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Venezuela´s Eco Regions

 

Llanos 

In South America the savanna ecosystem covers a total of 269 million ha. Most of it
(76%) belongs to the Cerrados of Brazil but about 11% (28 million ha) form the Venezuelan
Llanos and 6% (16-17 million ha) the "Llanos Orientales" of Colombia (Blydenstein, 1967;
Rippstein et al. 2001). These two areas, although belonging to different countries, form
a single ecoregion, the Llanos of the Orinoquia (latitude 3° to 10° N and longitude 62° to 74°
W). This is an area of extensive plains, covered mainly by savanna vegetation, of
great economic importance for both countries. This ecoregion is relatively young, perhaps
less than 10,000 years old, and developed in a great geosyncline between the Guiana Plateau
and the Andes Range. This extensive basin was, over time, filled with sediments from the
Guiana Plateau and the cordilleras during the tertiary. The ecoregion then experienced a series of subsidences resulting in a landscape made up mainly of alluvial plains and highlands (San José and Montes 1989; Blydenstein 1967). 

Location and General Description 

The llanos ecoregion covers a large elongated area 1200-1300 km long, that extends
in a gentle curve in a northeast direction, beginning at the foothills of the Oriental
Andes of Colombia and extending along the course of the Orinoco River almost to
its delta at the sea. The Llanos ecoregion is located in a great depression, limited by
the Andes in the west, the Venezuelan coastal range that isolates it from the
Caribbean Sea in the north, and the Guiana shield in the south. In Colombia they
occupy the departments of Meta, Arauca, Vichada and Casanare., and continue in
Venezuela in the states of Apure, Barinas, Portuguesa, Cojedes, Guarico,
Anzoategui and Monagas. The area of the lowlands of Colombia and Apure State
collects the rainfall from the Andes and the Guiana plateau and draining, due to the
presence of a slight downward slope in the north-east direction, through the Meta,
Arauca, Vichada, Cinaruco, Apure and Capanaparo Rivers, just to name a few, to
the Orinoco River. 

Throughout this long course the llanos ecoregion exhibits a high heterogeneity in landscapes
and types of vegetation. Huber and Alarcón (1988) divide the Venezuelan llanos into seven
areas, that listed from southwest to northeast are; a) the occidental llanos, b) the Apure llanos,
c) the low Central llanos, d) the high Central llanos, e) the Unare depression, f) the oriental
"mesas" and g) the oriental llanos of Monagas. For the Colombian part of the llanos ecoregion
several classifications have been proposed (Blydenstein 1962,1967; Etter 1998; Rangel et al.
1995; Rippstein et al. 2001). Rippstein et al. (2001) recognize three types of landscape: the
foothills, the alluvial plains and the highlands, that may be divided in well-drained highplains
and floodable highplains. Etter (1998) defines seven different zones; a) the highplains, b) very
dissected highplains, c) the sandy Guiana highplains, d) foothill non-flooded savannas, e)
bushy savannas on "medanos", f) flooded savannas of the eolic plains, g) patches of flooded
savannas and forests on the overflow plains. These classifications give an idea of the high
complexity of the area, given that each of these zones has a specific vegetation, soils,
topographic position and hydric regime.

Although there is some change in the climate as we move from the Colombian part of the
ecoregion with altitudes between 600-200 m, to the Venezuelan areas at less than 100 m. This
ecoregion has a typical savanna climate, with a well-defined wet and dry season and high
temperature all year round. According to PDVSA (1992) the climate of this area falls in the types
Awi (savannas and semi-dry tropophilus forests), Aw’i (savannas and semi-humid tropophilus
forests) and Aw’’i (savannas and humid tropophilus forests) according to Koeppen’s
classification. Total annual rainfall changes significantly as we move through the ecoregion
from the higher west-south with 2500 mm per year, to the center of the basin with 1200-1600 mm
per year in the Apure, and 800-1200 mm in the north-east end in the Llanos of Monagas State.
There is a definite rainy season in the middle of the year that may last as long as 10 months in
the south-west and seven months in the north-east. As much as 95% of all annual precipitation
falls between April and November, with over 400 mm of monthly rainfall during June-July. An
intense drought, 3 to 5 months long, occurs between December and April, with less than 50 mm
of monthly rainfall. This pattern causes some areas to flood during the rainy season and
become completely dry during the drought. Temperature also varies along the llanos ecoregion
being higher in the northeast. On average, the mean annual temperature is 27 °C, with minima in
June, July, December and January, and maxima in March and April, but the differences between
the coldest and hottest months are very small (2 ° C). In contrast, daily differences range from
13 to 17 ° C.

