Paraguana xeric scrub

Introduction

caption Bonaire, Venezuela (Photograph by Cayrn Ernst)

The ecoregion, called the Paraguaná xeric scrubland, is located in arid and semi-arid areas of northwestern Venezuela between 10° and 12° north latitude. This is paradoxical, however, because abundant precipitation would be expected with this geographic location. Walter suggests that this is due to the high drying power of the trade winds blowing constantly all along the coastal plain. Lahey attributes it to the combined effect of a high pressure center and the thermal contrast between the sea and the continent. Medina points out that besides reduced precipitation, there is also an effect on the distribution of rain, both on a daily and annual basis. Díaz concludes that there is erratic distribution in time and space. The high-velocity northeast trade winds combined with the presence of soils with high salt content, principally calcite, have given rise to a very specific composition of flora and fauna, with ecophysiological adaptations that allow them to survive in these extreme environments.

Location and General Description

This ecoregion lies within two physiographic provinces of the four existing in the northwest region of Venezuela: the coastal plains where the Paraguaná peninsula and the depression valleys of Lara and Falcón stand out, adjacent to the Andean piedmont. The xeric scrubland of Paraguaná develops on flat lands, with landscape on hills and the piedmont. This topographic variability generates contrasts in the composition of the flora in areas that are relatively close to each other. Thus, in the seaside plains herbaceous-bushy vegetation develops on complexes of mobile dune and saline depressions. Moving inland, xerophytic elements and deciduous trees develop, alternating with evergreens that sometimes form true woody masses.

Geologically, the peninsula was an island in the Pliocene that was joined to dry land by the isthmus of Médanos during the Holocene. The sands of the present-day dunes are very recent and are constantly renewed by material coming from the Vela Gulf. On the coast, sandy and sandy-silty beaches predominate, generally with very turbid waters. Toward the interior of the peninsula there is a hilly landscape where the Santa Ana Ridge stands out as the only elevation (830 meters (m) above sea level). The vegetation that develops there, due to the mass effect, includes the following types of vegetation: thorny plants located in the piedmont (0 a 300 m), deciduous basimontane tropophilous forests (300 to 550 m), cloud forests (550 a 700), Antillan scrubland (700 a 800), consisting of small trees with bush-like characteristics, and pseudo-paramo vegetation (800 to 830 m) with dwarf woody plants. With the exception of the first type, the others are far from forming a part of today’s ecoregion. These basimontane forests and cloud forests have heights varying from low to medium, and are generally very dense with two stories and developed undergrowth. The most common species include: Protium tovarense, Tetrochidium rubrivenium, Hieronyma moritziana var. fendlieri, Aichomea triplinervia, Qualea calophylla, Laplacea fruticosa, Graffenrieda latifolia, Clusia multiflora, Didymopanax glabratum, Ladenbergia moritziana, Carica microcarpa, Chamaedorea sp., Geonoma div sp. Catoblastus prae-morsus. Endemic species includes: Geonoma paraguanensis, Philodendrum holtonianum, Rodospatha falconensis.

The valleys of the Lara and Falcón depression include areas with intermediate or transitional relief between the two great mountain systems of Venezuela: the Andes and the coastal range. As on the peninsula, more favorable climatic conditions contribute to the persistence of low deciduous and/or evergreen trees in the bush lands, ranging from dense scrubland to small woody masses. Geologically, the valleys and depressions comprise important extensions of ancient Quaternary and recent sediments.

The climate is arid and although, as noted, it is erratic, there is a tendency for rainfall to cluster toward the end of the year. On the peninsula precipitation rarely exceeds 300 millimeters (mm) per year, as occurs all along the northern coast of Venezuela and the islands in the Caribbean. In the Lara and Falcón valleys, precipitation varies from 350 mm in the drier areas to some 1000 mm in the areas of the Andean piedmont. In this ecoregion, the predominant winds have velocities between 7 and 11 meters per square second (m/s2), especially in coastal areas. Temperatures at ground level and in exposed areas may reach 50 degrees Celsius (°C), although beneath the vegetation the temperature may reach 15°C to 20 °C.

