Location and General Description
Rivers in the countries of Colombia, Venezuela and Brazil bind the Negro-Blanco moist forest ecoregion. To the north in Colombia the border is formed by the Rio Vichada and in Venezuela by the Rio Ventuari. The southern border in Colombia follows the Guainía River to its headwaters where it then follows the middle Guaviare River, to the Guainia-Negro River along the Venezuela-Colombia border, and finally continues along the Rio Negro into Brazil east to the Rio Branco. From Colombia, the whitewater (carrying suspended organic and mineral material) Vichada, Guaviare and Inírida Rivers, and the clearwater (containing no suspended solids) Ventuari River in Venezuela, feed into the Orinoco Basin, which drains toward the Caribbean Sea. The blackwater (containing no suspended solids) Casiquiare River in Venezuela feeds the Guainia-Negro River, which drains into the Amazon Basin.
The Guyana Shield, underlying this ecoregion, consists of a variety of basement rock including igneous and metamorphic types and was formed during multiple geological events at different times beginning 1.8 billion years ago. The lowland plains of this ecoregion emerged only recently from lacustrine and marine environments in which sediments were deposited in layers over the landscape over millenia. The elevation ranges from 120 m in the western section to upwards of 400 m in Venezuela. A complex mixture of soil types that are generally poor in nutrients characterizes this ecoregion, and mostly podzols (white, sandy quartzite soils; strongly acidic, limited moisture and nutrient availability) are found on the ancient alluvial terraces. The vegetation consists of both flooded and non-flooded lowland moist forests, Amazon caatingas (tall to medium-tall sclerophyllous lowland forests), shrub communities in isolated patches, and herbaceous savanna-like meadows. Despite the predominantly poor soils, the ecoregion hosts a high level of plant diversity with considerable endemism at the generic level (Daly and Mitchell 2000). The forests are typical of the Guyana region and are biologically distinct from the classical Amazon forest (Berry et al. 1995). Families strongly represented here and not in the Amazon floristic province include Humiriaceae, Rapateaceae, Tepuianthaceae, Theaceae, and Xyridaceae.
In the Rio Negro Basin, because of the presence of a high density of blackwater streams and small rivers passing through the area, part of the ecoregion to the west of the confluence of the Rio Negro and Rio Branco hosts a large area of igapó forest. This forest type occurs on land that is seasonally flooded for five to six months each year by blackwater rivers and tends to occur on sandy, nutrient poor soils (Pires and Prance 1985). The canopy reaches to 35 meters with the most abundant tree species being Virola elongata, Eschweilera longipes, E. pachysepala, Aldina latifolia var. latifolia, and Pithecellobium amplissimum (Adis 1984).
One area within this ecoregion that has been well-studied is the level Casiquiare peneplain (lowland) in Venezuela which hosts a mosaic of forests, savannas, and other types of herbaceous vegetation. Both blackwater and whitewater rivers flow through this region whose flooded banks host two different forest types, igapó and várzea, respectively. Annual rainfall ranges from 2,000 to 3,000 mm. Both seasonally flooded and non-flooded tall (40 m) evergreen lowland forests are found (Huber 1995) as well as low (20 m) evergreen flooded palm forests. These low forests are dominated by species of Mauritia flexuosa or Mauritiella aculeata or by dense associations of Euterpe catinga var. catinga, Iriartea setigera, and Socratea exorrhiza. The non-flooded forest of about 40 m in height contains variable mixtures of Lecointea amazonica, Clathrotropis glaucophylla, Peltogyne venosa, and species in the genera Ocotea, Nectandra, Licania, Trichilia, Guarea, Toulicia, Erisma, and Ruizterania. Arboreal palms in the genera Oenocarpus, Socratea, Leopoldinia, and Bactris are found in the tall canopy forests, as well. Few epiphytes or lianas grow here compared to elsewhere in western Amazonia. Elements of the Rio Negro caatinga vegetation on white sands are found near the border of Venezuela and Colombia.
The economically important palm Leopoldinia piassaba is endemic to this ecoregion. This ecoregion is considered an area of high botanical endemism, including the genera Duckeanthus, Heteropetalum, Pseudephedranthus, Urospathella, Aquiaria, Angostyles, Astrococcus, and Chonocentrum.
Among the 194 mammal species found here, not many are exclusive to this ecoregion. The few species that are mainly restricted to this ecoregion include the endemic black uakari monkeys (Cacajao melanocephalus) and bearded sakis (Chiropotes satanas), opossums (Marmosops impavidus), bats (Micronycteris pusilla, Phyllostomus latifolius, Platyrrhinus aurarius), and several rodents (Neusticomys venezuelae, Rhipidomys macconnelli, Cavia guianae, and Proechimys simonsi). Widespread species occur here such as tapirs (Tapirus terrestris), peccarys (Tayassu tajacu), pale-throated sloths (Bradypus tridactylus), long-tailed weasels (Mustela frenata), two canids (Atelocynus microtis and Speothos venaticus), and three species of small cats Leopardus.
