Natural
The rainforest likely formed during the
Eocene era. It appeared following a global reduction of tropical temperatures when the Atlantic Ocean had widened sufficiently to provide a warm, moist climate to the Amazon basin. The rain forest has been in existence for at least 55 million years, and most of the region remained free of
savanna-type
biomes at least until the
current ice age, when the climate was drier and savanna more widespread.
[2][3]
Following the
Cretaceous–Paleogene extinction event, the extinction of the
dinosaurs and the wetter climate may have allowed the tropical rainforest to spread out across the continent. From 65–34
Mya, the rainforest extended as far south as
45°. Climate fluctuations during the last 34 million years have allowed savanna regions to expand into the tropics. During the
Oligocene, for example, the rainforest spanned a relatively narrow band that lay mostly above latitude
15°N. It expanded again during the
Middle Miocene, then retracted to a mostly inland formation at the
last glacial maximum.
[4] However, the rainforest still managed to thrive during these glacial periods, allowing for the survival and evolution of a broad diversity of species.
[5]
Aerial view of the Amazon rainforest.
During the
mid-Eocene, it is believed that the drainage basin of the Amazon was split along the middle of the continent by the
Purus Arch. Water on the eastern side flowed toward the Atlantic, while to the west water flowed toward the
Pacific across the
Amazonas Basin. As the
Andes Mountains rose, however, a large basin was created that enclosed a lake; now known as the
Solimões Basin. Within the last 5–10 million years, this accumulating water broke through the Purus Arch, joining the easterly flow toward the Atlantic.
[6][7]
There is evidence that there have been significant changes in Amazon rainforest vegetation over the last 21,000 years through the
Last Glacial Maximum (LGM) and subsequent deglaciation. Analyses of sediment deposits from Amazon basin paleolakes and from the Amazon Fan indicate that rainfall in the basin during the LGM was lower than for the present, and this was almost certainly associated with reduced moist tropical vegetation cover in the basin.
[8] There is debate, however, over how extensive this reduction was. Some scientists argue that the rainforest was reduced to small, isolated
refugiaseparated by open forest and grassland;
[9] other scientists argue that the rainforest remained largely intact but extended less far to the north, south, and east than is seen today.
[This debate has proved difficult to resolve because the practical limitations of working in the rainforest mean that data sampling is biased away from the center of the Amazon basin, and both explanations are reasonably well supported by the available data.
Human activity
Based on
archaeological evidence from an excavation at
Caverna da Pedra Pintada, human inhabitants first settled in the Amazon region at least 11,200 years ago.
[Subsequent development led to late-prehistoric settlements along the periphery of the forest by AD 1250, which induced alterations in the forest cover.
[14]
Geoglyphs on deforested land in the Amazon rainforest, Acre.
For a long time, it was thought that the Amazon rainforest was sparsely populated, as it was impossible to sustain a large population through
agriculture given the poor soil. Archeologist
Betty Meggers was a prominent proponent of this idea, as described in her book
Amazonia: Man and Culture in a Counterfeit Paradise. She claimed that a population density of 0.2 inhabitants per square kilometre (0.52 /sq mi) is the maximum that can be sustained in the rain forest through hunting, with agriculture needed to host a larger population.
[15]However, recent
anthropological findings have suggested that the region was actually densely populated. Some 5 million people may have lived in the Amazon region in AD 1500, divided between dense coastal settlements, such as that at
Marajó, and inland dwellers.
[16]By 1900 the population had fallen to 1 million and by the early 1980s it was less than 200,000.
The first European to travel the length of the
Amazon River was
Francisco de Orellana in 1542. The BBC's
Unnatural Historiespresents evidence that Orellana, rather than exaggerating his claims as previously thought, was correct in his observations that a complex civilization was flourishing along the Amazon in the 1540s. It is believed that the civilization was later devastated by the spread of diseases from Europe, such as
smallpox.
Left - nutrient-poor soil; right - terra preta
Since the 1970s, numerous
geoglyphs have been discovered on deforested land dating between AD 0–1250, furthering claims about
Pre-Columbian civilizations.
[Alceu Ranzi, a Brazilian geographer, is accredited with first discovering the geoglyphs whilst flying over
Acre.
[The BBC's
Unnatural Histories presented evidence that the Amazon rainforest, rather than being a pristine
wilderness, has been shaped by man for at least 11,000 years through practices such as
forest gardening and
terra preta.
Terra preta (black earth), which is distributed over large areas in the Amazon forest, is now widely accepted as a product of indigenous soil management. The development of this fertile soil allowed agriculture and
silviculture in the previously hostile environment; meaning that large portions of the Amazon rainforest are probably the result of centuries of human management, rather than naturally occurring as has previously been supposed.In the region of the
Xingu tribe, remains of some of these large settlements in the middle of the Amazon forest were found in 2003 by Michael Heckenberger and colleagues of the
University of Florida. Among those were evidence of roads, bridges and large plazas.
Biodiversity
Deforestation in the Amazon Rainforest threatens many species of tree frogs, which are very sensitive to environmental changes (pictured:
Giant leaf frog)
Wet tropical forests are the most species-rich
biome, and tropical forests in the Americas are consistently more species rich than the wet forests in Africa and Asia.
[24]As the largest tract of tropical rainforest in the Americas, the Amazonian rainforests have unparalleled
biodiversity. One in ten known species in the world lives in the Amazon Rainforest.This constitutes the largest collection of living plants and animal species in the world.
