Phosphorus drives the biogeographical distribution of sweet and bitter manioc
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Manioc (Manihot esculenta Crantz) or cassava comes in two varieties: bitter which is dangerous and sweet which is safe. Three basic questions about this tropical crop remain unanswered decades after they were first asked. First, does a distinct geographical distribution of sweet and bitter manioc exist? That is, are these two varieties partitioned into two discrete regions, or are they so intermixed that neither variety can be assigned to a specific territory? To answer that we compiled the preferences for sweet and bitter manioc among 127 Native Amazonian groups, the largest dataset yet created. Here we establish that sweet and bitter manioc have a well-defined geographical distribution. Second, what causes that distinct distribution? Here we introduce the Phosphorus hypothesis. We explain that distinct distribution in terms of differential levels of phosphorus, the limiting nutrient in Amazonia. And third, why do the majority of Amazonians choose to grow bitter manioc, a poisonous and labor-intensive crop rather than sweet manioc, which is harmless and quickly prepared? We solve this long-standing paradox by demonstrating how the distinct soil types and soil chemistries of different ecological regions play a pivotal role in determining what crops can and cannot be grown in Amazonian soils.
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M.A.