Location and General Description
The Orinoco wetlands are flooded grassland occurring in seven distinct patches, which are embedded in a mosaic of mangroves, swamp forest, moist forest, and llanos. The ecoregion is located in the north of the delta of the Orinoco River- the second largest river in the Neotropics - in eastern Venezuela. The blocks of flooded grassland occur north of the main flow of the Orinoco River, along the Boca Grande and San Juan Rivers, and within the alluvial fan of the Orinoco (Amacuro) delta. The core portion of the ecoregion occurs near Tucupita, along the Manamo River, which diverges from the Orinoco near Barrancas to form the western edge of the delta. The next largest patch occurs along the Canal Macarao. Others patches occur along the coast, between the mangrove and swamp forests.
Climate in this region is tropical and wet. Precipitation varies throughout the region, and fluctuates between 1,000 and 2,000mm annually. Rainfall is irregular and the wet season begins in April/May and usually last through December with a brief pause in July. Geographically this is a landscape of little relief. Elevations typically average one meter, but reach as high as 9 meters in the highest terra firma levies along the coast. The soils in this ecoregion are almost entirely alluvial deposits, originating as far away as the northern Andes of Colombia and Venezuela. Over the last century alone, more than a 1,000 km² has been added to the delta by alluvial deposits, which continues to extend into the Atlantic at a rate of 40 m per year over its entire 360 km coastline.
This delta region is characteristically impregnated with river systems, and hosts a great diversity of riparian features including permanent wetlands and marshes, large rivers, oxbow lakes, small gallery streams, levies, and the typical delta alluvial fan. The delta itself consists of increasingly partitioned tributaries, which become more confined and disjunct as they diverge from the main channel, then come together again as they move east towards the Atlantic Ocean (Conde 2001). In so doing these numerous rivers form a great number of islands. The primary river, the Orinoco only enters a small portion of this ecoregion, on the southern extension of the largest patch.
According to the broad vegetation classification of UNESCO (1981), the core of this ecoregion is considered tropical tall flooded grasslands with very few woody elements present. Others regional surveys by Huber et al (1975) and Holdridge (1977) provide similar assessments. These predominant vegetative features are tall grasses, including Jussieua sp., Lagenocarpus guianensis, Mesosetum sp., Nepunia sp., Paspalum repens, and Rhynchospora sp. Interspersed among the grasses are local concentrations of palms such as Attila sp., Euterpe cuatrecasana, Manicaria saccifera, Orbignya cuatrecasana, and Trithrinax sp. There are some areas in the savanna where the moriche palm (Mauritia flexosa) forms monotypic stands known as morichal. Transitional and fringe habitat will host components of the surrounding matrix, including mangrove, tropical ombrophilous swamp, and lowland forest. In the western portion, these flooded grasslands become drier and retain stands of evergreen broadleaf trees. Bordering these formations on terra firma, these grasslands have been colonized by the pioneer Cecropia sp.
Floodplains of large rivers, such as the Orinoco, are among the most productive ecosystems. Very little research has been conducted in the wetlands of the Orinoco Delta (Delta Amacuro), and species compositions are doubtlessly influenced by the surrounding terra firma moist forests, swamp forests, and mangroves. The monodominant stands of morichi palm (Mauritia flexosa) provide important food to a great number of species, including numerous primates, parrots, and rodents, and also provides nesting habitat for many bird species.
Among the threatened species (IUCN 2000) within this ecoregion are the Orinoco crocodile (Crocodylius intermedius CR), Amazon River dolphin (Inia geogffrensis VU), jaguar (Panthera onca LR), bush dog (Speothos venaticus VU), giant river otter (Pteronura brasiliensis EN), Orinoco goose (Neochen jubata LR), and the harpy eagle (Harpia harpyja LR).
The delta region of the Orinoco River has been declared an internationally significant wetland (IWCP 1991), and is extremely sensitive to ecological damages. Although still in relatively moderate condition ecologically, the number of threats have increased in recent years. The density of people in the ecoregion is low, although many small villages of the native Waraos Amerindians live along the riverbanks. An exception is the city of Tucupita and its surrounding towns. This small city is located on the fringe of the largest outlier of this ecoregion, along the Manamo River.
