Guyanan Savanna

• Category: Grassland, Tundra, and Human Biomes.
• Geographic Location: South America.
• Summary: This large region of mixed tropical savanna and forest is situated between the Amazon and Orinoco River basins.

The Guyanan savanna is a heterogeneous system of mixed tropical grasslands and forests covering approximately 21,236 square miles (55,000 square kilometers) of northeastern South America. Researcher J. G. Myers formally described the ecosystem in 1936 as the Rio Branco–Rupununi savannas, named for the two major river systems in the region. The Pakaraima Mountains (Sierra de Pacaraima in Spanish) bound the savannas to the north in Guyana and the west in Venezuela. The Amazon rainforest marks the southern boundary of the Guyanan savanna.

To the east, the Kanuku Mountains bisect the northern and southern portions of the Rupununi savanna, which eventually transition to the rainforests of interior Guyana. The region receives 59 to 79 inches (1,500 to 2,000 millimeters) of rainfall annually, seventy to eighty percent of which falls during the wet season (May to August). Although many biologists consider these savannas part of the Amazon region, they are geographically isolated from the southern Brazilian cerrados(savannas) and constitute a distinct ecosystem.

Vegetation

The vegetation of the Guyanan savanna is a mosaic of low-lying tropical grassland interspersed with low-statured solitary trees, small bush islands, and gallery forests bordering the rivers. In the Pakaraima and Kanuku Mountains, savanna vegetation transitions into deciduous and semi-evergreen forest, while cloud forests cover some of the higher peaks. Vast tracts of the savanna become inundated during the wet season, with floods often high enough that the waters from the Orinoco and Amazon basins intermingle. Wet-season inundation also favors the establishment of grasses and restricts forests to areas with more persistently dry soils.

During the dry season each year, frequent savanna fires—fueled by high grass abundance and steady trade winds—burn large areas of the region. These fires also maintain the boundaries between forest and savannas, by restricting the establishment of woody plants and favoring fire-tolerant plant species.

Wildlife

The Guyanan savanna is renowned for its abundant wildlife; the biome provides critical habitat for many remnant elements of neotropical megafauna. South America's largest mammals are relatively common here, including tapirs (Tapirusterrestris), giant anteaters (Myrmecophagatridactyla), giant armadillos (Priodontesmaximus), and capybara (Hydrochoerushydrochaeris). Big cats such as jaguars (Pantheraonca),pumas (Pumaconcolor), and ocelots (Leoparduspardalis) are also fairly abundant.

The habitat along the Rupununi River is especially notable for high densities of giant river otters (Pteronura brasiliensis) and arapaima (Arapaima gigas), one of the world's largest freshwater fish. Also common along the river are some of South America's largest reptile species, such as black caiman (Melanosuchus niger), green anacondas (Eunectes murinus), and giant river turtles (Podocnemis expansa).

The Kanuku Mountains provide important habitat for harpy eagles (Harpia harpyja), which are among the world's largest raptors; while scarlet macaws (Ara macao) and blue and yellow macaws (A. ararauna) occur in high numbers in the savannas. The seasonally flooded wetlands are globally important for supporting high densities of waterbirds such as stork, ibis, duck, heron, and many other avian types.

Human Activity

Several Indigenous groups exercise stewardship over lands in the Guyana savanna. The Mukushi (Macuxi) reside among scattered villages throughout the northern Rupununi savanna of Guyana and in Roraima state in Brazil. In the forested Pakaraima Mountains to the north, communities are mixed, featuring both Makushi and Patamona natives. The Waipishana make up the majority of the population in the south Rupununi savannas, while the Waiwai people live in the forest to the south.

Indigenous people have long played an important role in the ecology of the Guyanan savanna, primarily through the intentional use of fire. Burning was traditionally used to drive game and provide fresh grass for game species, reduce insect pests, prevent destructive fires, and clear land for farming.

At present, limited economic opportunities in Guyana lead many Indigenous people to seek work elsewhere, primarily on cattle ranches in Brazil. However, among the scattered communities, many still practice swidden cassava farming and rely on wildlife and fish resources for subsistence use. A developing ecotourism trade also contributes to the local economy.

One of the reasons for the Guyanan savanna's rich wildlife is the relative lack of development, especially in the Rupununi region. There are effects, however, from cattle ranching and soybean farms, the region's primary industries, with the latter posing a greater threat to the Guyanan savanna, in part due to the biofuels-driven boom in soy cultivation. The majority of development has occurred in the Brazilian state of Roraima, in the western part of the savannas, which is experiencing rapid economic growth.

By comparison, in Guyana, population density is much lower, and economic activities consist largely of a few scattered ranching operations. Natural rubber production was prevalent in Guyana from the late 1800s to the mid-1900s, through the harvesting of balata, the resin of bulletwood (Manilkara bidentata).

Although the region is rich in timber, minerals, and oil, Guyana's poor infrastructure has limited access and extraction. The route through the Guyanan savanna provides the only direct land access between the Amazon forests and the sea at Guyana's capital city of Georgetown. Thus, to expedite the export of timber and other resources, current development is quickly improving bridges and roads, which will fragment some habitat and open the entire region to exploitation. While much of the Guyanan savanna remains undeveloped, very little land area has been set aside for conservation. Gold and silver mines within the region cause a number of problems, including mercury pollution. Recent satellite datashows a 138 percent increase in gold mining deforestation in the Guyanan savanna region compared to the previous decade, driven by rising gold prices and improved road access.

The long-term effects of global warming are difficult to project for this region of South America. The complex interplay of temperature gradients, wind patterns, humidity, and precipitation seasonality are not well understood. However, it is believed that a less stable precipitation cycle, coupled with even moderate increase in average temperatures, will lead to greater threat from both natural and man-made fire in the Guyanan Savanna biome. If increased forest-understory and grassland damage from more severe fire events becomes a regular feature in the dry seasons in the Guyanan savanna, then habitat fragmentation will accelerate and many animal species will be pressed to migrate toward wetter niches, putting additional stress on those habitats.

Bibliography

Black, Jasmine E., et al. "Assessing Lignin Decomposition and Soil Organic Carbon Contents Across a Tropical Savannah-Rainforest Boundary in Guyana." Frontiers in Forests and Global Change, vol. 4, 2021, pp. 1-12. Accessed 9 Nov. 2024.

MAAP. "2023 Report: Mining Deforestation in the Guiana Shield." Monitoring of the Andean Amazon Project, 2023, maaproject.org/2023/guiana-shield-mining/. Accessed 9 Nov. 2024.

Myers, J. G. "Savannah and Forest Vegetation of the Interior Guiana Plateau." Journal of Ecology, vol. 24, no. 1, 1936, pp. 162-84.

Ter Steege, H. Plant Diversity in Guyana With Recommendations for a National Protected Area Strategy. Tropenbos Foundation, 2000.

Watkins, G. Rupununi: Rediscovering a Lost World. Earth in Focus Editions, 2010.