Location and General Description
The Sierra Madre del Sur is one of the main physiographic provinces of Mexico. The Sierra runs parallel to the Pacific Coast in a northwest direction. It covers an area of approximately 9,000 km2 in the Mexican South Pacific region; most of this ecoregion falls within the Guerrero and Oaxaca states. Characterized by complex geology and diverse topography with steep slopes on both sides of the mountains, the Sierra’s altitudinal range varies from 100 to 3,500 m (Ferrusquía 1993, Luna-Vega 1993). The climate at the highest elevations ranges from temperate to subhumid with heavy summer rains. The mean annual temperature is 24 oC with oscillations of 4.5 oC and extremes between 5 and 40 oC (Diego et al. 1997). The annual precipitation in the region is between 800 and 1,600 mm (Rzedowski 1988).
Pine-oak forests of the Sierra Madre del Sur grow on rocky soils of volcanic origin, although the lowest elevations consist mostly of karstic formations (García-Rendón, 1993). There are three main vegetation associations: oak forests growing at 1,900-2,500 m, cloud forests at 2,300 m, and pine-oak forests and some dispersed pine forests at elevations between 2,400 and 2,500. Quercus magnolifolia, Q. castanea and some Pinus montezumae individuals are common in oak-dominated areas, with abundant herbaceous elements as well as some epiphytes of the families Orchidaceae and Bromeliaceae. Pine-oak and pine forests are also inhabited by Q. obtusata and Arbutus xalapensis. Vascular epiphytes are not abundant, except for Tillandsia spp. Herbaceous elements are common. Pine trees that are dominant in pine forests are P. herrerai, P. pseudostrobus, P. pringlei, and P. ayacahuite. The endemic Pinus rzedowskii is classified as endangered species (Perry 1991). Vascular epiphytes are scarce, but moss and lichens grow abundantly. The cloud forest is characterized by Abies sp., Quercus uxoris, P. ayacahuite and Cupressus lindleyi. Here, the herbaceous stratum is well developed, and ferns and orchids grow virtually everywhere.
Cloud forests house a great portion of the biodiversity of the Sierra Madre del Sur: 48.7% of the species in the state of Guerrero inhabit montane forests, while 43.7% inhabit pine-oak forests. Montane forest coverage plays a crucial role in preventing erosion, primarily in the steep-sloped areas (Luna-Vega & Llorente-Bousquets 1993). Isolated from nearby environments as a consequence of their altitudinal levels and steep slopes, the temperate forests of the Sierra Madre del Sur ecoregion have recently been considered an unparalleled center of endemism and biodiversity (Fa & Morales1993). Hydrophyllic elements have diversified successfully (Luna & Llorente 1993), and have been involved in the evolution of many plant and animal endemics. The biological richness of the region is illustrated in the 195, of the 207, species of orchids in the state of Guerrero. The pre-Cenzoic tropical relict species, Peltogyne mexicana, is found in this ecoregion (Diego et al. 1997).
There are 595 species of plants, with seven endemic genera; and of the 161 plant species endemic to Mexico, 16 occur only in Guerrero (Salazar-Chávez 1993). The Canyon of the Zopilote, located in the northern section of the ecoregion, is considered a center of endemic plant species and the site of the diversification of Bursera species. Some of the endemic genera include Silviella, Omiltemia, Microspermum, Peyritschia, and Hintonella (Diego et al. 1997).
Many coleoptera of the family Pasalidae are also endemic to the Sierra Madre del Sur (Reyes-Castillo & Castillo 1993), which is also one the richest areas in butterfly species (161 species) in the Mexican Pacific (Martínez & Llorente-Bousquets 1993). Although the herpetofauna have not radiated extensively (39 species), half of the amphibians and 34% of the reptiles are endemic.
