Aim To describe patterns of crop richness at a global scale and investigate the potential influence of energy, water, habitat heterogeneity, area and economic commitment to agricultural practices on patterns of crop species richness.
Methods Ethnographic literature for 86 subsistence-oriented societies was surveyed; 55 of these societies grow crops and 31 do not. These 86 societies occupy the range of latitudes at which subsistence-oriented groups manage domesticated plant species. The number of domesticated plants that each society cultivates is reported. Regression analyses were conducted to evaluate predictions for relationships between crop richness, energy, area, habitat heterogeneity, commitment to cultivation and species taxonomic descriptions.
Results Crop richness is positively and significantly related to energy availability and commitment to cultivation. The warmer the environment in which cultivators live, the more crop species cultivators grow. The more subsistence farmers commit to cultivation, the more crop species subsistence farmers grow. Above a threshold of 500 m in elevation range, crop richness is positively related to topographic heterogeneity. Mean annual rainfall, elevation, area, commitment to domesticated animals and exchange are not significantly related to crop richness.
Main conclusions 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.