Analyzing the impact of agave cultivation on famine risk in arid pre-Hispanic northern Mexico

Cultivation of agave was common in pre-Hispanic northern Mexico and the American Southwest, and scholars generally accept that it was a strategy to ensure food supply during years of drought when the maize crop failed. Some even suggest that incorporating agave cultivation make large, nucleated settlements possible in arid northern Mexico ca. 500-900 CE. Yet the environmental circumstances under which farmers could reasonably expect such a strategy to decrease the chances of agricultural failure are not well understood. We explore the potential of this crop complementarity by assessing the risk of famine-induced migration events in different idealized environmental settings. We use Monte Carlo simulation to analyze a simple discrete-time, age-structured stochastic model for maize and agave agroecology, deriving the climatological conditions under which agave could have significantly reduced the probability of short- and long-term famine events. Investments in agave production made the most sense where average annual rainfall was between the levels that would ensure maximum maize yield and those that would mean loss of the maize crop due to drought-related mortality. Cultivating agave had little impact on famine risk at high (maize yields sufficient) and low (failure of both maize and agave) rainfall levels. Perhaps more surprisingly, it had its highest impact at moderate rainfall levels when variance in rainfall was relatively low. While a higher variance in rainfall increased the number of 'good' years for maize-where production would exceed demand and allow for storage-it also increased the probability of simultaneous failures in both agave and maize production. These findings are difficult to apply to specific times and places in the past, because rainfall distribution in complex, environments change, and it is difficult to take all relevant human interventions into account. The analysis does, however, offer support for the proposition that agave cultivation could have significantly enhanced survival probabilities of large, nucleated settlements in certain circumstances. It remains for further study to identify such circumstances more precisely geographically and temporally.