The [hydrographic network of this ecoregion can be broken down into two groups. On the Paraguaná peninsula there is a notable absence of important rivers and only intermittent waterways and streams. Inland, the situation is somewhat different; the Mitare river feeds the Pedregal and Paraíso rivers and empties into the Coro gulf. The Morere, Barigua and Bucares rivers come together to form the Tocuyo river, one of the most important rivers in the area, emptying into the Caribbean near the Triste Gulf. The Tocuyo river crosses the ecoregion called the Lara Falcón Dry Forest. In the valleys of the Lara-Falcón depression, the Turbio river is the most important due to its volume of water; its mouth is near the Orinoco river basin.

Within the ecoregion the following plant formations can be differentiated: coastal grasslands, brush, scrubland and cardonales and/or briars. The percentage of trees varies according to prevailing environmental conditions, giving rise to true mosaics of vegetation. It is for this reason that some authors recognize a still greater number of categories.

Based on the above, in an area limited to the coast there are halophytic and psamophilous grasslands. The first are characteristically low, open communities with no tree or bush elements; they generally have few flowers as they are located in salty depressions. Component plant species include: Atriplex pentandra, Heterostachya ritteriana, Salicornia fruticosa, Batis maritima, Sezuvium portulacastrum. The psamophilous grasslands also form herbaceous-bushy vegetation, with dense and variable cover; they develop on sand dunes along the shore and like the halophytic grasslands have few species. The most important include: Scaveola plumieri, Portulaca pilosa, Cakile lanceolata, Cyperus planifolium, Sporobolus virginicum, Sporobolus piramydatus, Ipomoea pescaprae. and Euphorbia buxifolia, Spartina patens, Lycium nodosus, Calotropis procera, Egletes prostrata, Argusia gnaphalodes, Tournefortia volubilis, Opuntia caracasana, Heterostachys ritteriana, Chamaesyce dioica, Chamaesyce mesembryanthemifolia, Crotom punctatus, Cenchrus echinatus, Tribulus zeyheri, and others. On some isolated dunes there are trees or bushes bent by the action of the wind; these include Conocarpus erectus, Prosopis juliflora and Acacia tortuosa. The coastal xerophilous grasslands are distributed on the peninsula and on the coastal strip between the towns of Coro and La Vela and Coro and Adícora, where they form a narrow, more or less continuous, strip of stabilized dunes. In addition there are the so-called Coro dunes that make up extensive mobile dunes to the north of Coro; they are highly developed and almost totally bereft of vegetation. Toward Cape San Román, the southernmost location on the Paraguaná peninsula, there are dispersed spots of psamophilous grasslands on coastal dunes. In addition, between Coro and Cujima there are extensive areas of mobile dunes with grasslands of this type that are, however, very sparse.

The bush lands are characterized by having primarily bushy plant species or trees under 5 meters high with an irregular canopy. Their physiognomy is variable depending on climatic conditions. They have no forest potential although they do provide firewood and building materials for the urban population. This type of vegetation represents an important factor in protecting the soil, fauna and the environment in general. It can be considered a type of transitional vegetation between the dry deciduous and semi-deciduous forests and the briars. They have a low, very homogeneous story consisting basically of Opuntia caracasana, Lippia oreganoides and Croton flavens. During the rainy season a large number of annuals develop. The most abundant species in the area are: Croton heliaster, Borreria cumanensis, Caesalpinia mollis, Randia gaumeri; Jacquinia. aristata, Caesalpinia. Coriaria, Pithecelobium. Dulce, Capparis odoratissima, Capparis linearis, Caesalpinia coriaria, Capparis teniusiliqua, Pereskia guamacho, Prosopis juliflora, Ritterocereus griseus, Malphigia sp. and Bursera tomentosa, Morisonia americana.

The scrublands represent low bushy communities between 3 and 8 meters high. They are generally, but not exclusively, very dense. There is a high density of armed species. They have been cataloged as degraded deciduous forests. The most notable species are: Prosopis juliflora, Castella erecta, Ritterocereus griseus, Opuntia caracasana, Crotom rhamnifolius, Ipomea carnea, Subpilosocereus attonis, Cercidium praecox, and others.

