Location and General Description
The Gurupa várzea ecoregion is named for the large alluvial island, Ilha Grande de Gurupá that occurs in the mouth of the Amazon River. Extending from the mouth of the Tapajós River at the city of Santarém to the Xingu River, which drains into the mouth of the Amazon itself. It is distinct from the flanking várzea ecoregions in that a majority of the landscape is savanna rather than dense tropical forest. The Xingú, Jari, and Tapajós Rivers, which drain into this lowest section of the Amazon River, are blackwater rivers that carry little, if any, sediment. The ochre-colored Amazon River is considered a whitewater river because it carries suspended organic and inorganic sediment loosed from the Andes. The term várzea refers to land that is inundated by overflow from whitewater rivers .
The flooding in this region is influenced by both the seasonal inundation’s of rivers caused by rainfall in the drainage basins along the Amazon and its tributaries and by the daily tide that pushes a large volume of river discharge upriver and over the landscape. The daily flood level ranges from 4 to 7 m at the height of the seasonal flood pulse and from 2 to 3 m at the low season. This area is called the "region of the islands" because of the intricate labyrinth of sedimentary islands and channels resulting from constant fluvial action. Sediments settle out near the margins of the river, building levees upon which tropical humid forest grows to an average height of 25 m. Behind these, on clay soils, lie robust savannas (canarana) and open lakes, both of which flood (Pires, 1985 #659). The lakes swell and retract according to the flood cycle, sometimes covering tens of square kilometers. The substrate of the várzea is alluvial and fluvial recent Holocene sediments (less than 10,000 years old) loosed from the eastern slopes of the Andes Mountains and carried by the powerful current of these mighty rivers. The elevation along this region is no higher than 5 m. Average annual rainfall is less than 2,500 mm.
Both the geomorphology and biogeography in this region are influenced by very active fluvial dynamics. The deposition of nutrient-rich sediments renders the várzea much more fertile than the adjacent terra firme. The várzea forest performs critical ecological functions, such as capturing and rapidly cycling nutrients, hosting a great diversity of freshwater fish and aquatic mammals, stabilizing the flooded soils and landscapes, and perhaps providing a source of new taxa that colonize the surrounding terra firme . The diversity of vegetation types found on the várzea reflects the heterogeneity of the landscape resulting from constant erosion and deposition activities of the flood pulse .
Large grasses found on the flooded savanna include Echinochloa polystachya, E. spectabilis, Hymenachne amplexicaulis and H. donacifolia, Leersia hexandra, Paspalum platyaxix, Luziola spruceana, Panicum elephantipes, Paspalum fasciculatum, and various species of Oryza. Some of these grasses occur on abandoned river channels as well. Much sedge is found amongst the grasses including Scirpus cubensis, Cyperus luzulae, C. ferax, and Scleria geniculata. On the transition areas between forest and river, shrubs and small vines of the following species occur: Artemisia artemisiifolia, Ipomoea fistulosa, Polygonum punctatusm, Justicia obtusifolia, Alternanthera philoxeroides, Capironia fistulosa, Sesbania exasperata, Mimosa pigra, Montrichardia linifolia, Clamatis aculeata, Cassia reticulata, Phaseolus lineatus, Rhabdadenia macrostoma, and Clitonia triquetum. Large trees of the forest include Hura crepitans, Triplaris surinamensis, Calycophyllum spruceanum, Cedrela oderata, Pseudobombax munguba, Virola surinamensis, and Ceiba pentandra.
The humid tropical floodplain forests host a diversity of trees that produce fleshy fruit and are critical to the survival of fruit-eating species of fish, primates and bats that enter the forest understory during the flood. Some of these important trees include yellow mombim (Spondias mombim), palms such as buriti (Mauritia flexuosa) and açaí (Euterpe oleraceae), socoró (Mouriri ulei), and tarumã (Vitex cymosa). The várzea is critical habitat for reproductive and nursery grounds for fishes and many invertebrates.
This ecoregion hosts 148 mammal species, the largest of which include ocelots (Leopardus pardalis), margays (Leopardus wiedii), tapirs (Tapirus terrestris), capybaras (Hydrochaeris hydrochaeris), kinkajous (Potos flavus), and white-lipped peccaries (Tayassu pecari). Primates with a widespread distribution that occur here include spider monkeys (Ateles paniscus) and red howler monkeys (Alouatta seniculus). Mammals found only here or in few other Amazonian regions include primates such as tamarins (Saguinus midas), squirrel monkeys (Saimiri ustus), endangered titi monkeys (Callicebus moloch), marmosets (Callithrix argentatado), a number of rodents such as Coendou koopmani, Myoprocta acouchy, and arboreal rats (Echimys chrysurus and E. grandis), savanna foxes (Cerdocyon thous), and many bats.
