Location and General Description
This ecoregion in Brazil comprises portions of the low, seasonally inundated river basins of the central and lower Amazon, much of the length of the Madeira River Basin, and the mouth of the Purus River where it joins the Solimões (Amazon), as well as several smaller tributaries to these. An isolated patch occurs on the border of Brazil and Bolivia along the Mamoré River. The western limit of this ecoregion lies at an extensive area of várzea at the confluence of the Purus and Amazon Rivers about 300 km southwest of Manaus. The eastern limit is at the mouth of the Tapajós River that feeds into the Amazon.
The large urban centers of Manaus, Itacoatiara, Coari, and Óbidos lie in or near the várzea in this ecoregion. These flooded forests are called várzea because they are seasonally inundated by overflow from whitewater rivers . The Amazon, Purus, and Madeira Rivers are designated whitewater rivers because they carry a great deal of suspended organic and inorganic sediment from the Andes and have an ochre color. The elevation along this region ranges from 80 m on the Madeira to 15 m at the confluence of the Tapajós with the Amazon River. Average annual rainfall varies from 2,500 mm on some parts of the Madeira River to less than 2,000 mm in the easternmost sections of this ecoregion.
The substrate of the várzea is composed of alluvial and fluvial recent Holocene sediments (less than10,000 years old) loosed from the eastern slopes of the Andes Mountains and carried by the powerful current of these mighty rivers. The rivers swell and exceed their banks each year from heavy seasonal rainfall in the watershed area. The river floods the surrounding forest under 6 to 12 m for periods up to eight months each year. The nutrient-rich sediments deposited on the landscape constantly renew the mineral richness of the várzea soils rendering it much more fertile than the adjacent terra firme.
Both the geomorphology and biogeography in this region are influenced by very active fluvial dynamics. Over time (decades to centuries) the river course meanders across the floodplain making the várzea an ever-changing and heterogeneous landscape with a diversity of fluvial elements such as oxbow lakes, levees, meander swails, and point bars. The diversity of vegetation types found on the várzea reflects this landscape heterogeneity . Unlike the Upper Amazon Basin where the boundary between the várzea and terra firme is relatively indistinct, on the central Amazon floodplain the ecological difference between these forest types is dramatic because of distinct soil quality, forest structure, and floristics of non-flooded versus flooded land. On the várzea four vegetation types are defined and are delimited by the present and historical influence of the flood cycle. Three are on floodplain meanders, all resulting from the instability of the river courses: (1) sequences of successional vegetation, (2) forest mosaics, and (3) aquatic vegetation in poorly drained areas. The fourth type is permanent swamp vegetation in flooded river basins . 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 flooded soils and landscapes, and perhaps providing a source of new taxa that colonize the surrounding terra firme .
Although it comprises a mosaic of vegetation types, the central Amazon várzea is mostly characterized by seasonally indundated evergreen moist tropical forest. Because of the landscape heterogeneity caused by river dynamics, these forests are found in numerous seral stages and with topographically or edaphically distinct floristics. The várzea forests tend to have fewer tree species than the terra firme forest. They host some species restricted to wetland areas, such as Virola surinamensis, Calycophyllum spruceanum, and Euterpe oleracea, although many tree species are also found in terra firme forest. The trees are generally shorter in the várzea, with a canopy reaching to 25 m. Some palms are present and a there is often a dense understory of large-leaved herbaceous plants in the Zingiberaceae, Maranthaceae, and Heliconiaceae. These várzea host patches of grassland locally called canarana (Pires and Prance, 1986) from the mouth of the Rio Negro down and along the Madeira River.
The topography of these várzeas is such that the land is highest on the river margin, where the deposition of sediment has formed levees. Behind these levees, the land slopes down toward the interior forest to a point at which no flooding occurs. Behind these levees the canarana exist with open lakes that swell and retract according to the flood cycle and host large grasses such as species in the genera Echinochloa and Hymenachne. The youngest vegetation, lying along the river margin, is dominated by tall grasses such as Gynerium sagittatum, Paspalum repens, and Echinocloa polystachya. On one inventory transect below Manaus the low-lying shrub community included Coccoloba ovata, Eugenia inundata, Ruprechtia ternifolia, and Symmeria paniculata. Two meters higher in elevation, early succession trees were found such as Acosmium nitens, Buchenavia macrophylla, Cecropia latiloba, Crateva benthamii, Ficus anthelminthica, Machaerium leiophyllum, Macrolobium angustifolium, Piranhea trifoliata, Pseudobombax munguba, and Tabebuia barbata. The tree community on highest ground, but still flooded, comprised Calycophyllum spruceanum, Ceiba pentandra, Couroupita guianensis, Hura crepitans, Pirahnea trifoliata, Virola surinamensis, Genipa americana, Hevea brasilienses, Lecointea amazonica, Sterculia elata, and Rheedia brasiliensis. Economically important palms that are common on the floodplain are present such as Astrocaryum jauari, Mauritia flexuosa, and Euterpe oleraceae.
