Most national parks have a dual role as offering a refuge for wildlife and as serving as popular tourist areas. Managing the potential for conflict between these two roles can become problematic, particularly as tourists often generate revenue for the parks which, in turn, are spent on conservation projects. Parks also serve as reserves for substantial natural resources, such as timber, minerals and other valuable commodities. The balance of the demand for extraction of these resources, against the damage this might cause, is often a very important challenge in national park management. National parks have been subject to illegal logging and other exploitation, sometimes because of political corruption. This threatens the integrity of many valuable habitats.