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.
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.
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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Prepared by: E. M. Pérez & L. Bulla
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