San Lucan Xeric Scrub

The Espíritu Santo antelope squirrel, sometimes considered a subspecies of white-tailed antelope squirrel, is found only on the island of Espíritu Santo in the Gulf of California. Here it enjoys some of the benefits of being isolated away from predators, although this also means being restricted to only 83 km² of dry shrubland habitat far from the mainland. The squirrels are highly specialized to survive in habitats with limited to no surface water.

The San Lucan Xeric Scrub ecoregion includes the southernmost part of the Baja California peninsula. The area originated in the Miocene as an isolated portion of land before joining the peninsula, and so is considered an “island” of vegetation. The San Lucan Xeric Scrub contains a diversity of landscapes, including granitic mountains, valleys, canyons, and plateaus. The ecoregion occupies the plateaus between the coast and the lower limits of the dry forests, which begin around 250 m above sea level. Precipitation is approximately 400 mm annually. 

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. Some elements of dry forest are present in this community, but traits characteristic of species adapted to dry climates prevail. Cacti, elephant trees, yucca, and a variety of succulents and herbaceous plants proliferate throughout the region. 

There are 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. Dominant flora species are creosote and desert burr sage, but other species are also found, including Jatropha cinerea, palo fierro, 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, Typha domingensis, Phragmites communis and Phoenix dactylifera. The oases are remnants of mesic environments that existed in the peninsula in past times.

Although the ecoregion has not been studied intensively, it is known to house 31 of 48 of the reptile species for the Cape Region. Almost a third of the region’s recorded species of collembolans and spiders (30 of 138 species) are found in this ecoregion. More than 10% of animal and plant species found at Sierra de la Laguna are found exclusively in the San Lucan Xeric Scrub, and the ecoregion is contained within the extensive Baja California Endemic Bird Area. The isolation of this region has also contributed to the scarcity of predators and to the poor competitive ability of some animals. 

Rodents and other small, herbivorous mammals are virtually absent in this region, favoring certain species of animals that would otherwise be subject to competition. This is the case with the acorn woodpecker, for example. The Cape Region has also served as a natural refuge for species migrating from neighboring areas undergoing long-term geological change. 

The Mexican state of Baja California Sur contains the highest proportion of intact xeric scrub of any Mexican state. Large areas of habitat remain connected, as the topography makes this ecoregion difficult to exploit. In June 1994, the region was established as a Protected Natural Area, with pine-oak forests and subtropical dry forest as the key areas for protection. 

The fragile nature of this region requires extended legal protection to protect it from disturbances caused by human overpopulation and exploitation of forest resources for livestock. Native villagers often hunt wildlife (mostly predators) that are considered threats to their domestic animals. If this continues, it could alter the natural processes maintaining biodiversity in the area. 

Although human disturbance in the 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 has produced such a unique vegetation assemblage in the middle of an enormous desert. 

The priority conservation actions for the next decade will be to: 1) expand legal protection of the natural areas in this ecoregion; 2) limit the exploitation of the region’s natural resources; and 3) reduce human-wildlife conflict by developing “wildlife friendly” agricultural and grazing practices that support coexistence of people and carnivores.

Citations

1. Valero, A., J. Schipper, and T. Allnutt. 2018. Southern North America: Southern Baja California in western Mexico. https://www.worldwildlife.org/ecoregions/nt1314. Accessed June 15, 2018.
2. Arriaga, L. and A. Ortega (editors). 1988. Características generales. Pages 15-26 in La Sierra de la Laguna de Baja California Sur. Centro de Investigaciones Biológicas de Baja California Sur, México.
3. Rzedowski, J. 1978. Vegetación de Mexico. Editorial Limusa. Mexico, D.F., Mexico.
4. Rodríguez-Estrella, R. 1988. Avifauna. Pages 185-208 in L. Arriaga and A. Ortega (editors), La Sierra de la Laguna de Baja California Sur. Centro de Investigaciones Biológicas de Baja California Sur, México.
5. Linzey, A.V., Timm, R., Álvarez-Castañeda, S.T., Castro-Arellano, I. & Lacher, T. 2016. Ammospermophilus leucurus (errata version published in 2017). The IUCN Red List of Threatened Species 2016: e.T42452A115189458. http://dx.doi.org/10.2305/IUCN.UK.2016-3.RLTS.T42452A22251719.en. Downloaded on 30 August 2018.