Amazon Forest

The majority of the 7 million km2 of the Amazon Forest is constituted by a dry land forest ("Floresta de Terra Firme"). This is a forest that is never flooded, spreaded across a great plain of up to 130-200 meters of altitude, up to the bottom of the mountains. This great plain corresponds to the sediments left by the lake "Belterra", which occupied the greatest part of the Amazon basin during the Miocene and the Pliocene, between 25 thousand and 1.8 million years ago. The silt and clay deposited in this old lake have been submitted to a gentle mass elevation movement, while the Andes rose and modern rivers began to excavate their beds. Thus the three kinds of Amazonian forests have appeared: the highland forests of the Andes, the dry land forests and the flooded river forests, both the last ones in the Brazilian Amazon.

The climatic fluctuations of the Pleistocene Age were a repeated succession of cold / dry - hot / humid - hot / dry climates. The last cold / dry phase dates from 18 thousand to 12 thousand years ago, when the Amazon climate was semi-arid, with an average temperature reduced by up to 5ºC. Following it, there was a return to the hot / humid climate, which reaches at the maximum around 7 thousand years ago. Since then, with many oscillations of a lesser scale, we live a relatively hot / dry climate.

Very important was the fact that during the semi-arid phase the great dry land forest was divided and fragmented by open vegetation formations, of the type of "cerrados", "caatingas" and "campinaranas" (open prairie-land which changes little by little into woodland), all better adapted to the dry climate. The forest survived in "refuges" situated in areas with higher soils and better water supply. In returning to the more humid climate, the forest expanded again, in detriment to the "cerrado" vegetation. Nowadays, the "cerrado" survives in its own "refuges", within the immensity of the "matas de terra firme". This floating process will undoubtedly repeat itself, unless man interferes in the situation.

The "matas de terra firme" has countless adaptations to the clay and podzol soils, poor in nutrients. The trees that form it are capable of stocking up with nitrates through nitrogen fixing bacterias, which are linked to their roots. Besides that, a great variety of fungi, also symbiont of the roots, called mycorrhizas, rapidly recycle the organic material before it is leached. The ground litter (formed by the debris vegetation that fall on the soil) is rapidly recycled by the fauna rich in insects, especially beetles, ants and termites. The insects constitute the majority of the animal biological mass in the dry land forest.

This forest, especially rich in epiphytal araceaes, is relatively poor in bromeliads and orchids, in comparison to the Atlantic Forest. Among these epiphytes are the myrmecophites, plants which have a symbiotic relation with ants. In the sub-woods of the forest stand out especially palm trees and lianas; large ferns are rare.

The macrofauna of the forest grounds is relatively poor. The frogs found there have various adaptations in order to guarantee the necessary water for the development of the tadpoles. The large mammals include tapirs, "catetos" (peccaries) and "queixadas" (wild boars), and among the ground birds of distinction are "mutuns" (curassow) and inambus. Close to the ground there are also many ant-eating birds ("papa-formigas") profiting from the enormous migrations of the army ants.

The great animal diversity is found in the tree tops, between 30 and 50 meters high, an environment of difficult access to the researcher. The bird fauna is rich there, with parrots, toucans and woodpeckers. Especially eye-catching are the "pavãozinho do pará" and the "cigana" (hoatzin). The predominant mammals living in the tree tops are marsupials, bats, rodents and monkeys. The primates have very differentiated niches. The "bugio" monkey is a daytime animal, and feeds preferably on leaves. The night monkey "Aotus" is the only nocturnal monkey. There are many species and sub-species of voracious insectivorous sagoins, differentiated by the colors and shape of their faces. Alongside the classic pollinators - bees, butterflies and birds - the Amazon forest monkeys also have an important role in the pollination. The birds, bats and frugivorous monkeys of the dry land forest have an important role on disseminating the fruits and seeds from the trees.

