Location and General Description
The savanna encompasses the treeless and tree patch mosaic of the Gran Sabana, and occurs as three distinct outliers: the largest spanning northern Brazil, southeastern Venezuela, and southeastern Guyana (also several small patches extending north along the Pakarima footfills); a smaller patch bordering northern Brazil and extending into southern Suriname, and the smallest and most elongate outlier, that occurs in eastern Brazil north of the Amazon extending from near Macapa to near Calcoene.
The Gran Sabana uplands are gently rolling high plains, formed by sediments of the Roraima Formation, which overlie discordantly the rocks of the Guyana Shield (Dezzeo 1994). Geologically, the Guiana Shield is an ancient Precambrian land mass (4 billion to 590 million years old) made up of varied formations of sedimentary and igneous origin especially granites and gneisses (Huber 1995). This basement was formed during different orogenic phases characterized by large and long-lasting tectonic thermal events that occurred repeatedly during Archean and Proterozoic times (Huber 1995). The Roraima formation consists on pink, yellow and white sandstones, red quarzitess, green, black and red shales, conclomerates and boulder beds (Fanshawe, 1952 in Boggan et al. 1997).
Oxisols are frequently found under savanna vegetation (Hubber 1995). This soil has experienced intense meteorization and high weathering rates, losing aluminum and silicates. The additional low content of organic matter and low capacity of cationic interchange, makes these soils poor in phosphorus and other nutrients (Hernandez 1987). High accumulations of toxic aluminum compounds have often been measured in the subsoil, which severely affect the nutrient balance of the vegetation growing on them (Fölster 1986 en Huber 1995).
The northern area of this ecoregion belongs to the eastern basin of the Río Orinoco. This sector is drained mainly by the upper Río Caroní called the Río Kuquenán, the Río Yuruaní, and Río Arabopó. The southern and eastern savannas belong to the Río Branco basin. The rivers of the region are black-water rivers, characterized by their typically low concentration of electrolytes and the dark brown color of the water due to the fluvic acids (Dezzeo, 1994). Low to extremely low nutrient content of these rivers indicates pronounced nutrient deficiencies in the ecosystems of the area (Briceño and Marti 1986 in Dezzeo 1994).
This region shows a submesothermic climate (20–24°C). Average temperatures are around 20°C and average rainfalls are between 2,000 and 3,000 mm. There is a weak dry season from December to March in the northern portion. In the Guyana at least one dry season occurs in a year and two during most years (Boggan et al. 1997). During most of the year, NE and SE trade winds are predominant in the area. The relative air humidity is generally high in the entire region, with mean annual readings between 75–85% (Dezzeo, 1994).
The plant cover of the Gran Sabana is an intricate mosaic, constituted by numerous types of vegetation. With the exception of the continuous forests at the foot of the Tepuis, forests occur in patches or islands, encircled by extensive grasslands and meadows, as well as by shrub formations (Dezzeo, 1994). The savannas dominated by grasses are essentially free of shrubs and trees; but in some cases, low shrubby or suffruticose elements may be present, thus classifying as shrubby meadows of scrub savannas (Dezzeo 1994). The most common plant species in the Venezuelan savannas are: Euphorbia guianensis, Humiria balsamifera, Clusia sp., Calliandra sp., Chamaecrista sp., Bonnetia sessilis, Myrcia sp., Ternstroemia pungens (scrublands), Axonopus pruinosus, A. kaietukensis, Trachypogon plumosus, Echinolaena inflexa, Bulbostylis paradoxa, Rhynchospora globosa, Hypolytrum pulchrum (open savannas), Hypogynium virgatum, Andropogon sp., Panicum sp., Byttneria genistella, Miconia stephananthera, Mahurea exstiputata and Mauritia flexuosa (palm savannas), Chalepophyllum guianense, Digomphia laurifolia, Tococa nitens and Poecilandra retusa (meadows) (Huber and Alarcon 1988; Dezzeo 1994). Most of the elements of the flora reported to Venezuelan savannas are also present in northern Brazil, Guyana and Surinam (Steyermark, 1977; Boggan et al. 1997).
As many other savanna regions, the most obvious and recurrent dynamic element in the Gran Sabana is fire, which plays a very important role in the culture of the indigenous people living in the area for centuries (Dezzeo 1994). Nevertheless, the susceptibility to fire and its lasting effects are not typical of a humid tropical forest environment; instead, these can only be explained by very particular conditions of ecological instability, such as the reduced ability of the ecosystem to withstand external impacts (fire and extreme droughts), as well as unfavorable internal factors, such as oligotrophic and hydric stress (Folster, 1986 in Dezzeo, 1994). The main consequence of this ecological instability has been originated the gradual (ancient and recent) degradation of remaining forest, and its substitution by savannas (Worbes 1999).
