Location and General Description
The region is composed of two main mangrove areas located in Mexico, one on the Pacific Coast and one on the Gulf of California Coast. The mangroves of Bahía Magdalena lie on the Pacific Coast of Baja California Sur, and the west side of Sierra la Giganta. The mangroves off the coast of Sonora are located in the delta of three Mexican rivers: the Yaqui, the Mayo, and the Fuerte. Additional smaller mangrove areas include those associated with the lagoons of San Ignacio and Ojo de Liebre, which are part of the El Vizcaino Biosphere Reserve. The climate in both areas is dry with occasional rains in summer and winter, averaging <200 mm/year. Soils are derived from igneous rocks in Baja California Sur, and from sedimentary rocks in Sonora. These mangroves lie on coastal plains where altitude ranges between 50 -100 m.
The dominant species are Rhizophora mangle, Laguncularia racemosa, and a few individuals of Avicennia germinans, in Baja California Sur. While R. mangle, L. racemosa, A. germinans and Conocarpus erectus dominate in Sonora. The herbaceous stratum is not well developed, due to its intolerance to permanent floods (Lot et al. 1993). Behind the mangroves and salt marshes expanses of low sandy plains hold species such as Salicornia subterminalis, Atriplex barclayana, Sorobolus contractus and others sparsely cover the ground (Seeliger 1977).
The northwestern coast mangroves are the northernmost representatives of mangroves on the Pacific Coast of North America, and are found along the zone of transition between tropical and subtropical habitats. The region is an important point in the migratory routes of many birds (Cervantes 1994; Calderón-Aguilera et al. 1993) and endangered grey whales that use the lagoons as a mating and calving ground. Mangroves are a natural refuge for diverse species of aquatic birds, as they harbor many aquatic organisms on which birds feed such as oysters, crabs and invertebrate larvae to name but a few. The large invertebrate community in turn, feeds on the enormous deposits of organic matter provided by mangrove trees; thus, the mangroves are natural promoters of biodiversity in aquatic environments.
According to Kjerfve (1997) the leaves of the mangrove tree produce the most organic matter in these ecosystems. Mangroves synthesize organic matter and filter nutrients, making them crucial for the high productivity in tropical coastal zones (Gallegos 1986). Odum & de la Cruz (1967) stated that 2/3 of the fish populations worldwide depend on mangrove ecosystems during at least one or more stages of their lives. Mangrove associations have long been recognized as good soil retainers (Miranda 1998; Tomlinson 1994) that also allow for the continuous formation of soil, which supports the terrestrial communities associated with mangroves (Rzedowsi 1988). Mangroves also promote and maintain the biodiversity of terrestrial and aquatic environments by preventing erosion of the coast (Gobierno del Estado de Chiapas & TNC 1993). CONABIO has identified the Planicies de Magdalena terrestrial priority area, which shares part of its territory with this ecoregion (Arriaga et al. 2000). The Bahía Magdalena-Almejas important bird area has also been identified within this ecoregion (Benitez et al. 1999).
Patches of mangrove ecosystem, which fall within the Sonora province also fall with in the North-west Mexican Pacific Slopes Endemic Bird Area. Two birds restricted to this EBA utilize mangrove habitat and this ecoregion including the San Blas jay (Cyanocorax sanblasianus) and the purplish-backed jay (C. beecheii) (Stattersfield et al. 1998). These mangrove patches also form important habitat for use by migrating songbirds, raptors and shorebirds (Olson et al. 1996). According to Olson et al. (1996) mangrove patches serve as both rest areas and winter residence for birds.
Mangroves that occur in arid environments such as this ecoregion also serve as refuge and habitat to mammals and herpetofauna depending on the availability of resources such as water and shade in the surrounding habitats (Olson 1996). Some of these species may include populations of primates, cats, coatis, boas and other wildlife that will use this ecoregion's mangrove ecosystems year round or seasonally.
This ecoregion was listed as one in seven out of more than 30 ecoregions in need of conservation, or within the next 10-20 years it will experience some of the most sever conversion and degradation (Olson 1996). There is no information on exactly how well preserved the ecoregion is or how intact the habitat remains. Water pollution due to human dumping of organic and inorganic waste threatens the survival of many aquatic species. Human population in the region is growing rapidly, which could displace natural habitat and increase pollution entering mangrove communities.
Some mangrove patches of this ecoregion are included in the El Vizcaino Biosphere Reserve, which also supports several marine mammals, birds, and marine turtles. Important solutions include managing the mangroves as well as regulating the illegal extraction of wildlife, and water pollution by humans and by industrial settlements.
Types and Severity of Threats
Mangroves have been gradually eliminated. The most severe threats currently are from fuel-wood collection, sedimentation, non-sustainable hunting, shrimp farming, changes in the water flow patterns by road building, diversion or other activities that alter the freshwater inflow to the mangrove areas. Clearing of mangrove trees for development, agriculture or salt and charcoal production are also concerns (Olson 1996). Illegal extraction of sea turtles, cacti, and reptiles in Baja California Sur mangroves could result in the loss of these already endangered species.
Long-term threats to the region include overexploitation of resources, and continuous industrial pollution of the waters due to human overpopulation (CONABIO 1998). Artificial drainage of mangroves for the creation of cultivable lands, and highways, and construction of dams that alter hydrological regimes, are among the major threats facing mangroves. Drainage also leads to loss of the nutrients stored by mangroves, which destabilizes the communities that depend on them. In Sonora, water pollution is aggravated by the accumulation of heavy metals in the waters, and also by extensive logging.
Justification of Ecoregion Delineation
Classification and linework for all mangrove ecoregions in Latin America and the Caribbean follow the results of a mangrove ecoregion workshop (1994) and subsequent report (Olson et al. 1996).
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Prepared by: Alejandra Valero
Reviewed by: In process