The species and sub-species of monkeys, sloths, squirrels and others are often separated by the large tributary rivers of the Amazon. The biogeographical units formed by these rivers basins explain, in part, the great diversity of the Amazon biota. Also relevant are the areas of the forest which served as a refuge to the various differentiated populations during past periods of arid climate mentioned above, when large areas of "cerrado" used to fragment the Amazon Forest. Nowadays it is the indiscriminate deforestation that has been fragmenting the dry land forest. Without the necessary precautions, whole faunistic provinces and former centers of formation of species risk being obliterated forever.

The flooded forests are within reach of annual floods of the Amazon River and its closest tributaries. The fluctuations of the water level can get to ten meters or more. From March to September large portions of the riverside forest are flooded. The plants and animals of the flooded Amazonian Forest live because of the several special adaptations for surviving the floods.

The Amazonian waters have different characteristics, as a result of the geology of its river basins. The rivers called white or clouded water rivers, like the Solimões and the Madeira, traverse lands rich in minerals and organic suspensions. The so-called black water rivers, such as the Negro, originating from sandy soils poor in minerals, are transparent and colored brown by the humid substances. There are also rivers of clear waters like the Tapajós, which originate in areas of the old continental shields, also poor in minerals and nutrients.

The woodlands bathed by the white waters are normally called "florestas de várzea" (meadow forests) and those bathed by the clear and black waters are called "igapó" forests. The "várzea" vegetation is much richer than the "igapó" vegetation, due to the fertility of the white waters and the alluvial soils carried by them. The same is noticed in the fauna of the two kinds of forests, especially in terms of aquatic life. The white water rivers are rich in fish, while the black water rivers are the "hunger rivers". The areas where both kinds of water mix, like for instance, the area near Manaus, are considered especially rich.

The flooded forest trees contain many morphologic and physiologic adaptations in order to survive partially submersed, for instance respiratory and buttress roots. The trees are poor in epiphytal plants and the sub-forest is practically inexistent. In its place there is a rich herbaceous flora, such as the "capim-mori", the "canarana" and the wild rice. During the flood season the grass stands out and forms real floating islands. Other floating plants, the "vitória-régia" (giant water lily) and the "aguapé" (aquatic plants), also follow the water level.

The flooded forest mammals - tapirs, capybaras and others - are all good swimmers. Even sloths can swim. The fauna of monkeys and other tree inhabiting mammals is generally poor, in comparison to the dry land fauna. However, many species of aquatic mammals can be found in the "várzea" rivers, like "botos" (fresh water dolphins), "peixe-boi" (cow fish), "ariranhas" (Brazilian otters) and others. The primates' fauna is minimal. The vegetarian cowfish and the predator dolphins are, however, very rare in the black and clear waters of the "igapós", poor in aquatic vegetation and fish.

In the relatively poor avifauna of the "igapó" forests, the aquatic birds are predominant: herons, "biguás" (cormorants), "jaçanãs", "mucurungos" and ducks.

The flooded forest waters are rich in aquatic reptiles. The turtles are important herbivores of the aquatic vegetation and widely hunt. The "real" turtle (Podocnemis expansa) is in danger of being extinguished; the "cabeçuda" (P. dumeriliana) and the "tracajá" (P. unifilis) are also well appreciated by the hunters. The Phrynops turtles are more often found in the rapids. Among the alligators, the "jacaretinga" (Palaeosuchus trigonatus), a genus with only one endemic species in the Amazon, is threatened by extinction. The "jacaré-açu" (Melanosuchus niger) is the most common in the area. Many authors attribute to the predator alligators an important role of "regulators" of the "várzeas". The large Amazonian boa ("jibóia") must also be mentioned.

