Quantifying direct vs. indirect effects of nectar robbers on male and female components of plant fitness

SummaryPlants interact simultaneously with both mutualists and antagonists. While webs of plant–animal interactions in natural systems can be highly complex, most interactions can be simplified into those that are either direct (mediated through pairwise interactions) or indirect (mediated through third‐party species). Mechanistic studies of the direct and indirect pathways by which foliar herbivores affect plants have been well explored; however, mechanistic explorations of how floral herbivores, such as nectar robbers, affect total plant fitness via direct vs. indirect pathways have received less attention.The goal of this study was to assess the importance of direct vs. pollinator‐mediated indirect effects of a floral antagonist on female and male components of plant fitness. We focused on the hummingbird‐pollinated plant scarlet gilia,Ipomopsis aggregata, which is nectar‐robbed by the bumblebeeBombus occidentalis. Prior studies have found evidence for pollinator‐mediated indirect effects of robbing on female and male components ofI. aggregatafitness, but the mechanisms by which these indirect effects occur, and experimental evidence supporting or refuting direct effects of robbing, have been lacking.We found no evidence for direct effects of robbing on plant fitness. Robbers did not act as pollinators ofI. aggregatanor did they directly affect seed production by making nectar‐robber holes or removing nectar in hand‐pollinated flowers. Moreover, robbing had no direct effect on pollen production per flower or the ability of pollen from robbed flowers to sire seeds in hand pollinations.However, nectar robbing had indirect effects on plant reproduction mediated through per‐visit pollinator effectiveness at depositing pollen in robbed vs. unrobbed flowers. A simple model of a plant‐robber‐pollinator system suggested that robbing effects in general may occur through more indirect mechanisms when nectar removal by robbers is high relative to nectar replenishment, and that compensation for robbing is then more profitable through the production of additional flowers.Synthesis. Our results highlight the importance of indirect effects in mediating the fitness consequences of species interactions.