It is difficult to determine the time scale (e.g. 10, 50, or 100 years) as different systems may require different time scales. In ecology, Ten Brink et al. (1991), who firstly developed the AMOEBA model (AMOEBA is the Dutch acronym for ‘‘a general method of ecosystem description and assessment’’), examine the ecological situation in the sea and a major river over 58 years (1930–1988). In agricultural sustainability, according to Bell and Morse (1999), pest problems are best looked at scales over 5–20 years, while land degradation requires scales of 20–100 years. Tourism sustainability requires a much shorter time scale (e.g. 5–10 years) as tourist destinations tend to be influenced very sensitively by internal (e.g. ecological base of many tourism enterprises) or external (e.g. terrors or SARS) factors.