The Distribution Pattern And Characteristics Of The Invasive Plant Species In China

Biological invasions have attracted extensive attention from ecologists because of their significant ecological impacts and economic costs worldwide. Biological invasions are one of the global problems of the natural ecosystems; it is one of the main causes of biodiversity loss and threatening the ecological environment and ecological development globally. What habitats will be easily invaded and what species will become invasive are the two key questions of invasion ecology.Researches on the characteristics of the recipient regions are essential to understand the process of plant invasion. However, few previous studies on invasibility of habitats concerned social factors, even though human activities are critical during the process of plant invasion. China is a vast country with high plant species diversity and a long history of introduction of alien plant species, which is particularly vulnerable to invasive plant species. Currently, alien plant species are widespread in the country, and some of which are costly in economic. Therefore the study of invasive plants in China is urgent in practice and important for developing invasion ecology. For the present study, 126 species were selected to represent the major invasive plant species in China. We then collected data on their species richness in 31 provincial administrative units of China and performed Spearman's rank correlations between the species richness and possible natural and socio-economic factors. We found that socio-economic factors, such as human density and GDP. correlated positively with the species richness of invasive plants in China. In conjunction with the natural and socio-economic correlations in the study of regional distribution pattern of the major invasive plants, we discussed the factors influencing the regional distribution pattern of the major invasive plants in China. The results suggested that native plant species richness was mainly determined by the naturalconditions of the regions, while invasive species richness was influenced by natural conditions and human disturbance together. The results had two important implications: (1) The positive corrections between species richness of invasive plants and native plants result from the natural conditions that influencing both of them simultaneously; (2) The human disturbance has important influence on the distribution of the invasive plant species.Biological invasions are always difficult to eradicate, or are economically costly. In order to decrease or avoid introduction invasive plant species, it is paramount to analyse alien plants before introduction so that the invasive plant species can be identifed beforehand. Unfortunately, to date there is no consensus on invasion mechanisms and predictive models. Controversies range from whether we can reliably predict which species may become invasive to which species characteristics (e.g., life history, taxonomic groups, or geographic origin) contribute to the invasion processes. We examined 126 invasive plant species in China to understand the role of clonality and geographical origin on their invasion success. These species were categorized into three groups based on their invasiveness. The invasiveness decreases from group I, group II to group III. Clonal plants consisted of almost half (44%) of the 126 invasive species studied, and 66% of 32 the most invasive alien plant species (Group I). The positive correlation between clonality and the invasiveness was significant. 68% of the 126 species studied originated in the continent of America (North and/or South America). These preliminary findings supported that America was the primary geographical origin of invasive alien plant species in China and that clonality of the invasive plant species contributed significantly to the their invasiveness. The results suggest an urgent need to investigate the mechanisms whereby plant clonal growth influences plant invasions, and the need for a focus at regional scale to examine factors affecting the exchange of invasive plant species between America and China.Clonal integration and clonal plasticity are used by clonal plants in response to spatial heterogeneity of habitats in abiotic and biotic factors that exists extensively in the nature. Integration among ramets may also improve the ability of clonal plants to tolerate abiotic stress and thereby improve the invasion success of individual ramets. Alligator weed (Alternanthera philoxeroides (Mart.) Griseb), one of the important invasive clonal plant species, was chosen to do the case study. Using the method ofexperimental ecology, the relationship be'tween clonalty and invasiveness was investigated and the causes that it becomes invasive were discussed.The experiments found that the clonal integration could help the alligator weed to endure and utilize the poor nutrient habitats. The integration intensity was different in different direction. Within fragments, proximal (older) sections growing in high nutrient conditions could give the distal (offspring) sections growing in poor nutrient significant support. On the other hand, distal sections with ample nutrient gave less support to the proximal sections growing in sterile conditions. In heterogeneous habitats, clonal integration could also contribute to the increase of the invasiveness-related traits, such as the total length of stolons and the number of ramets. These results suggested that clonal integration could help alligator weed tolerate abiotic stress and thereby improve the invasion success of individual ramets. On the other hand, alligator weed was plastic to nutrient in the experiment. The plasticity of the ratio of root to shoot and specific leaf area was non-local response at the level of ramet pair. The ramet pair level plasticity could use resources more effectively and improve its adaptability. The alligator weed in China has very low genetic diversity, so the plasticity is its strategy to deal with different habitats it invaded. Another experiment found that the severing of stolons promoted the growth of alligator weed in homogeneous high nutrient,which suggested that the species was adapted to the stolon breakage. This trait can contribute to the adaptation to the human disturbance.The photosynthesis index confirmed that alligator weed could share some of the nutrients by clonal integration. The ramets in poor nutrient but connected with high nutrient ramets could then have comparatively high photosynthesis rate, and the fitness of the ramet pair was boosted. Clonal integration was the ecological strategy in response to spatial heterogeneous habitats. In the homogenous poor nutrient, the chlorophyll of the plant was maintained like in the high nutrients, which ensured the plants to resume high photosynthesis rate when the condition would become appropriate. This is the ecological strategy adapted to temporal heterogeneityIn conclusion, human disturbance has important influence on the distribution pattern of the invasive plant species in China. The statistical study using 126 invasive plant species in China and the case study both show that clonality can contribute to the invasiveness of some alien plants. The alligator weed has traits to adapt toheterogeneous habitat and human disturbance, which may be the cause for its invasion in China.