Fish Species at FishChannel.com
   Sponsored by:

Click here to learn more.

South America - A Continent of Extreme Contrasts

The vast majority of South American fishes hail from tropical rainforest drainages, principally Amazonia.

By Wayne Leibel

Printer Friendly

Mention South America and you conjure up images of oppressively hot, steamy jungles: the "green hell" of fiction. In fact, the Amazon basin, an enormous flat drainage area of some 2.7 million square miles (roughly 2200 by 1200 miles: 40 percent of the continent), contains the greatest continuous tropical rain forest in the world through which flows the second longest river in the world, the Amazon River. However, the South American continent is a land of geographic, climatic and biological diversity. The rainforest is only one part of the picture.

South America, as we know it today, is half the size of Africa and one-third the size of Eurasia, some 4500 miles long and 3000 miles across at its widest point. This represents an area of approximately 7 million square miles, of which two-thirds are not more than 1000 feet above sea level. These interior plains, the Amazon basin, are bounded by the Andes Mountains on the west, the Brazilian Highlands on the south and east, and the Guiana Highlands to the north.

Although the Andes are not the highest mountains in the world (the highest South American peak is Argentina's Mt. Aconcagua at 22,834 feet, compared with Mt. Everest of the Himalayas at 29,028 feet), they do constitute the longest continuous mountain chain. Indeed, the Andes run along the entire western Pacific slope from Venezuela to the tip of the continent, Tierra del Fuego, some 5000 miles distant.

The altiplano, the high Andes of Peru, is a land of perpetual snow, with an average elevation of 12,250 to 14,000 feet. The mountains rise dramatically from the Pacific slope and drop off steeply to the east. The tradewinds carry moisture-laden clouds from the east up to, but not beyond, this massive mountain range, where they drop their watery burden on the eastern front range to fuel the mighty Amazon, with its many freshwater fish species, below.

The Andes present a wealth of life zones from alpine to tropical cloud forest and everything in between. To the west, on the Pacific slope of Peru and Chile, great deserts — 1600 miles of them — extend from the mountains down to the sea. The largest of these, the Atacama desert (600 miles long), gets virtually no rain — perhaps a shower every 10 years or so — because the Andes effectively block rain-bearing clouds from the east.

There are other rainforests in South America aside from Amazonia proper. These include the Guiana Highlands to the north and the lands ringing the Amazon drainage (southern and eastern Venezuela and the eastern slopes of Colombia, Ecuador and Peru south to Bolivia). Additionally, the Atlantic coast of Brazil down to Rio de Janeiro and a large area between Rio and Buenos Aires, as well as the Pacific coast of Colombia and Ecuador just west of the Andes, support luxuriant tropical forests.

However, most of the continent south of Amazonia is anything but rainforest. The Brazilian Highlands to the south and east — formed of ancient crystalline rock and covering 580,000 square miles — is primarily arid savannah consisting of scrub forest and grasslands. The Mato Grosso is the most famous of the Brazilian campos cerrados.

As the plateaus diminish in altitude toward the south, the tropical deciduous forest and scrub of the Gran Chaco dominate the landscape of Bolivia, Paraguay and northern Argentina. The chaco grades southward into the Argentine pampas: great arid grassland regions (covering 300,000 square miles) much like our American midwest — rolling grass-covered plains. South along the eastern base of the Argentine Andes is a large desert with shrubs and cacti much like our own state of Arizona.

To the south and west, in the shadow of the Chilean Andes, humid temperate (subtropical) forests, not unlike those of Washington and Oregon, characterize the region known as Patagonia, which increasingly resembles Alaska as you travel southward — complete with glaciers! To the east, on the Patagonian Atlantic slope, are desolate desert-like steppes where rain is scarce and high winds blow most of the time. This finally gives way to subantarctic beaches at the tip of the continent, Tierra del Fuego. Southernmost South America is decidedly cold and humid and is the winter home to several species of Antarctic penguins.