The briars are predominantly stunted bushy communities with a broad range of thorny species; when columnar cacti are predominant they are called cardonales. The density of the cover can be very variable, from very sparse to totally dense. They develop in areas where the climate corresponds to the driest extremes of the arid and semi-arid zones. The species are late-growing although they are a first-class resource for maintaining wildlife. The most frequent species are: Castella erecta, Prosopis juliflora, Cercidium praecox, Borreria cumanensis, Pithecelobium dulce, Acacia tortuosa, A. fluexuosa, Ritterocerus deficiens, Opuntia caribea, Ipomea carnea, Crotom rhamnifolius, Ipomea carnea, Indigosfera suffruticosa, Teprosia senna, Aristida venesuelae, Calotropis procera, Capraria biflora, etc. Notable cactaceae include: Acanthocereus tetragonus, Cereus hexagonus, Opuntia eliator, Pilosocereus lanuginosus, and others.

In general, the coastal grasslands have clearly sandy soils. The bush lands go to clearly clayey-sandy soil in the xerophilous bush or scrublands, passing through intermediate forms in the briars. All are low in organic content and phosphorous, with pH close to neutral, whereas the content of exchangeable cations Ca++, K+, Na+, Mg++ is relatively high, with a base saturation percentage close to 80%.

In these arid and semi-arid zones, the imbalance existing between evaporation and precipitation proves to be a limitation in terms of water available for the development of vegetation. However, there is a certain buffer effect due to the existence of clear clayey-sandy soils, which in times of good water supply, allows for greater availability of water for plants. In this regard, Medina states that in arid areas sandy soils maintain a more productive vegetation as compared to clayey soils, primarily due to the lesser force with which they retain water molecules. In the xerophilous grasslands there are shallow soils and abundant outcrops of limestone rock. The intermediate textures favor the recovery of plant cover during the rainy season, both in the undergrowth and in bush level.

Biodiversity Features

Despite being in the tropics, this ecoregion does not exactly distinguish itself as having a high number of species. This is offset by development within the species living there of many ecophysiological characteristics that allow them to sustain themselves in these extreme environments. Floristically, the ecoregion is considered one of the best known both in terms of area explored and systematically. Unfortunately, despite the enormous effort made, no regional systematic flora has been published, although we know of preliminary lists. This fact makes it difficult to obtain reliable data on biodiversity. It should be noted that there are recent publications based on data from earlier works regarding biodiversity but focused more on land use than on the protection of endemic species.

In addition, Huber et. al. indicate that reliable information on endemic plants and the degree of plant endemism in Venezuela is sparse, dispersed and difficult to access, in that geographic, plant and taxonomic data are needed that are not yet available. However, Steyermark specified that the great centers of endemisms in Venezuela are associated with the high mountains and that in the case of lowlands, where climatic and ecological conditions are more homogeneous, diversity and endemisms fall off abruptly, with the exception of some areas with edaphic or ecological characteristics that favor speciation or isolation. Although this work was written more than 20 years ago, the criteria expressed coincide with what has been observed for the ecoregion. In effect, even the Santa Ana Ridge itself has been assigned a low level of endemism. Currently, with the development of modern taxonomic techniques, the status recognized for some regions of the country, including these arid zones, could change.

It has been pointed out that the ecoregion is located in an extremely arid area with dunes occasionally populated by coastal grasslands, bush, scrubland or briars as well as fresh water marshes. Although the latter are an exception in extremely dry areas, when surrounded by antagonistic types of vegetation they have given rise to the maintenance of endemic species. They have been a refuge and dormitory for various species of autochthonous and migratory birds. Plant species that are very scarce in the area grow there; this is true of the ferns: Acrostichium aurem, very common in the delta areas of the Orinoco and in the mangrove areas, Marsilea ancylopoda, Nephrolepis hirsutula, Pitygramma trifoliata, as well as the water lily Eichornia crassipes. It is very interesting to learn what the factors were that confined these species and reduced their density within these marshes, in that in other areas outside the arid zones they have been cataloged as species that constitute a veritable plague. This would have interesting implications in terms of both ecology and economics, and useful in controlling them. Other scarce and little-studied species that are found in these marsh areas are: Amoreuxia wrightii, Ipomoea wrightiii, Pluchea odorata, Pluchea sagittalia and Eleocharis mutata. In the sand dune areas, precisely where the birthplace of these topographical formations lies, between Coro and La Vela, a clear coastal area of shore grasses and xerophilous bush free of grazing was maintained for more than 20 years. This allowed the growth of scarce and very interesting plant species, including Sezuvium edmonstoneei, which appears in the literature as endemic to the Galápagos Islands. Other species like Oxycarpha suaedifolia and Atriplex oestophora are very rare species almost endemic to the state of Falcón.

