Ecological Aspects of Stability in Polycultures Versus Sets of Monocultures of Annual Crops (Intercrop, Risk, Variation).

Abstract

Relative variation (as expressed by the coefficient of variation) in yields or economic returns, in polycultures of annual crops versus sets of monocultures of the same species, was studied in theoretical models and in experimental polycultures grown near Ann Arbor, Michigan. The risk of obtaining disastrously low values was also discussed. Methods were developed for finding the combination of monoculture crops that should be least variable (minimum CV) in total value (combined yields), for comparison as an alternative to polycultures. A review of the statistical literature showed that the use of the median-Levene's test was both the most appropriate and the simplest test for significant differences in variation. Biological factors affecting stability in polycultures versus monocultures are discussed, including effects of crop pests, the area of l and devoted to each system, and competition between the polyculture crops. Models are used to explore potential effects of competition, with the conclusion that competition will tend to reduce stability in polycultures, relative to sets of monocultures, especially for each crop species considered by itself. Negative correlation between effects of environmental variation on the crop species is expected to reduce the relative stability of polycultures. In experimental polycultures of tomatoes (Lycopersicon esculentum) and cucumbers (Cucumis sativus) and of carrots (Daucus carota) and beans (Phaseolus vulgaris), each crop species was either significantly more variable when in a polyculture than as a monoculture; or there was no significant difference between cropping systems. No crops were significantly more variable as a monoculture. There were no consistent differences in variability in combined yields. Thus competition should tend to be a destabilizing factor in polycultures, although other effects which stabilize polycultures may occur. Advantages as well as disadvantages of variations are discussed. Finally, sociopolitical implications of technological solutions for reducing variation and risk are examined. Stability will tend to benefit smaller producers more than larger ones, but can also invite outside control or acquisition by large investors.