Invasive plants strongly affect physical and biotic environments of native ecosystems. Insects and other arthropods as one of the major components of many ecosystems are very sensitive to subtle changes in abiotic and biotic environments. We examined the effects of exotic Spartina alterniflora invasion on community structure and diets of arthropods in a saltmarsh previously dominated by native Phragmites australis in Yangtze River estuary through net sweeping and plant harvesting methods and stable isotope analysis. Our results showed that diversity indices were not significantly different between exotic and native plant communities, but the total abundance of insects estimated through plant harvesting method was found to be lower in Spartina monoculture than that in Phragmites monoculture. Community structure of insects in Spartina monoculture was dissimilar to that in Phragmites monoculture and Phragmites-Spartina mixture. Moreover, stable carbon isotope patterns of arthropods were significantly different between Phragmites and Spartina monocultures. Although some native arthropods (perhaps generalists) shifted their diets, many native taxa did prefer Phragmites to Spartina even in Spartina monoculture. Spartina invasions resulted in reduced abundances of some arthropds, and increased dominance of others feeding preferably on Spartina. This study provides evidence that invasive plants can change the community structure and diets of native arthropods, which will eventually alter the arthropod food web, and affect the integrity and functioning of native ecosystems within a nature reserve that has been set aside for conserving the native biodiversity and maintaining the ecosystem integrity. In this sense, Spartina invasions in the Yangtze River estuary need to be managed appropriately.

Abstract. Although many studies have been conducted on the interspecific competition between new arrivals and native plants, few of them have demonstrated how these processes interact with non-resource factors to determine vegetation pattern. This study investigated how salt stress mediates competition between native Phragmites australis and invasive Spartina alterniflora and thus changes plant communities in Dongtan, a Chinese coast salt marsh. The experiments revealed that the growth and reproduction of the native species declined with increasing salinity but that the invasive species performed well in the salinity range of 0-20%, illustrating why the invader could proliferate in the high salinity mudflats in Dongtan. Moreover, the native had a high growth rate and therefore exhibited a competitive dominance over the invader at low salinity of ca. 7%. Thus, the invader could not displace the native, and the native communities were stable in the low salinity zones. In contrast, the growth rate of the invader became higher when salinity increased; correspondingly, it gained the competitive dominance at high salinity of ca. 11%. As a result, the invader colonising the native communities in high salinity zones performed better and could displace the natives over time. Consequently, after invasive S. alterniflora colonisation, the vegetation pattern of Dongtan marsh gradually changed from ''mudflat-sedge-P. australis'' to ''mudflat-S. alterniflora'' and ''mudflat-S. alterniflora and natives'' along the elevation gradients. The findings of the case study demonstrated that if a new arrival has a wide tolerance range to major non-resource stress in an ecosystem, it can not only displace natives by interspecific competition in high stress zones but can also spread into the zones without natives; on the other hand, natives with a narrow ecological amplitude in relation to the non-resource stress can only persist in low stress zones. Therefore, the distribution area of new arrivals increases as the distribution area of natives decreases. Because some non-resource stresses have substantial positive effects on native community invasibility, the practices that change the level of non-resource stress and create favorable conditions for invasive species should be stopped.

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Physical conditions and biotic interactions are believed to be the determinants of plant zonation in saltmarshes. However, in rapidly developing estuarine marshes, succession is regarded as the primary process responsible for plant zonation and it is controlled mainly by environmental factors. Salinity and inundation are two important factors responsible for the distribution pattern of dominant plants in coastal saltmarshes. Here we conducted a common garden experiment as well as a field transplanting to examine the responses of four dominant saltmarsh plants (native Scirpus mariqueter, Scirpus triqueter and Phragmites australis, and exotic Spartina alterniflora) in the Yangtze River estuary to environmental gradients, which may help us understand their current and potential zonation. The results showed that Scirpus adapted to freshwater and less inundated habitats, Phragmites performed well in brackish or freshwater environments with less inundation, and Spartina tolerated the highest salinity and deepest inundation. In the harshest environments (the highest salinity and water level), only Spartina performed well. In the mild environments, however, there were only minor differences in the performances among the four species. The potential ranges of Phragmites and Spartina were predicted to be larger than their current ones, and their lower boundaries might be set by tidal scour rather than edaphic factors. With the saltmarsh succession, invasive Spartina in the Yangtze River estuary might ultimately replace Scirpus, and alter the zonal patterns of native saltmarsh plants, which will lead to severe ecosystem consequences. Thus, proper management measures (e.g., repeated mowing) need to be implemented to control this invasive exotic plant, and restore the vulnerable ecosystems invaded by Spartina in the Yangtze River estuary.

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