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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.

Summary An increasingly recognized impact of plant invaders is the disruption of positive interactions between native plants and their belowground mutualistic mycorrhizal fungi. We reviewed 112 studies from 61 publications that report invader impacts on mycorrhizal fungi. We describe emerging patterns on the frequency of negative, neutral and positive invader effects on mycorrhizal fungal abundance, richness and community composition, and we evaluate how these outcomes vary with invasion age. We also describe the evidence for three mechanisms of disruption: (1) plant‐plant competition that alters host quantity or quality (2) changes in soil properties such as nutrient availability and (3) allelopathy. Invaders can disrupt native mutualisms if they are non‐mycorrhizal, associate with a different type of mycorrhiza, or associate with different taxa of the same type of mycorrhiza as the native plants. Invaders that enrich soil nutrients can cause declines in mycorrhizal abundance, shifts in fungal community composition, and cause native plants to reduce their dependence on mycorrhizas. Invaders that produce allelopathic compounds cause mycorrhizal abundance to decrease and alter community composition because of variation in the sensitivity of different fungi to toxins. While the evidence for disruption of the mycorrhizal mutualism with invasion is strong, temporal patterns have not yet emerged from the literature. Invasion age was not clearly associated with extent of disruption, and the timing of recovery following invader removal was highly variable. Differences in the biology of native and invasive plants, their interactions with mycorrhizal fungi and the surrounding landscape contributed to this variation. Synthesis. Our systematic review suggests that invaders frequently decrease mycorrhizal abundance and alter fungal community composition while only occasionally affecting fungal species richness. The development of invader impacts, as well as the restoration of native communities after invader removal, are influenced by a suite of plant, fungal and environmental traits that change over time. However, few studies have examined the temporal dynamics of mycorrhizal disruption, and results are variable. Future research should focus on the temporal scale of mutualism disruption while considering plant phylogenetics and demography, fungal functional traits such as spore longevity and dispersal, and soil chemistry.