The humid tropical floodplain forests host a diversity of trees that produce fleshy fruit that are critical to the survival of fruit-eating fish that enter the forest understory during the flood. Some of these important trees include yellow mombim (Spondias mombim), the palms mentioned above, socoró (Mouriri ulei), and tarumã (Vitex cymosa). The várzea is critical habitat for reproductive and nursery grounds for fishes and many invertebrates. The central Amazon flooded forests host 200 mammal species including jaguars (Panthera onca), ocelots (Leopardus pardalis), tapirs (Tapirus terrestris), capybaras (Hydrochaeris hydrochaeris), kinkajous (Potos flavus), and white-lipped peccaries (Tayassu pecari). Primates that live in young floodplain forests and are widespread include, among others, various species of spider monkeys (Ateles spp.), red howler monkeys (Alouatta seniculus), and squirrel monkeys (Saimiri sciureuas, S. ustus and S. midas). Mammals found only here or in few other Amazonian regions include three species of endemic titi monkeys (Callicebus hoffmannsi, C. dubius, C. cinerascens), endemic marmosets (Callithrix humeralifer), locally endemic white-faced sakis (Pithecia albicans), a number of rodents such as South American field mice (Bolomys lasiurus), and bats Saccopteryx gymnura.
Large reptiles in the area include black caimans (Melanosuchus niger), spectacled caimans (Caiman crocodilus), and anacondas (Eunectes murinus). Aquatic mammals include Amazon river dolphins (Inia geoffrensis, pink, and Sotalia fluviatilis, grey) and endangered manatees (Trichechus inunguis).
The region hosts 681 reported bird species. Many aquatic birds are here, such as herons and egrets (Egretta, Ardea), ducks (Dendrocygna), ibis (Cercibis, Theristicus), and the rosette spoonbill (Ajaia ajaia). Many birds that are found only here or in few other places include ash-throated crakes (Porzana albicollis), plain-breasted ground-doves (Columbina minuta), red-shouldered macaws (Ara nobilis), sun parakeets (Aratinga solstitialis), green-rumped parrotlets (Forpus passerinus), scaled ground-cookoos (Neomorphus squamiger), and stygian owls (Asio stygius).
Amazon várzeas have a long history of human occupation because of their high productivity and accessibility. Principal activities in this region are subsistence agriculture, fishing, and selective logging. The production systems of smallholder 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 várzea forest is intact in this region, although much of it is managed or unmanaged secondary forest. Nevertheless, there is considerable degraded deforested habitat, particularly where cattle are grazed. Few protected areas are located in this ecoregion. The Lago Aiapuá State Reserve (6,100 km2) lies at the edge of the vast expanse of várzea near the mouth of the Purus River.
Types and Severity of Threats
Contamination of the rivers and fish by gold mining activities, and heavy sedimentation resulting from forest destruction, such as logging, threaten this transitional terrestrial/aquatic habitat and the people, plants, and animals that live there. Ranchers clear and burn vast patches of floodplain forests to encourage native grasses for livestock grazing, destroying important habitat and sources of food for fish and both terrestrial and aquatic mammals.
Justification of Ecoregion Delineation
This várzea is situated along the middle and eastern Solimões (Amazon) River, from near the confluence of the Tapajós River (Monte Alegre Arc) extending westward to the Purus Arc (Daly and Prance 1989, da Silva 1998) near the confluence of the Purus River, and also extends up the Madeira River as far south as the Beni savannas of Bolivia. The extend of these inundated forests and northern and southern linework follow the IBGE (1993) classification of "alluvial vegetation" and were modifies by expert opinion (da Silva 1998). Portions extending into Bolivia were mapped according to Ribera et al. (1994).
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Prepared by: Robin Sears
Reviewed by: In process