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Venezuela´s Eco Regions

 

Gran savanna 

This ecoregion occupies an area within the Roraima formation distinguished by extensive
savannas and schrubby vegetation. The region is traversed by streams with gallery
forests, and extensive savannas. In this ecoregion, recurrent fires and extremely poor
soils are the most important factor in the advance of savannas in place of forest and the
processes that are derived by these changes. Comparing with Guyanan Tepuis, endemism is
low; however, an important number of endemics are found. 

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).

Biodiversity Features

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).

Current Status 

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). 


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