In addition, species like Crossopetalum rhacoma (Celastraceae), a very rare bush 1 to 3 meters high that grows on calcareous outcrop, has only been reported in the Morrocoy National Park in the ecoregion of the Lara and Falcón forests and in Paraguaná Specifically for Cactaceae family, Ponce and Trujillo, following the IUCN criteria, evaluated 44 species indicated for Venezuela and found that 21 of them (47.75%) were distributed among the genera Acanthocereus, Epiphyllum, Mammillaria, Melocactus, Opuntia, Pilosocerues, Disocactus, Rhipsalis, Strophocactus, Subpilosocereus and Wittiocactus. These are endangered to varying degrees, most of them in the rare category although none of them is at risk of immediate extinction. Among the indicated species, we have: Wittiocactus panamensis, Rhipsalis pittieri, Epiphyllum columbiense, Disocactus ramulosus, all of which are protected in different national parks. Although the new World Conservation Union (IUCN) classification does not consider the category of rare, it is clear that habitat destruction as a basic threat is a parameter that must be considered in one of its units.

With respect to fauna, the most notable avian endemisms are those relating to the red siskin (Carduelis cucullata), a small bird with bright red and black plumage that is considered one of the most threatened species in Venezuela. Its current distribution is less than 20% of the original distribution, which included this ecoregion. Currently there are only two known nuclei and one of them is located in the Lara and Falcón valleys. This species is critically endangered. Fortunately, Venezuelan environmental regulations include Resolution No. 439 (12/07/82) that provides for its protection. However, this is not enough as the greatest threats to this species are capture and later sale. The yellow-shouldered amazon (Amazona barbadensis) is in danger of extinction. In fact, the populations in Paraguaná are considered virtually extinct in that only isolated individuals and no more than five individuals together have been reported. Again, the basic cause relates to habitat destruction in the arid areas of Falcón.

Notable among the mammals is the dryland mouse opossum (Marmosa xerophila), which has been recorded for some localities in the state of Falcón, although it is likely that its distribution extends throughout the ecoregion. This a species endemic to Venezuela and Colombia, confined to dry forests. It is believed that a portion of its distribution is found within the Coro Dunes National Park. Rodríguez and Rojas-Suárez stated that at the time knowledge was inadequate but it did not appear to be under imminent threat. In any case, given that it is a species with restricted distribution and the conversion of its habitat is likely, they recommended that natural history studies be conducted in order to design control and management measures based on certainties. In this regard, it is known that many institutions in the area concluded related studies but it is not known whether their findings are being considered. Unfortunately, one year later, this species appears in the IUCN Red Book as critically endangered. Other species indicated as vulnerable are: the ocelot (Leopardus pardalis), puma (Puma concolor) and jaguar (Panthera onca). Given the characteristics of these felines, their presence in the ecoregion is more associated with the ecotonal areas with ecoregions that are more complex both in terms of structure and vegetation such as on the Santa Ana Ridge.

Recently incorporated in the catalogue of new species is the anuro (species of poison-arrow frog) Mannophryne lamarcai, which lives in a very reduced area of the Socopó Ridge (520 m, state of Falcón). Although its presence is limited to relicts of cloud forests, they are found in enclaves within the ecoregion, where expansion of the agricultural frontier in search of land seems imminent along with a decline in rainfall and consequently in relative humidity, as pointed out by Mijares-Urrutia and Arends.

Current Status

The xeric scrublands of Paraguaná are distributed in two states in northwest Venezuela, which based on the country’s political division are Lara and Falcón. The state of Falcón has 12 areas with varying protection status; of these 7 are highly restrictive categories such asNational Park, Natural Monument and Fauna Refuge. In the state of Ara there are 16 protection areas, 6 of which are in the indicated categories. Of these, the following are located in or near the ecoregion: the Coro Dunes National Park with 91,280 square kilometers (km2), the Santa Ana Ridge Natural Monument with 1,900 hectares (ha), both created in 1974, the León Hill Natural Monument with 7,275 hectares and created in 1989.

