Location and General Description
This region is delineated by its most prominent feature, the mountain range of Sierra of La Giganta. La Giganta runs north south on the eastern portion of the Baja California peninsula. Igneous (piroclastic, basaltic, and andesitic lava) and sedimentary rocks from the tertiary make up the region (Ferrusquía-Villafranca 1993). Its altitudes vary between 200-1000 m, and in some localized portions they reach more than 1000 m. The climate is dry hot in the narrow coastal plains on the east of the peninsula, but it becomes dry semi-hot as the distance from the coast increases. Precipitation reaches some of the lowest levels of all Mexico (<100 mm/year). Temperature oscilations are extreme.
Dominant flora species are creosote (Larrea tridentata) and desert burr sage (Ambrosia dumosa) (Rzedowski 1988), but other species also found are: Jatropha cinerea, palo fierro (Olneya tesota), Acacia brandegeana, Cercidium floridum, and Pithecellobium undulatum. Species of more mesic habitats occur on the many oases that are present in the peninsula: palma de taco (Washingtonia robusta), Typha domingensis, Phragmites communis and Phoenix dactylifera. The oases are remnants of mesic environments that existed in the peninsula in past times (Maya et al. 1997); they consist of regular-sized bodies of water dispersed throughout the peninsula, and are surrounded by vegetation that belongs to wetlands interspersed with common elements of the xeric scrub (Arriaga et al. 1997).
The Gulf of California xeric scrub represents one of the largest well-preserved extensions of arid lands in Mexico. It is home to many endemic and endangered species. The isolation of the Baja California Peninsula from other deserts and from the continent is in part responsible for the high levels of endemism and diversity of organisms in this region: 238 species of plants, 32 reptiles and amphibians, 199 birds (2 endemic), and 64 mammals (12 endemic). There are 20 endemic genera of plants, and 20-25% of the plant species are endemic; 9 species of the herpetofauna are endemic and 10 are exclusive to oases (Alvarez et al. 1997); 37 of the birds are migrants that come from the U.S. and Canada. The xeric scrubs of Baja California house the oldest tree of any desert: the palo fierro or ironwood (Olneya tesota). More than 160 plant species, depend upon legumes such as ironwood and mesquite (Prosopis pubescens) for their regeneration in arid lands (Nabham & Plotkin 1994), as well as endangered wildlife species like the desert bighorn (Ovis canadensis) and masked bobwhite quail (Colinus virginianus ridewayi), that use ironwood both as shelter and as aforage resource. Ironwood nursing relationships promote diversification and increase richness of other plants in arid ecosystems (Búrquez & Quintana 1994; Tewksbury & Petrovich 1994). Unlike its continental counterpart -the Sonoran desert- biodiversity in the Gulf of California xeric scrub occurs mostly in various oases that remain in this arid region. Oases represent a relict of the ancestral vegetation that once dominated Baja California (Jiménez et al 1997); they serve as refuges for endangered and endemic species or for those species that were extirpated from less dry environments in the peninsula. They also serve as "stopovers" for migrant birds that resume migration after foraging on the rich sources of food that oases provide (Rodríguez-Estrella et al. 1997). Arid lands are characterized by high number of endemic taxa; despite their apparent "aridity", these ecosystems are highly valuable for conservation in terms of the biological uniqueness of biotas they support.
The Gulf of California xeric scrub remains partially intact, despite intensive human activity in and around the area. Large portions of habitat are well preserved, but the system is considered fragile and in high danger of perturbation due to cattle grazing, agricultural fires, and extraction of water from the oases.
Types and Severity of Threats
Cattle has effectively displaced populations of pronghorn, mule deer, and mountain sheep, and hunting of the puma (Felis concolor) has been a common practice among villagers; both phenomena have reduced the populations of the aforementioned mammals. Habitat destruction for agriculture and human settlement threatens the xeric scrub and its many endemic cacti; as a consequence, the introduction of animal and plant species could displace native fauna and flora through direct competition. Tourism activities in the region have become extensive, enhancing the dangers of pollution and other disturbing effects on the flora. The introduction of buffel grass (Cechrus ciliaris) to feed cattle has been especially harmful, because it accumulates combustible litter that causes the complete burning of ironwood and other native plants. As arid grasslands replace the xeric scrub, the recruitment of perennials may be lower or completely non-existent; if this practice remains uncontrolled, the landscape of the region could change dramatically (Búrquez & Quintana 1994). Populations of the peninsular yellowthroat (Geothlypis beldingii) have already disappeared from some of the oases due to human pressure on the habitat. Rodríguez-Estrella et al. (1997) estimate that the same could happen with other birds in the future if habitat perturbation persists. A reserve has been proposed to protect the region (SEMARNAP 1996). Management plans for the area are needed, as fires and logging threaten the habitat and human exploitation of resources and irregular settlements are growing at a quick pace.
Justification of Ecoregion Delineation
This xeric scrub ecoregion is limited to the Gulf of California coast of the Baja Peninsula (and Gulf islands) in Mexico, and is thus isolated from other similar habitats. Delineation’s for this ecoregion follow the of Sierra of La Giganta and the vegeation classifications of INEGI (1996), from which we lumped the following classifications: "sarcocaulous matorral", "crasicaulous matorral", and portions of "agricultural landuse". Reference was also made to Rzedowski (1978), and reviews and revisions to the linework were done by expert opinion at several ecoregional priority setting workshops (CONABIO 1996 and 1997).
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Rodríguez-Estrella, R., L. Rubio, y E. Pineda. 1997. Los oasis como parches atractivos para las aves terrestres residentes e invernantes. Pages 157-196 in L. Arriaga, y R. Rodríguez-Estrella (editors). Los Oasis de la Península de Baja California. México: CIBNOR.
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Tewksbury, J. J., y C. A. Petrovich. 1994. The influences of ironwood as a habitat modifier species: A case study on the Sonoran Desert coast of the Sea of Cortez. Pages 5-8 in G. P. Nabhan, y J. L. Carr (editors). Ironwood: An Ecological and Cultural Keystone of the Sonoran Desert. Occasional Papers in Conservation Biology No. 1. Washington, D.C. USA: Conservation International.
Prepared by: Alejandra Valero, Jan Schipper, and Tom Allnutt
Reviewed by: In process