Urbanization-mediated context dependence in the effect of floral neighborhood on pollinator visitation.

Abstract

Context dependence in ecological interactions is widespread, but our ability to predict how environmental variation will mediate the effects of a given interaction remains poor. Co-flowering plants can influence visitation of shared pollinators to one another; the effect of these interactions varies with ecological context. While research has investigated the effect of local biotic conditions on such interactions, little is known about how land use change, specifically urbanization, affects them. I tested how the interaction of urbanization and neighborhood floral density (NFD) and richness affected pollinator visitation rates to two crop species, cucumber and sunflower by placing experimental arrays of each species in paired high- (garden) and low- (lawn) floral density neighborhoods along an urban-to-rural gradient. Pollinator visitation to flowers was monitored over 2 years, as was NFD and richness. The two plant species showed contrasting responses to both urbanization and floral neighborhood density, with only cucumber experiencing context dependence in the effect of floral neighborhood. These contrasting responses to urbanization and floral neighborhood are likely due to differences between species in floral visitor community composition. Plants grown in gardens experienced higher pollinator visitation regardless of floral neighborhood. This study highlights the need for better understanding of genus- or species-specific pollinator responses to urbanization to predict the effect of urbanization on plant-pollinator interactions.