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Sierra de la Laguna dry forests

Located at the southern end of the Baja California peninsula, this region was once an isolated island and contains a high number of endemic species. After joining the mainland, this region gave way to speciation and is high in species diversity; this portion of the peninsula contains the majority of the species found here. This subtropical dry forest is threatened by cattle grazing and the hunting of predators. Designated as a Protected Natural Area (PNA), hopefully this unique and important ecoregion will be saved from fragmentation.

  • Scientific Code
    (NT0227)
  • Ecoregion Category
    Neotropical
  • Size
    1,500 square miles
  • Status
    Vulnerable
  • Habitats

Description
Location and General Description
This ecoregion is contained in a larger area known as the Cape Region, and constitutes the southernmost part of the Baja California peninsula. The area is considered an island of vegetation (Arriaga & Ortega 1988, Challenger 1998) due to its particular origin as an isolated land area, ten million years ago (during the Miocene), which later rejoined the more desert-like peninsula. The region is shaped by a vast complex of granitic mountains, running southward from the Gulf of California to the Pacific. These mountains are intersected by valleys and canyons, and surrounded by vast plateaus. The topographical features and geological events that gave rise to The Cape Region are responsible for the diversity of climates and of vegetation in this area. Subtropical dry forests, with < 500mm of precipitation per year, dominate lower portions of the mountains (300m – 800m). The trees in this region endure a long dry season during which, the majority of them lose leaves (Rzedowski 1988). The forest is transitional both with the pine-oak forests at higher elevations, and with the xeric scrub at lower portions. The dry forest of Sierra de la Laguna is characterized by abundance of low trees and scrubs, and poor vertical stratification (Arriaga & León 1989). The main tree species in the subtropical forest are Mauto (Lysiloma divaricata), Palo blanco (L. candida), Cajalosucho (Bursera microphylla) and Palo zorrillo (Albizia occidentalis). Herbaceous elements are poorly developed, but their representatives are Caribe (Cnidoscolus angustidens), Buena mujer (Aster spinosus), Solanum spp., and cacti such as Biznaga (Ferocactus spp). Given its biological and climatic conditions, Sierra de la Laguna dry forest is not analogous to the more continental dry forests located in Sonora and other parts of Mexico (Arriaga & Ortega 1988).

Biodiversity Features
The past and present isolation of Sierra de la Laguna from the rest of the peninsula has played a major role in producing an extraordinary array of unique species. Isolation and their alleged history of contractions and expansion, has promoted high levels of speciation in dry forests of many tropical regions (Ceballos 1995). There are 224 species of vascular plants inhabiting Sierra de la Laguna dry forests. Of 138 species of spiders and collembola, 38 (27%) are found in the dry forest (Jiménez, 1988; Vázquez, 1988). Half the species the reptiles and amphibians of Sierra de la Laguna inhabit the dry forest habitat (Alvarez-Cárdenas et al. 1988), over 29 of 194 species of birds (Rodríguez-Estrella 1988), and 29 of 40 species of mammals (Gallina-Tessaro & Gonzalez 1988). The degree of endemism is high (Arriaga & Ortega 1988), and this is well demonstrated by the proportion of endemic species with respect to total recorded species: over ten percent of animal and plant species found at Sierra de la Laguna are endemic. This corresponds well with the fact that endemism in tropical dry forests is higher than in moist forests (Rzedowski 1993). The isolation of this region has contributed to a scarcity of predators, and to the poor competitive ability of some animals (Arriaga & Ortega 1988). Rodents and lagomorphs are virtually absent from the region, favoring the abundance of species that would otherwise be abated by direct competition with them (such is the case with the acorn woodpecker, Melanerpes formicivorus)( Rodríguez-Estrella 1988). Large areas of habitat still remain intact, as the topographical features make this ecoregion difficult to explore and exploit (Arriaga 1988).

Current Status
Subtropical dry forests have been considered one of the most threatened of the tropical ecosystems (Janzen 1988), and in the state of Baja California Sur they hold most of the biodiversity for the state (León de la Luz et al. 1988). Portions of the Sierra de Laguna dry forest ecoregion still remain intact. However, accessible areas are being converted for cattle grazing. There is still no recognizable fragmentation of the habitat, mostly due to the reduced exploitation of forest resources (Ortega & Arriaga 1988). In June 1994 the region was established as a Protected Natural Area (PNA) with pine-oak forests and subtropical dry forest as the key areas for protection.

Types and Severity of Threats
The fragile nature of this region requires legal protection over a long period, in order to prevent it from disturbances caused by human overpopulation and exploitation of forest resources for livestock. Native villagers often killed wild species (mostly predators) that are considered as threats to their domestic animals (Ortega & Arriaga 1988). If this continues, it could alter the natural processes maintaining biodiversity in the area. Although human disturbance in Sierra de la Laguna has been kept to a minimum, any uncontrolled perturbation to this fragile ecosystem could create an imbalance. This would ultimately cause the disappearance of an extraordinary array of evolutionary phenomena that produce such a unique vegetation assemblage in the middle of an enormous desert.