Clearly, South America is a continent of extreme contrasts: tropical rainforests that occupy nearly half of the landmass, mountains and deserts, huge grassland savannahs and subtropical deciduous forests. The average yearly temperature of Amazonia is 81 degrees Fahrenheit (27 degrees Celsius), with extremes of 69 degrees Fahrenheit (21 degrees Celsius) and 98 degrees Fahrenheit (37 degrees Celsius) recorded at Manaus, Brazil. Uruguay to the south is decidedly temperate, averaging 50 degrees Fahrenheit (10 degrees Celsius) in winter and 71 degrees Fahrenheit (22 degrees Celsius) in summer. Average yearly temperature variations from 32 to 49 degrees Fahrenheit (0 to 9 degrees Celsius) have been reported at the tip of the continent at Tierra del Fuego.

Not surprisingly, South America consists of a wide variety of "life zones" supporting a wide diversity of organisms adapted to the highly variable topography and associated climate. Because rivers penetrate most of the South American continent, it should come as no surprise that the fishes of South America are equally diverse and adapted to their own particular life zones.

Ultimately, it is in the tropical rainforests, principally Amazonia, where Mother Nature has gone completely wild! Goulding (1990) reports an estimated 2500 to 3000 species of fish (only 2000 of which have been formally described) from the Amazon drainage. With a world total of 6650 described freshwater fish, the Amazon basin is the world's richest ichthyological region, with nearly ten times as many fish species as all of Europe and two to three times as many as the Congo (Zaire) river system.

In a 20-mile radius around Manaus, 700 fish species have been found, more than have been described from the whole of North America! It is also the richest bird region in the world, with over 900 reported species, fully one-tenth of all known avian species on this planet.

But it is the insects that truly dominate. Goulding (1990) writes: "No one knows for sure how many insect species there might be in the Amazonian rainforest, by far the richest entomological region in the world, though whatever the number, it is in the millions." And the plants that exploit those insects for pollination are equally impressive in variety. Bates (1964) offers these anecdotes: A 2-acre patch of rainforest will contain 30 to 40 different kinds of trees, compared with the five or six often found in mature deciduous forests of the United States. A square mile of rainforest may contain as many as 3000 species of trees and shrubs, not counting undescribed species that inevitably are part of the census.

The sheer organismal diversity and evolutionary ingenuity have impressed all who have spent time in Amazonia, from Wallace to Bates to Darwin, whose classical written accounts are well worth the time and effort to read. Clearly, the relatively stable tropical climate has allowed for explosive diversification. Amazonia is an evolutionary laboratory and some of its experiments have involved fishes.

The Mighty Amazon
Because the vast majority of South American fishes hail from tropical rainforest drainages, principally Amazonia, it is not unreasonable to spend some time describing this biotope. It will provide a useful starting point for our discussions of cichlid fish maintenance.

The Amazon is the second longest river in the world at 3,915 miles, second only to the Nile measuring just over 4000 miles. However, the Amazon is the largest river in terms of volume of water discharge. It delivers about one-fifth of the total riverine freshwater discharge of this planet, which is five times that of the Congo (Zaire) and seven to 10 times that of the Mississippi.

The Andean sediments carried by the river are dropped as much as 200 miles beyond the actual mouth of the Amazon, making it effectively longer than the Nile. It drains four-tenths of South America, 2.5 million square miles, including Peru, Ecuador, Colombia, Venezuela, the Guianas and, of course, Brazil. It boasts more than 1000 tributaries, 17 of them over 1000 miles long. It is deep enough (295 feet in places) to permit ocean-going ships access to the Peruvian port of Iquitos, some 2300 miles inland at the foot of the Andes. It is as wide as 7 miles across at some points. Its mouth at the Atlantic is fully 200 miles across.

The Amazon didn't always flow eastward from the Andes to empty into the Atlantic. South America was once part of Gondwana, an ancient "supercontinent" that is believed to have contained what is now Africa, Australia, India, Iran and Antarctica. The convex eastern coast of South America initially nestled into the concave western coast of Africa.

About 140 million years ago, Gondwana broke apart and the component plates comprising it drifted away to become our present continents. South America sailed westward as an island continent, eventually banging into the Pacific Plate, which resulted in the upfolding (crumpling) of its western margin into the Andes mountains about 100 million years ago.

Initially, until approximately 250 million years ago, the Amazon Basin was a vast inland sea: a gulf opened wide on the Pacific coast and seawater flooded the area now occupied by the rainforest. In fact, the modern freshwater dolphins, manatees and rays of the Amazon are relics of this epoch.

