The Amazon basin is, above all, our planet's greatest celebration of biodiversity, where for hundreds of millions of years environmental conditions have favored an overall increase in the numbers of species. It is not clear why this has been so. Perhaps the principal factor has been a warm and humid climate, at least in large parts of the Amazon Basin.
Tracing the Amazonian landscape through time, we see huge rivers shifting their courses, rainforests contracting and expanding to the pulse of climatic change, and the evolution of several major ecosystems within the reaches of the world's greatest river network. These factors and others allowed the diversification of both animal and plant species. Indeed, flowering plants have been diversifying there for at least 65 million years. Although the plant species are far less diverse than the animals, it is the structure of the tropical rainforest that supports the millions of animal species found there.
The giant rivers of the Amazon weave together the ecosystems that support our planet's greatest profusion of life. The reflection of the trees that we see splashed across the water's surface is both literally and ecologically a reminder of how closely linked are the rivers and rainforest. In the Amazon one is unimaginable without the other. Flowing through vast floodplains, the Amazon's various river types include muddy, clear, and black-water tributaries, each with its own chemistry and unique combination of species.
The geography on which the rich biodiversity in the Amazon is expressed embraces five main geologic-ecologic regions: the Andes, Amazon Lowlands, Brazilian Shield, Guiana Shield, and estuary. The shield regions are the ancient but now highly eroded uplands (usually below 3,300 feet) north and south of the Amazon River. They are found mostly in Brazil within the Amazon drainage. The diversification of the Amazon flora and fauna is most prevalent here.
Large areas of the Amazon Lowlands have very sandy soils. These sandy regions discharge waters that, depending on light conditions and exact chemistry, appear amber, black, or brownish in color. They are generally called black-water rivers, of which the Rio Negro is the largest and most famous. Most of the rivers in the western part of the Amazon Basin that do not receive Andean sediments discharge black water. In fact the Amazon River may have been a black-water river before the rise of the Andes some 15 million years ago. Black water is usually very acidic in contrast to that found in muddy rivers, such as the Amazon River, whose pH is near neutral. A combination of highly sandy soils and acidic water has led to the evolution of unique flora and fauna in black-water river basins, which are almost biological islands within the larger rainforest.
Approximately 95 percent of the Amazon Basin consists of uplands and 5 percent is wetlands. Five percent might not sound like much, but because the Amazon Basin is nearly the size of the continental United States that means that an area more than two times the size of Florida is inundated to some extent each year. Under natural conditions rainforest covers about one-third to one-half of floodplains. In most of the Amazon lowlands, the floodplains are inundated with water for about six months each year from depths of four to 20 feet. During this time, the rich diversity of arboreal and aquatic life interacts via the flooded forest. Fishes, for example, swim among the flooded rainforest trees.
The Amazon Basin claims the world's richest concentration of flora, with approximately one-third of South America's floristic diversity and one-tenth of that of the planet. There are perhaps as many as 5,000 plant species on Amazon floodplains, including those in rainforest streams.
Because arthropods (jointed invertebrates, which are mostly insects) have been so little studied in the Amazon Basin it is not possible to make an accurate estimate of the total number of animal species that might be present. Estimates range from one to more than 20 million species.
Fishes and birds are the most diverse vertebrate groups in the Amazon. Fishes are by far the least known. Based on taxonomic work in the last four decades, a reasonable guess is that there are at least 3,500 fish species in the Amazon, compared to 800 native species in North America. The Amazon has the richest freshwater fish fauna in the world and also the richest bird fauna of any river valley. At the present rate of taxonomic progress it would take at least another century for Amazon fish species to be described and classified, and distributions known, to the level of those of their North American counterparts.
Amazonian birds are well known and few new species are described each year. The Amazon Basin claims approximately one-tenth of the world's birds, or about 950 of the 10,000 known species. There are many migratory species that contribute to the incredible avian diversity in the Amazon. Bats, with perhaps 150 species, are the richest mammal group in the Amazon, followed by rodents. The Amazon also has the most diverse primate fauna in the New World.
Humans have been in the Amazon Basin for at least 12,000 years, which is not very long compared to their presence on most of the other continents. Their impact on biodiversity, however, has received considerable attention from social scientists, and it is often hypothesized that indigenous peoples greatly modified rainforest and savanna ecosystems before the arrival of Europeans.
