Comparative pollinator limitation of two non-native shrubs: do mutualisms influence invasions?

While interactions between invaders and resident species have received a great deal of attention recently, the role of mutualists in facilitating or constraining invasions is rarely considered. We investigated the reproductive ecology of two closely related, woody legumes, Cytisus scoparius (Scotch broom) and Genista monspessulana (French broom), invading the same sites. Both species are considered noxious non-native weeds in California, and are considered to be ecologically similar, but Genista has much smaller flowers than Cytisus. Neither species showed appreciable levels of autogamous selfing. When experimentally self-pollinated, Genista demonstrated less depression of fruit set and seed set relative to outcrossed flowers than did Cytisus. At two sites on the Marin peninsula, Calif., Genista flowers were consistently less likely to be pollinated than Cytisus flowers. Genista was significantly pollen limited at both sites, while Cytisus was pollen limited at only the site with lower visitation rates. In the three populations with demonstrable pollen limitation, we found a significant relationship between fruit production and natural pollinator visitation at the level of the individual plant. However, we did not find that overall patterns of fecundity were strongly predicted by differences in pollen limitation between species or between sites. While a previous study found a tight link between patterns of pollinator visitation and patterns of reproduction in Cytisus in Washington State, we conclude that a more complex and variable environment (in terms of resources, herbivores, and florivores) on the Marin Peninsula de-coupled the relationship between pollinators and fruit production in these invaders. Our results suggest that the role of mutualisms in promoting or constraining invasions is likely to vary considerably among invaded communities.