Bolivian Ecoregions: Bryophyte Diversity and Composition
One of the many questions one can ask about biodiversity is – “what is the contribution to diversity from the major ecoregions or vegetation zones?” With a focus on taxonomic diversity, further inquiries are of interest including the composition – the common and rare species found in each ecoregion, and the similarities and differences within and between ecoregions with regard to families, genera, and species. The objective of this section is to address these various questions about bryophyte diversity in Bolivia
Origin of the Bolivian Landscape
The entire land surface is the product of the Andean orogany that commenced just before the onset of the Cenozoic (Lamb 2004). Approximately 65 million years ago along the western margin of what is today the border of Bolivia and Chile the first volcanoes appeared, a product of the Nazca Pacific plate making contact and subsequent subduction under the continental South America plate. Directly to the east of this developing chain of isolated (or possibly connected in part) volcanic range was a large inland sea or lake, and to the east of this expanse of water vast lowlands. Within approximately 15 million years the lake was replaced by debris out-washed from the western volcanic range and the newly formed eastern range, the latter range a product of up-thrusting and horizontal compression. Further development involved increased sedimentation between the older western and younger eastern mountain ranges, and lateral “growth” eastward of the eastern range. The developing and expanding eastern range came in contact and overrode the Brazilian shield leading to a series of parallel minor ranges along the eastern flank. Thus today the western cordillera was and is the zone of volcanic activity. The detritus flowing in and filling up the one-time sea from both the western and eastern ranges now forms the impressive 4000 meter high plateau of the Altiplano. The eastern cordillera and eastern flanking sub-Andean range run perpendicular northwest to southeast to near the city of Santa Cruz, then oriented directly north-south, this abrupt turn is referred to as the “knee” or “elbow” of the Bolivian Andes.
Present Bolivian Landscape
The present landscape of Bolivia can be divided between the Andean highlands occupying ca. 40% of the land surface in the southwest, and the lowlands with 60% of the land surface fanning out to the north and east.
Highlands. The southwestern portion of the country consists of parallel cordilleras and elevated plateau. The Bolivian Andes consists of two mountain ranges, the western range (Cordillera Occidental) and eastern range (Cordillera Oriental) with the arid and expansive Altiplano plateau lying between (ca. 800 kilometers long and 130 kilometers wide). The highland vegetation ranges from dry to wet grassland to wet and dry forest. The Yungas montane forest extend from Peru’s eastern cordillera and cut diagonally along a narrow band to the southeast about midway through the country, and then replaced by Tucumano-Boliviano montane forest that turns sharply south and extend into northernmost Argentina. Dry forest borders the isolated Tucumano-Boliviano forest on either side. Humid puna, similar if not equal to the pramo of the northern Andes, borders the Yungas montane forest to the southeast, and dry puna extends further south.
Lowlands. The eastern and northeastern portion of Bolivia contains the vast lowlands, referred to as the Oriente. Elevation is mostly below 500 m (lowest point - Río Paraguay at 90 m elevation). The principle lowland vegetation contains both forest and grasslands. Humidity and moisture decreases from north to south. Amazon forest in the north gives way to the central region of semi Chaco-deciduous Chiquitano forest and both open Cerrado and savanna vegetation, and in the south, the dry forest.
There are two recent maps and accompanying text for the vegetation of Bolivia. Each presents a slightly different classification and definition of the vegetation zones (or ecoregions) in Bolivia. Ibisch and Mérida (2003, 2004) recognize 12 major ecoregions (vegetation zones) with 19 subecoregions or subzones (see map). Navarro and Ferreira (2004; see also Navarro & Maldonado. 2002) also recognize 12 major vegetation zones but with 38 subzones (see map and legend). Both interpretations share much in common in defining and delimiting the boundaries of the vegetation in Bolivia, differing in some names and subdivisions. Each is equally useful in gaining an understanding of the complexity of vegetation types represented in Bolivia. The advantage of the classification presented by Navarro and Ferreira (2004) is a more precise definition and distribution based on geographical features, e.g., river basins; a further asset is a more detailed list of species comprising the different plant communities or formations within each of the 38 subzones.
Map of the Ecoregions of Bolivia. Courtesy of FAN, Fundación Amigos de la Naturaleza), Santa Cruz, Bolivia.