Dominant soils in the area belong mainly to the orders ultisols and oxisols (Berroterán 1988;
Ramia 1993; Rippstein et al., 2001), but with a great diversity of types according mainly to their
topographic position. Analysis of texture indicates that in the llanos ecoregion north of the
Orinoco River soils are mostly of a group of sandy-clay soils. The high llanos soils have a
higher proportion of sand than the lowlands, with very low fertility due to leaching, acid (pH
between 4.5 and 5.5), low organic matter content, C/N relation between 15 and 20, low CIC, high
levels of Fe and Al, and deficiency of P and Ca. A cemented hardpan or plinthite of iron
concretions is present in many areas near the surface of the soil, affecting water percolation and
making difficult the establishment of trees (Foldats and Rutkis, 1975; San José and Fariñas,
1983). In the seasonally flooded lowland savannas, soils are somewhat richer, with a higher
proportion of silt and organic matter and higher fertility. Tejos et al. (1990) gives a good
description of the soils of this periodically flooded system and its potential use. In these areas,
the organic matter production is much higher than in the highlands (Bulla et al. 1990). The soils
of the several types of forests found in the area differ among them and with the savannas
(Stergios et al. 1998). For a more detailed description of the rich variety of soils in the llanos and
their relationship with the topography and vegetation types see Berroterán (1988), Rippstein et
al. (2001), Blydenstein (1962, 1967), Sarmiento (1990), Zinck (1980), Ramia (1993), Garcia
Miragaya et al. (1991), Medina and Silva (1990), Tejos et al. (1990) and Stergios et al. (1998).

According to Holdridge’s classification, a deciduous dry tropical forest should cover the llanos
(PDVSA 1992). The dominant vegetation type in this ecoregion; however, is savanna creating a
contradiction that has not yet been satisfactorily explained. In fact, the llanos are a very
floristically heterogeneous ecoregion. Huber and Alarcón (1988) recognize 29 different types of
vegetation for the Venezuelan llanos, including ten different types of savannas, 9 types of
forests. Blydenstein (1967), Rangel et al. (1995) and Rippstein et al. (2001) describe a similar
degree of complexity for the Colombian llanos. Below, we will try to give a short description of
the more important vegetation types, although the fact that the scientists working in Colombia
and Venezuela haven't homogenized the nomenclature makes difficult their comparison between
both countries. 

