Changes in pollinator fauna cause spatial variation in pollen limitation

Abstract

1. Pollen limitation may be a consequence of changes in pollinator abundance, diversity and identity. However, no empirical evidence exists concerning the consequences that the spatial variation in pollinator fauna has on pollen limitation intensity and plant reproduction. In this study, we test the effect that changes in flower-visitor abundance, diversity and identity exert on the occurrence and strength of pollen limitation by experimentally quantifying pollen limitation in eight populations of Erysimummediohispanicum, a pollination-generalist plant native to the Iberian Peninsula. 2. Pollen limitation was accounted for by using a comprehensive estimator, the net reproductive rate (R0). Nevertheless, we also determined which components of plant reproduction, from ovule fertilization to seedling survival, were more intensely pollen-limited. Finally, we explored whether the spatial variation in pollen limitation intensity was related to among-population changes in flower-visitor abundance, diversity and identity. 3. The whole reproductive cycle of E. mediohispanicum was pollen-limited, although pollen limitation occurred more strongly during the ovule fertilization and seed-production phases than during fruit ripening or seedling emergence and establishment. 4. There was a significant among-population difference in pollen limitation intensity. Pollen limitation strength was associated with variations in flower-visitor diversity, and identity. Populations with lower flower-visitor diversity and with many low-efficiency pollinators (i.e. beetles) showed stronger pollen limitation. 5. Synthesis. Our study shows that the intensity of pollen limitation at the population level may depend on several characteristics of the assemblage of flower-visiting insects, such as their abundance, diversity and identity. Our results suggest that any impoverishment of pollinator diversity or any alteration in the specific composition of the pollinator assemblage may exacerbate pollen limitation.