Latitudinal Differentiation In Responses Of Native Insect Communities To Spartina Alterniflora(Poaceae) Invasion And Its Driving Forces In China's Coastal Wetlands

The exotic invasive plant Spartina alterniflora,which is originated from North America,has exerted a great impact on coastal wetland ecosystems in China.As an important part of biodiversity,insects play important roles in sustaining the ecological functions of coastal wetlands.The study on interactions between S.alterniflora and native insect communities is significative for the exploration of invasive mechanisms and consequences of this exotic plant.However,knowledge about the latitudinal differentiation in responses of native insect communities to S.alterniflora invasion is still scarce.This study focuses on the latitudinal and temporal variations in diversity and community compositions of three insect feeding functional groups,namely herbivorous,predatory-parasitic and detritivorous-algophagous groups,collected respectively from exotic S.alterniflora and native Phragmites australis.To reveal the different roles of environmental variables including plant structural characteristics,soil and climatic factors in driving the latitudinal variations of species richness of three insect functional groups,the generalized estimating equation analysis was performed.By doing this,the heterogeneity in responses of native insect communities to S.alterniflora invasion at a broad spatial scale and its driving factors are expected to be explored.Results of this study are as following:?1?The functional composition of insect communities were different between two plants.Specially,the species percentage of predatory-parasitic insects was distinctly lower in exotic S.alterniflora than in native P.australis.Moreover,species richness and abundance of herbivorous and predatory-parasitic insects were distinctly higher in autumn than in summer on P.australis,but less temporally varied on S.alterniflora.?2?Generally the mean species richness and abundance of all three insect groups were significantly lower on exotic S.alterniflora than on native P.australis,which indicates that S.alterniflora invasion had a generally negative influence on the biodiversities of native insect communities.The analysis of linear mixed model showed that the responses of native insects to S.alterniflora invasion could vary among functional groups and change with sampling time and latitude.?3?The curve fitting analysis showed that the relationship between species richness of herbivorous and predatory-parasitic insects and latitude actually differed between seasons.The species richness of these insects exhibited a hump-shaped correlation with latitude in summer,but linearly declined with latitude in autumn.And insects on S.alterniflora generally showed a relatively weak latitudinal differentiation trend than those on native P.australis in both seasons.By contrast,the latitudinal differentiation trend in species richness of detritivorous-algophagous insects on P.australis was similar in summer and autumn,both of which decreased with increasing latitude,while such trend was absent in insects on S.alterniflora.?4?The curve fitting analysis showed that the Bray-Curtis dissimilarity index(d _BC)of herbivorous and predatory-parasitic insect communities on P.australis was significantly increased with the latitudinal distance between them in summer and autumn.However,such correlations observed on S.alterniflora were relatively weak.For detritivorous-algophagous insects,the species turnover rates were generally less increased with increasing latitudinal distance.?5?The driving factors for species richness of insects from different functional groups in different seasons was analyzed by generalized estimating equation.Results showed that the species richness of insects on P.australis significantly fluctuated between years.Comparatively,annual fluctuation in insects on S.alterniflora was less significant in autumn.The effects of plant structural characteristics,soil and climatic factors on insect species richness were also different between two plants,which depended on insect functional groups and sampling seasons.Among these factors,the effects of plant structural characteristics?e.g.shoot height and diameter?and soil salinity were significant on herbivorous and predatory-parasitic insects on S.alterniflora and varied with seasons.And annual maximum and minimum temperatures played a more important role respectively in influencing insect species richness in summer and autumn.?6?Both the principal components analysis and detrended correspondence analysis were adopted to reveal seasonal and latitudinal?high and low?differences in insect community composition on two plants.It was suggested that,in comparison with those on P.australis,herbivorous and predatory-parasitic insects on S.alterniflora were distinctly less affected by sampling seasons and latitudinal regions.Different from the other two functional groups,detritivorous-algophagous insects on both plants generally had less different community structures in different seasons and latitudinal regions.In conclusion,results of this thesis suggested that S.alterniflora invasion had a general negative impact on the insect biodiversity in China's coastal wetlands.However,different insect functional groups responded differently to S.alterniflora invasion.Furthermore,S.alterniflora invasion had altered the latitudinal differentiations of species richness and species turnover rate of native insects and the models for the action of driving factors.The effects of season and latitude on insect community compositions were also relatively weakened by the invasion of this exotic plant.These results would be significative for the assessment of ecological consequences of S.alterniflora invasion at large spatial scale.