Crop richness in subsistence-oriented farming systems is conditioned by energy availability via the physiological tolerances of crop species. Crop richness is secondarily conditioned by topographic heterogeneity, which may serve as an estimate of spatial climatic heterogeneity. Where energy availability is a relatively weak constraint on crop production, commitment to cultivation for subsistence strongly conditions how much effort producers invest in diversifying their crop base. It is postulated that tolerance to freezing temperatures limits the ranges of domesticated plant species. This range limitation might be the result of niche conservation among domesticated crop species and a legacy of greater rates of speciation and lower rates of extinction among ‘free living’ species in tropical settings.