| Coastal wetlands in China have a great diversity of native burrowing crabs.As physical ecosystem engineers,crabs play important roles in the intertidal ecosystems.Crabs burrow into the sediment and introduce air and plant nutrients that are vital to the mangroves.Spartina alterniflora(hereafter Spartina),which is invading the entire Chinese coast,occupying mudflats throughout this range and displacing mangroves in the upper intertidal of southern China,is bound to affect the biotic interactions,resulting in changes of community structure and ecological functions.The ecotone between mangroves and Spartina provides a useful study system for understanding the interaction between native burrowing crabs and exotic Spartina.In this study,we monitored the distribution and community structure of two dominant species of Parasesarma plicata and Metaplex longipes in mangrove,Spartina,and unvegetated mudflat in Zhangjiang Estuary,Fujian,China.We conducted field manipulation experiments and laboratory feeding assays,in order to disentangle the roles of the biotic(vegetation,predator,and food source)and abiotic(light,temperature,salinity,and sediment)factors on the habitat selection of crabs.The main results are as follows:1.P.plicata and M.longipes showed totally opposite patterns of distribution in the three different habitats.The densities of P.plicata in the mangroves and Spartina habitats were significantly greater than those in the mudflat.On the contrary,the densities of M.longipes in the unvegetated mudflat were significantly greater than those in other two habitats.The number of P.plicata in mangrove and Spartina habitats showed some differences in different seasons,with more in Spartina than those in mangroves in Spring and Winter,more in mangroves than those in Spartina in Summer,and no difference in Autumn.Both of the two crabs showed higher densities in Spring,Summer,Autumn than those in Winter.Otherwise,there were no significant differences in different habitats in every season for the maximum carapace width of P.plicata,while the maximum carapace width of M.longipes in Spring and Summer were bigger than that in Autumn and Winter.And the number of males were more than that of females in general for both of the two kinds of crabs.2.Vegetation is the key factor influencing the distributions of P.plicata and M.longipes.When unvegetated mudflat plots were occupied by the rapid expansion of Spartina,the dominant crabs shifted from M.longipes to P.plicata within only one growing season.On the opposite,the dominant crabs shifted from P.plicata to M.longipes after removing Spartina within one growing season.3.Mangrove and Spartina could significantly decrease light density and temperature as well as provide compatible food for P.plicata,thus influencing the distributions of crabs.M.longipes preferred to inhabit of high light intensity compared with P.plicata and its tolerance to high temperature was also significantly higher than P.plicata.P.plicata can feed on mangrove and Spartina leaves,but we did not observed the phenomena of M.longipes feeding on plant leaves.4.Soil water content and soil salinity were not the main factors influencing the distribution of crabs for there were no significant difference among mangroves,Spartina,and unvegetated mudflat.5.The predation of P.plicata and M.longipes by their predator in mangroves and Spartina were not less than that in unvegetated mudflat,and the prey of P.plicata was higher than M.longipes in Spartina habitat.So,in this study,the results of tethering studies did not 'support the hypothesis that mangrove or Spartina provided refuge for crabs,and predators were not the main factors influencing the distribution of this two kinds of crabs.6.Both Spartina and crabs are regarded as ecosystem engineer in the intertidal zones.In China,the distribution of mangrove vegetation has been changed by the invasion of Spartina,which resulted in the rapid respond of crabs.Invasive Spartina is increasing the population of P.plicata by providing novel habitat and food.While M.longipes that depend on the mudflat habitat will be inhibited by Spartina invasion. |
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