Orinoco Piedmont

The Orinoco Piedmont is the hinge between two worlds: the cold, boulder-strewn torrents falling off the Colombian Eastern Cordillera and the Venezuelan Mérida Andes, and the flat, grass-covered llanos below. FEOW maps it as ecoregion 306, a montane-freshwater transition that runs along the western and northern rim of the Orinoco basin from the Serranía de la Macarena north to the Cordillera de la Costa. Its rivers are the upper reaches of the Guaviare, Meta, Arauca and Apure — Andean whitewater, heavy with the sediment the young mountains shed. The fauna is a mix of hill-stream specialists and lowland Orinoco fishes meeting at the foot of the mountains, and it is poorly explored, fast-changing, and under pressure from oil, cattle and the plow.

Geography & hydrology

Ecoregion 306 traces the eastern slope of the Eastern Cordillera of the Colombian Andes and the Mérida Andes across into the Cordillera de la Costa, the coastal range of northern Venezuela. It sits between two neighbors that define it by contrast: the high, cold streams of the Orinoco High Andes (ecoregion 305) above, and the vast flat savannas of the Orinoco Llanos (ecoregion 307) below. Elevations span a steep range, from about 486 ft along the fringing floodplains where the rivers spill onto the llanos to over 2,656 ft on the Andean flank. The piedmont is the belt where that gradient breaks — where a stream stops behaving like a mountain torrent and starts behaving like a lowland river (FEOW Ecoregion 306; Petry & Hales).

The rivers here are the Andean headwaters of the western Orinoco's largest tributaries: the upper Guaviare in the south, then the upper Meta, the upper Arauca, and the upper Apure to the north. The Meta is the main Colombian tributary of the Orinoco, draining roughly 10,830 km2 along the Andean front, fed by a fan of piedmont rivers — the Casanare, Ariporo, Cusiana, Cravo Sur, Pauto, Túa and Upía among them — that all run off the cordillera within a few tens of kilometers of one another. The Casanare, to take one, rises in El Cocuy National Park at about 3,2247 ft and has dropped to roughly 279 ft by the time it meets the Ariporo on the savanna, a fall of more than three kilometers over its length (Zamudio et al., 2017; Urbano-Bonilla et al., 2018). Lago Valencia, the largest lake in the ecoregion, sits apart from all of this as an endorheic basin in the northern coastal range, with no outlet to the sea (FEOW Ecoregion 306).

Morphologically the piedmont rivers come in two forms. The upland trunk streams in the Apure, Meta and Guaviare systems are shallow and rocky, running fast over gravel, cobble and bedrock; the larger streams, lower down where the slope eases, carry heavy sediment loads and begin to braid and meander as they reach the llanos. The climate is strongly seasonal and, on the Colombian side, unimodal: rain peaks around June and July and bottoms out in January and February, so the rivers swing between a high-water and a low-water state on a single annual cycle. That flood pulse is the organizing rhythm of the system, connecting the piedmont to the floodplains downstream and cueing the migrations and spawning of its fishes (Zamudio et al., 2017; Rodríguez et al., 2007).

Water & habitat

These are whitewater rivers in the limnological sense Harald Sioli gave the term — not white-foaming rapids, but waters carrying a heavy load of dissolved and suspended solids produced by rapid weathering in a young, still-rising mountain range. The Andes shed sediment quickly, and the piedmont is where the rivers pick most of it up. Compared with the blackwaters draining the ancient, leached Guayana Shield on the far side of the Orinoco, the western whitewaters of the Apure and Meta run turbid, near-neutral to slightly alkaline, and comparatively high in conductivity and nutrients. Water type is one of the strongest predictors of which fishes live where in the Orinoco: whitewater assemblages (higher pH, higher conductivity, low transparency) are distinct from blackwater and clearwater ones, and in many cases it is the water chemistry itself, not a physical barrier, that limits how species spread (Rodríguez et al., 2007).

Up close the piedmont is more of a mosaic than a single water type. Field surveys in the Casanare drainage sampled, within a few kilometers of each other, fast turbid piedmont rivers, clear gravel-bed creeks, tea-colored blackwater caños draining swampy patches, piedmont lagoons and morichal palm swamps — each with its own chemistry and its own fishes. The sediment-laden trunk rivers grade into clearer, rockier tributaries as you climb, and the substrate shifts from the sand and silt of the lowland reaches to the gravel, cobble and exposed bedrock of the upland streams. That hard, oxygen-rich, fast water is the habitat the hill-stream specialists need — the suckermouth Astroblepus catfishes clinging to rock, the Chaetostoma 'bristlenose' loricariids grazing biofilm off stones (Zamudio et al., 2017; Urbano-Bonilla et al., 2018).

Everything is governed by the wet-and-dry seasonal regime. At high water the rivers swell, turn browner with suspended load, and push out across their floodplains; in the dry months they shrink back to clear, shallow, well-defined channels, and the smaller piedmont streams can nearly stop flowing. The seasonal swing drives the long-distance migrations that tie the piedmont to the lowlands — the coporo (Prochilodus mariae) and other migratory characiforms run up out of the western llanos toward the Andean-piedmont rivers to spawn on the rising water, then drop back down to feed in the rain-swollen lowlands. A river that runs dry too early, or whose flood is blunted, breaks that cycle (Rodríguez et al., 2007; Zamudio et al., 2017).

