Monte Alegre Varzea forests

Category: Forest Biomes.

Geographic Location: South America.

Summary: These periodically flooded forests line the Amazon River along its central reaches and contain many oddly adapted animal species.

The Monte Alegre Várzea Forests biome represents the flooded forests within the central and lower Amazon River Basin, roughly from the confluence of the Tapajos River westward (upstream) to the confluence of the Purus River, including nearly the entire length of the Madeira River Basin. The city of Manaus, Brazil, lies roughly in the middle of the biome, which also includes the confluence of the Solimões River (“white water”) and Rio Negro (“black water”) near Manaus.

The Monte Alegre section of the region is characterized in particular by white-water rivers—tributaries that are clouded by sediments brought downstream from the Andes. This biome comprises a vast amount of the central Amazonian region, as much as 25,000 square miles (65,000 square kilometers) in total area, although it is in many cases not contiguous but rather contorted or distended. The topographic relief of the multiple channels, rivulets, meanders, and gorges that riddle the central stretches of the Amazon form the eccentric contours of this biome, defined by the very nature of its relation between dry gradient and seasonal stream overflow.

Flora

Generally speaking, várzea biomes represent a complex and seasonal interface between freshwater and terrestrial ecosystems—where the river systems flood their banks and become the understory of a flooded forest. Várzea biomes are harsh environments, where flood levels can reach 20 to 40 feet (6 to 12 meters), and they are unusual in that they are characterized by their intimate mixture of terrestrial and aquatic elements. These include fish that act as seed dispersers for trees, and the region’s iconic pink river dolphins that hunt between the giant buttresses of riparian trees.

Tropical and subtropical evergreen and moist broadleaf forest types share the landscape here, depending on factors including elevation, slope alignment, and inundation patterns. The canopy generally reaches to about 80 feet (24 meters), topped by such members as capirona, cumala or wild nutmeg, acai palm, and aguajé or Mauriti palm. The understory features such river-friendly shrubs as eugenia, the evergreen chaparro de agua or symmeria, and isabgol or psyllium. Grasses abounding here include bitter cane, millet, and water paspalum.

While large portions of this biome have faced development pressure from logging, monoculture farming and plantation forest cultivation, ranching, mining, and other human activities, the periodic inundations have effectively barred large areas from such impacts.

Fauna

The Monte Alegre Várzea, as part of the network of biomes that make up the lowland Amazon Basin, has very high biodiversity but low endemism, that is, few species that uniquely evolved here. Many of the flooded-forest-adapted species occur along much of the Amazon River and its tributaries, and during the flood season there is a massive turnover in animal species composition as water replaces the forest understory. During flood stages, it is not uncommon to have complete overlap of terrestrial and aquatic elements in virtually the same space. For example, the brown bearded saki monkey and toco toucans feed in the rainforest canopy, while arawana, arapaima, and several types of piranha feed among the inundated lower branches of the same trees.

Mammals that typify the biome are both permanent residents and seasonal visitors. Aquatic mammals including the giant otter, manatee, and Amazon River dolphin are present all year in the rivers and streams, but enter the forest only during flood stages. On the other hand, terrestrial mammals like the short-ear dog, jaguar, and ocelot tend to stick to dry land at all times. Arboreal mammals, such as the brown-throated three-toed-sloth, a variety of primates, and bats such as the Amazonia sac-winged bat, remain in the forest throughout the year, as they have little trouble staying dry in the canopy during flood season. The capybara, however, is a highly adapted large rodent perfectly at home in and out of the water in the varzea—as is the yapok, or water opossum.

Among the richest of the vertebrate fauna here are birds, with many species well adapted for land, air, or water. Along the river banks and into the forest, it is not uncommon to see the neotropical river warbler and fork-tailed wood nymph, while Amazon kingfisher typically perch on overhanging branches looking for small fish. The harpy eagle, among the largest raptors in the Americas, is a well adapted and opportunistic hunter of small mammals and other prey.

The ancient hoatzin, however, is the most characteristic and perhaps best-adapted bird of the flooded forests. Hoatzin chicks feature claws on two of their wing digits, which help them crawl up and down trees to and from the water. The claws are retained until the chicks mature enough to use flight as their preferred mode of movement within the varzea. More colorful are the red-and-green macaw, toco toucan, and numerous hummingbirds.

The flooded nature of these forests makes ideal habitat for many amphibians and reptiles. Among the amphibians are a number of species whose behavior is as curious as their names imply; examples include the Rio Mamore robber frog, nauta mushroom-tongue salamander, and Manaus slender-legged treefrog. Among the reptiles are a vast number of turtles and tortoises who take advantage of the freshwater, while green and tropical thornytail iguana lizards bask on the branches above. Along the river bank, it is not uncommon to see both dwarf and black caiman.

Threats and Conservation

Among the ongoing effects of the combined anthropogenic impacts noted above, the Monte Alegre Várzea Forests biome is vulnerable to habitat fragmentation. While many of the species here are robustly adapted to a natural environment with inherently dramatic changes on a seasonal basis, there are severe disruptions of the soil, water, and vegetation base that can quickly undermine many of these finely-tuned adaptations. Gross sedimentation from the erosion and runoff released by clear-cut logging, clearing for roads, mining, and monoculture plantation farming can cause immediate habitat destruction by blocking oxygen availability, upsetting nutrient mix, and inputting toxic materials.

Climate change looms as a complex challenge to this ecosystem. Precipitation, wind currents, temperatures, and air pressure are all subject to long-term variance under different models for predicting the effects of global warming on the Amazon. Combinations of pronounced changes in some or all of these vectors are expected to put stress on many of the unique local habitat niches that are already under pressure from aggressive human activities. Accelerated—or delayed—onset of seasonal flooding or of its drawdown is a core pattern to be monitored as these coming changes become more clear in their general trends and relative severity.

The production of açaí palm, a cash crop used in the production of juices, is adding to the environmental stress of the region. Cultivation areas are heavily pruned to ensure the viability of the açaí palm. This practice can be detrimental to other native species of vegetation and native animal populations. Other human economic activities in the region include fisheries, timber cultivation, and cattle and buffalo ranching. These are leading to the destruction of habitats for species such as turtles, manatees, and caimans.

Bibliography

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