In terms of biodiversity, the Gran Sabana has been recognized as an important plant refuge and dispersal center. Steyermark (1979) reports several endemic plants in the Gran Sabana. Picon (1995) (in Huber et al, 1998), registered 204 species including endemic species of Sierra de Lema and Cerro Venamo in the Venezuelan portion. Some of this taxa occur in the open savanna in swamps, sometimes on dry rocky terrain, or in the gallery forests or forested quebradas, which at different altitudinal levels traverse savanna landscape (Steyermark 1979).
Endemic bird areas of the Guyanan Highland includes 36 totally restricted to the vicinity of the tepui mountains, most of the endemics are to be found on the Gran Sabana (Huber 1997). They are primarily montane species occurring in the humid forest on the piedmont slopes above 600 m (Huber 1997). Some examples are the Tepui Swift (Cypseloides phelpsi) that inhabits montane evergreen forest, cliffs, rocky canyon, grasslands and savannas. The Tepui Goldenthroat (Polytmus milleri) lives in the forest edge and low, seasonally wet grassland and scrub, and the Tepui Wren (Troglodites rufulus) occurs in montane evergreen forest edge, elfin forest, scrub and savanna (Stattersfield et al. 1998)
Comparing with the Guyanan Tepuis, the Gran Sabana has a relative low number of endemic anurans. Most of the endemic species of this area are restricted to the forest of La Escalera including Colostetus parkerae, Stefania scalae, Cinax danae, Tepuihyla rodriguenzi, and Eleutherodactylus pulvinatus. Tepuihyla galani is found in savannas and certain tepuis (Frost, 1985; Gorzula and Señaris, 1998). Some species found only in savannas are Scinax exiguus and Leptodactylus sabanensis (Frost 1985; Gorzula and Señaris 1998).
The Gran Sabana lies entirely in the eastern section of the Canaima National Park that embraces about 30,000 km2. Mount Roraima National Park at Brazil, covering 1,160 km2, and the Parque Indigena Tucumaque protect other portions of the savannas. However, a great area of this ecoregion still remain unprotected and more effective controls and enforcement of the regulations are needed in protected areas (Huber 1997).
Types and Severity of Threats
Degradation of the remaining forests into grasslands, due to either natural or intentional fires, is the most important threat to vegetation (Huber 1996). Habitat fragmentation caused by fires, results in the gradual extinction of the species that cannot adapt to the degraded habitats (Gorzula and Señaris, 1998). The loss of vegetation cover and soil erosion affects microhidrology of small streams causing them to become intermittent during the dry season. These impacts have caused the decline of amphibians that depend on this microsites (Gorzula and Señaris 1998).
Vegetation fires form a dense layer of smoke that affect the climatic conditions. This occurrence could create a local greenhouse effect during the hottest time of the year, leading to even hotter conditions and more intensive damage by the fires (Huber 1995).
Gold and diamond mines are found along the region. Although the direct impact caused by this activity on the vegetation is usually small, the side effects can be severe. Some of these side effects are mercury pollution, an increase in the sediment load of the rivers, overhunting, and the frequent fires (Huber 1995). Extensive tourism is already present in Venezuelan savannas. The construction of the paved road from La Escalera to Santa Elena de Uairén has caused several problems including littering, illegal gathering of plants and animals, and fires (Huber 1995).
Justification of Ecoregion Delineation
This savanna ecoregion occurs in three large patches across northern Brazil and extending into Guyana, Suriname, and Venezuela. Smaller outliers also occur along the northwestern portions. These grasslands are surrounded by moist forest and are isolated from each other and other similar habitats – and contain a number of endemic species. Linework follows national vegetation coverage maps. In Venezuela we followed Huber and Alarcon (1988) by lumping the following classifications: "open graminoid savannas", "Graminoid savannas with Mauritia flexuosa", and "sub tepuian herbaceous and savanna complex". In Brazil linework follows INGE (1993), from which we lump the following classifications: "woody-grass (Gramineae)", "savanna – ombrophilous forest transition", "dense wooded savanna", and small portions of "ombrophilous forest-seasonal forest" which fell within the greater delineation. In Suriname we followed the line extensions from Brazil and according to OAS (1988). Within Guyana we followed the Huber et al. (1995) classification of "lowland shrub savanna", "upland shrub savanna", and "open lowland savanna". All border areas were blended by closest approximation to the maps listed above. Classification follows Silva (1998).
Boggan, J., V. Funk, C. Kelloff, M. Hoff, G.Cremers, and C. Feuillet. 1997. Chechlist of the Plants of the Guianas (Guyana, Surinam, French Guinana). 2nd Edition. Biological Diversity of the Guianas Program, Department of Botany, National Museum of Natural History, Smithsonian Institution, Washington, D.C. USA.
