Regional contrasts in cultural diversity – Environmental influences ?

The rich biological diversity of the tropics by comparison to higher latitudes, the Forster effect, is well known. Apparently far less well known is the fact that cultural diversity shows the same latitudinal pattern. In tropical South and Central America, for instance, we find over 480 languages, compared to the USA?s 176.
Textbooks of physical anthropology and biogeography largely ignore this geographic contrast in cultural diversity, perhaps because they see it as a cultural phenomenon, not a biological one. Following others, though, I suggest that the Forster effect in humans has the same biogeographical explanations as it does among other animals.
Biogeographers have produced multiple explanations for the Forster effect. One explanation that seems most applicable to humans is that high environmental productivity in the tropics compared to higher latitudes allows populations of sustainable size to live in smaller areas than they can in the less productive temperate regions. More cultures thus occupy a given area in the tropics than they do at higher latitudes.
Evidence for the explanation includes not just the higher productivity, smaller territory size, and higher population densities of tropical peoples, but also, for example, the fact that tropical hunter-gatherers move camp less often and move less far each time than do hunter-gatherers of higher latitudes.
The question remains, I suggest, of why successful tropical societies – and if they are there, they are successful – do not expand their territories to the sizes of temperate latitude societies. The answer might have something to do with another manifestation of the Forster effect, namely that parasites and pathogens are also more diverse in the tropics. Consequently, a population that expanded could quickly find itself among parasites and pathogens to which it was not adapted.