Types and Severity of Threats

A significant portion of the ecoregion lies within the limits of the Coro Dunes National Park. Thus, representative samples and singular landscapes unaffected by human action should be under protection. However, this national park has been subject to numerous invasions carried out by third parties as well as by regional governments themselves. Thus, the Municipal Dump operates within the borders of the park. In addition, many pipes of untreated water empty into areas belonging to the park. Privately owned salt extraction facilities are also found within the park’s jurisdiction.

Numerous areas have been invaded by unplanned urban and tourism development. This becomes considerably serious as extensive areas of scrubland have already been destroyed, leading to the movement of dunes that once gone leave only a sea of sand. There is a tourism complex that has tried to establish itself in the area for more than 20 years. During the most recent attempt in 1992, an entire movement of organized communities with support from the Francisco de Miranda National Experimental University (UNEFM) was created and succeeded in reining in its aspirations. However, the threat is still latent and in the latest attempt extensive areas of vegetation typical of the ecoregion were destroyed, with the resulting activation of new dunes. Also, some areas of the marsh have been drained for urban development, leading to the destruction of exceptional species in these areas, as already noted.

Extensive livestock and some crops represent a threat to the integrity of the ecoregion. On the peninsula, in the northern strip in the state of Falcón and in the state of Lara, wide areas of xeric scrubland have been destroyed to make way for the planting of vegetables. In this regard, the Centers for Research on Ecology and Arid Zones (CIEZA) has made available to the communities its experience with crops under partial shade, recommending the sowing of some traditional crops that utilize the shade of nearby trees. In addition, regional universities and some government agencies have undertaken various projects with a view to sustainable development.

Besides poaching, particularly of small mammals, there is significant pressure on vegetation in the natural environment. Forty years ago there were incipient timber operations where the demand for cutting has been increasing. This activity resulted in destruction of the dry forests, of which there are only vestiges in the area of Paraguaná and the valleys of the Lara and Falcón depression. Witness to this is the fact that currently only hundred-year-old trees of Guajacum officinale with trunks up to 3 meters DCH (diameter at chest height) can be found. Curiously, Bisbal did not indicate this zone as being particularly affected by the removal of timber. However, his work points out that his analysis was based on official statistics and this activity is specifically carried out furtively but intensively by the most economically deprived population. The timber is used for small-scale building of furniture. This has thus had an effect on different species of columnar cacti and another species attractive for these purposes is Caesalpinia coriaria. Proposis juliflora is used intensively for firewood and for making shelving, fences, etc. Quite probably in his analysis, this type of habitat conversion was considered as part of deforestation activity. Added to this activity is the impact of extensive raising of goats that browse on populations of cují (Prosopis juliflora), the name used locally. Some authors have even pointed out that although the species is far from being considered extinct in that is already a cosmopolitan species to judge from its worldwide distribution, the arboreal forms are disappearing.

In his work, Bisbal considers the following activities in the region that can modify or degrade habitat: deforestation as mentioned, building of dams, petroleum operations, mining operations and road construction. From a merely numerical analysis, it can be concluded that the types of vegetation most affected are those located in lowlands with woody characteristics. The scrublands, bush and briars as such are subject to average pressures. Another form of habitat conversion other than those pointed out by Bisbal is that associated with the area’s climatic conditions and the scarcity of waterways on the peninsula. This has led to indiscriminate removal of aquifers, bringing about their salinization.

Justification of Ecoregion Delineation

The initial delineation of this ecoregion follows the classification of Huber and Alarcon, who classify this region as "xeric spiny shrublands", with areas of agriculture and cultivation engrained. Final linework however follows Smith. This ecoregion represents a unique and distinctive assemblage of species (see description above), with a number of endemics.

Additional Information on this Ecoregion




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Glossary

Citation

Fund, W. (2007). Paraguana xeric scrub. Retrieved from http://www.eoearth.org/view/article/155138

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