During the Carboniferous period, the continent rose and tilted, draining the waters toward the west. The upwelling of the Andes effectively dammed this exit and created a large lake from the basin. Eventually, the continent tipped eastward and drained the lake to the Atlantic through the gap between the Guyanan and Brazilian shields. The eroded sediment washed down from the Andes, filled the basin and eventually created the rivers that drain the area today.

A second consequence of the colliding plates was the creation of the land bridge that connects South and Central America. No direct connection existed between South America and any other continent for about 70 million years. The upfolding and subsequent vulcanism that occurred during the Cretaceous and Eocene periods resulted in the first South-Central American land bridge.

Central America consists of a chain of volcanos stretching some 700 miles. The original bridge is believed to have sunk, but was then reestablished in the Late Miocene. The Isthmus of Panama was only completed approximately two to three million years ago. Some geologists believe the chain of Caribbean islands are what is left of the original land bridge.

Not surprisingly, the fish fauna, specifically cichlid fish, of Central America is primarily of South American emigrant origin. Ancestral cichlids from the south entered nuclear Central America at the time of the first land bridge and then diversified rapidly in isolation when the bridge sunk. Today, the Panamanian ichthyofauna is a mixture of South and Central American species as befits a relatively recent contact zone.

During the rainy season, from November to June in Amazonia, the mighty river rises as much as 50 feet and spills over its banks to flood adjacent forest, called igapo, as much as 50 to 60 miles beyond its normal channel, encompassing an area of some 38,600 square miles — about 2 percent of the total Amazonian rainforest and an area larger than England. This happens because the Amazon basin, the ichthyofaunal region described by Gery (1984), is so flat, rising no more than 650 feet above sea level at its highest point. Thus, the river channel is unable to contain the huge increase in water volume caused by the rains and the melting ice as the moisture-laden trade winds condense at high altitude on the eastern side of the Andes. The seasonal floods cover vast expanses of Amazonia for four to seven months each year.

This seasonal flooding, more than anything else, dictates the rhythm of life in Amazonia. The flooded forests are extremely important spawning grounds for most Amazonian fish, providing both food, in the form of insects and fallen fruits, and cover for newly hatched fry. In fact, fish, particularly the pacu (Colossoma sp.) and its relatives, are important as agents of seed dispersal.

After the peak of the flooding, some fish simply escape the draining forest by returning to the river channel. Others remain in marshes or varzea lakes that may or may not dry out depending on the length of the subsequent dry season. There they are hunted by birds or other predators as the water level drops and the occupants become more and more concentrated within the shrinking pool. I heartily recommend Goulding's (1990) book, Amazon, the Flooded Forest, and a Public Broadcasting System film of the same name, for a riveting and popularized account of this highly important ecosystem.

Annotated References and Additional Reading

Popular
Bates, M. 1964. The Land and Wildlife of South America. Time Inc., NY.

Dorst, J. 1967. South America and Central America: A Natural History. Random House, NY. (An exceptional "geography" of South and Central America that considers each of the main biotopes in separate chapters.)

Goulding, M. 1990. Amazon: The Flooded Forest. Sterling Publ. Co., NY. (Excellent popularized introduction to this amazing ecosystem.)

Sterling, T. 1975. The Amazon. Time-Life International, Netherlands.

Exploration
Agassiz, Louis. 1867. A Journey in Brazil.

Bates, Henry Walter. 1863. The Naturalist on the River Amazon. (Paperback reprint 1962, University of California Press.)

Keller, Franz. 1875. The Amazon and Madeira Rivers.

Shoumatoff, Alex. 1978. The Rivers Amazon. Sierra Club Press.

Wallace, Alfred Russel. 1853. Travels on the Amazon and Rio Negro.

Technical
Fink, W.L. and S.V. Fink. 1979. Central Amazonia and Its Fishes. Comparative Biochem. Physiol. Vol. 62A:13-29.

Gery, J. 1984. The Fishes of Amazonia. In: Sioli, H., ed., The Amazon: Limnology and Landscape Ecology of a Mighty Tropical River and Its Basin. Dr. W. Junk, Pub., Netherlands.

Kullander, S.O. 1986. Cichlid Fishes of the Amazon River Drainage of Peru. Monograph. Swed. Mus. Nat. Hist. 431 Pp.

Lowe-McConnell, R.H. 1984. Ecological Studies in Tropical Fish Communities. Cambridge Univ. Press, NY.