In the Gurupa várzea, 558 bird species are reported, including many aquatics such as herons and egrets (Egretta and Ardea), ducks (Dendrocygna spp.), ibis (Cercibis spp., Theristicus spp.), and rosette spoonbills (Ajaia ajaia). Dark-winged trumpeters (Psophia viridis), eared doves (Zenaida auriculata), crimson topaz (Topaza pella), endemic scaled spinetails (Cranioleuca muelleri), and bare-eyed robins (Turdus nudigenis) are found here but are not common elsewhere.
These whitewater rivers, subjected to tidal floods, are rich in fish and turtles, including giant river turtles (Podocnemis expansa). Some of the largest fish are the pacu (Metynnis and Mylossoma), tambaqui (Colossoma macropomum), pirarucu (Arapaima gigas), sardinha (Triportheus angulatus), and the smaller carnivorous pirana (Serrasalmus spp.). Many beautiful aquarium fish come from these rivers and blackwater tributaries and lakes in this region, including the discus fish (Symphsodon aequifasciata sp.), hundreds of cichlids, such as in the genus Cichlasoma, assorted characins (family Anostomidae), tetras such as those in the genera Hemigrammus and Hyphessobrycon, and many catfish (families Aspredinidae, Corydoradinae, Doradidae, and Loricariidae).
The Amazon várzea has a long history of human occupation because of their high productivity and accessibility. Principle activities on the Gurupa várzea are subsistence agriculture, fishing, selective logging, and ranching. Much of the forest along these banks of the Lower Amazon have been cleared and the natural savannas altered to expand pasture area for cattle or water buffalo. The production systems of small holder farmers tend to be biologically diverse agroecosystems with a strong tree component, so land degradation does not generally occur where small-scale farmers live. Much of the forest that remains is managed or unmanaged secondary forest. A few urban centers such as Monte Alegre are located on the riverbank in this region, and urban sprawl has replaced some natural habitat. The major commercial timber species such as Virola surinamensis and Ceiba pentandra are all but depleted in this region.
Types and Severity of Threats
Habitat is threatened mostly by anthropogenic alteration. The flooded savannas are stressed by grazing cattle and water buffalo. The fertile soil of this inundated area has also attracted large-scale agricultural projects such as jute (in the past) and mechanized rice production today which degrade the natural habitat. Commercial logging and fisheries extraction poses a threat to populations of all species.
Justification of Ecoregion Delineation
This seasonally flooded ecoregion was delineated according to the presence of ancient crystalline arches which distinguish these várzea ecoregions (Daly and Prance 1989) based on basement rock composition. Linework is approximating the presence of two arches – the Gurupa arch to the east (near the Tapjós confluence), and the Monte Alegre Arch to the west (near the confluence of the Xingu rivers). The importance of these arches in distinguishing várzea ecoregions is discussed by da Silva (1989).
Daly, D. C., and J. D. Mitchell. 2000. Lowland vegetation of tropical South America. Pages 391-453 in D. L. Lentz, editor, Imperfect Balance: Landscape transformations in the Precolumbian Americas. New York: Columbia University Press.
Daly, D.C., and G.T. Prance. 1989. Brazilian Amazon. Pages 401-426 in D.G. Campbell, and H.D. Hammond, editors. Floristic inventory of tropical countries. New York Botanical Garden, Bronx, New York, USA.
Silva, J.M. C. 1998. Um método para o estabelecimento de áreas prioritárias para a conservação na Amazônia Legal. Report prepared for WWF-Brazil. 17 pp.
Pires, J. M., and G. T. Prance. 1985. The vegetation types of the Brazilian Amazon. Pages 109-145 in G. T. Prance and T. E. Lovejoy, editors, Key Environments: Amazonia. New York: Pergamon.
Prance, G. T. 1979. Notes on the vegetation of Amazonia III. The terminology of Amazonian forest types subject to inundation. Brittonia 31: 26-38.
Prepared by: Robin Sears
Reviewed by: In process