Boundaries

This is a distinctive and large region of low-lying savanna grasslands covering the northwestern part of the Orinoco Basin of Venezuela and Colombia, from the Orinoco Delta upstream to the upper limits of the fringing floodplains (200 m elevation). The ecoregion includes the Orinoco’s western and northern tributary drainages between the Río Inirida in the south and the Morichal Largo drainage in the north. It is bounded to the east by the Guiana Shield contact zone, to the south by the Orinoco-Río Negro divide, and to the west and northwest by the piedmonts of the Eastern Andes.

Topography

This ecoregion has a varied landscape (< 200 m asl) that Rippstein et al. (2001) divides into three general categories: foothills, alluvial plains, and highlands, which can be further divided into well-drained high plains and floodable high plains.

Freshwater habitats

Although several types of rivers characterize the Orinoco system, this ecoregion primarily comprises the leftbank tributaries of the Orinoco, such as the Guaviere, Meta (mean annual discharge, 5600 m³/s), and Apure (mean annual discharge, 2300 m³/s). These whitewater rivers carry high sediment loads from the Andes and are actively braided upstream and meandering downstream, forming anastomosing patterns even further downstream. These whitewater rivers tend to have dynamic channels at their confluences. An example of this is the internal delta at the confluence of the Apure and Orinoco.

The ecoregion also contains low gradient rivers, streams, lakes, and marshes that are subject to extensive seasonal flooding. Large river channels have well developed fringing floodplains, such as the internal delta at the confluence of the Apure and Orinoco. While the Orinoco experiences annual river fluctuations between 10-15 m, floodplains in the llanos absorb any excess discharge, resulting in lower fluctuations along the tributaries.

Terrestrial habitats

Most of the ecoregion is composed of llanos, which is heterogeneous. Although many types have been described, they can be generally classified as non-flooded savannas and two types of seasonally flooded savannas. There are also a number of forest types in the llanos, including gallery forests that line rivers and streams.

Description of endemic fishes

The most recent taxonomic accounts list 60 endemic species. The distributions of a number of species center on the Venezuelan and Colombian llanos and only slightly (if at all) extend into the adjacent Andean piedmont and/or Orinoco Guiana Shield. Examples of near-endemic species that do extend into the piedmont and Guiana Shield include Curimata cerasina (Curimatidae), Orinocodoras eigenmanni (Doradidae), Hypostomus ammophilus (Loricariidae), and several species of Apistogramma (Cichlidae). 

At least six species of annual killifishes (Rivulidae) are known from the Venezuelan llanos and exhibit interesting distributional patterns (Thomerson and Taphorn 1992b).  Three species, Pterolebias hoignei, Llanolebias stellifer, and the strikingly beautiful saberfin killie (Terranatos dolichopterus) appear to be endemic to the Venezeulan llanos.  Endemic genera include Schultzites, Gymnotichthys, and Terranatos.

Other noteworthy fishes

The flooded llanos serve as important breeding and nursery grounds for a large number of Orinocan fishes. Its main rivers serve as migration routes for many large river species, including Brachyplatystoma species like the paraíba (B. filamentosum), Laulao catfish (Brachyplatystoma vaillantii), and dourada (B. rousseauxii).  Several fishes typical of the Orinoco llanos also occur or have close relatives in the western Amazon.  Examples include the Leptodoras acipenserinus-complex (Doradidae; Sabaj, unpubl. data) and Panaque species (Loricariidae; Schaefer and Stewart 1993).

Ecological phenomena

Large migrations of characiformes and pimelodid catfishes such as Pseudoplatystoma metaense and P. orinocoense occur in the ecoregion. There is also a unique assemblage of deep channel species, dominated by a high diversity of specialized gymnotiformes.

Justification for delineation

This ecoregion contains a unique composition of rich and diverse assemblages of large river lowland fauna affiliated with the extensive seasonally flooded floodplain of the Orinoco River. It forms a large part of the Orinoco ichthyographic province outlined by Ringuelet (1975) and Gery (1969).

Level of taxonomic exploration

Good throughout most of the river network, with poor coverage in the southernmost tributaries.