Plant invasions — the role of mutualisms

Abstract

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 animalmediated 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 improv...

Biodiversity as a Bulwark Against Invasion: Conceptual Threads Since Elton

“…Darlington ' s (1965) arguments were fi rst articulated by Darwin. In the last chapter of On the Origin of Species (1859) , Darwin wrote that ' As natural selection acts by competition, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates; so that we need feel no surprise at the inhabitants of any one country, although on the current state of the hypothesis seems to be that some researchers would prefer to emphasize the role that community diversity plays, even if weak (see, for example, Stachowicz & Tilman 2005 ), whereas others view the poor ability of diversity in natural systems to predict invasibility as damning evidence against the DIH paradigm (Rejm á nek 1996 ; Williamson 1996 ;Huston 2004 ;Davis 2009 ). Still another perspective is the possibility that high levels of diversity imply not only many potential competitors, but also many potential mutualists (Richardson et al 2000 ). Several studies in marine systems in particular have demonstrated links between diversity and facilitative interactions in invasions (see, for example, Stachowicz & Byrnes 2006 ).…”

Mutualisms: Key Drivers of Invasions … Key Casualties of Invasions

“…In fact, if plant invasions are facilitated by a diverse array of pollinators, dispersers, fungi and bacteria. So, if disturbance and/or fl uctuations in resource availability create windows of opportunity, we should expect invasibility to be positively correlated with native species richness (Richardson et al 2000a ). capacity for autonomous self -pollination), and therefore requires pollinators to set seeds.…”

“…Much evidence has accumulated in the last decade from terrestrial and aquatic ecosystems to show that native species frequently promote (facilitate) the colonization and establishment of introduced species through a variety of mechanisms (Richardson et al 2000a ;Bruno et al 2005 ;Badano et al 2007 ;Milton et al 2007 ;Olyarnik et al 2008 ). Simberloff and von Holle (1999) were the fi rst to incorporate facilitation explicitly in an invasion biology framework; they coined the term ' invasional meltdown ' for the process whereby two or more introduced species facilitate establishment and/or spread of each other (and potentially other species).…”

“…Native and alien animals clearly assist the spread of alien plants by pollinating their fl owers or by dispersing their seeds (reviewed in Richardson et al 2000a ). Native and alien plants also facilitate the spread of alien animals (pollinators and seed dispersers) by providing them with important food resources (pollen, nectar, resins, fruit pulp, etc.).…”

Soil Biota and Plant Invasions: Biogeographical Effects on Plant–Microbe Interactions

“…This interest is associated with recent sea changes in plant ecology in which the effects of positive interactions (i.e. facilitation and mutualisms) are emphasized in shaping natural and invaded communities (Bruno et al 2003 ;Richardson et al 2000 ). Perhaps successful invasions are due in part to the lack of coevolved interactions with soil biota, thus invaders are interacting with microbial symbionts in ways that are more benefi cial through positive feedbacks (Klironomos 2002 ;Packer & Clay 2000 ;Reinhart et al 2003Reinhart et al , 2005Callaway et al 2004a,b ), increased soil nutrient pools (Liao et al 2008 ;Rodgers et al 2008 ;Rout & Chrzanowski 2009 ), and perhaps the accumulation of more beneficial mutualisms.…”