The archaeological evidence is sufficient to demonstrate wide occupation of the Amazon Basin and relatively dense human populations in some areas, such as on Marajó Island at the mouth of the Amazon River and in the savanna regions of eastern Bolivia. Some anthropologists have even suggested that the great diversity witnessed in the Amazon today is the result of human-caused forest fragmentation that led to genetic isolation and thus the evolution of new plant and animal species. There is no sound biological data to support this, however, and the evidence now suggests that the Amazon rainforest is very old and was highly diverse before humans arrived. Indigenous peoples undoubtedly had significant impacts on local biodiversity but probably not on regional biodiversity because of the wide distribution of most species.
With the arrival of Europeans, some Amazon plant and animal species began to be heavily exploited for food, but none were driven to extinction. The giant river turtle, manatee, and pirarucu fish were the three most important species exploited until about 1960. The giant river turtles were killed for meat and their eggs were being gathered in the millions by mid-19th century. Their populations plummeted and they have not been allowed to recover to their natural levels. The Amazon manatee was overexploited for meat, hide, and oil by the end of the 19th century. The large pirarucu, a fish species that grows to more than 250 pounds and 10 feet in length, was used as a substitute for salted cod in Brazilian cuisine. All three animals continue to be exploited, though perhaps the manatee is the most threatened.
When South American cities began to grow rapidly after the 1950s, fish became the main protein source of those urban populations. The Manaus and Belém fishing fleets grew exponentially and both began to exploit rivers as far away as 1,200 miles. There were few, if any, regulations. By the 1980s it became obvious that some fish stocks had been overexploited, but more alarming was floodplain deforestation and the introduction of large numbers of cattle and water buffalo, which were eliminating fish habitats that included flooded forests and floating meadows. Unfortunately there is still too much zeal to regulate fishermen and too little effort to protect the habitats on which commercial fish species depend.
Each of the Amazon's five main ecosystem regions faces its own set of biodiversity threats. Perhaps the greatest threats to biodiversity come in the Brazilian Shield and Guiana Shield regions and the floodplain forests of the Amazon lowlands. Large-scale agriculture, such as soybean farming and cattle ranching, has been rapidly expanding in upland areas toward the central Amazon from both the south and north. Brazilian Shield rivers, such as the Tocantins and Tapajós, are becoming more turbid because of increased erosion as the result of large-scale agriculture. Higher turbidity, along with increased pesticides, will undoubtedly threaten aquatic biodiversity in the long run. Floodplain deforestation has been heavy in the past few decades along the lower 1,200 miles of the Amazon River, and this has undoubtedly hurt fisheries.
Many headwater areas of the Andean slopes have been heavily deforested in Colombia, Ecuador, Peru, and Bolivia, though there is no evidence that this has yet altered rainfall and river-level patterns, impacts that might seriously affect biodiversity. Alluvial gold mining in Peru and Bolivia has increased the sediment load of the Madeira River, a naturally turbid river anyway. Mercury from gold-mining operations has been a serious concern for more than two decades, but fortunately it has not shown up in dangerous levels in fish food chains.
To date there is no evidence that any species in the Amazon Basin has become extinct as a result of human activities, but extinction is probably a poor measure for most Amazon species, as so many of them are widely distributed and have recognizable subspecies. The most likely scenario is that, by the end of the 21st century, the populations of many subspecies will be greatly reduced and in some cases driven to extinction. This, more than loss of a species per se, will decrease the genetic diversity in the Amazon.
The greatest concern that we should now have regarding Amazon biodiversity is our lack of understanding it. At present there are only a few scientists who study some of the huge regions discussed earlier in this article. There has been an arrogance of late; many believe that enough is already known about Amazon biodiversity and therefore that large-scale investment in cataloguing flora and fauna and studying their ecology is not worth the resources required. That naiveté spread across governmental organizations, NGOs, and benevolent foundations, is the major threat to the development of a meaningful conversation on Amazon biodiversity. Until we better understand the extent of the biodiversity out there and how it is distributed, it will be virtually impossible to develop strategies to protect it, given the far-reaching economic development that will take place during this century.