A latitudinal gradient in the density of human languages in North America

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Abstract
The number of different species found in a given area of the Earth increases from the poles towards the equator for a wide range of terrestrial and marine organisms. A similar but opposite latitudinal gradient is also generally found in the size of the geographic range over which a species is found: range sizes are larger in more northern latitudes becoming smaller in the south, a phenomenon known as Rapaport's rule. Here we show that the density of human language-cultural groups in North America, at the time of contact by colonizing Europeans, also followed a strong latitudinal gradient qualitatively similar to that found in North American mammals. Six times, or more, different languages were spoken in a given area in southern latitudes, compared to the density of language groups nearer to the poles. In addition, the amount of territory over which the speakers of a given language were found increases with latitude: linguistic-cultural groups conform to Rapaport's rule. Finally, based upon a categorization of North America into 23 distinct major habitat types, we find greater linguistic diversity in areas of greater habitat diversity, independently of latitude. `Linguistic ecology', or the examination of ecological factors associated with diversity in the number of linguistic groups, may shed new light on some features of cultural and linguistic evolution.