Abstract:

Background and Aims The growing frequency and impact of biological invasions worldwide threaten biodiversity, ecosystem functioning, resource availability, national economies, and human health. Alternanthera philoxeroides is one of the most harmful invasive species and has caused great economic and ecological losses. This species has caused great harm to agriculture, fisheries, as well as to landscapes and native biodiversity, and, at times, has completely overrun and dominated an entire terrestrial ecosystem. It also can form dense tangled mats that float on the water surface of ponds or lakes and shade indigenous aquatic vegetation from sunlight, and thus replace native species. Previous studies have shown that A. philoxeroides can reduce the biodiversity of plants on paddy field ridges, but there have been no other studies on the effects of A. philoxeroides on biodiversity in other habitats. In this study, we conducted a survey of the species and their distributions in different weedy habitats that were invaded by A. philoxeroides in Nanjing during the spring and early summer season in order to interpret the relationship between its occurrence and the environments and elucidate the impact of its invasion on plant diversity.
Methods The abundance, coverage and frequency of each weed population were quantified using the square intercept method in 27 plots. The importance value of each weed was calculated as the sum of the relative abundance, relative coverage and relative frequency. We recorded 142 weed species that belonged to thirty-six families. The data were analysed using principal component analysis (PCA) in SPSS. Thirty-eight out of 142 weed species that had more than 20% frequency occurred in 27 samples were used in the analysis.
Key Results The results showed that the most important factors affecting the occurrence and distribution of weed species were the levels of human disturbance and the soil humidity. According to these two factors, all samples were divided into four groups. Group I included samples collected from experimental fields, and the dominant weed species were Capsella burss-pastoris, Veronica persica and Stellaria media. Group II included samples located in rape farmland which have high soil humidity, and the dominant weed species were Galium aparine var. tenerum, Conyza bonariensis, and Roegneria kamoji. Group III included samples located in public parks or under trees, and the dominant weed species were Vicia sativa, Beckmannia syzigachne, and Geranium carolinianum. The last group was mainly composed of samples distributed on wastelands and waysides which were non-managed or non-irrigated farmland, and the dominant weed species were Erigeron annuus, and Veronica arvensis. The change in the density of Alligator weed and other weeds in the different habitats were analysed. The results showed that the importance value of Alligator weed was high when the soil humidity was relatively high and the intensity of human disturbance was not too great, indicating that Alligator weed has a strong ability to adapt to different habitats. There were different dominants in the different plant communities, which means human disturbance and soil humidity had a key effect on the communities that were composed of a mixture of Alligator and other weeds. When species diversity indices of each group were compared, it was found that although species richness and Simpson's index increased slightly from group I to group IV, but the Shannon-Wiener index and the Pielou evenness index decreased with an increase in the average importance value of Alligator weed.
Conclusions These results indicated that the species composition of the weed community had changed, and the species diversity gradually decreased with increasing dominance of the invasive Alligator weed. These results clearly showed that the invasion of Alligator weed had a negative affect on biodiversity.

Key words: Alternanthera philoxeroides, Weed distribution, Principal component analysis, Cluster analysis, Species diversity