Effects Of Crabs On The Interspecific Relationship Of Plants In Different Elevation Of Chongming Dongtan

Salt marsh is one of the ecosystems with highest primary productivity. Zonation of plant species along elevation gradient is a common characteristic of salt marshes. Previous studies have shown that herbivory animals may change the community structure and the relationship between plants. The Bertness and Callaway (1994) Stress Gradient Hypothesis (SGH) showed the plant relationship responses to the shifting physical and biotic environments. However, there is few study have tested if this hypothesis can explain the plant relationship at local scale with the consideration of both abiotic and herbivory pressures. The Yangtze estuary salt marsh from high to low elevation can be divided into Phragmites australis community and Cyperaceae plant community. In addition, there are a large number of herbivory animals such as crabs. Therefore it provides an ideal experiment system to verify the SGH model, and the probability to explain the salt marsh plant zonation pattern. The effect of crabs on plant intercations and plant growth is a worthwhile study, because it may provide a scientific basis for marsh protection and management. In this study, we designed a transplant experiment to test the effect of two dominant crabs, Chiromantes dehaani and Helice tientsinensis, on the interaction between P. australis and Cyperaceae plant along a local-scale elevation gradient.1. Densities of crabs was significantly different along the elevational gradient (Repeated-ANOVA analysis result:location factor,p<0.001). At the highest location, the density of crabs was up to21.47±1.99ind. per square meter. While at the lowest location, the density of crabs was down to4.40±0.97ind. per square meter.2. There was no clear trend of RII (Relative Interaction Index) for ramet number, ramet height, above-ground biomass, under-ground biomass of P. australis at different locations. But the crab herbivory changed RⅡ at different locations and showed a trend along elevational gradient. At locations of higher elevation, RII of P. australis biomass increased when crabs were present. At the location of lowest elevation, the positive effect of crabs on RII values was not obvious, because the density of crab was rather low. 3. The Eh-value of sediment gradually increased with the elevation declined. When the density of crabs was high, crabs could significantly enhance the Eh of sediment. The electrical conductivity of sediment also tended to increase with the presence of crabs.4. The Curculionidae larva density in neighbors left intact treatment was91%higher than in neighbors removed treatment. Impact of crabs on Curculionidae density changed along the elevation gradient. Crab herbivory depressed the density of Curculionidae at higher elevational locations when plant neighbors left intact.5. Feeding preference experiment showed that plant materials of all four plant species were consumed by crabs C. dehaani and H. tientsinensis. Comparatively, both crabs prefer to Cyperaceae plants.In conclusion, the effects of crab herbivory on plant-plant interactions varied along the elevational gradient. The crab herbivory improved the positive associations or reduced competitions between P. australis and Cyperaceae plants at the higher elevational locations. However, the positive effect of crabs on plant interactions was not obvious at the lowest elevational location. The varying effects of crab herbivory on plant interactions along the elevational gradient were probably due to crab density differences, feeding preference, effects on sediment properties and other herbivory animals.The study implied that the effects of crabs are not the main driver of the plant zonation at the Yangtze estuary marshes, but the crabs can mediate the plant distributions. Therefore, the effect of crabs and other herbivory animals should be taken into consideration for the protection and restoration of marsh vegetation.