Portions of this ecoregion are represented in a number of national parks, indigenous areas, and a biosphere reserve, which offer varying degrees of protection to these fragile wetlands. The Delta del Orinoco biosphere reserve (9¸21'N - 60¸56'W) was established in 1991 by WCMC & UNEP, and is the largest protected area within the delta region. This national biosphere reserve has been given IUCN category VI status and covers an area of 876,500 ha - only portions of which are represented by this wetlands ecoregion. The Delta del Orinoco National Park (9¸25'N - 61¸30'W), created in 1991 also offers protection with increasing levels of enforcement. This national park is given IUCN Category II status and covers an area of 331,000 ha. Turuïpano National Park (10¸34'N - 62¸43'W), established in 1991 protects portions of the northern extent of this ecoregion. This 72,600 ha park is given an IUCN Category II status. Finally, Mariusa National Park (9¸30'N - 61¸30'W ) along the northern coastal delta offers protection to some of the smaller outliers of this ecoregion, especially along the Maracao River. This 265,000 ha park has an IUCN category II status and was created in 1991.
Types and Severity of Threats
Threats within the ecoregion are moderate. On the larger scale, oil extraction and exploration, water diversion projects and dam construction upstream are of immediate concern (Dinerstein et al. 1995). Rapidly increasing human populations resulting in urban sprawl and increased settlement along the numerous rivers represent a growing concern. Population pressure is increasing threats to the delta fisheries, triggering overhunting in areas close to towns and villages and along rivers. The floodplains of large rivers are among the first to be altered by economic development and population growth because of their high productivity. However they are also among the most vulnerable, not only to direct change such as agriculture and settlement, but also to changes in the hydrology and water quality upstream (Lewis et al. 2000).
Portions of this wetland ecoregion have been severely altered as a consequence of a flood control program initiated in the 1960s when the Caño Mánamo was dammed. The reduction of seasonal flooding was done with the intention of making the land more suitable for cattle farming. There was, however, a number of severe and unanticipated side effects to this plan, which have dramatically affected wetlands in particular. The reduced water levels in the upper delta have caused the region to become tidal, and as a result the water levels now rise and fall by 1-2 m daily. This has also caused the salinity to increase dramatically, and in turn has impacted the flora and fauna, which are able to survive in both wetlands and the river and its tributaries.
The Raul Leoni dam on the Caroní River upstream is another dam that influences the area. This dam forms the biggest reservoir in Venezuela, Embalse de Guri, which retains the water and interrupts the vital seasonal flooding.
Oil exploration and extraction are potentially the greatest threat to the Orinoco Delta at large, affecting these fragile wetland, the surrounding forests, and the native Warao people who still inhabit the region (Tahbou pers. comm.).
Justification of Ecoregion Delineation
These wetlands of the Orinoco Delta region occur in an archipelago of patches extending from near the Araya and Paria Peninsula southwards to the Amacuro Delta. Linework for this ecoregion follows Huber & Alarcon (1988), who classify these patches as "Delta plains and coastal wetlands" and UNESCO (1980). From the Huber & Alarcon map (1988) we lumped the following subregions to derive our initial linework: "innundated woodland savannas (with palms) of the Upper Delta", "wetlands of the Middle Delta", "swamp wetlands of the Lower Delta". Further, we added additional patches of wetlands in the area from the UNESCO (1980) classification of "tropical tall flooded grasslands".
Conde, J. E. 2001. The Orinoco River Delta, Venezuela. Pages 61-70 in U. Seeliger, B. Kjerfve, editors. Ecological Studies 144: coastal marine ecosystems of Latin America. Springer-Verlag, New York.
Dinerstein, E., D. M. Olson, D. L. Graham, A. L. Webster, S. A. Primm, M. P. Bookbinder, and G. Ledec. 1995. A conservation assessment of the terrestrial ecoregions of Latin America and the Caribbean. DC.: WWF and The World Bank.
Lewis, W. M. Jr., S. K. Hamilton, M. A. Lasi, M. Rodríguez, and J. F. Saunders III. 2000. Ecological Determinism on the Orinoco Floodplain. BioScience, Vol. 50 No. 6. P. 681-692.
Huber, O., and C. Alarcon. 1988. Mapa de vegetación de Venezuela. 1:2,000,000.
Ministerio del Ambiente y de los Recursos Naturales Renovables, Caracas, Venezuela.
Tabou, A. Tucupita Expeditions C.A. On site contact. Personal comments 07/01/2001.
UNESCO. 1980. Vegetation map of South America. Map 1:5,000,000. Institut de la Carte Internationale de Tapis Vegetal. Toulouse, France.
Prepared by: Jan Schipper
Reviewed by: In process