The state of Campeche has a large indigenous population. Of them, the group that is largest and that has the most significant history, is the Yucatec Mayas, who are concentrated in the oldest settlements in the northern area of the state. This group lives on most of the Yucatan Peninsula, with 11% in Campeche, 76.25% in Yucatan and 12.73% in Quintana Roo. Campeche also has a high concentration of Guatemalan indigenous refugees who live in refugee camps, and the southern part of the state has received migrants from other ethnic groups. The 1990 census shows that there are 45 different languages represented in the state; among them, the most important are Ch'ol, Tzeltal, Kanjobal and Mam. Most of Campeche's territory is characterized by a subhumid climate and a dense tropical forest. There is also a lowlands region with lakes and a riparian area. The "milpa" system, which predominates in the state, requires a deep understanding of specific ecological systems, including the cycles of various plants and the rain cycle, as well as the fallow period required by its soils. Hunting and gathering are supplementary subsistence activities. The local natural resources are used as materials to build houses and for energy needs. Together, these activities along with forestry and livestock activities, have had a devastating impact on the tropical forest, and attempts are being made to reverse the impact through sustainable development programs focused on reforestation and wildlife management. Preservation policy has brought about the creation of various protected areas, among them the Calakmul Biosphere Reserve--notable for the number of indigenous migrants who live within the reserve's 723,185 hectares of forest and also for plans to develop ecological and archaeological tourism opportunities.