This first effort to characterize the diversity and composition of Bolivian bryophytes recognizes seven major ecoregions (vegetation) zones. Lowland ecoregions include: the Amazon, Chiquitano, and Chaco (or Gran Chaco); highland ecoregions include: the Yungas, Tucumano-Boliviano, Dry Inter-Andean Valleys, and Puna. Present knowledge is not sufficiently specific, i.e., label data, to fully characterize all species present within a given ecoregions. The availability of a more standardized vegetation classification will allow a more precise description for future bryophyte studies and collections.
Map and ledged of: Zonas de Vegetation Potencial de Bolivia. Courtesy of G. Navarro.
The seven ecoregions recognized with the links given below are at different stages of preparation. The Amazon, Dry Inter-Andean Valleys, and Tucumano-Boliviano, and Puna are relatively complete. The Yungas, certainly the most diverse and challenging of the ecoregions, will be completed in late 2007. Further inventories are required for the Chiquitano and Chaco (also the Chaco Serrano not yet considered). Cerrado has not been yet considered for lack of data. The Cerrado vegetation occurs in isolated regions of the Amazon, Chiquitano, and Chaco. While it may be assumed that the affinities of the Cerrado bryophytes may reside with each of these three surrounding ecoregions, only further detailed fieldwork can address this question.
Important phytogeographical units within Bolivia
Virtually every politically defined country exhibits to one extent or another limits or boundaries of phytogeographical or vegetational interest. Within the borders of Bolivia there are several of note. The Yungas (true) montane forest (including the premontane forest) reaches its southernmost range in Bolivia. It is most assuredly Bolivia’s richest region for bryophyte diversity in only 7% of the country. The Tucumano-Boliviano montane forests are only known from Argentina and Bolivia. The most extensive and well conserve forests are in Bolivia. This is now considered Bolivia’s most endangered forest type (Ibisch & Mérida, 2003). The Amazonian forests reach their southwestern range in central Bolivia. The Chaco forests reach their northern limits in southeast Bolivia. This dry vegetation is well represented, if not in better conserved condition than either Paraguay or Argentina.
Analysis and Limitations
Data drawn from specimen labels should be one of the major sources in compiling a list of the species from the various ecoregions. The other source would be simply personal field experience. We have both sources, but much more is still required. Many collections lack precise data on the type of vegetation, in part due to a lack of a uniform vegetation classification that was generally accepted by field botanists, and not all field botanists provided consistent or adequate label data. Personal experience requires a period of not only gaining familiarity with bryophyte taxa (a good long learning curve), but also gaining familiarity with the general types of vegetation. Given that we are now at a point of having a reasonable knowledge of many of the bryophytes and a general classification (and terminology), it seems reasonable that future attempts in the analysis of Bolivian bryophytes and associated ecoregions can be attained. Links to the Bolivian Ecoregions:
Dry Inter-Andean Valleys
Permission to reproduce maps has been kindly provided by the following: Fundación Amigos de la Naturaleza (FAN), Santa Cruz de la Sierra (maps of the country and ecoregions), and G. Navarro (see Navarro & Ferreira. 2004), Universidad Mayor de San Simón, Cochabamba (map and legend of Bolivian vegetation).
Lamb, S. 2004. Devil in the Mountains: A Search for the Origin of the Andes. Princeton University Press, Princeton.
Ibisch, P. L. & G. Mérida (eds.). 2003. Biodiversidad: La Riqueza de Bolivia. Editorial FAN, Santa Cruz de la Sierra, Bolivia. [The most recent detailed account of the biodiversity and conservation for Bolivia. Excellent maps, photographs and discussions of the Bolivian vegetation. The English version given below.] Ibisch, P. L. & G. Mérida (eds.). 2004. Biodiversity: The Richness of Bolivia. Editorial FAN, Santa Cruz de la Sierra, Bolivia.
Navarro, G. & W. Ferreira. 2004. Zonas de vegetación potencial de Bolivia: Una base para el anlisis de vacíos de conservación. Revista Boliviana de Ecología y Conservación Ambiental 15: 1-40.
Navarro, G. & M. Maldonado. 2002. Geografía Ecológica de Bolivia: Vegetación y Ambientes Acuticos. Centro de Ecología Simón I. Patiño-Departamento de Difusión. Imprenta Sirena Color, Santa Cruz. xv, 719 pp. [Provides a detailed description of plant communities, vegetation and characteristic species of each. Excellent photographs provide a good impression of the types of vegetation and representative species.]