Mechanisms Of Exotic Plant Invasion And Responses To Habitat Characteristics

With increasing economic, trading, traveling and other human activities, invasions of exotic species has become one of the most important global-scale problems, which not noly alters ecological functioning and threatens biodiversity, but also causes huge economic losses and ecological threats. More than 50 years have past, however, biological invasions are still scarce of uniform theoretical and predictive frame.Researches on biological invasions are mainly discussed by two angles, one focuses on three key problems on biological invasions, i. e. species invasiveness, invisibility of community and pest (impact); the other emphasizes the invasion process by exotic species, i. e. four stages of introduction, establishment, spread and stabilization (impact/pest), which makes the research on invasion mechanism much easier. However, these studies on species invasions have tended to focus on species that are more likely to be successful colonizers than those that quickly disappear from the invaded communites. Additionally, surviors producing highly visible ecological and economic impacts are more likely to be studied than inconspicuous or innocuous species. Given this tendency, failed or non-visible colonists are underrecorded, and this creates a significant bias in the data on invasive species. So in this paper, invasion mechanism by exotic species was investigated by differtiating exotic clonal plant species (rhizomatic plant and stoloniferous plant) and exotic sexual plant species producing slight and small seeds. The study not only can exhibit various life history traits and spatial patterns of two plant types, but also can involve the "three focus" and "four stages" involved in biological invasions. Also, the model results are tested by population dynamics of exotic species Spartina alterniflora through indoor experiment, field investigations and data analysis, which accords well.Innovative models, biological invasion disturbance model and cellular automata model, were set up in the paper, which principly disclosed the mechanisms of biological invasions and response to various habitat traits. Six conclusions can be drawn by a great deal of numerical simulations, theoretical analysis and field investigations: 1) Exotic species invasions can result in one of the following four possibilities according to various dispersal abilities: a) rejection failure, b) coexistence, c) indirect failure and d) rejection; 2) The optimal spread of exotic clonal plant is in 6 directions; 3) Superior exotic species can not necessarilly invade native ecosystem, which is associated with biology characteristics and habitat conditions besides dispersal abilities. In virtue of clonal advantage, inferior exotic species can precolonize available habitat (First colonize, first develop rule). And a certain habitat increase and habitat destruction can clear off ability of competition; 4) Effect of habitat destruction on exotic species invasions is determined by dispersal and competitive ability and variated by plant types as well. For exotic clonal plant, habitat destruction lowers invasiveness by exotic plant species and clonal plant spreads in 6 neighboring directions, which limits their survivals, as serious habitat destructions increase species mortality and propagule pressure in the transportation. Severe habitat destruction slacks invasibility by exotic clonal plant species. For seed plant, competition between native species and colonizers can affect colonizer success and estabilishment. When superior exotic species are introduced, slight destruction has no obvious effect on population dynamics, but median habitat destruction can protect native species 1 to some certain extent. When the introduced exotic speces is inferior, serious destruction decreases the abundance of native species 1 and 2 while increases the colonization of exotic species with high dispersal abilities; 5) Habitat increases in favor of the occurance of edge effect. Inferior species (native or exotic) can escape superior ones in the edges, which is decided by the comparison between the speed of habitat increases and species spread; 6) Exotic invasion is sensitive to its main life history traits. Results from cellular automata model showed that growth stages from seedlings to adults made the greatest contributions to population growth of exotic species, while the adult survival probability can not significantly affect it. The variation of dispersal distance and the age of reproduction maturity show that exotic species abundance grew logarithmitically with dispersal distance and decreased logarithmically with reproductive maturity age for those species than two years old.