Boundaries

This area covers the lowland and highland areas of the Guiana Shield drained by tributaries of the Orinoco. It is limited in the south by the drainage divide between the Upper Orinoco and the Río Negro basin excluding the western half of the Casiquiare Canal or the Río Negro. It is bounded to the east by the drainage divide of the Río Branco basin and to the north by the divide of the Cuyuni drainage (Essequibo basin).  The western limit is the main stem of the Río Orinoco.

Topography

The landscape includes forested lowlands, rounded hills, undulating high plains, and tepuis with peaks that extend to 2800 m. These are the remnants of an ancient sandstone tableland overlying the even older granitic basement of the Guiana Shield.

Freshwater habitats

The ecoregion contains mostly clear and blackwater streams, morichales, and rivers, many with rapids, cataracts, and waterfalls; some lakes, flooded forests, and savannas; and inselbergs and tepuis with isolated wet areas. Angel Falls on the Río Churún, a tributary of the Caroní, is the world’s highest waterfall with a total drop of 979 m. Headwater streams, particularly in Bolivar State, Venezuela, are threatened by small-scale hydraulic mining, which increases erosion and suspended sediments, alters substrates and flow patterns, and releases toxic mercury. The Orinoco basin is linked to the Amazon basin by the Río Casiquiare, which is a distributary of the Orinoco that flows southwest toward the Negro during high water.

Terrestrial habitats

This ecoregion includes moist forests, open savannas, flooded forests, ancient uplands, and highlands. Vegetation on tepuis are distinct from the surrounding moist forests and contain high floristic and ecological diversity.

Description of endemic fishes

The number of endemics in this ecoregion is relatively low with less than 10% endemism at the species level. Of the 49 endemics, more than 40% are loricariids and nearly 20% are characids. The ecoregion, however, contains some highly distinctive endemics and near-endemics: Neblinichthys roraima (Loricariidae), the monotypic Acanthobunocephalus nicoi (Aspredinidae), and Ammoglanis pulex (Trichomycteridae) (Ferraris et al. 1986; Friel 1995; Provenzano et al. 1995; Ferraris 1996; de Pinna and Winemiller 2000).  Ammoglanis pulex is a miniature catfish that reaches sexual maturity at about 10 mm SL, ranking it among the smallest (and probably among the lightest) of all vertebrates (de Pinna and Winemiller 2000).  Bryconops colanegra (Characidae) is endemic to the Río Caroni (Chernoff & Machado-Allison 1999); Melanocharacidium compressus (Characidae) is endemic to the Upper Orinoco (Buckup 1993).

Other noteworthy fishes

The distributions of many species extend among Orinoco tributaries that drain the Guiana Shield and occasionally the Orinoco itself; but, the distributions do not extend significantly into the neighboring llanos region bordering the left bank of the Orinoco in Venezuela and Colombia.  Examples include Melanocharacidium melanopteron (Characidae); Pseudolithoxus anthrax, P. dumus and P. tigris (Loricariidae); and Geophagus grammepareius and G. taeniopareius (Cichlidae) (Kullander et al. 1992; Buckup 1993; Armbruster and Provenzano 2000).  Another example is the brilliantly colored Aphyocharax colifax (Characidae), which occurs in the Caura and Caroni Rivers and is apparently replaced by A. alburnus in the rest of the Orinoco basin (Taphorn and Thomerson 1986). 

Some species or higher groups may occur or have close relatives in parts of the Guiana Shield drained by other river systems, such as the Río Negro (Amazonas drainage) [ecoregion 314] and the Essequibo River [ecoregion 310].  For instance, the distributions of many fishes span the upper Orinoco, Orinoco tributaries draining the Guiana Shield, the Río Negro and the Río Casiquiare, which connects the Orinoco and Negro basins.  Examples include the anostomid Leporinus brunneus, ctenoluciid Boulengerella lateristriga, characids Leptocharacidium omospilus, Ammocryptocharax minutus (the smallest species of its genus), Heterocharax leptogrammus and Hydrolycus wallacei, doradid Anduzedoras oxyrhynchus, and loricariids Acestridium martini and possibly A. dichromum.  Another species known from the upper Orinoco and Río Negro is the morphologically and ecologically unusual Glanapteryx anguilla, a rarely collected eel-like catfish in the family Trichomycteridae (Nico and de Pinna 1996).  A second distribution pattern involves both the upper Caroni (Río Orinoco drainage) and tributaries of the Essequibo River draining the eastern Guiana Shield (e.g., Cuyuní–Mazaruni, Potaro).  Examples of this pattern include the characid Ammocryptocharax vintonae, trichomycterid Trichomycterus guianense, and rivulid Rivulus gransabanae (Lasso et al. 1990; Lasso et al. 1992; Buckup 1993).  Distributions of several species (e.g., doradids Leptodoras hasemani and Nemadoras leporhinus) incorporate all three of the aforementioned ecoregions: Orinoco Guiana Shield [308], Essequibo [310], and Negro [314].  Finally, the low-lying floodplains along the right bank of the Río Orinoco are inhabited by a number of species otherwise typical of the opposite bank in the adjacent llanos ecoregion [307].

Ecological phenomena

Pimelodid catfish like the piraíba or kumakuma (Brachyplatystoma filamentosum), Laulao catfish (B. vaillantii), dourada (B. rousseauxii), and jaú or gilded catfish (Zungaro zungaro) undergo large spawning migrations from the Orinoco estuary into tributaries and headwaters in this ecoregion and the llanos [307].

Justification for delineation

This ecoregion lies within the Orinoco ichthyographic province outlined in Ringuelet (1975) and Gery (1969) and includes a unique assemblage of western slopes of the Guiana Shield draining into the Orinoco basin, as well as many endemic taxa of significant zoogeographic importance.

Level of taxonomic exploration

The level of taxonomic exploration is fair to poor in several headwaters. There is a high probability of discovery of many endemic forms.