Invasive Scotch broom (Cytisus scoparius, Fabaceae) and the pollination success of three Garry oak-associated plant species

A growing number of studies have reported an effect of invasive species on the pollination and reproductive success of co-flowering plants, over and above direct competition for resources. In this study, we investigate the effect of the invader Scotch broom (Cytisus scoparius) on the visitation, pollen deposition, and female reproductive output of three co-flowering species (two native, one exotic) of the critically endangered Garry oak grassland ecosystem on the Saanich peninsula of Vancouver Island. The presence of C. scoparius was largely neutral, with the exception of some facilitation of pollen deposition to the native Camassia leichtlinii, the one species exhibiting pollinator overlap with Scotch broom. Yet, this pattern occurred despite a decreased visitation rate from pollinators. There was little observed effect of the invader on the native Collinsia parviflora or the exotic Geranium molle. Because broom was not favourited by any of the observed pollinators, this study provides evidence that the spread of Scotch broom is not due to the reduction of pollination success of natives nor is C. scoparius likely to be facilitating the pollination of other exotics in Garry oak ecosystem remnants.