The bird species richness is not particularly high for Amazonia at 486 species. A number of birds occur only here or are shared with a few other ecoregions. These include the endemic grey-legged tinamous (Crypturellus duidae), crestless curassows (Mitu tomentosa), double-striped thick-knees (Burhinus bistriatus), oilbirds (Steatornis caripensis), tawny-tufted toucanets (Selenidera nattereri), endemic orinoco piculets (Picumnus pumilus), two endemic antbirds (yapacana, Myrmeciza disjuncta, and grey-bellied, Myrmeciza pelzelni), spot-tailed nightjars (Caprimulgus maculicaudus), azure-naped jays (Cyanocorax heilprini), and white-naped seedeaters (Dolospingus fringilloides).
Reptiles and amphibians are abundant. The more famous snakes that occur here include fer-de-lance (Bothrops asper), palm pit-vipers (Bothriechis spp.), coral snakes (Micrurus spp.), boa constrictors (Boa constrictor), and bushmasters (Lachesis muta). Iguanas (Iguana iguana) are ubiquitous and tegus lizards (Tupinambis) common.
Due to the inaccessibility of this ecoregion, the forest remains largely intact. No paved roads exist here although unpaved roads exist to the north and west of the ecoregion in Colombia. People living in settlements along the rivers practice small-scale rotational agriculture, and boat traffic along the rivers brings loggers and merchants. Brazil nut collectors set fire to the lower portion of the forest as a management practice. This alters the understory in some stands, but this is a localized practice. The largest biosphere reserve in the tropics, the Alto Orinoco-Casiquiare Biosphere Reserve, lies mostly in this ecoregion. In Brazil a small portion of this ecoregion lies within the Pico da Neblina National Park.
Types and Severity of Threats
The threats to this ecoregion are few and small-scale. In some areas, peasant farmers or cattle ranchers move into the areas along the rivers and put some pressure on the environment. Logging occurs along the Rio Negro, but does not pose a serious a threat to the habitat as of yet. The leaves of the palm Leopoldinia piassaba are harvested destructively to make brooms for international markets, and this species may be overexploited.
Justification of Ecoregion Delineation
This moist forest ecoregion is distinct in that it forms both the transitional zone between the floristic provinces of Amazonia and Orinoco, as well as between Andean and Guinan Shield influences. The delineation’s are based on unique and endemic species. The southern arm of this ecoregion occurs between the northern bank of the Negro River and the distinct campinarana and Guyanan highlands to the north, and the eastern boundary follows the Branco rivers western shores from its confluence with the Negro northwards. This southern arm is interspersed with campinarana ecoregions. Eastern boundaries follow the elevational transition of vegetation types between lowland and highland forests. In the western portion, southern delineation’s into Colombia follow the north bank of the Guainía River, then breaks off following expert opinion (Robin Sears pers. comm. and da Silva 1998). Northern linework follows the Vichada River.
Adis, J. 1984. 'Seasonal igapó'-forests of Central Amazonian blackwater rivers and their terrestrial arthropod fauna. Pages 245-268 in H. Sioli, editor, The Amazon -- Limnology and landscape ecology of a mighty tropical river and its basin. Junk, Dordrecht.
Berry, P. E., O. Huber, and B. K. Holst. 1995. Floristic analysis and phytogeography. Pages 161-191 in P. E. Berry, B. K. Holst, and K. Yatskievych, editors, Flora of the Venezuelan Guayana. St. Louis: Missouri Botanical Garden and Timber Press.
Daly, D. C., and J. D. Mitchell. 2000. Lowland vegetation of tropical South America. Pages 391-453 in D. L. Lentz, editors, Imperfect balance: Landscape transformations in the Precolumbian Americas. New York: Columbia University Press.
Foster, R. Personal communications.
Fundação Instituto Brasilero de Geografia Estatástica-IBGE. 1993. Mapa de vegetação do Brasil. Map 1:5,000,000. Rio de Janeiro, Brazil.
Huber, O. 1995. Vegetation. Pages 97-160 in P. E. Berry, B. K. Holst, and K. Yatskievych, editors, Flora of the Venezuelan Guayana. St. Louis: Missouri Botanical Garden and Timber Press.
Pires, J. M., and G. T. Prance. 1985. The vegetation types of the Brazilian Amazon. Pages 109-145 in G. T. Prance and T. E. Lovejoy, editors, Key Environments: Amazonia. New York: Pergamon.
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.
Prepared by: Robin Sears and César Marín
Reviewed by: In process