Location and General Description
Volcanic activity from the Pleistocene gave rise to the four Revillagigedo Islands: Socorro, Clarión, San Benedicto, and Roca Partida. Socorro Island is the largest of the four; it first formed as a series of small explosions from the Evermann volcano, at 1,150 m. above sea level. Socorro and Clarion share an abrupt topography of deep canyons and valleys. Rocky or sandy beaches can be found on all islands. In Clarion, the highest elevations are at 350 m above sea level. The climate is tropical subhumid with summer rains throughout the archipelago. Socorro Island also has a tropical semi-dry climate zone (between 0-400 m above sea level) and a subtropical subhumid climate zone (between 400-1150 m above sea level) with occasional summer rains. In general, the four islands are dry, with a mean annual precipitation of 600 mm/year. Soils are of volcanic origin; Clarion’s soil is older and deeper than Socorro’s. Both islands share an abundance of igneous rocks, mostly of basalt and cineritic cones. The four islands are covered by dry forest and share climatic and ecological characteristics that are responsible for the different vegetation associations found at the islands. The coastline is dominated by mangle botoncillo (Conocarpus erecta), and Hibiscus pernambucensis, and herbaceous elements are abundant. At elevations between 0-250 m., growing on basalt spills, the scrub Croton masonii is the dominant species. On top of the Croton scrub, a secondary type of vegetal community has developed due to extensive erosion. Here Ficus cotinifolia, Psidium spp, and Guettarda insularis dominate the forest. Herbaceous elements are also abundant. The most dominant vegetation association in the islands is composed of Dodonea viscosa herbs, pygmy Guettarda insularis, Prunus serotina and the endemic cactus Opuntia sp., where the fern, Pteridium caudatum is very abundant. On elevations of 250-500 m. in Socorro, a dense forest of amate (Ficus cotinifolia) is the dominant vegetation; higher in the mountains (>500 m) other species substitute Ficus cotinifolia, including Bumelia socorrensis, Ilex socorrensis, and Psidium socorrense. Climbers and epiphytes are more abundant in these forests. Above 700 m, Meliosma nesites, Oreopanax xalapense, and Prunus capuli dominate the landscape; humidity levels in this area are higher than in any other part of the island, and therefore lichens and ferns (e.g. Adiantopsis radiata, Polypodium alfredii, and Asplenium formosum) grow abundantly. A prairie association of Centaurium pacificum, Hypericum eastwoodianum and Heterotoma cordifolia, among many others, covers the Evermann volcano, at 1,100 m above sea level.
The isolation of the Revillagigedo Islands in the Pacific Ocean has favored the radiation of many species making the islands a place of unparalleled endemism. Of 117 species of native plants, 31.6% are endemic on Socorro island, 26% on Clarión, and 45% on San Benedicto (Challenger 1998). All of the terrestrial vertebrates are endemic to the islands, as well as 14 out of 16 terrestrial birds (Brattstrom 1990). The Revillagigedo Islands are considered as a priority area for conservation by IUCN (1980), and as an Endemic Bird Area (EBA) by ICBP (1992). Forest associations house the highest number of endemic bird species in the islands (León de la Luz et al. 1994). The islands constitute one of the most important nesting, breeding, and foraging sites for four sea turtle species that are in need of special protection: leatherback turtle (Dermochelys coriacea), olive ridley turtle (Lepidochelys olivacea), hawksbill turtle (Eretmochelys imbricata ) and the green turtle (Chelonia mydas). Its isolation from the continent makes Revillagigedo Islands one of the few ecosystems that have unique species of flora and fauna worldwide (Jiménez et al. 1994).
Distinctive species on these islands include the following endemic and restricted range species: Socorro Island tree lizard (Urosaurus auriculatus), a brown recluse spider (Loxosceles reclusa), Socorro dove (Zenaida graysoni), Socorro parakeet (Arahuga brevipes), Socorro wren (Thryomanes sissonii), and Socorro Mockingbird (Mimodes graysoni) (Stattersfield et al. 1998).
A great portion of the original vegetation of the Revillagigedo Islands has been destroyed over the last 50 years. Domestic sheep were introduced to the island in the 1960’s and 70’s, contributing to the loss of at least 1% of the native vegetation (Castellanos & Rodríguez-Estrella 1992). Feral cats have also contributed to the reduction of native bird populations (Ortega et al. 1992); as a consequence of this and other pressures, the Socorro dove became extinct in the wild between 1958-1978 (Jehl & Parkes 1982, CONABIO-INE 1995).
Types and Severity of Threats
Main threats to the islands include destruction and perturbation of native vegetation, soil erosion caused by introduced ungulates, and the introduction of exotic birds and mammals. Naval officers and their families, who are responsible for the gradual destruction of the vegetation, inhabit the islands. They maintain the populations of introduced sheep, but do not keep adequate control of them. Since most of the flora and fauna is endemic to the islands and the ecological relationships among members of the biota are complex, the gradual loss of some elements from the ecosystem could lead directly to extinction. The islands have received federal protection since 1994, yet an adequate management program is needed: sheep populations should be caged and appropriate control of their grazing habits should be monitored to prevent loss of vegetation. Elimination of the feral cat population is also recommended, as is the creation of a biological station for monitoring the islands’ status.
Justification of Ecoregion Delineation
This island ecoregion is justified from its distance from shore and subsequent endemic species (Stattersfield et al 1998). We consulted Rzedowski (1978) for classification, and linework and delineation’s encompass all of the islands in the Islas de Revillagigedo group.
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Prepared by: Alejandra Valero, Jan Schipper, and Tom Allnutt
Reviewed by: In process