Measuring the Effects of Pollinators and Herbivores: Evidence for Non-Additivity in a Perennial Herb

Using an experimental approach, this study addresses the following two questions for the perennial herb Paeonia broteroi (Paeoniaceae) at a location in southeastern Spain. (1) What are the relative magnitudes of the effects of pollinators and herbivores (invertebrates and vertebrates feeding on flowers and developing fruits) on maternal fecundity (total seed production per plant)? (2) Are the effects of pollinators and herbivores on fecundity additive, or is there some significant interaction between them? A two-factor (“Pollinators” and “Herbivores,” each with two levels, “present” and “excluded”) experimental design was used, with individual plants being treated as experimental units and maternal fecundity as the response variable. On average, the effects of pollinators and herbivores were of opposite sign and roughly similar absolute magnitude (∼7.5 seeds per plant), thus approximately canceling each other. A significant interaction (non-additivity) between factors did exist, with plants exposed to pollinators experiencing a disproportionately higher incidence of herbivores than those from which pollinators had been excluded. This was mainly due to mammals browsing only on the larger fruits from flowers that had been exposed to pollinators. Only in the absence of herbivores did pollinators account for a significant amount of between-plant variance in maternal fecundity. It is concluded that the interaction between pollinators and herbivores will lead to variable, herbivore-dependent “opportunity for selection” (sensu Arnold and Wade 1984) on P. broteroi by its pollinators. Regional variation in herbivore incidence experienced by P. broteroi populations will presumably generate local variation in the degree of adaptedness of the plant to its pollinators.