Scarborough said large rivers made it easy for states to grow and feed a rapidly expanding population. However, early states were also emerging in tropical areas. They're unique, because they take longer to develop. In the tropics, there is tremendous species variation, but each locale is unique. Populations grow rapidly in the semi-arid regions with irrigation, but less so in the tropics.
Centralization is a hallmark in civilization, and they're already having trouble with that.
Scarborough said the problems and solutions are consistent across early tropical states from Bali to Belize and including Sri Lanka and Cambodia as well. Civilizations are dispersed across the landscape, but the residents use the local topography and in the Maya case develop hilltop reservoirs to capture rainwater for use in the dry seasons. Water power quickly gives rise to political power. Rain is seasonal instead of temperature in these areas. Where the dry season availability of water is limited, semitropical states build reservoirs.