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 conductive 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.

/pdf/plant-invasions-the-role-of-mutualisms-wxlrtt363j.pdf

Plant invasions--the role of mutualisms.

Myrica faya, an introduced actinorhizal nitrogen fixer, in invading young volcanic sites in Hawaii Volcanoes National Park. We examined the population biology of the invader and ecosystem-level consequences of its invasion in open-canopied forests resulting from volcanic cinder-fall. Although Myrica faya is nominally dioecious, both males and females produce large amounts of fruit that are utilized by a number of exotic and native birds, particularly the exotic Zosterops japonica. In areas of active colonization, Myrica seed rain under perch trees of the dominant native Metrosideros polymorpha ranged from 6 to 60 seeds m{sup {minus}2} yr{sup {minus}1}; no seeds were captured in the open. Planted seeds of Myrica also germinated an established better under isolated individuals of Metrosideros than in the open. Diameter growth of Myrica is > 15-fold greater than that of Metrosideros, and the Myrica population is increasing rapidly. Rates of nitrogen fixation were measured using the acetylene reduction assay calibrated with {sup 15}N. Myrica nodules reduced acetylene at between 5 and 20 {mu}mol g{sup {minus}1} h{sup {minus}1}, a rate that extrapolated to nitrogen fixation of 18 kg ha{sup {minus}1} in a densely colonized site. By comparison, all native sources of nitrogen fixation summed to 0.2 kg ha{sup {minus}1}more » yr{sup {minus}1}, and precipitation added < 4 kg ha{sup {minus}1} yr{sup {minus}1}. Measurements of litter decomposition and nitrogen release, soil nitrogen mineralization, and plant growth in bioassays all demonstrated that nitrogen fixed by Myrica becomes available to other organisms as well. We conclude that biological invasion by Myrica faya alters ecosystem-level properties in this young volcanic area; at least in this case, the demography and physiology of one species controls characteristics of a whole ecosystem.« less

Biological invasion by Myrica faya in Hawai'i: plant demography, nitrogen fixation, ecosystem effects

Much of the recent progress in ecosystem ecology can be traced to studies which have examined the responses of ecosystems to disturbance (Odum 1969). For example, early studies of forest clear-felling (cf. Hesselman 1917, in Stalfelt (1960) demonstrated that soil nutrient availability is usually enhanced in harvested sites. More recently, studies at the Hubbard Brook Experimental Forest (Likens et al. 1970, Bormann and Likens 1979), in Sweden (Tamm et al. 1974, Wiklander 1981), and elsewhere documented that forest cutting alters watershed-level hydrology and nutrient losses; longer-term measurements have documented the reestablishment of biotic regulation of water and nutrient cycling during secondary succession (Bormann and Likens 1979).

Biological invasions and ecosystem processes : towards an integration of population biology and ecosystem studies

The Colletotrichum gloeosporioides species complex

Plant responses to atmospheric CO2 enrichment with emphasis on roots and the rhizosphere.

Two species of Rubus occur naturally in the Hawaiian Islands, R. hazvaiensis and R. macraei. It has been previously thought that R. spectabilis, a western North American species, and R. hawaiensis share a recent common ancestor based on morphological similarity, and that R. macraei was a later derivative of R. hawaiensis. DNA sequences of the chloroplast gene ndhF were used to reconstruct the phylogeny of these species with non-Hawaiian members of subgenus Idaeobatus. These findings strongly contradict the previous hypothesis for the origin of the Hawaiian Rubus. Although R. hazvaiensis is closely related to R. spectabilis, R. macraei is distantly related to these species. Sequence identity, genetic distance, and phylogenetic analysis all suggest that R. macraei is distantly related to New World and Asian species of subg. Idaeobatus. Consequently, R. macraei and R. hawaiensis may have arisen from two separate colonizations in the Hawaiian Islands rather than one as previously assumed. The ancestry of R. macraei may trace to other Pacific Rim islands.

Phylogeny of Rubus Subgenus Idaeobatus (Rosaceae) and its Implications Toward Colonization of the Hawaiian Islands

An evaluation of the wilt-causing bacterium Ralstonia solanacearum as a potential biological control agent for the alien kahili ginger (Hedychium gardnerianum) in Hawaiian forests.

Dieback of Acacia koa in Hawaii: ecological and pathological characteristics of affected stands

Acacia koa seedling wilt caused by Fusarium oxysporum f. sp. koae, f. sp. nov.

Fusarium oxysporum f. sp. passiflorae, the cause of vascular wilt of passionfruit (Passiflora edulis f. edulis), also attacked inoculated banana poka (P. mollissima) seedlings. Other noncrop Passiflora spp. (P. ligularis and P. foetida) were susceptible, whereas P. e. f. flavicarpa, the cultivated passionfruit in Hawaii, and P. suberosa were resistant. This work is part of an initial evaluation of possible biocontrol agents for banana poka

Donald E. Gardner

Papers

Phylogeny of Rubus Subgenus Idaeobatus (Rosaceae) and its Implications Toward Colonization of the Hawaiian Islands

An evaluation of the wilt-causing bacterium Ralstonia solanacearum as a potential biological control agent for the alien kahili ginger (Hedychium gardnerianum) in Hawaiian forests.

Dieback of Acacia koa in Hawaii: ecological and pathological characteristics of affected stands

Acacia koa seedling wilt caused by Fusarium oxysporum f. sp. koae, f. sp. nov.

Pathogenicity of Fusarium oxysporum f. sp. passiflorae to banana poka and other Passiflora spp. in Hawaii