Justification of Ecoregion Delineation
The delineation for this ecoregion were derived according to the current landcover classifications of INEGI (1996) and compared with Rzedowski (1978). Following this classification, we lumped oak-pine forests, subtropical matorral ("crasicaule" and "sarcocaule"), and "selva baja cadulifolia" with all encompased human modified habitats to derive our initial linework. This was then reviewed and modified by expert opinion during several ecoegional workshops (CONABIO 1996 and 1997).

References
Alvarez-Cárdenas, S., P. Gallina-Tessaro, y A. González. 1988. Herpetofauna. Pages 167-184 in L. Arriaga, and A. Ortega, editors, La Sierra de la Laguna de Baja California Sur (México: Centro de Investigaciones Biológicas de Baja California Sur.

Arriaga, L. 1988. Importancia ecológica de las perturbaciones exógenas en un bosque de pino-encino. Pages 115-132 in L. Arriaga, and A. Ortega, editors, La Sierra de la Laguna de Baja California Sur,México: Centro de Investigaciones Biológicas de Baja California Sur.

Arriaga, L., y J. L. León.1989. The Mexican tropical decidous forest of Baja California Sur: A floristic and structural approach. Vegetatio 84: 45-52.

Arriaga, L., y A. Ortega. 1988. Características generales. Pages 15-26 in L. Arriaga, and A. Ortega, editors, La Sierra de la Laguna de Baja California Sur, México: Centro de Investigaciones Biológicas de Baja California Sur.

Ceballos, G. 1995. Vertebrate diversity in Neotropical decidous forests. Pages 195-220 in S.H. Bullock, H.A. Mooney, y E. Medina, editors, Seasonally dry tropical forests. Cambridge, U.K.: Cambridge University Press.

Challenger, A. 1998. Utilización y conservación de los ecosistemas terrestres de México. Pasado, presente y futuro. México: Conabio, IBUNAM y Agrupación Sierra Madre.

CONABIO Workshop, 17-16 September, 1996. Informe de Resultados del Taller de Ecoregionalización para la Conservación de México.

CONABIO Workshop, Mexico, D.F., November 1997. Ecological and Biogeographical Regionalization of Mexico.

Gallina-Tessaro, P., y A.González. 1988. Mastofauna. Pages 209-228 in L. Arriaga, and A. Ortega, editors, La Sierra de la Laguna de Baja California Sur. México: Centro de Investigaciones Biológicas de Baja California Sur.

INEGI Map (1996) Comision Nacional Para el Conocimiento y Uso de la Biodiversidad (CONABIO) habitat and land use classification database derived from ground truthed remote sensing data Insitituto Nacional de Estastica, Geografia, e Informática (INEGI). Map at a scale of 1:1,000,000.

Janzen, D. H. 1988. Tropical dry forests. The most endangered major tropical ecosystem. Pages 130-137 in E. O. Wilson, editor, Biodiversity. Washington: National Academy Press.

Jiménez, M. L. 1988. Aspectos ecológicos de las arañas. Pages 149-166 in L. Arriaga, and A. Ortega, editors, La Sierra de la Laguna de Baja California Sur. México: Centro de Investigaciones Biológicas de Baja California Sur.

León de la Luz, J. L., R. Domínguez, y R. Coria. 1988. Aspectos florísticos. Pages 83-114 in L. Arriaga, and A. Ortega, editors, La Sierra de la Laguna de Baja California Sur. México: Centro de Investigaciones Biológicas de Baja California Sur.

Ortega, A., y L. Arriaga. 1988. Consideraciones finales. Pages 229-237 in L. Arriaga, and A. Ortega, editors, La Sierra de la Laguna de Baja California Sur. México: Centro de Investigaciones Biológicas de Baja California Sur.

Rodríguez-Estrella, R. 1988. Avifauna. Pages 185-208 in L. Arriaga, and A. Ortega, editors, La Sierra de la Laguna de Baja California Sur. México: Centro de Investigaciones Biológicas de Baja California Sur.

Rzedowski, J. 1978. Vegetación de Mexico. Editorial Limusa. Mexico, D.F., Mexico.

Rzedowski, J. pers.comm. at CONABIO Workshop, 17-16 September, 1996. Informe de Resultados del Taller de Ecoregionalización para la Conservación de México.

Rzedowski, J. 1993. Diversity and origins of the fanerogamic flora of México. Pages 129-144 in T.P. Ramamoorthy, R. Bye, A. Lot, y J. Fa, editors, Biological diversity of Mexico: Origins and distribution. New York: Oxford University Press.

Vázquez, M. M. 1988. Fauna colembológica de hojarasca y suelo. Pages 133-148 in L. Arriaga, and A. Ortega, editors, La Sierra de la Laguna de Baja California Sur. México: Centro de Investigaciones Biológicas de Baja California Sur.

Prepared by: Alejandra Valero, Jan Schipper, and Tom Allnutt
Reviewed by: In process

 

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