In Omiltemi, Guerrero, 9% of the 54 species of mammals are endemic including the critically endangered Omiltemi cottontail (Sylvilagus insonus); although this species is considered extinct by many, there have been possible recent sightings since the specimens were collected in the 1800's. Lying within the Sierra Madre del Sur ecoregion the state of Guerrero has the fourth highest bird richness in the world (Navarro & Benitez 1993) with 160 species, 28 of which are endemic to the Sierra Madre. The short-crested coquette (Lophornis brachylopha), white-tailed hummingbird (Eupherusa poliocerca), oaxaca hummingbird (Eupherusa cyanophrys) and white-throated jay (Cyanolyca mirabilis) are all endangered and endemic to the Sierra Madre del Sur ecoregion (Stattersfield et al. 1998). Also found in this ecoregion; amethyst-throated hummingbird (Lampornis amethystinus), Steller’s jay (Cyanocitta stelleri), ruddy foliage-gleaner (Automlus rubigihosus), russet-nightingale-thrush (Catharus occidentalis), and collared towhee (Pipilo ocai).
Though in danger of extinction (Cárdenas-Hernández et al. 1994), portions of temperate forests still remain in the Sierra Madre del Sur. According to Toledo & Ordoñez (1993), the states of Guerrero and Oaxaca still have considerable portions of forests in good state. What is more, Toledo et al. (1989) specify that 60% of the temperate forests in Mexico remain intact. According to them, 37% of the pine-oak forests in Mexico have succumbed to agricultural pressure and logging.
Until five years ago, federal protection covered only 470 km2 (< 1%) of the temperate forests of the Sierra Madre del Sur. In recent years however, proposals have been considered covering up to 4% or even 10% of the unprotected area. Protected areas in this ecoregion include the Omiltemi State Ecological Park, which covers 96km2, while the Reserva de la Biosfera Sierra de Manantlan protects a small percentage. According to Stattersfield et al. (1998) there needs to be at least two new protected areas created to protect the restricted range birds. If all specialized organisms are considered the protected area needed is not even estimated but would be assumed to be much larger.
Types and Severity of Threats
The main threats to this ecoregion are all associated with the removal of vegetation and the subsequent reactions of the habitat to these conversions, which trickle down through the food chain of the invertebrates and vertebrate species. Clearing of forest for large-scale agriculture projects is devastating and threatens this ecoregion with complete destruction (Stattersfield et al. 1998). Lower montane forest is being cleared for agricultural use such as growing corn, citrus fruits and coffee. Cloud forests are also being cleared for coffee plantations and the pine; oak and fir forests are being removed for lumber (Stattersfield et al. 1998). One of the after effects of deforestation is the subsequent invasion by grass, which also affects the distribution and numbers of mammals (Jiménez-Almaraz et al. 1993). Livestock also displace native mammals and directly competes with them.
Game hunting affects big and medium-sized mammals, and road expansion makes mammals easy to find by hunters. Although pine-oak forests and montane forests of the Sierra Madre del Sur are well preserved, they risk extinction due to an increase in human settlements in the region caused by both urbanization and tourism. Intensive exploitation of forest resources, accompanied by new road openings, could severely damage the native communities, and endanger many of the endemic and restricted-range taxa that are unique to the Sierra Madre del Sur.
Justification of Ecoregion Delineation
These montane pine and oak forests of the Sierra Madre de Sur occur along ridge tops, valleys and slopes in a patchwork distribution and are host to a number of endemic species. Linework for this ecoregion follows the INEGI (1996) current landcover maps, encompassing all "pine-oak forests", "oak with pine forests", and "pine forest" classifications within the Sierra de la Laguna region, as well as portions of "low open forest", "mesophyll montane forest", " "low deciduous forest", "matorral", and agricultural activities which fall within these parameters. Classification and justification follow Rzedowski (19789). Linework was reviewed by experts during ecoregional priority setting workshops (CONABIO 1996 and 1997) in Mexico.
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Prepared by: Alejandra Valero, Jan Schipper, and Tom Allnutt
Reviewed by: In Process