The ten different types of Venezuelan savannas described by Huber and Alarcón differ from
one another in: a) the topographic position, that separates them in "llanos altos" or high llanos,
that never get flooded, and the "llanos bajos" or lowlands that get flooded during the rainy
season; b) the presence or absence of trees and bushes; and c) the floristic composition of the
herbaceous layer. However, it should be emphasized that these three factors are
interdependent; each type of vegetation appears in a specific topographic position and is
usually associated to a certain type of soil. About 65% of the Venezuelan savannas, a total of
28 million ha, in the Orinoquia are the Trachypogon species savannas (San José and Montes,
1989). These are a somewhat floristically heterogeneous group of savannas, whose main
common characteristic is the dominance of this grass species genus, however it has been
subdivided by Blydenstein (for Colombia) and other authors (for Venezuela), into several
sub-types. These are non-flooded savannas, that grow mainly on the "llanos altos" or
highplains, over poor soils with very low nutrient content, many times with a lateritic hardpan
layer near or at the surface of the soil. The herbaceous layer has a height of 30-100 cm and the
tussocks are separated by a distance of 10 to 30 cm. San José and Montes (1989) reported a
total of 285 species of angiosperms belonging to 55 families for these savannas. In a typical
Trachypogon spp. savanna characteristic species are Trachypogon plumosus, T. vestitus,
Axonopus canescens, A. anceps, Andropogon selloanus, several species of the genus Aristida,
Leptocoryphium lanatum, Paspalum carinatum, Sporobolus indicus, S. cubensis, sedges of
the genera Rhynchospora and Bulbostylis, and a good variety of legumes of the genera
Mimosa, Cassia, Desmodium, Eriosema, Galactia, Indigofera, Phaseolus, Stylosanthes,
Tephrosia, and Zornia. Scattered trees belonging mostly to two species, the "manteco"
(Byrsonima crassifolia) and the "chaparro" (Curatella americana) occur rather frequently, as
does the "alcornoque" (Bodwichia virgilioides). Groups of trees, usually called "matas", are
common, with sizes that vary between less than 12 m in diameter to one ha or more. They are
considered remnants of the deciduous dry forest that covered much larger areas some years
ago, but humans are rapidly destroying these. Several subtypes of this Trachypogon species
savanna occur and differ in their floristic composition. These savannas have been used
traditionally for extensive cattle raising, but their grasses are of poor quality and productivity is
low. 

The other two Ramia's types are seasonally flooded savannas, which support some level of
inundation for a few to several months a year. They comprise the Paspalum fasciculatum
savannas and the savannas of "banco, bajio and estero". The Paspalum fasciculatum
savannas, locally called "gamelotales", are almost monospecific communities, that support over
two meters of water at peak rainfall, grow over much better soils than the Trachypogon
savannas, have high productivity (up to 25 tn/ha) and provide good pastures during the
drought (Escobar 1977). They comprise about 15% of all Venezuelan savannas. The second
important type of flooded savanna are the "banco, bajio, and estero" savannas that, in
Venezuela, represent about 20% of the llanos. These savannas derive their name from the
topography of the place were they grow, a series of gentle slopes with scarcely two meters level
difference between its upper and lower parts. The "banco" is the higher area, originally the bank
of a former river that has a changed course. The bancos are elongated areas, with sandy soils
and many of them keep remnants of their former vegetation, from the gallery forest. They have a
rich flora dominated by grasses (Gonzalez and Escobar 1977; Bulla et al. 1990); occupy 60-80%
of these savannas and are flooded with 5-20 cm of water at peak rainfall. They have a mixture of
C3 and C4 grasses. Finally the "esteros" occupy the lower part of these savannas, where water
accumulates during the rainy season reaching 50-80 cm depth. They are covered by C3
hydrophilus grasses. Two special cases of flooded savannas that cover comparatively small
areas are the so called by Huber and Alarcón (1988) open flooded savannas ("Estero de
Camaguan"), flooded during most of the year with 30-100 cm of water and characterized by the
presence of the palm Copernicia tectorum; and the "congriales" or bushy flooded savannas of
the Orinoco vegas.

For Colombia, Blydenstein (1967) proposed a more detailed floristic classification of savannas
that includes the following types; a) the Melinis minutiflora savannas (an introduced African
grass), b) the Trachypogon ligularis-Paspalum carinatum savannas; c) the Paspalum
carinatum savanna; d) the Trachypogon vestitus savanna; e) The Paspalum pectinatum
savanna; f) the T. vestitus-Axonopus purpusii savanna; g) the T. ligularis savanna; h) the
Leptocoryphium lanatum savanna; i) the Mesosetum savanna and j) the Andropogon savanna.
Many of them are sub-types of the Trachypogon spp. savanna sensu Ramia (1967). 

