Southwestern Amazonian moist forests

Southwestern Amazonian moist forests

The Southwestern Amazonian Moist Forests (a "Global 200 Ecoregion" - SWA G200), contains some of the richest and largest tracts of intact tropical rainforest found in the entire Amazon basin. These forests are the habitat of emblematic species like the "jaguar" and "harpy eagle", highly threatened in other Amazonian regions, dozens of indigenous groups, some of whom have not been contacted by Western civilization, and scattered populations of traditional Brazil nut gatherers and rubber tappers. In spite of its relative isolation, the SWA G200 forests are threatened by the opening and paving of roads that provide access to a growing population of small farmers, oil and gas exploration, as well as large-scale cattle ranching and agribusiness.

Covering 2,093,811 km² of northern Bolivia, southeastern Peru and western and central Brazil, these forests still contain some of the largest tracts of intact tropical rainforest within the entire Amazon basin. Reaching into the foothills of the "Andes", the Southwestern Amazonian Moist Forests are influenced by pre-montane conditions that produce diverse climatic and edaphic conditions. The high rainfall, complex topography, and influence of southern temperate winds produce a mosaic of habitats and ecosystems within the lowland tropical forest, contributing to the overall high levels of biodiversity. Such conditions contribute to the formation of an ecologically unique southwestern arc of lowland "tropical forests", harboring some of the richest and most intact forest communities on Earth.

The large expanses of relatively undisturbed habitat provide refuge for many species that are highly threatened elsewhere: Jaguar ("Panthera onca"), Harpy eagle ("Harpia harpyia"), the Giant river otter ("Pteronura brasiliensis"), Black caiman ("Melanosuchus niger"), White-lipped peccary ("Tayassu tajacu") and several species of macaws ("Ara" spp.), guans (Cracidae), and curassows ("Crax" spp.). While largely understudied biologically, various inventories demonstrate that the region has some of the highest recorded species diversity index in the world for plants, birds, fish, and butterflies. Many of these species are "endemic" to the Southwestern Amazonian Moist Forests or are no longer found in other regions due to hunting pressure, destruction of intact forest communities, and expansion of “development” programs.

Recent decades have seen the development of the Southwestern Amazon lowland, with the opening of the Brazilian frontier to slash and burn agriculture and large-scale ranching funded by the governments to promote the development of this region and relieve the pressure of the growing populations of small farmers in the northeast and south of Brazil. In addition, colonists from the poor Andean communities of Peru and Bolivia have settled in the lowlands, repeating the process of overexploitation of resources that were eventually depleted in their abandoned highland communities. A network of roads to promote these migrations, as well as allow access to high value woods, oil, gas, and forest resources (especially Brazil nuts and rubber), have increased the colonization of the lowland forests and added stress to the ecological systems dependent on the decreasing area of intact and closed-canopy forests. Decreased rainfall, due primarily to local deforestation and increased human presence, has increased the opportunity for extensive forest fires, having reached a high in the dry season of 1999 as millions of hectares in Rondonia and Acre (Brazil) were engulfed by flames. Tourism, while an opportunity for conservation, is also proving to be a threat to the fragile ecosystems of the Tambopata-Candamo region along the Madre de Dios River basin, and along the Beni River in Bolivia (Rurrenabaque).

The Southwestern Amazonian Moist Forests Global 200 Ecoregion is around 209,381,146 ha and encompasses four ecoregions: Southwest Amazon moist forests, Jurua Purus moist forests, Purus Madeira moist forests and Madeira Tapajos moist forests.

outhwest Amazon moist forests (NT0166)

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 [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: vegetación húmeda tropical en el llano subandino. Turku: PAUT and ONERN.] . 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 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 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 [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.] . 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") [Ducke, A., and G. A. Black. 1953. Phytogeographical notes on the Brazilian Amazon. Anais da Academia Brasileira de Ciências 25: 1-46.] . 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 Kapok ("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" [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.] . 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.

Biodiversity Features

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 led 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 (Henderson 1995). 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 birds 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"), capybaras ("Hydrochoeris hydrochaeris")—the world’s largest living rodents, 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's Marmosets ("Callimico goeldii"), pacaranas ("Dinomys branickii"), and olingos ("Bassaricyon gabbii") are found here, but not in regions to the east [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.] . 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") [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.] . 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 Brown-throated Three-toed 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" [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.] .

Conservation Status and Threats

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 Bahuaja-Sonene National Park and Tambopata National Reserve protect 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.
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 [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.] . 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").


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