The contribution of pollinator-mediated versus pollinator-independent interactions to plant reproduction

Abstract

Pollinator-mediated and pollinator-independent interactions both affect plant reproductive success but are often studied independently. Evaluating the separate and cumulative effect of both types of interactions is necessary to understand population dynamics and species coexistence. Here, we ask how interactions during growth and flowering contribute to pollinator-mediated and pollinator-independent density dependence in components of reproduction and total fecundity in communities of Clarkia species. Using experimental plots embedded in natural communities of forbs and grasses, we examine the response of flower number, ovule number per flower, seed set (% of ovules in a fruit that are filled seed), and total fecundity (total seed number per plant) of focal plants of four Clarkia species to varying densities of background Clarkia, forbs, and grasses, with (control) or without supplemental pollination of focal flowers. A comparison of seed set and total fecundity between control and pollen-supplemented flowers provided an estimate of pollen limitation to reproduction, which was largely pollinator mediated in this study. Forbs and grasses exerted a density-dependent, pollinator-independent competitive effect on all reproductive components and on total fecundity. By contrast, interactions between focal and background Clarkia were entirely density-independent, pollinator-mediated, and affected only seed set. Pollinator-mediated effects on seed set between pairs of focal and background Clarkia species were largely competitive, and in line with the known pollination biology of Clarkia species. Our results point to the importance of evaluating pollinator-mediated interactions in the context of natural communities, and that pollinator-mediated interactions between Clarkia species, while strong, are not likely to affect population dynamics at the scale of the small local neighborhood but may do so at larger spatial and/or temporal scales.