Boundaries

This ecoregion encompasses a transition zone between the Amazon lowlands and the Andes in the western portion of the Amazon, including drainages between the Apaporis drainage in the north and the Marañón drainage to the south.

Topography

The ecoregion lies along the foothills of the eastern Andes on a platform that slopes from roughly 800 m elevation in the west to 250 m in the east into the world’s largest humid tropical alluvial plain. The ecoregion’s northern boundary follows the swell of the Vaupés Arch, which forms the divide between the upper Amazon and upper Orinoco drainages (Lundberg et al. 1998). Sediments carried from high gradient streams in the Andes are volcanic in origin.

Freshwater habitats

One of the main rivers in the ecoregion is the Río Caquetá in Colombia (Rio Japurá in Brazil). Its middle course is lined with numerous cataracts, deep gorges, rocky outcrops, broad meanders, and floodplains. Like the Putumayo, it is a whitewater river that carries nutrient-rich suspended organic and inorganic sediments from the eastern slopes of the Andes. In contrast, clearwater and blackwater tributaries such as the Apaporis tend to originate from the lowlands or Guiana Shield and are nutrient-poor with few sediments. The Apaporis, in particular, contains notable waterfalls and rapids along the contact zone between the shield and the lowlands. The Napo is a whitewater river in its lower course, but like other rivers in the Andes, is a seasonal clearwater river in its upper reaches.

Some of the ecoregion’s rivers overflow their banks each year from heavy seasonal rainfall, leaving the surrounding forest under six to twelve meters of water for a majority of the year. This produces a dynamic environment for a large diversity of fishes. The meandering rivers produce a number of fluvial elements such as oxbow lakes, levees, meander swales, and point bars.

Terrestrial habitats

The moist forests that stretch across this landscape include terra firme forests with families such as Leguminosae, Lauraceae, Myristicaceae, Sapotaceae, and Moraceae. There are also large areas of várzea (seasonally flooded by whitewater rivers) and igapó (permanently or seasonally flooded by blackwater rivers) forests, as well as permanent swamp forests with buriti (Mauritia flexuosa) palms.

Description of endemic fishes

More than 100 species in 18 families are endemic to this ecoregion. Nearly half of the loracariids are endemic, including a large number of species in the genera Chaetostoma, Hypostomus, and Panaque. A number of genera are also endemic, including Nannoglanis and Horiomyzon (Heptapteridae); Tridens, Malacoglanis, and Stenolicmus (Trichomycteridae); Othonocheirodus (Characidae); and Heroina (Cichlidae).

Other noteworthy fishes

The marbled swamp eel (Synbranchus marmoratus) is an air breather and can reach one meter in length.

Ecological phenomena

The flooding cycle produces a heterogeneous environment that contributes to the high diversity of fishes.

Goliath catfish like the piraíba (Brachyplatystoma filamentosum) and Laulao catfish (B. vaillantii) undergo spawning migrations from the Amazon estuary as far west as this ecoregion. One of the most notable phenomena is the migration of the dourada (Brachyplatystoma rousseauxii), which migrates 4000-5000 km to spawn in the foothills of the Andes.

Justification for delineation

This ecoregion lies in the Amazonas ichthyographic province outlined in Ringuelet (1975), and more broadly within the Guyanan-Amazonian region (Gery 1969; Ringuelet 1975).

Level of taxonomic exploration

Fair