Plant invasions - the role of mutualisms

Many introduced plant species rely on mutualisms in their new habitats to o vercome barriers to establishment and to become naturalized and, in some ca ses, invasive. Mutualisms involving animal-mediated pollination and seed di spersal, and symbioses between plant roots and microbiota often facilitate invasions. The spread of many alien plants, particularly woody ones, depend s 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 conditi ons relating to Ficus and orchids are described). Seeds of many of the most notorious plant invaders are dispersed by animals, mainly birds and mammal s. Our review supports the view that tightly coevolved, plant-vertebrate se ed dispersal systems are extremely rare. Vertebrate-dispersed plants are ge nerally not limited reproductively by the lack of dispersers. Most mycorrhi zal plants form associations with arbuscular mycorrhizal fungi which, becau se of their low specificity, do not seem to play a major role in facilitati ng 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 plant s, notably for Pinus spp. in parts of the southern hemisphere. The roles of nitrogen-fixing associations between legumes and rhizobia and between acti norhizal plants and Frankia spp. in promoting or hindering invasions have b een virtually ignored in the invasions literature. Symbionts required to in duce nitrogen fixation in many plants are extremely widespread, but intenti onal introductions of symbionts have altered the invasibility of many, if n ot most, systems. Some of the world's worst invasive alien species only inv aded after the introduction of symbionts. Mutualisms in the new environment sometimes re-unite the same species that form partnerships in the native r ange of the plant. Very often, however, different species are involved, emp hasizing the diffuse nature of many (most) mutualisms. Mutualisms in new ha bitats usually duplicate functions or strategies that exist in the natural range of the plant. Occasionally, mutualisms forge totally novel combinatio ns, 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 i nvolving plant roots and fungi). Many ecosystems are becoming more suscepti ble to invasion by introduced plants because: (a) they contain an increasin g array of potential mutualistic partners (e.g. generalist frugivores and p ollinators, mycorrhizal fungi with wide host ranges, rhizobia strains with infectivity across genera); and (b) conditions conducive for the establishm ent of various alien/alien synergisms are becoming more abundant. Incorpora ting perspectives on mutualisms in screening protocols will improve (but no t perfect) our ability to predict whether a given plant species could invad e a particular habitat.