1. Hawkins J. P, Roberts C. M. (1997). Estimating the carrying capacity of coral reefs for SCUBA diving. IN. Proceedings of the 8th International Coral Reef Symposium, Panama: Smithsonian Tropical Research Institute, Balboa, 1923–1926.
2. Tourism in the high north: management challenges and recreation opportunity spectrum in Svalbard, Norway;Kaltenborn;Environmental Management,1993
3. Karaj environment;Kianfar,2000
4. What to do about crowding and solitude in parks and wilderness? A reply to Stewart and Cole;Manning;Journal of Leisure Research,2003
5. Limits of acceptable change for tourism in the OKAVANGO delta;Mbaiwa,2002