Besides these savanna areas, the llanos have a wide variety of forests (Huber and Alarcón
1988; Etter 1998). The most important of these are: a) gallery forests of various types that follow
the courses of the streams and rivers. In some cases the rivers overflow their banks limiting the
gallery forest so it coincides with the extent of the flooding, behaving as a seasonal swamp
forest. A special case is the "morichales" characterized by the presence of the palm Mauritia
flexuosa, and the Orinoco "vegas", evergreen forests of 8 to 20 m high whose more common
species are Inga spp., Combretum frangulifolium, Gustavia augusta, Pterocarpus sp.,
Etaballia dubia, Spondias mombin, Copaifera pubiflora, etc. In other cases the forest occurs
on the higher banks where they avoid flooding and most trees are semideciduous, of medium
height (12-15 m), with a well-developed understory. A recent description of the floristic
composition and diversity of these gallery forests may be found in Stergios et al. (1998); b)
deciduous dry forests probably covered most of the northern part of the central high
Venezuelan llanos, but have been reduced to isolated patches or even very small "matas".
These are deciduous woods 8-15 m high, very dense, with well developed understories of
semi-deciduous shrub stratum. Although their floristic composition varies, frequent species are
Tabebuia billbergii, Godmania aesculifolia, Cassia moschata, Spondias mombin, Copaifera
pubiflora, Bourreria cumanensis, several species of Cordia, Bursera simaruba,
Cochlospermum vitifolium, Hura crepitans, Acacia glomerosa, etc. The c) "matorrales" or
bushlands are 5-8 m high, deciduous and semideciduous, and is most likely secondary
vegetation that developed in zones formerly occupied by deciduous dry forest (Huber and
Alarcón, 1988). They now cover extensive areas in the north of the Venezuelan central llanos.
Characteristic species are Bourreria cumanensis, Randia aculeata, Godmania aesculifolia,
Pereskia guamacho, Prosopis spp., Xylosma benthamii, Erytroxylum sp. and Cereus
hexagonus (Ramia 1974). For the Colombian llanos, Rangel et al. (1995) reported 2,126 species of
plants belonging to 807 genera and 180 families. The highest diversity corresponds to the
Rubiaceae with over seven hundred species, the Leguminosae (255), Poaceae (214) and
Cyperaceae (96). Geographically, the highest diversity is found in the highplains area of the
ecoregion with over 1,500 species.

Biodiversity Features

The llanos ecoregion has less biotic diversity and fewer endemic species than the
adjacent ecoregions; most biodiversity is found in the forests (Ojasti 1990; Rangel et
al. 1995; Stotz et al. 1996; Péfaur y Rivero 2000). There is a small number of
endemic plant species in the llanos. For the savannas, Huber & Alarcon (1988) list
Vernonia aristeguietae, Bourreria aristeguietana, Stilpnopappus pittieri, S.
apurensis, Hymenocallis venezuelensis , Eriocaulon rubescens, Limnosipanea
ternifolia; for the gallery forests, Gustavia acuta. The open savannas are the least
used habitat by the megafauna of this ecoregion, and most of the faunistic richness is
concentrated around permanent and temporary water sources (Pérez and Ojasti
1996).