The region is home to about 2.5 million
insect species,tens of thousands of plants, and some 2,000 birds and
mammals. To date, at least 40,000 plant species, 2,200
fishes,1,294 birds, 427 mammals, 428 amphibians, and 378 reptiles have been scientifically classified in the region.
[28]One in five of all the bird species in the world live in the rainforests of the Amazon, and one in five of the fish species live in Amazonian rivers and streams. Scientists have described between 96,660 and 128,843
invertebratespecies in Brazil alone.
[29]
The biodiversity of plant species is the highest on Earth with some experts estimating that one square kilometer (247 acres) may contain more than a thousand types of trees and thousands of species of other higher plants. According to a 2001 study, a quarter square kilometer (62 acres) of Ecuadorian rainforest supports more than 1,100 tree species.
One square kilometer (247 acres) of Amazon rainforest can contain about 90,790 tonnes of living plants. The average plant biomass is estimated at 356 ± 47 tonnes per hectare To date, an estimated 438,000 species of plants of economic and social interest have been registered in the region with many more remaining to be discovered or catalogued.
The green leaf area of plants and trees in the rainforest varies by about 25% as a result of seasonal changes. Leaves expand during the dry season when sunlight is at a maximum, then undergo abscission in the cloudy wet season. These changes provide a balance of carbon between photosynthesis and respiration.
Deforestation
Deforestation is the conversion of forested areas to non-forested areas. The main sources of deforestation in the Amazon are human settlement and development of the land. Prior to the early 1960s, access to the forest's interior was highly restricted, and the forest remained basically intact.Farms established during the 1960s were based on crop cultivation and the
slash and burn method. However, the colonists were unable to manage their fields and the crops because of the loss of
soil fertility and weed invasion.The soils in the Amazon are productive for just a short period of time, so farmers are constantly moving to new areas and clearing more land.These farming practices led to deforestation and caused extensive environmental damage Deforestation is considerable, and areas cleared of forest are visible to the naked eye from outer space.
Between 1991 and 2000, the total area of forest lost in the Amazon rose from 415,000 to 587,000 square kilometres (160,000 to 227,000 sq mi), with most of the lost forest becoming pasture for cattle.Seventy percent of formerly forested land in the Amazon, and 91% of land deforested since 1970, is used for livestock
pasture. In addition, Brazil is currently the second-largest global producer of
soybeans after the United States. The needs of soy farmers have been used to validate many of the controversial transportation projects that are currently developing in the Amazon. The first two highways successfully opened up the rain forest and led to increased settlement and deforestation. The mean annual deforestation rate from 2000 to 2005 (22,392 km
2 or 8,646 sq mi per year) was 18% higher than in the previous five years (19,018 km
2 or 7,343 sq mi per year).Deforestation has declined significantly in the Brazilian Amazon since 2004.
NASA satellite observation of deforestation in the Mato Grosso state of Brazil. The transformation from forest to farm is evident by the paler square shaped areas under development.
Fires and Deforestation in the state of
Rondônia.
One consequence of forest clearing in the Amazon: thick smoke that hangs over the forest.
Conservation and climate change
Environmentalists are concerned about loss of biodiversity that will result from destruction of the forest, and also about the release of the
carboncontained within the vegetation, which could accelerate
global warming. Amazonian evergreen forests account for about 10% of the world's terrestrial primary productivity and 10% of the carbon stores in ecosystems
[45]—of the order of 1.1 × 10
11 metric tonnes of carbon.
[46]Amazonian forests are estimated to have accumulated 0.62 ± 0.37 tons of carbon per hectare per year between 1975 and 1996.
[46]
One
computer model of future
climate change caused by
greenhouse gasemissions shows that the Amazon rainforest could become unsustainable under conditions of severely reduced rainfall and increased temperatures, leading to an almost complete loss of rainforest cover in the basin by 2100.
[47][48] However, simulations of Amazon basin climate change across many different models are not consistent in their estimation of any rainfall response, ranging from weak increases to strong decreases.
[49] The result indicates that the rainforest could be threatened though the 21st century by climate change in addition to deforestation.
In 1989, environmentalist C.M. Peters and two colleagues stated there is economic as well as biological incentive to protecting the rainforest. One hectare in the
Peruvian Amazon has been calculated to have a value of $6820 if intact forest is sustainably harvested for fruits, latex, and timber; $1000 if clear-cut for commercial timber (not sustainably harvested); or $148 if used as cattle pasture.
[50]
As indigenous territories continue to be destroyed by deforestation and
ecocide, such as in the
Peruvian Amazon[51] indigenous peoples' rainforest communities continue to disappear, while others, like the
Urarinacontinue to struggle to fight for their cultural survival and the fate of their forested territories. Meanwhile, the relationship between non-human primates in the subsistence and symbolism of indigenous lowland South American peoples has gained increased attention, as has ethno-biology and
community-based conservationefforts.
From 2002 to 2006, the conserved land in the Amazon rainforest has almost tripled and deforestation rates have dropped up to 60%. About 1,000,000 square kilometres (250,000,000 acres) have been put onto some sort of conservation, which adds up to a current amount of 1,730,000 square kilometres (430,000,000 acres).
[52]
Anthropogenic emission of greenhouse gases broken down by sector for the year 2000.
Aerosols over the Amazon each September for four burning seasons (2005 through 2008). The
aerosol scale (yellow to dark reddish-brown) indicates the relative amount of particles that absorb sunlight.
Aerial roots of red mangrove on an Amazonian river.
_12ago2007.jpg)
