Non-linear relationship between intensity of plant-animal interactions and selection strength

The shape of the relationship between intensity of biotic interactions and strength of selection is important for spatial variation in selection, but is little explored. We quantified interactions and selection in 69 populations of the short-lived herb Primula farinosa. As predicted because of saturation and depletion effects, the strength of selection on a discrete and on a continuously varying floral display trait were in several cases significantly non-linearly related to the mean intensity of mutualistic and antagonistic interactions. Strength of selection was strongest at low levels of fruit initiation and at high intensities of seed predation. Seed predation varied more among populations than did fruit initiation and could explain a larger proportion of the among-population variation in strength of selection. Our results support the contention that interaction intensity affects selection strength, and suggests that for mutualistic and antagonistic interactions that can be saturated or depleted, this relationship is sometimes non-linear.