In the Amazon, there are around ten thousand species of fish. Here we have only mentioned some species related to the flooded forest. These are the frugivorous fish which evolved in close co-evolution with the Amazonian trees and shrubs: the fruit fall in the water, are swallowed by the fish and the seeds, resistant to gastric enzymes, are transported far away. Many fish, especially of the large order of the Characinoidea, have specialized teeth for certain kinds of fruit. The "tambaqui" (Collosoma macropomum) is a specialist eater of the fruits of the Hevea spruceana. "Pacus" (a kind of snapper), of the genus Mylossoma, Myleus and Broco, are also important eaters of the fruit of the palm trees, "embaúbas" (trumpet trees) and other trees. The "piranheira" (piranha tree) is a plant preferred by some piranha species. The dispersal of plants by the "várzea" and "igapó" fish has an importance comparable to the classic dispersal of seeds by birds and mammals in dry land forests. The "tambaquis", "pacus" and "pirarucus" (Arapaima gigas) are the fish of most commercial importance in the Amazon. Nothing illustrates better the important ecologic role of the frugivorous fish. The "tambaqui" is widely sought by tourist fishermen.

The frugivorous fish is only one of the types of fish in the "várzeas", but their role is particularly important in the black and clear waters. Due to the excessive absence in these waters of phyto and zoo-plankton, the trees are the suppliers of most of the food. Even so, the Negro river fish are smaller than the ones of the same species in the Solimões river. The shoals are also smaller.

The fauna of insects is mainly related to the floating vegetation. The few species of termites and ants accompany the ascent and descent of the waters along the tree trunks. Several insect varieties live on the floating vegetation, whilst in the waters there are enormous populations of mosquitoes and other irritating diptera. The black water rivers are free of this affliction.

The flooded forests have several species of trees of economic use, besides hardwoods. The rubber tree, the "sorva", the "andiruba", the "macaranduba", the "buriti" and the "tiucum" produce rubber, food, oils, resins and fibers of economic importance. The "várzeas" are especially rich and productive. It is there that large indigenous concentrations used to be found, and where nowadays large agricultural and cattle-raising projects are developed.

Specific of the "igapós" of sandy soils and black waters are the "piranheiras" (Piranhea trifoliata), the "oeirana" (Alcornea castaniifolia), various species of Inga and Eugenia, the Copaifera martii ("copaiba") and the Leopoldinia. Some trees have a big resistance to the prolonged flood, such as the Myrciaria dubia, the Eugenia inundata ("araca de igapó") and, finally, the Salix humboldtiana, which survive many years of permanent submersion.

Many of the "várzea" species are under the threat of extinction, due to the rapid development of urban areas, the building of dams, the mercury pollution from the prospects etc. The non-regulated hunting and fishing in the "várzeas" have already put in risk the existence of several large sized aquatic vertebrates. The list of species in extinction is headed by "botos", cow fish, Brazilian otter, real turtle, "jacaretinga" and others. The fish under threat include the "pirarucu", the biggest fresh water fish in the world.

The high productivity of the "várzeas" has allowed the existence of a dense indigenous population at the time of the discovery. The banks of the large river have held many villages and thousands of inhabitants. The population density reached 14,6 inhabitants per km2. The riverside population used to cultivate maize and manioc in the rich alluvian soils, they collected wild rice and took advantage of the rich fishing. These indians had an organization of social classes and used slave work.

The black water rivers, on the contrary, were considered "hunger rivers" and historically have been little populated. However, due to an absence of irritant diptera, such as mosquitoes, "borrachudos" and "mutucas", the new settlers preferred to live on the banks of the black water rivers. For a short period of time, the capital of the region went to Barcelos, in the middle Negro River, but rapidly moved to Manaus, close to the "várzeas", rich in fish. We still have to consider that the fertile soils in the Amazon are the "várzea" soils, exactly where the large urban centers tend to be, together with a supply basis.

There is an ecological station which is entirely situated in the environment of the "igapós": the Federal Ecological Station of the Anavilhanas archipelago, in the lower Negro river. In the floods, the archipelago of hundreds of islands is practically submersed. The Station's research laboratory is on the floating houses which follow the level of the waters. Another station, Mamirauá, is located in the "várzea", close to Tefe. The National Institute for Research in the Amazon (INPA), in Manaus, and the Goeldi Museum, in Belém, maintain various reserves and research areas in the dry land forests. In Santarém there is a center for fish research.

by Vera Imperatriz Fonseca and Francis Dov Por