Modulation Of Crabs On Growth And Recruitment Between An Invasive Species Spartina Alterniflora And The Native Species Phragmites Australis,in Fengxian Salt Marsh

Since the introduction of the invasive species,Spartina alterniflora,it has expanded into the coastal mudflats and wetlands,and formed a strong competition situation with native plant species,which has exerted a far-reaching impact on the local biodiversity and ecosystem functioning.Crabs,as one of the common macro-benthic animals in salt marsh,have a strong “ecosystem engineer” effect on the biological population,community and even ecosystem with their herbivory and burrowing behaviors.In addition,their feeding preference for different plants will affect their inter-specific relationship with other plants.Therefore,in this study,we selected Fengxian salt marsh which is located in the North Hangzhou Bay,as the research area,and two dominant plant species,Spartina alterniflora and Phragmites australis as research objects,then we established 3 dynamic vegetation transects in late March 2018.Within each transect,we selected 3 different kinds of habitats including Spartina monoculture,Phragmites monoculture and ecotone,and then set up totally 27 quadrats which contain different crab treatments.We had been regularly monitoring the growth performance and seedling recruitment dynamics of Spartina and Phragmites in 2018 and 2019 for two consecutive years,during which the survey on the richness of crabs in the study area,the laboratory feeding trail of crabs and the measurement of feeding related traits were completed.According to our research,the main results can be showed as follows:(1)Crabs had a promoting effect on the maximum plant height of Spartina both in low intertidal zone and middle intertidal zone,while they had a strong inhibiting effect on the maximum plant height of Phragmites in middle intertidal zone,and the results in 2018 and 2019 were generally consistent.The average relative growth rate of Spartina was significantly higher than that of Phragmites,and the difference was more remarkable in 2019.Considering the effect of crabs on the relative growth rate of the two plants,the results showed that the relative growth rate of Spartina was significantly reduced without crabs,but the effect on the relative growth rate of Phragmites was non-significant.In addition,crab exclusion significantly reduced theaboveground biomass of Spartina in low and middle intertidal zone,while increased the aboveground biomass of Phragmites in the middle intertidal zone,indicating that crabs had opposite effects on the accumulation of aboveground biomass between Spartina and Phragmites.Based on the above results,it was concluded that crabs had a promoting effect on the growth of invasive species Spartina,but a strong inhibiting effect on the growth of native Phragmites.(2)In low intertidal zone and middle intertidal zone,the exclusion of crabs significantly reduced the seedling density of Spartina within the quadrat,but increased the seedling density of Phragmites in middle and high intertidal zone,showing that crabs had a facilitating effect on the seedling recruitment of invasive species Spartina,while exerted a intense inhibiting effect on the seedling recruitment of indigenous species Phragmites.In addition,this effect was much stronger in summer and autumn than that of in spring and winter,which can be found the similar trend both in 2018 and 2019.(3)The investigation of crab density by trapping method showed that the dominant crab species in Fengxian wetland were Sesarma plicata and Helice tientsinensis.Different seasons and habitats had significant influence on the distribution of Helice,but the habitat factors had non-significant effect on the density of Sesarma.In addition,the density of both crabs was higher in summer and autumn,but lower in spring and winter.The two kinds of crabs have different habitat choices in different seasons.In spring and winter,Helice were distributed mainly in the low intertidal zone,while merely in the middle or high intertidal zone.However,in summer and autumn,they were widely distributed in the Spartina zone and Phragmites zone,which was different from the distribution of Sesarma.(4)The laboratory feeding preference trail showed that Sesarma and Helice both fed on Spartina and Phragmites,while the average consumption of Phragmites was significantly higher than that of Spartina.In addition,both crabs showed a feeding preference to Phragmites seedlings,which consumed more Phragmites seedlings than Spartina seedlings.Furthermore,experiments on feeding preference of various plant organs in different life history stages indicated that crabs prefer to fed onmature leaves,young leaves and young stems of Phragmites,compared to the invasive species,Spartina.(5)The nitrogen content of Phragmites was significantly higher than that of Spartina,while the phosphorus content and toughness were significantly lower than that of Spartina.In addition,the stoichiometric ratio of each nutrient content in the two plants was significantly different,and the carbon-nitrogen ratio of the invasive species Spartina was significantly higher than that of Phragmites,while the nitrogenphosphorus ratio was significantly lower than that of Phragmites.Combined with the laboratory feeding preference experiments,the results showed that toughness,nitrogen content and carbon-nitrogen ratio of plants were the main factors affecting the feeding process of crabs,while the water content of plants may not be the main factor accounting for the feeding preference of crabs.Based on the above results,it was concluded that biological factors such as crab herbivory had significant effects on the growth and recruitment of invasive species Spartina and native species Phragmites.In addition,physical and chemical factors such as toughness and nutrient content can further explain the reasons why crabs showed different feeding preference to different plant species.Our research mainly explored the biological control effect of crabs on different plant species in Fengxian salt marsh,through the combination of field investigation and laboratory controlled experiments,which has important reference value for the restoration and management of degraded wetland vegetation,as well as the prevention of invasive plants.In addition,the study provides a new research perspective for further revealing the invasion mechanism of invasive plants.