According to Ojasti (1990), there are 102 species of mammals in the Venezuelan llanos; about
31% of the terrestrial mammal fauna of Venezuela (Linares 1998). Most of them are 59 Chiroptera,
but there are also 17 Rodentia, 11 Carnivora, 5 Edentada, 4 Marsupialia, 2 Primates, 2
Artiodactyla, 1 Perissodactyla and 1 Lagomorpha. The mammalian fauna of neotropical
savannas is rather poor, considering their geographical extent. A surprising characteristic of the
llanos fauna is the almost complete absence of native ungulates, especially in comparison with
the African savannas. Almost all African ungulates are specialized for the savanna ecosystem,
whereas in the Orinoquia savannas only the white-tailed deer (Odocoileus virginianus) is
found, and even this species reachs its highest densities in the gallery forest and the
savanna-forest ecotone (Eisenberg 1999). In the wet and flooded savannas, the large herbivore
ecological niche is occupied by the largest existing rodent, the capybara (Hydrochoerus
hydrochaeris), that reachs weights over 50 kg (Ojasti, 1993). Besides this species, the mammals
more commonly found in the open savannas are the savanna rabbit (Sylvilagus floridanus), and
several species of rodents like Sigmodon alstoni, S. hispidus, Zygondotomys brevicauda, and
Orizomys bicolor (Ojasti, 1990). In the gallery forest a much greater diversity of large and
medium-size mammals: pecaríes (Tayassu tajacu and T. pecari), tapirs (Tapirus terrestris), deer
(Odocoileus virginianus, Mazama americana), monkeys (Cebus nigrivittatus, Alouatta
seniculus), large rodents (Agouti paca, Dasyprocta spp, Coendou prehensilis), and several
felides like pumas (Puma concolor), jaguars (Panthera onca), and ocelots (Leopardus
pardalis).

Colombia has the richest avifauna of any country in the world (more than 1700 bird species), but
less than 40% of them are found at the Colombian llanos (Rangel et al 1995). Roughly, at least
half of the 1,313 bird species recorded in Venezuela (Phelps and Meyer 1978) include the llanos
in theiir distribution. Over one hundred of the birds reported for the Orinoquia are migratory
birds that winter in the llanos (Stotz et al. 1996). Most of the birds of the llanos inhabit and are
usually restricted the gallery forest (Stotz et al. 1996). In contrast, habitat specialization is rare in
savanna birds, and many of them are able to proliferate in agricultural areas as is the case of
almost all seed-eater birds (pigeons, doves, finches, sparrows, crested bobwhite). Wading and
aquatic birds represent a large portion of the total bird fauna in the flooded savannas (Pinowsky
and Morales 1981; Gomez-Dallmeier and Cringan 1989). They are one of the major tourist
attractions in the ecoregion, given many of them are large colorful birds that form large
aggregations around water sources. 

A fairly large number of herpetological fauna exisits in this ecoregion; mainly in the forests and
the "bancos, bajíos, and esteros" savannas, but is comparatively poor in Trachypogon
savannas (Rivero-Blanco and Dixon 1979). A total of 36 amphibians and 75 reptiles have been
reported for the Venezuelan llanos (Péfaur and Rivero 2000), whereas 28 amphibians and 119
reptiles are included in the list of species for the Colombian llanos (Rangel et al. 1995). Some
reptile species deserve mention: Arrau sideneck or Orinoco turtle (Podocnemis expansa), the
largest american fluvial turtle, reaching weights of over 50 kg; the Orinoco cocodrile
(Crocodylus intermedius) which is the only species of cocodrile restricted to a single river
basin, and the red-footed tortoise (Geochelone carbonaria) which is the wild species more
frequently used as food for rural populations in the area (Ojasti, 1993)

Some 300 fish species have been reported for the Venezuelan llanos (Machado-Allison, 1993).
The aquatic fauna greatly increases, both in abundance and in number of species, during the
rainy season when there is a substantial expansion of the area covered by water and changes in
the level of some rivers that may reach 8 m. In contrast, during the drought, only some fishes
with respiratory adaptations to breath atmospheric oxygen can survive outside the rivers given
that the high temperature (30-40 °C) and poor water circulation cause a drastic reduction in the
amount of oxygen in the water (Machado-Allison 1993). 

The number of endemic vertebrates is even lower. There are no endemic birds restricted to the
llanos ecoregion (Wege and Long 1995), and only two mammals: the marsupial Monodelphis
orinoci and the edentate Dasypus sabanicola (Eisenberg and Polisar 1999). Herpetological
endemism in the llanos is very low in comparison with adjacent ecoregions (Péfaur and Rivero
2000). One of them is the Orinoco crocodile (Crocodylus intermedius) one of the most world’s
endangered crocodilians (Muñoz and Thorbjarnarson 2000).

