Many introduced plant species rely on mutualisms in their new habitats to overcome barriers to
establishment and to become naturalized and, in some cases, invasive. Mutualisms involving animal-mediated pollination and seed dispersal, and symbioses between plant roots and microbiota often facilitate
invasions. The spread of many alien plants, particularly woody ones, depends on pollinator mutualisms.
Most alien plants are well served by generalist pollinators (insects and birds), and pollinator limitation does
not appear to be a major barrier for the spread of introduced plants (special conditions relating to Ficus and
orchids are described). Seeds of many of the most notorious plant invaders are dispersed by animals, mainly
birds and mammals. Our review supports the view that tightly coevolved, plant-vertebrate seed dispersal
systems are extremely rare. Vertebrate-dispersed plants are generally not limited reproductively by the lack
of dispersers. Most mycorrhizal plants form associations with arbuscular mycorrhizal fungi which, because
of their low specificity, do not seem to play a major role in facilitating or hindering plant invasions (except
possibly on remote islands such as the Galapagos which are poor in arbuscular mycorrhizal fungi). The lack
of symbionts has, however, been a major barrier for many ectomycorrhizal plants, notably for Pinus spp. in
parts of the southern hemisphere. The roles of nitrogen-fixing associations between legumes and rhizobia and
between actinorhizal plants and Frankia spp. in promoting or hindering invasions have been virtually ignored
in the invasions literature. Symbionts required to induce nitrogen fixation in many plants are extremely
widespread, but intentional introductions of symbionts have altered the invasibility of many, if not most,
systems. Some of the world's worst invasive alien species only invaded after the introduction of symbionts.
Mutualisms in the new environment sometimes re-unite the same species that form partnerships in the native
range of the plant. Very often, however, different species are involved, emphasizing the diffuse nature of
many (most) mutualisms. Mutualisms in new habitats usually duplicate functions or strategies that exist in
the natural range of the plant. Occasionally, mutualisms forge totally novel combinations, with profound
implications for the behaviour of the introduced plant in the new environment (examples are seed dispersal
mutualisms involving wind-dispersed pines and cockatoos in Australia; and mycorrhizal associations
involving plant roots and fungi). Many ecosystems are becoming more susceptible to invasion by introduced
plants because: (a) they contain an increasing array of potential mutualistic partners (e.g. generalist
frugivores and pollinators, mycorrhizal fungi with wide host ranges, rhizobia strains with infectivity across
genera); and (b) conditions conducive for the establishment of various alien/alien synergisms are becoming
more abundant. Incorporating perspectives on mutualisms in screening protocols will improve (but not
perfect) our ability to predict whether a given plant species could invade a particular habitat.