The cichlid fauna

The cichlids of the piedmont are mostly lowland Orinoco fishes reaching the foot of the mountains, sorted by the same water-chemistry and habitat filters that organize the rest of the fauna. The dwarf cichlids are the signature group. Apistogramma macmasteri and the closely related A. hoignei belong to the middle Orinoco and Meta drainages, living in small streams and creeks that run through savanna and gallery forest — soft sandy bottoms, leaf litter and tangles of submerged root. A. minima rounds out the roster's Apistogramma, and the blue-finned dwarf Mikrogeophagus ramirezi — the ram, one of the most familiar aquarium cichlids in the world — turns up in the quiet, warm, shallow margins of these same piedmont and savanna waters; both it and Apistogramma have been recorded directly in piedmont drainages such as the Casanare (Cichlid News, 2014; Zamudio et al., 2017).

Acaras and their relatives carry several of the regional names. Andinoacara metae and Aequidens metae are named for the Meta itself; Bujurquina mariae is a mouthbrooding acara of the western Orinoco; and Acaronia vultuosa is the big-eyed, ambush-feeding 'owl-eye' acara of the basin. Bigger predators are here too: Astronotus mikoljii, the Orinoco oscar described as a species distinct from the Amazonian A. ocellatus only in 2022, ranges through the Orinoco and Gulf of Paria basins; Cichlasoma orinocense is the basin's representative of that genus; and the pike cichlid Crenicichla sveni hunts from cover in the Meta and neighboring drainages. Heros severus, the severum, and the flag cichlid Mesonauta insignis fill the deeper, slower, more vegetated reaches (Lasso-Alcalá et al., 2022; Zamudio et al., 2017).

The eartheaters and their kin make up the rest. Satanoperca daemon and Biotodoma wavrini are sand-sifting geophagines that mouth through soft substrate for invertebrates, and the tiny Biotoecus dicentrarchus is a secretive dwarf of the same lineage. The piedmont is poorly explored, by FEOW's own assessment, and that shows in the fish lists: surveys of single drainages keep multiplying the known fauna severalfold and turning up undescribed species, including an undescribed Andinoacara from the Casanare. The ecoregion's nine strict endemics are catfishes, characins, a knifefish and a silverside rather than cichlids — the cichlids here are shared with the wider Orinoco, the piedmont being one end of their range rather than a cradle of their own (FEOW Ecoregion 306; Zamudio et al., 2017; Urbano-Bonilla et al., 2018).

Conservation

The piedmont carries some of the heaviest human pressure in the whole Orinoco basin, because the belt at the foot of the Andes is where the people, the roads and the industry are. Deforestation has the most wide-reaching effect on the rivers. Stripping the Andean piedmont of forest drives erosion and siltation, exposes streams to more sun, dries up the smaller ones, and rewrites the in-stream habitat and food webs; agricultural clearing along the Andean front has loaded the headwater rivers with sediment and lengthened and sharpened the dry season, which lets fishers strip the shrinking streams nearly bare (Rodríguez et al., 2007; Lasso et al., 2016).

Oil is the other defining pressure. The llanos foothills are an active petroleum province, and the exploration and extraction of crude is accelerating across exactly the piedmont and savanna belt these fishes occupy; water pollution and the effects of deforestation in the llanos and foothills, alongside the spread of petroleum operations, are flagged as principal threats to the Orinoco's aquatic biodiversity (Lasso et al., 2016). Cattle ranching, rice and oil-palm plantations push into the same ground — large-scale agriculture has been encroaching on the wetlands of Casanare, one of the region's most important floodplain systems — and the land conversion and habitat loss that come with it fall hardest on the piedmont and savanna, where most of the basin's threatened fishes live (Zamudio et al., 2017; World Bank, 2019).

Dams and the blunting of the flood pulse close the loop. Highland rivers cut by dams, dykes and irrigation canals have lost large parts of their migratory prochilodontid runs, because barriers and altered flow break the rising-water spawning cue these fishes depend on; reservoirs trap the sediment and nutrients that whitewater systems run on, and in places mining has put mercury into the food chain. The predatory saltador, once common in the Andean-piedmont rivers, was nearly wiped out by the combination of overfishing, deforestation, siltation and dams — a warning of how quickly a piedmont fish can vanish when several of these pressures land at once. The Meta drainage already lists fifteen or more species as endangered, and the piedmont's poor exploration means some are likely being lost before they are even recorded (Rodríguez et al., 2007; Zamudio et al., 2017; Lasso et al., 2016).

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Acaronia · 1

Aequidens · 1

Apistogramma · 3

Astronotus · 1

Biotodoma · 1

Biotoecus · 1

Bujurquina · 1

Cichlasoma · 1

Heros · 1

Mesonauta · 1

Mikrogeophagus · 1

Satanoperca · 1

Saxatilia · 1