Southern Pacific dry forests
The Southern Pacific dry forests ecoregion is situated along the southeastern versant of the Sierra Madre del Sur Mountains including the Pacific Ocean coastal plain. These forests are a key locus of endemism for butterflies, and has the greatest diversity of scorpions and spiders in the entirety of Mexico. This ecoregion is classified in the Tropical and Subtropical Dry Broadleaf Forests biome. The Southern Pacific dry forests exhibit a moderate to high faunal species richness; for example, there are a total of 744 vertebrate taxa recorded in the ecoregion, with a particularly large number of endemic reptiles.
However, little of the original habitat is extant, since agriculture development for fruit and coffee plantations has occurred on a broad scale. Also, overgrazing by sheep has severely degraded vegetation and produced widespread soil erosion.
Location and general depiction
The Southern Pacific dry forests ecoregion is positioned on the southeastern slopes of the Sierra Madre del Sur Mountains, which yield to a narrow fringe of coastal plains on which the dry forests of this ecoregion are found. The elevation ranges from sea level to 1400 metres (m). The climate is tropical and dry, with precipitation levels of 800 millimetres (mm) per annum. There is an extended arid season, which factor drives the prevalence of deciduous vegetation. The forests grow chiefly on shallow, well-drained soils derived from limestone. Closer to the base of the Sierra Madre del Sur Mountains, the soils are more rocky, and are derived of igneous rocks.
The dominant plant species include Mauto (Lysiloma divaricatum), Bursera excelsa and Fragrant Bursera (B. fagaroides), which are typically found in association with Pochote (Ceiba aesculifolia), Comocladia engleriana, and Trichilia americana. In the Mexican state of Michoacán, the macro plant species more generally in evidence are Ficus insipida, F. pertusa, Breadnut (Brosimum alicastrum), Licania arborea, Sideroxylon capiri and Elephant Ear (Enterolobium cyclocarpum).. In the states of Guerrero and Oaxaca, the composition of the forests transforms and the more abundant species are Brosimum alicastrum, Sideroxylon persimile, Godmania aesculifolia, Sapodilla (Manilkara zapota), Pterocarpus acapulcensis, Licania arborea, Purple Tabebuia (Tabebuia impetiginosa), Bombax palmeri, Shaving-brush Tree (B. ellipticum) and Templetree (Plumeria rubra). Herbaceous taxa and epiphytes are rather scarce. In the area of the border between the states of Chiapas and Oaxaca, the climate is tropical subhumid (the precipitation is 1040 to 1600 mm/yr) and is dominated by Mexican Alvaradoa (Alvaradoa amorphoides), Cacho de Toro (Bucida macrostachya), Mexican Logwood (Haematoxylum brasiletto), False Chiggergrape (Coccoloba venosa), Gumbo Limbo (Bursera simaruba), Copalillo (B. bipinnata) and Ironwood (Prosopis juliflora).
The dry forests of southern Pacific Mexico consist of a narrow belt of vegetation along the Pacific Ocean coastal plain, manifesting a unique assemblage of species, including many endemics. The genera Acacia, Ipomoea and Euphorbia have more endemic species in these forests than anywhere else in Mexico. A high number of endemic plants are found in the portions of this ecoregion which occur in Oaxaca. There are numerous special status organisms found in the Southern Pacific dry forests ecoregion, denoted variously as Near Threatened (NT), Vulnerable (VU), Endangered (EN), or Critically Endangered (CR).
Butterflies of the Papilionidae family also have an important center of endemism in this region, with eleven endemic species. This region also has the greatest diversity of scorpions in Mexico, including the large Centaurea centauroides. 311 spider taxa have been recorded in the ecoregion, constituting the richest site in Mexico. The region the second for endemic reptiles, after the trans-volcanic belt region. It is also one of the few parts of Mexico where one observes the Mexican Beaded lizard (Heloderma horridum), a highly venomous reptile.
There are a number of anuran species present in the ecoregion, including: Blunt-toed Chirping Frog (Eleutherodactylus modestus VU); Cloud Forest Stream Frog (Ptychohyla euthysanota NT), found from southeast Oaxaca to Guatemala and eastern El Salvador; Matuda's Spikethumb Frog (Plectrohyla matudai VU). A special status caecilian found in the ecoregion is the Mexican Caecilian (Dermophis mexicanus VU), a fossorial species that can attain lengths up to sixty centimetres. A special status salamander found in the ecoregion is the Sierra Juarez Salamander (Pseudoeurycea juarezi CR), a near-endemic known only between Cerro Pelón and Vista Hermosa in the Sierra de Juarez, north-central Oaxaca. The White-lipped Peeping Frog (Eleutherodactylus albolabris CR), a near-endemic known chiefly from Agua del Obispo, central Guerrero.
