Effect Of Spartina Alterniflora Invasions On Nematode Communities In Salt Marshes Of The Yangtze River Estuary: Patterns And Mechanisms

Invasions of exotic species have caused serious environmental problems in the world. As increasing attention has been paid to underground in recent years, the impact of exotic plant invasions on the composition and function of soil communities has emerged as an essential issue in invasion ecology. Coastal and estuarine salt marshes are the critical transition zones between terrestrial and aquatic ecosystems which play major roles in conserving biodiversity. Compared with other ecosystems, the salt marshes have widely been recognized as one of the most heavily invaded ecosystems. In Yangtze River estuarine salt marshes, the ecosystems are now heavily infested with an introduced exotic plant Spartina alterniflora. S. alterniflora spreads to most of the wetlands and is competing with native plant species by forming dense monocultures. This study aims to evaluate the impacts of S. alterniflora invasions on soil biodiversity by using nematodes as indicators and to explore the mechanisms by which S. alterniflora affect the structure and function of soil nematodes. Nematodes are selected as indicators because they are the most abundant soil metazoan taxon, closely linked with plants and mineralization processes, and can provide unique insights into many aspects of ecosystem processes.During the study, a total of 70 nematode genera were found from the salt marsh wetlands of the Yangtze River estuary belonging to 39 families, 7 orders and 2 classes. Photographs of 37 common genera were presented to show their main characteristics.To assess the effect of S. alterniflora invasion on soil nematodes, I compared the nematode communities in marshes respectively dominated by invasive S. alterniflora and native Scirpus mariqueter and Phragmites australis at three local sites in the Yangtze River estuary over two seasons. S. alterniflora stands had generally lower nematode trophic diversity than the stands of the 2 native plants, suggesting that the exotic plant led to a simplified benthic food web. The relative abundance of bacterial-feeding nematodes tended to increase in S. alterniflora marshes compared to P. australis marshes. The increased bacterial-feeding nematodes in S. alterniflora stands are likely to reflect the altered decomposition processes, rates and pathways, which may, in turn, modify belowground nutrient cycling of the estuarine ecosystems. The dissimilarity in nematode community structure between S. alterniflora and S. mariqueter marshes was smaller than that between S. alterniflora and P. australis marshes, suggesting that the detection of the ecological consequences of plant invasions depends on which native plant species is considered. In addition, site effects were generally detected in the comparison of sediment properties and nematodes among 3 plant marshes.To assess the effect of S. alterniflora invasion on epiphytic nematodes, I compared epiphytic nematode communities associated with standing live and dead stems of P. australis and S. alterniflora at Dongtan of Chongming Island across three seasones. In each sampling season, the dissimilarities in epiphytic nematode communities between P. australis and S. alterniflora were significant. S. alterniflora stems supported more abundant epiphytic nematodes comparing to P. australis stems. These results suggest that the invasion of S. alterniflora increase the epiphytic nematode abundance and altered the epiphytic community composition. Greater abundances of epiphytic bacterial nematodes were found on dead stems of S. alterniflora than on those of P. australis, which was mainly due to the decomposing nematode Diplolaimelloides. It indicates that the dissimilarity of stem litter quality between S. alterniflora and P. australis may be important in shaping nematode communities associated with dead stems.To test whether the exotic plant invasions affect soil nematode communities through litter inputs, I compared mass loss and nematode colonization during the stem litter decomposition of invasive S. alterniflora and native P. australis in salt marshes of the Yangtze River estuary. With higher nitrogen content and lower C:N ratio, stem litter of the invasive S. alterniflora decayed faster than the native P. australis. The total nematode abundance was the highest in S. alterniflora litter at each experimental stage, followed by P. australis and control. The total nematode abundance remained low in control containers throughout the experiment. Most changes of nematode abundance were due to the change in bacterivores. Compared to P. australis, the greater nematode abundance in S. alterniflora was mainly due to two dominant genera of bacterial nematodes. Diplolaimelloides and Diplolaimella were dominant genera in both S. alterniflora and P. australis litter, contributing 53% and 28% to the total abundance, respectively. Lower values of maturity index and structure index in S. alterniflora than in P. australis litter indicate that a more degraded food web condition resulted from the invasion of S. alterniflora by producing higher quality of litter than the native P. australis.To test whether the exotic plant invasions affect soil nematode communities through altering root inputs, I conducted a two-year pot experiment grown with S. alterniflora and two native plant monocultures. The plant species identity did not affect the biomass, genus richness and diversity, trophic diversity and community structure of rhizosphere nematodes, which suggests a relatively weak rhizosphere effect. Contrary to the findings from litter decomposition experiment, the abundance of bacterial nematodes in rhizosphere of P. australis significantly increased than that of S. alterniflora. The abundance of plant-feeding nematodes was found significantly decreased in rhizosphere of S. alterniflora compared to S. mariquete, suggesting that the invasive S. alterniflora is less vulnerable than the native S. mariquete.