Abstract
Farmer management of plant germplasm pre-dates crop domestication, but humans’ role in crop evolution and diversity remains largely undocumented and often contested. Seemingly inexplicable practices observed throughout agricultural history, such as exchanging or replacing seed, continue to structure crop populations across the developing world. Seed management practices can be construed as events in the life history of crops and management data used to model crop demography, but this requires suitable quantitative data. As a prerequisite to addressing the causes and implications of maize seed management, we describe its patterns of variation across Mexico by drawing from the literature and new analysis. We find that rates of seed replacement, introduction and diffusion differ significantly across regions and altitudinal zones, but interactions among explanatory factors can obscure patterns of variation. The type, source, geographic origin and ownership of seed help explain observed rates. Yet, controlling for the characteristics of germplasm barely reduces interregional differences vastly exceeding variation across elevations. With few exceptions, monotonic altitudinal trends are absent. Causal relationships between management practices and the physical environment could determine farmers’ wellbeing and crop conservation in the face of climate change. Scarce and inconsistent data on management nevertheless could prevent an understanding of these relationships. Current conceptions on the management and dynamics of maize diversity are founded on a patchwork of observations in surprisingly few and dissimilar environments. Our estimates of management practices should shed light on differences in maize population dynamics across Mexico. Consistency with previous studies spanning over a decade suggests that common sets of forces are present within large areas, but causal associations remain unknown. The next step in explaining crop diversity should address variation in seed management across space and time simultaneously while identifying farmers’ values and motivations as underlying forces.
Highlights
Adaptive radiation is perhaps the most obvious expression today of the evolution of maize, and one of its manifestations is a wide array of patterns associated with altitude [1,2,3,4]
Associations of crop diversity with altitude have been observed from the morphological to the molecular level. It has been known for over half a century that most maize races are restricted to certain altitudinal zones, while some sub-races are adapted to different elevations [1,5,6]
Management practices can be construed as events in the life history of crops and quantitative data used to estimate demographic parameters that can shed light on their population dynamics [55,56]; this requires that data be reported on a specific basis
Summary
Adaptive radiation is perhaps the most obvious expression today of the evolution of maize, and one of its manifestations is a wide array of patterns associated with altitude [1,2,3,4]. Associations of crop diversity with altitude have been observed from the morphological to the molecular level It has been known for over half a century that most maize races are restricted to certain altitudinal zones, while some sub-races are adapted to different elevations [1,5,6]. Systematic analysis of maize collections later confirmed that the environment—as defined by altitude and geographic location—could explain the distribution of all known races of maize in Mexico [2]. The explanation for these patterns has been that, in Mexico, environmental conditions (e.g., temperature, precipitation, moisture and length of growing season) co-vary with altitude to a greater extent than either latitude or longitude [8,9]. After more than two decades of social research, evidence supporting this hypothesis remains hard to find [11,12,13,14,15]
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