Lowe-McConnell, R.H. 1991. Ecology of Cichlids in South American and African Waters, excluding the African Great Lakes. In: M.H.A. Keenleyside, ed., Cichlid Fishes: Behavior, Ecology and Evaluation. Chapman & Hall. Chap. 3, Pp. 60-85.

Where Do Amazonian Cichlids Live?
How many cichlids are there in the Amazon? Lowe-McConnell (1984) estimates that there are approximately 110 species of South American cichlid fish, 82 of which are found in the Amazon drainage. (This estimate is low, particularly in view of recent work by the cichlid taxonomist Sven O. Kullander. A more recent estimate of 300 appears in Lowe-McConnell's most recent [1991] contributions.) Of these, 45 species are endemic; that is, found only there.

Cichlid fish make up only about six to 10 percent of the total diversity. Others, such as characoids (tetras, silver dollars and relatives; 43%) and siluroids (catfishes; 39%), are much more numerous. Cichlids, being primarily lentic fishes (inhabitants of slow-moving or stagnant waters), do not frequent the rapidly flowing Amazon proper. They are actually found in smaller bodies of water — tributaries or associated pools, marshes and so on — according to Kullander (1986), who has collected them exhaustively in Peru.

It's also important to note that all of the rivers in the Amazon drainage are not chemically equivalent. Three major types of Amazonian rivers are recognized by limnologists. These are whitewater, clearwater and blackwater rivers. The Amazon proper (known as the Solimoes in Brazil and the Maranon in Peru) is a typical whitewater river loaded with suspended sediment, usually clays from the Andes. Because of the turbidity, the water appears "white" or, more accurately, "cafe-au-lait" in coloration.

Because of their limited transparency, whitewaters do not support much in the way of aquatic plants. However, because they are nutrient-rich, extensive "floating meadows" of water hyacinth and floating grasses arise in the calmer backwaters and support large communities of insects, and thus fish.

Clearwater rivers are typically those draining the ancient mountains of Brazil and the Guianas. Given the relative stability of the pre-Cambrian rock they drain, there is little in the way of erodible materials. Hence, there are few sediments or nutrients in these very transparent, sometimes greenish waters. The Rios Toncantins and Xingu, which arise from the Brazilian plateau south of Belem, are two classic examples.

The blackwater rivers, often tea-colored but highly transparent, derive their peculiar coloration from humic acids leached from flooded vegetation growing on the white sands of the lowland flood plains through which they flow. These sandy soils are too nutrient poor to support the microorganisms that would ordinarily degrade the leached organics, so they remain in the water, coloring it brown.

Blackwater rivers, like the Rio Negro, are usually highly acidic (pH 4.0 to 5.0) and nearly as soft as distilled water. They are also the least productive. Few aquatic insects survive the extreme conditions, and blackwater fishes depend largely on food items that fall into the water from the forest.

All of these water types present the quiet, still-water (lentic) habitats preferred by cichlid fish. The populations of fish are generally poor in the main rivers. These lentic habitats include swamps (aguajales), backwater pools (charcos), oxbow lakes (cochas) and streams that originate from swamps (quebradas) that drain lakes (canos) or that connect rivers (sacaritas).

Most of these habitats have mud bottoms with abundant leaf litter and felled, submerged trees/ trunks/logs/branches. Sometimes, the mud bottoms support plants (macrophytes), particularly in lakes. You will be hard pressed to find rocks or gravel, although there is occasionally sand.

Of course, these habitats blend together during the seasonal flooding. Although the water temperature in the main river fluctuates no more than 1 degree from the normal temperature of 84 degrees Fahrenheit (29 degrees Celsius), temperatures in associated water bodies may vary from a low of 74 degrees Fahrenheit (23 degrees Celsius) in small forest streams to as high as 93 degrees Fahrenheit (34 degrees Celsius) in stagnant pools.

The Orinoco-Venezuelan region to the northwest, another ichthyofaunal region described by Gery (1984), is connected to the Amazon system via the Casiquaire canal (Rio Negro [Brazil] and Rio Orinoco [Venezuela]). Venezuela is a land of great plains — the llanos or Orinoco basin.

The Orinoco basin comprises the third largest river system in South America, occupying 360,000 square miles, nearly four-fifths of Venezuela and one-quarter of Colombia. The Rio Orinoco proper is 1,281 miles long, rising in the Parima mountains near the Rio Negro in the Brazilian frontier and flowing first west and then north to the Caribbean sea. Some of its major tributaries include the Rios Meta and Apure.

