The Selva Zoque (), which includes the Chimalapas rain forest, is an area of great ecological importance in Mexico. Most of the forest lies in the state of Oaxaca but parts are in Chiapas and Veracruz. It is the largest tract of tropical rainforest in Mexico, and contains the majority of terrestrial biodiversity in the country. The forest includes the Selva El Ocote, a federally-protected biosphere reserve, but is otherwise not yet protected. Despite the rich ecology of the region, a 2003 study that focused on bird populations stated that "the fauna of the heart of the Chimalapas, including its vast rainforests, have seen little or no study". As it is an impoverished region, efforts to preserve the ecology are often at odds with demands to improve the economy. Geography The forest lies between 16°00'32"N, 17°32'00"N, 93°21'40"W and 94°53'53"W in the eastern part of the mountain range that forms the spine of the Isthmus of Tehuantepec. The terrain is rugged and includes a complex mixture of forest types at different levels. Geologically, the region dates from the Upper Cretaceous and the Cenozoic. Elevations range from 100m to 2,700m. Average monthly temperatures in most areas range between 18 °C and 22 °C, and annual rainfall between 500 and 2,500 mm, with high humidity throughout most of the region although there are dry interior valleys. In some of the higher areas, mean annual temperatures are as low as 13 °C. Excluding the Uxpanapa and La Sepultura extensions, the forest region covers about one million hectares. The main Chimalapas region in Oaxaca covers approximately 600,000 hectares. The forest contains the headwaters of five hydrological systems: the El Corte River watershed, the Negro River watershed, which feeds the Nezahualcoyotl (Malpaso) Dam, the Uxpanapa River watershed and the Coatzacoalcos River which flow into Veracruz's southern wetlands and the Gulf of Mexico; and the Espiritu Santo River watershed which supplies the lagoon system to the south in the Pacific Ocean. Vegetation The region has highly diverse plant communities. For example, over 300 orchid species have been identified, 60% of all orchid genera in Mexico. Over 200 canopy tree species have been found in the Uxpanapa lowlands, and the entire region is estimated to have 3,500 vascular plant species. Several genera and one family are only found in this area of Mexico. Valuable plants include the tropical red cedar (Cedrela odorata) and bigleaf mahogany (Swietenia macrophylla). Palmita