Contrary to what happens in the south of South America, no mammals were intentionally
introduced in the Orinoquia, except domestic animals. However, the cosmopolitan rodents Mus
musculus y Rattus rattus are an important plague in cereal crops. The most relevant example of
the introduction of a fish in the Venezuelan llanos is the "mojarra" (Caquetaia kraussii), a
native species to other parts of Venezuela that was introduced in the flooded savannas and is
now a dominant species in the area (Machado-Allison, 1993). 

According to the red book of the Venezuelan fauna (Rodriguez and Rojas-Suarez, 1999) and the
apendix III of CITES for Colombia, the following species that inhabit the llanos are at risk of
extintion: the giant armadillo (Priodontes maximus) virtually extinct north of the Orinoco; the
giant river otter (Pteronura brasiliensis), a species that till the sixties was common in the
Orinoco River and its tributaries, but today is one of the most endangered otter species of Latin
America; the ocelot (Leopardus pardalis) that although severely affected in the llanos persists
in the forests south of the Orinoco river; the jaguar (Pantera onca), the largest american felidae
which has been severely hunted in the llanos both for sport and because ocassionally may
attack cattle; the tapir (Tapirus terrestris), very abundant in the past but now drastically
reduced to some scattered areas; the manati (Trichechus manatus), still abundant in some areas
of the high Orinoco, but intensively hunted; the Arrau sideneck (Podocnemis expansa) whose
populations have fallen to alarmant levels in spite of the efforts made for its protection; and
finally, the Orinoco crocodile (Crocodylus intermedius), considered in critical risk of extinction,
but fortunately, several captive-breeding stations have been established which released over
1500 animals during the last decade (Muñoz and Thorbjarnarson, 2000). In contrast, there are no
birds in serious risk of extinction in the llanos (Wege and Long, 1995). Bird species of the area
listed as vulnerable are: sharp-tailed ibis (Cerbibis oxycerca) whose distribution is restricted to
the llanos and is the most scarce ibis species found in Colombia and Venezuela; and the scarlet
macaw (Ara macao), the macaw most used as a pet.

Current Status 

According to White et al. 2001, 71% of the South American savannas have been
converted to croplands and 5% are now urban areas. Most of this devastating
transition took place in the Brazilian Cerrados during the last forty years, but the
modification of the Orinoquia has been significant (Bisbal 1988; Rippstein et al.
2000) and will continue to increase in the future because this ecoregion is the center
of agricultural production, and more recently, of oil production for both countries. 

A total of 1.2 million ha are protected in the Colombian Orinoquia as National Parks of
"Cordillera de Los Picachos", "El Tuparro" and "Tinigua" (Rangel et al., 1995). In the
Venezuelan llanos also 1.2 million ha are protected in the National Parks of "Aguaro-Guariquito"
in the high llanos, "Cinaruco-Caparo" in the lowlands of Apure State, and "Río Viejo". Besides,
there are four fauna refugies: "Tortuga Arrau", "Caño Guaritico", "Estero de Chiriguare" and
"Morichal Largo".

Types and Severity of Threats 

The llanos ecoregion is being affected by several transformations; below we list the
most important: (a) Agriculture: Cattle raising is by far the main activity in the
ecoregion and it is responsable of many changes in the area. There are 15 million
head of cattle in the ecoregion (MAC 1998; Pardo et al. 1999). Given the low
quality of the native grasses, fire is used regularly to increase their quality, the forests
are cleared to increase pasture lands and natural savannas are being replaced by
introduced pastures. There are 1.3 million ha being used as introduced pastures in
the Colombian llanos (Pardo et al. 1998), and about 4 million in its Venezuelan
counterpart (MAC 1998). Besides, a rapidly increasing area is being cultivated with
different crops, especially corn and rice. The 200,000 ha dedicated to the rice crops
in the western Venezuelan llanos attract huge flocks of migrant birds like the whistling
ducks (Dendrocygna viduata, D. autumnalis, and D. bicolor) and the dickcissel
(Spiza americana). These birds cause serious damage to the crops that in some
cases may reach 100% of the harvest (Gómez-Dallmeier and Cringan 1989)
because of this the ranch owners kill these birds in large numbers. Dickcissel is now
considered an endangered species (Stotz et al. 1996) due to the rapid decrease in its
population numbers caused in part by this massive annihilation in the rice crops of
Venezuela (Audubon, dickcissel research project). 

