The effect of a flower-dwelling predator on a specialized pollination system

Abstract Plant species vary greatly in the number and diversity of floral visitors with which they interact. Even so, pollination ecologists have focused mainly on direct pairwise interactions (mutualists), overlooking indirect effects produced by different agents, such as floral larcenists and flower-dwelling predators. In our study area in Brazil, the pollen-flowers of Chamaecrista ramosa (Fabaceae) harbour Misumenops sp. (Thomisidae) spiders, a flower-dwelling predator with unknown signalling strategy. We measured the effect of Misumenops on the foraging behaviour of three bee species, one behaving as a pollen robber (Trigona spinipes) and two as pollinators (Xylocopa ordinaria and X. hirsutissima), and the consequences for plant fitness. The presence of Misumenops reduced the frequency of Trigona bee visits and increased the proportion of undamaged anthers, and thus pollen available to Xylocopa bees, which seemed unaffected by spider presence. However, spider presence (detectable by both Trigona and Xylocopa bees through achromatic and chromatic contrasts) had no effect on fruit and seed set when compared to flowers without spiders. Thus, the apparently antagonistic interaction between Trigona and flowers can represent a commensalism, modulated by differences between pollinators and larcenists regarding the window of foraging activity, as well as regarding foraging efficiency. The results also indicate that the quantitative impact of predators of floral visitor on plant fitness will be highly dependent on context in species with specialized pollination systems.