Dezzeo, N. 1994. Ecología de la Altiplanicie de la Gran Sabana (Guayana Venezolana). Investigaciones sobre la Dinámica Bosque-Sabana en el Sector Sureste: Subcuencas de los Ríos Yuruaní, Arabopó y Alto Kukenan. Scientia Guaianae, No. 4.
Frost, D., editor. 1985. Amphibian species of the world. A taxonomic and Biogeographic Reference. Allen Press Inc. and The Association of Systematics Collections. Lawrence, Kansas, USA.
Fundação Instituto Brasilero de Geografia Estatástica-IBGE. 1993. Mapa de vegetação do Brasil. Map 1:5,000,000. Rio de Janeiro, Brazil.
Gorzula, E., and J.C. Señaris. 1998. Contribution to the herpetofauna of the Venezuelan Guyana I. A database. Scientia Guaianae 8:xviii+270+32 pp. Caracas, Venezuela.
Hernandez, R. 1987. Geografía del Estado Bolívar. Academia Nacional de la Historia, SIDOR, Caracas, Venezuela.
Huber, O.1995. Geographical and phisical features. In: Flora of the Venezuelan Guayana. Vol. 1. Introducction. P.E. Berry, K.H. Bruce, and K. Yatskievych. Missouri Botanical Garden (1995). Timber Press, Portland USA.
Huber, O. 1996. Sabanas de Guayana-Surinam, Guyana, Brasil, Venezuela. In D. Olson, E. Dinerstein, G. Castro, and E. Maraví, editors, Identificación de vacíos de información botánica para la conservación de la biodiversidad en América Latina y el Caribe. Memorias del taller llevado a cabo entre el 7 y el 9 de abril de 1996 en Washington D.C. y consultas con los especialistas de la región.
Huber, O., R. Duno, R. Riina, F. Staufer, L. Pappaterra, A. Jiménez, S. Llasmozas, and G. Orsini. 1998. Estado actual del conocimiento de la flora de Venezuela. Documentos técnicos de la Estrategia Nacional Diversidad Biológica No. 1. Fundación Instituto Botánico de Venezuela. Ministerio del Ambiente y los Recursos Naturales Renovables. Estrategia Nacional de Diversidad Biológica, Caracas, Venezuela.
Huber, O.1997. Pantepui Region of Venezuela. In: Centres of plant diversity: a guide and strategy for their conservation. Vol. 3. The Americas. Pages 308-311 in S.L.D. Davis, V.H. Heywood, O. Herrera-MacBryde, J. Villa-Lobos y A.C. Hamilton, editors, World Widelife Fundation for Narure (WWF) y The World Conservation Union-IUCN. IUCN Publications Unit, Cambridge, U.K.
Huber, O., G. Gharbarran, and V. Funk. 1995. Vegetation map of Guyana (preliminary version). 1:1,000,000. Centre for the Study of Biological Diversity, University of Guyana, Georgetown, Guyana
Huber, O., and C. Alarcón. 1988. Mapa de vegetación de Venezuela.1:2,000,000. Ministerio del Ambiente y de los Recursos Naturales Renovables and Fundación BIOMA, Caracas, Venezuela.
OAS & National Planning Office of Suriname. 1988. Suriname Planatlas. Organization of American States, Executive Secretariat for Economic and Social Affairs Department of Regional Development. Washington, D.C., USA.
Silva, J.M.C. 1998.Un métedo para o estabelecimento de äreas prioritrias para a conservação na Amazönia legal. Report prepared for World Wildlife Fund – Brazil. 17 pp.
Stattersfield, A.J., M.J. Crosby, A.J. Long, and D.C. Wege. 1998. Endemic bird areas of the world. Priorities for biodiversity conservation. BirdLife International. Cambridge, U.K.
Steyermark, J.A. 1977. Future outlook for threatened and endangered species in Venezuela. In G.T. Prance, and T. S. Elias, editors, Extintion is forever. The New York Botanical Garden, New York
Steyermark, J.A.1979. Plant refuge and dispersal centres in Venezuela: their relict and endemic element. Pages 185-221 in K. Larsen y L.B. Holm-Nielsen, editors, Tropical botany Academic Press, London, Great Britain.
Worbes, M. 1999. Degradación e Historia de la Vegetación Boscosa de la Gran Sabana. In L. Hernandez, editor, Ecología de la Altiplanicie de la Gran Sabana (Guayana Venezolana) II. Estructura, Diversidad, Crecimiento y Adaptación en Bosques de Subcuencas de los Ríos Yuruaní y alto Kukenan. Scientia Guaianae, No. 9.
Prepared by: Elisa Bonaccorso
Reviewed by: In review