stands dominated by one or several species occur. The first are vast areas (more than 180,000 km2) dominated by the highly competitive arborescent bamboos Guadua sarcocarpa and G. weberbaueri near Acre, Brazil extending into Peru and Bolivia. Other monodominant stands include swamp forests of the economically important palms Mauritia flexuosa and Jessenia bataua.
In the north of the region, some of the best known plants yield products of commercial value, such as rubber (Hevea brasiliensis), mahogany (Swietenia macrophylla), balsam wood (Myroxylon balsamum), timber and essential oil (Amburana acreana), tagua nut (Phytelephas microcarpa), and strychnine (Strychnos asperula). An area representative of the southern part of this region, in the north of Bolivia, hosts a seasonal humid high forest to 35 m with some emergents reaching 40 m in height and many buttressed trunks. The largest trees are Ceiba pentandra, Poulsenia armata, Calycophyllum spruceanum, Swietenia macrophylla, and Dipteryx odorata. Other trees typical in this area are Calycophyllum acreanum, Terminalia amazonica, Combretum laxum, Mezilaurus itauba, Didymopanax morototoni, Jacaranda copaia, Aspidosperma megalocarpon, Vochisia vismiaefolia, Hirtella lightioides, and Hura crepitans. Palms include, among others, members of the genera Astrocaryum, Iriartea and Sheelea, Oenocarpus mapora, Chelyocarpus chuco, Phytelephas macrocarpa, Euterpe precatoria, and Jessenia bataua. Lianas are common with about 43 species present. Many Amazonian species reach the southern limit of their distribution here. The Brazil nut tree (Bertholletia excelsa) is present in the south, but is likely not native this far west in Amazonia.
What is distinctive about this region is the diversity of habitats created by edaphic, topographic and climatic variability. Habitat heterogeneity, along with a complex geological and climatic history has lead to a high cumulative biotic richness. Endemism and overall richness is high in vascular plants, invertebrates and vertebrate animals. This is the Amazon Basin’s center of diversity for palms. The rare palm Itaya amicorum is found on the Upper Javari River. This ecoregion has the highest number of mammals recorded for the Amazonian biogeographic realm: 257 with 11 endemics. Bird richness is also highest here with 782 species and 17 endemics. In the southern part of the Tambopata Reserve, one area that is 50 km2 holds the record for bird species: 554. On the white sand areas in the north, plants endemic to this soil type include Jacqueshuberia loretensis, Ambelania occidentalis, Spathelia terminalioides, and Hirtella revillae.
Many widespread Amazonian mammals and reptiles find a home in this region. These include tapirs (Tapirus terrestris), jaguars (Panthera onca), the world’s largest living rodents, capybaras (Hydrochoeris hydrochaeris), kinkajous (Potos flavus), and white-lipped peccaries (Tayassu pecari). Some of the globally threatened animals found in this region include black caimans (Melanosuchus niger) and spectacled caimans (Caiman crocodilus crocodilus), woolly monkeys (Lagothrix lagotricha), giant otters (Pteronura brasiliensis), giant anteaters (Myrmecophaga tridactyla), and ocelots (Leopardus pardalis).
Pygmy marmosets (Cebuella pygmaea), Goeldi marmosets (Callimico goeldii), pacaranas (Dinomys branickii), and olingos (Bassaricyon gabbii) are found here, but not in regions to the east. Other primates present include tamarins (Saguinus fuscicollis and S. imperator), brown pale-fronted capuchins (Cebus albifrons), squirrel monkeys (Saimiri sciureus), white-faced sakis (Pithecia irrorata), and black spider monkeys (Ateles paniscus). The rare red uakari monkeys (Cacajao calvus) are found in the north in swamp forests. Nocturnal two-toed sloths (Choloepus hoffmanni) are well distributed throughout this region along with the widespread three-toes sloths (Bradypus variegatus). The Amazon River is a barrier to a number of animals such as the tamarins Saguinus nigricollis, which occur on the north side, and Saguinus mystax, which occurs on the southwest side of the Amazon-Ucayali system.
In the region of Manu, 68 species of reptiles and 68 species of amphibians have been reported for the lowland areas while 113 species of amphibians and 118 species of reptiles are reported from Madre de Dios, including the rare and interesting pit-vipers (Bothriopsis bilineata, Bothrops brazili), and frogs such as Dendrophidion sp., Rhadinaea occipitalis, and Xenopholis scalaris.
Much of the natural habitat of the region remains intact, protected by sheer inaccessibility. People have dwelled along the major rivers for millennia and have subtly altered the forests on a small scale, but around the urban centers development proceeds. Very few roads exist in the region, limiting development. Intense deforestation is constrained to the few roads that do exist or around urban centers such as Iquitos, Puerto Maldonado, and Rio Branco.