(b) Deforestación and farming for the wood industry: The Venezuelan llanos have the highest
deforestation rate in the country (Bisbal 1988). Between 1950 and 1975, 1.3 million ha were
deforested in the western Venezuelan llanos (Veillon 1977); from this date to present the
average deforestation rate in all Venezuelan llanos has been 34,000 ha/year (Bisbal 1988). A
similar situation occurs in the foothills of the Colombian Orinoquia where deforestation reached
figures of 4.4% between 1979 and 1988 (Viña and Cavelier 1999). In contrast, half a million ha of
savannas in the llanos of Monagas have been transformed to Pinus caribeae plantations
during the last 30 years, and about 100,000 ha more will be sowed at Guarico State in the next
years. The pines completely eliminate the original savanna vegetation, a fact that greatly affects
the fauna of the area (Bulla and Bach 1999). In places were the pines were harvested, there is
some indication that a comparatively fast recovery of the savanna takes place, but a minimum of
20 years seems neccessary to achieve a near natural condition. 

(c) Oil industry: Almost 3 million ha of Venezuelan llanos has been affected by the oil industry
(Bisbal 1988). This is also one of the main threats in the Colombian Orinoquia, because it may
produce a wide spectrum of disturbances, such as deforestation, habitat fragmentation by
roads, increment in human settlements, as well as air and water contamination (Rangel et al.
1995). 

(d) Dikes and ponds: The llanos ecoregion is also the most affected by the construction of
dikes in Venezuela (Bisbal 1988). All over the ecoregion there are thousands of small permanent
ponds made by the land owners to provide water to their livestock during the drought, which
also benefits wildlife. In the "banco, bajío, and estero" savannas, an area of 190.000 ha has been
covered by a network of low dikes, the so-called "Modulos de Apure", whose purpose is to
control flooding during the rainy season and save water for the cattle during the drought. This
transformation completely altered the hydrologic flood/drought cycle of these savannas,
artificially increasing the level of flooding and almost eliminating drought. These changes
greatly impact the vegetation reducing its diversity by half (Bulla et al. 1990), but it benefited
the livestock (Tejos et al 1990), as well as the aquatic and wetland fauna (Ramos et al 1981;
Pinowski and Morales 1981). 

According to Baruch (1996) there are four African grasses that behave as very agressive
invaders in Venezuelan savannas. These are Melinis minutiflora, very successful in savannas
above 600 m.a.s.l. and rather abundant in Colombia; Hyparrenia rufa, in lowland savannas with
poor soils and marked dry season; Panicum maximum, in humid and relatively fertil areas, and
Brachiaria mutica in periodically flooded savannas. All these species generally occur on the
wetter (but not inundable) and/or more fertile habitats of the savanna, and are consequently
favored by the fertilizers used for the agriculture.

Justification of Ecoregion Delineation

These vast savanna shrublands dominate the northern Orinoco River Watershed, and
form the transitional zone from the xeric habitats to the north, and the moist forests to
the south. This is a nationally and internationally recognized ecoregion – and our
linework follows the classifications of Huber & Alarcon (1988). From their map we
lumped many of their fine scale delineation’s (subregion B.2 llanos) to meet our
broader classification, including: "high central llanos", "low central llanos",
"southwestern llanos (Apure llanos)", "eastern mesas", "eastern llanos", and portions
of "western llanos". Southern portions of the "western llanos" classification were
lumped into dry forest ecoregions for their dry climate and unique species
associations. 
                     


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