versant of the Isthmus of Tehuantepec, Niltepec, Oaxaca; Halberg's Cloud Forest Snake (Cryophis hallbergi), found only in northern Oaxaca, at Sierra de Juarez and Sierra Mazateca; Isthmian Earth Snake (Geophis isthmicus), known only from the vicinity of Tehuantepec, Mexico; Macdougall's Spiny Lizard (Sceloporus macdougalli), found only in Oaxaca within the ecoregion; Macdougall's Tropical Night Lizard (Lepidophyma dontomasi), known solely from the type locality on Cerro Lachiguiri, Oaxaca; Macrinius' Anole (Norops macrinii), known solely from the foothills of the Sierra Madre del Sur near Pochutla, Oaxaca; Bogert's Arboreal Alligator Lizard (Abronia bogerti), known only from north of Niltepec, between Cerro Atravesado and Sierra Madre, Oaxaca; Tehuantepec Anole (Norops isthmicus), restricted to foothills northwest of Tehuantepec, Oaxaca. Other reptiles found in the ecoregion include: Green Iguana (Iguana iguana) and Black Iguana (Ctenosaura similis).The Southern Pacific dry forests contain numerous reptilian taxa, including the following endemics: Bocourt's Anole (Norops baccatus); Taylor's Anole (Norops taylori), known only to Puerto Marquez area, in northern Acapulco, Guerrero; Simmons' Anole (Anolis simmonsi), restricted to the vicinity of Pinotepa Nacional, Oaxaca; Stegneger's Blackcollar Spiny Lizard (Sceloporus stejnegeri), restricted to the Pacific versant in the state of Guerrero, Mexico; Red Earth Snake (Geophis russatus), found in a very narrow range outside of Putla, Oaxaca; Sierra Mije Earth Snake (Geophis anocularis), known only from around Totontepec on the Atlantic versant of the Sierra Mixe, Oaxaca; Ramirez`s Hooknose Snake (Ficimia ramirezi), restricted to the Pacific
Characteristic mammalian fauna includes the endemic Oaxacan Pocket Gopher (Orthogeomys cuniculus), restricted to several sites on the Isthmus of Tehuantepec, Oaxaca. Other mammals seen in the ecoregion include the: Lesser Long-nosed Bat (Leptonycteris yerbabuenae VU), Tropical Hare (Lepus flavigularis EN), restricted to Salina Cruz, Oaxaca to the extreme west of Chiapas; Greater Bulldog Bat (Noctilio leporinus), Coati (Nasua narica), Buller’s Pocket Gopher (Pappogeomys bulleri), Javelina (Tayassu tajacu), and Mexican Long-tongued Bat (Choeronycteris mexicana NT).
A gamut of avian taxa are found in the Southern Pacific dry forests, including: Red-crowned Ant Tanager (Habia rubica), Elegant Quail (Callipepla douglasii), Olive-sided Flycatcher (Contopus cooperi NT).
For several hundred years, the deciduous forests of the Mexican Pacific have been severely exploited and disturbed by man, commencing at least as early as the arrival of Spaniards. Agricultural expansion, primarily for fruit and coffee plantations have replaced vast expanses of forest. This impacts native biodiversity, directly through habitat destruction, and indirectly through erosion and soil loss.
Ecological threat profile
Sheep farming is a major threat to the ecoregion. Furthermore, tourism is increasing in the coastal zone of the ecoregion, leading to considerable land development pressure. The only remaining intact dry forest occurs along the coastal plain of the state of Michoacán, an area of rather difficult human access. Illegal hunting is a further threat, and has severely reduced the populations of green iguana (Iguana iguana) and black iguana (Ctenosaura similis). Despite the biological importance of this region and the present threats that it faces, only two protected areas have been established, one along the coast of the state of Oaxaca and the other in Guerrero. Another eight areas have been proposed for protection. Recently, a number of terrestrial priority areas were nominated, a number of which overlap with this ecoregion, including Sierra Sur y Costa de Oaxaca, El Triunfo-La Encrucijada-Palo Blanco, and Bajo Rio Verde-Chacahua. Several Important Bird Areas have also been identified here, including Sierra Norte, Sireea de Miahuatlan, and La Sepultura.
Justification of ecoregion delineation
The dry forests of this ecoregion flank the southern valleys and foothills of the Sierra Madre del Sur, across much of the southern Pacific versant of Mexico. Initial linework follows the current landcover classifications of Instituto Nacional de Estadística Geografía e Informática (INEGI). From this map the following classifications are lumped: "subevergreen lowland forest", "subevergreen middle elevation forest", "evergreen lowland forest", "evergreen middle elevation forest", patches of savanna, and all subsequent agricultural activity. Later modification has occurred from expert opinions, following suggestions from Rzedowski at a regional ecoregion delineation workshop.
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