Manú National Park, a World Heritage Site, protects 15,328 km2 of pristine lowland forest in southern Peru, a large part of which falls into this ecoregion. The nearby Tambopata Reserve protects seven major forest types. This reserve offers refuge to game species that have been over-hunted in other areas such as tapirs, spider monkeys, jaguars, capybaras, white-lipped peccaries, monkeys, caimans and river turtles. The Manuripi Heath National Reserve is located in the southernmost area of this region in Bolivia covering 18,900 km2 of dense tropical forest. Several extractive reserves, the largest being Chico Mendes and Alto Juruá, are actively managed in Brazil. Other protected areas include national parks (Serra do Divisor, Madidi, Isoboro Secure, Bahuaja-Sonene), national forests (Macauã), Rio Acre Ecological Station, Antimari State Forest, Apurimac Reserve Zone, among others. Most protected areas suffer from insufficient administration and patrol.
Types and Severity of Threats
Hunting may be threatening populations of the tapir (Tapirus terrestris) and large primates in the north. Some habitat is threatened by expansion of the agricultural and pastoral frontier, gold mining, and selective logging that erodes the genetic diversity of a few valuable timber species. The economically important palm Euterpe precatoria is being depleted in some areas by unsustainable palm heart extraction. A dramatic problem that exists in the Brazilian State of Acre and in the adjacent area of Peru is the spread of the invasive Guadua bamboo forests. This highly competitive bamboo invades and dominates abandoned clearings and threatens to dominate the disturbed areas in this region. Logging along major rivers and near urban centers has decimated populations of mahogany (Swietenia macrophylla), tropical cedar (Cedrela odorata), and kapok (Ceiba pentandra).
Justification of Ecoregion Delineation
These moist forests of the upper Amazon Basin host a great number of endemic species of both flora and fauna and unique forest characteristics described below. Delineations for this ecoregion follow several national vegetation coverage maps. In Peru our linework follows the Instituto Geográfico Nacional map classification of "Amazon tropical forests". From this broader classification we used only the forests east of the Ucayali River and south of the Solimões (Amazon) River for this ecoregion – as these rivers are formidable barriers to the dispersal of many species. In Brazil we followed the classification and linework of IBGE, lumping the "lowland ombrophilous open forests" with "lowland Amazonian ombrophilous dense forests" south of the Solimões (Amazon) River in westernmost Brazil. In Bolivia, we followed the linework and classification of Ribera et al., lumping the "humid seasonal Amazon forest", "evergreen seasonal moist forests", and "riparian forest of major erosional activity" along the Mamoré River and its tributaries. Because of the broader scope of our study, in the southwestern portion we lumped into this ecoregional classification the "Maniqui-Chichiguambo wetlands" and portions of "semideciduos forests" which were enclosed by these previous classifications – specifically those along the Grande River basin. Further justifications are given by da Silva.
Additional Information on this Ecoregion
- For a shorter summary of this entry, see the WWF WildWorld profile of this ecoregion.
- To see the species that live in this ecoregion, including images and threat levels, see the WWF Wildfinder description of this ecoregion.
- World Wildlife Fund Homepage
- Daly, D. C., and J. D. Mitchell. 2000. Lowland vegetation of tropical South America. Pages 391-453 in D. L. Lentz, editor, Imperfect Balance: Landscape Transformations in the Precolumbian Americas. New York: Columbia University Press. ISBN: 0231111568
- Ducke, A., and G. A. Black. 1953. Phytogeographical Notes on the Brazilian Amazon. Anais da Academia Brasileira de Ciências 25: 1-46.
- Ergueta S.P., and J. Sarmiento. 1992. Fauna silvestre de Bolivia: diversidad y conservación. Pages 113-163 in M. Marconi, editor, Conservación de la Diversidad Biológica en Bolivia. La Paz, Bolivia: CDC-Boliva and USAID.
- Fundação Instituto Brasilero de Geografia Estatástica-IBGE. 1993. Mapa de vegetação do Brasil. Map 1:5,000,000. Rio de Janeiro, Brazil.
- Henderson, A. 1995. The Palms of the Amazon. New York: Oxford University Press. ISBN: 0195083113
- Instituto Geográfico Nacional. 1987. Ecoregiones del Peru. Map 1:5,000,000. Atlas del Peru, Lima, Peru.
- Pacheco, V., and E. Vivar. 1996. Annotated checklist of the non-flying mammals at Pakitza, Manu Reserve Zone, Manu National Park, Perú. Pages 577-592 in D. E. Wilson and A. Sandoval, editors, Manu: The Biodiversity of Southeastern Peru. Washington, DC: Smithsonian Institution.
- 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, editors, Mammals of the Neotropics: the Central Neotropics. Chicago: University of Chicago Press. ISBN: 0226195422
- Räsänen, M. 1993. La geohistória y geología de la Amazonia Peruana. Pages 43-67 in R. Kalliola, M. Puhakka, and W. Danjoy, editors, Amazonia Peruana: vegetacióon húmeda tropical en el llano subandino. Turku: PAUT and ONERN.
- Ribera Arismendi, M. 1992. Regiones ecológicas. Pages 9-71 in M. Marconi, editor, Conservación de la Diversidad Biológica en Bolivia. La Paz, Bolivia: CDC-Boliva and USAID.
- Ribera, M.O., M. Libermann, S. Beck, and M. Moraes. 1994. Mapa de la vegetacion y areas protegidea de Bolivia. 1:1,500,000. Centro de Investigaciones y Manejo de Recursos Naturales (CIMAR) and Universidad Autónoma Gabriel Rene Moreno (UAGRM), La Paz, Bolivia.
- 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.
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