Subsistence systems in insular Southeast Asia and the western Pacific are geographically patterned: from west to east, grain crops disappear while root and tree crops become more important, and south of the Torres Strait no crops are grown. Previous explanations for these patterns have assumed either a historical cause (the expansion of root and tree crops before grain crops) or some form of environmental filter. Paralleling the agricultural pattern is a westtoeast pattern of increasing variability in interannual rainfall variation. I propose that this variability limited the utility of annual crops and increased reliance on longlived plants whose food production averages across longer periods. The range of human responses to these difficult conditions is illustrated by discussion of the anomalies represented by the unique qualities of the agricultural regimes of Botel Tobago and eastern Melanesia and by the distribution of crops and subsistence systems across the Torres Strait.