The llanos are flat — often marshy — and when the rivers are filled with rushing water during the April to October rainy season, they flood. The water often rises as much as 50 to 60 feet and the nutrients deposited by the floods support the grasslands. Because of the interconnection of these basins across the Casiquaire, the Orinoco and Amazon drainages also share many species.

The Magdalenean and Trans-Andean regions characterize the remainder of northwestern South America: Colombia and Ecuador. The Andes, which to the south form one single grand mountain mass, split into three distinct chains in Colombia — the Cordilleras Occidental, Central and Oriental, which separate Colombia from Panama. Two distinct basins, the Rio Magdalena and the Rio Cauca, are defined by these ranges. While the mountains are cold or temperate, the valleys are tropical. To the east of the Andes is hot jungle. The fish fauna of the isolated Magdalenean region is relatively poor, as it is in the trans-Andean regions of far western Colombia and Ecuador. These are therefore "minor" regions with respect to their fishes.

A third major ichthyofaunal region described by Gery is called the Paranean. It is the second largest drainage in South America and comprises the La Plata-Uruguay-Parana-Paraguai system to the south of the Amazon basin (Paraguay, Uruguay, Argentina). In fact, the confluence of the Rios Paraguai, Parana and Uruguay at La Plata (Rio de la Plata), make this "river" the second largest in the world, after the Amazon, in terms of total volume discharge. This ichthyofaunal region is very dry and seasonally swampy. Vast marshes, known as the Pantanal, flood annually in May or June. Although isolated today, there were former connections with the Amazon. Thus, many of the larger fish species are now widely distributed from the Orinoco down to this essentially temperate zone. However, there are also many endemic fish species.

To the north and east of the Paranean region, and forming the eastern boundary of Amazonia (the Brazilian Highlands), is the East Brazilian ichthyofaunal region, characterized by smaller rivers that flow eastward to the Atlantic coast. One of these, the Rio Sao Francisco, is the third largest river of the continent. It rises in the Brazilian Highlands in southern Brazil and flows northward about 1000 miles before curving eastward into the Atlantic. It is about 2000 miles long and is navigable for almost 1000 miles. This ichthyofaunal region is discontinuous and includes the area immediately surrounding Porto Alegre. Although the immediate coast is humid and characterized by dense forest, the highlands themselves are arid plateaus that are largely scrub forest, even desert.

The remaining two icthyofaunal regions, the Andean and Patagonian, are — as might be expected — lacking in fishes. The Andean fish fauna is principally comprised of specialized torrent fishes and a few weird toothcarps (Orestes) that inhabit the high lakes. The Patagonian region in the far south is populated by a completely different fauna of Antarctic peripheral fishes.

Conclusion
As cichlid aquarists, we deal primarily with fishes from the Guyanan-Amazonian, Paranean, Orinoco-Venezuelan and, to a lesser extent, East Brazilian, Magdalenean and Trans-Andean ichthyofaunal regions. There is much we can learn about the maintenance and captive propagation of our cichlid fish by considering the ecological biotopes from whence they come.

This ready availability of cichlid fish species in the hobby reflects not only the relative richness of the respective ichthyofaunal regions but also the location of the main points for commercial collecting and exportation. Both Amazonia (Manaus, Brazil and Iquitos, Peru) and Guyana have been reliable long-term sources of aquarium tropical fishes for decades. More recently, Colombian, Ecuadorian, Venezuelan and Argentine (Paraguay) exporters, as well as new stations in Brazil (e.g., Belem, Santarem) have come on line, providing American, European and Japanese aquarists with the "latest" fish.

Similarly, the proliferation of "bring 'em back alive" expeditions mounted by European and, more recently, American hobbyists have supplied the hobby with even rarer, more obscure species from sites that are not collected commercially. Given the sheer biotopic diversity of South America, I know we have not yet seen the last "new species" — both those "new to science" and certainly those "new to the hobby."

In this, the amazing diversity and radiation of fishes, lies the charm of South America to the aquarist. In my "Goin' South" series of articles, I hope to undertake an exploration of that amazing diversity in the context of the fishes I love best: the cichlids of South America.

Printer Friendly



Top Products

ADS BY GOOGLE