An illustration of the covariance effects. Plant nutrient-defense (A) or defense-defense (B) covariance effects on herbivore performance in the cases of a hypothetical antagonistic effect between the plant nutrient and chemical defense concentrations and two synergistically acting defense traits, respectively. When herbivores feed on a plant population with both nutritive and defense variance (the two types of plants on the x-axis), the Jensen’s effect on performance, , depends on whether the covariance between the traits is positive (blue), negative (red) or null (purple). See methods for procedure.