Biotic Homogenization Caused By Alien Plant Invasions

Biotic homogenization is one of important research agendas on ecology,which has been causing serious challenge to biodiversity conservation and economy worldwide.Many factors many lead to biotic homogenization,in particular for those referring to human activity.With high levels of available resources and anthropogenic disturbance,agricultural areas are commonly susceptible to alien plant invasions,which may quickly cause biotic homogenization.To accumulate a firsthand data of weed homogenization in arable lands in China,as well as support valuable information to the establishment of scientific management of weed homogenization,the present studies conducted field surveys on weed communities of sunmmer croplands in Anhui Province,China,as well as compared these weed communties with the relative datasets for historical references,and thus studied the pattern and trend of weed homogenization in the surveyed arable areas.Moreover,a serial of field surveys on the communities with S.canadensis and the control sites(adjacent to the S.canadensis precent sites but without the species)in both the native and introduced ranges were conducted,and the biotic homogenization caused by S.canadensis distribution were studied.The main findings were:Biotic homogenization in the structure of weed community in croplands:visible biotic homogenization were found in the weed communities in summer croplands in Anhui Province The composition of dominant weed species changed and the sturcture of weed community shifted from mono-dominant to co-dominant by several principal weed speceies.Both of the richness and dominance value of overall alien invasive plant species increased significantly,which caused the similarities among different weed communities increased significantly,as well,the weed communities in lands with dry crop rotation and those in lands with rice-wheat(oilseed rape)shared increasing similarities,which were distinctly different in the historical datasets.Weed community in summer croplands in Anhui were still divided into three groups:1)weed communities in croplands with dry crop rotation in North Anuui,2)those in croplands with dry crop rotation in South Anuui,and 3)those in croplands with rice-wheat(oil rape)rotation.However,the dominance values and composition of principal weed species changed.In lands with dry crop rotation,the dominance of Galium aparine var.tenerum decreased significantly,while Geranium carolinianum,Conyza canadensis,Euphorbia helioscopia and Alternanthera philoxeroides become principal weed species.In croplands with dry crop rotation in North Anhui,Avena fatua become most dominant weed species,while in the same sort of croplands in South Anhui,the dorminance value of Avena fatua decreased significantly,while a wet plant speceis-Alopecurus aequalis become one of principal weed species.In rice-wheat(oilseed rape)roation system,Beckmannia syzigachne became the most dominant weed species,in lieu of Alopecurus aequalis,and the dominance values of several alien invasive plant speceis including Geranium caroli,nianum,Conyza canadensis,Vicia sativa,Erigeron annuus,and Veronica persica,as well as some xerophytic native weed species all increased significantly,which led to the decrease of mono-dominance in the relative weed communities.Weed homogenization caused by alien plant invasions:during the past 20 years,for alien crop weeds in this area,total number of species increased from 24 to 35,richness per site increased by 110%,a-diversity increased by 120%and total occurrence increased by 89%.Moreover,the dorminance of many alien invasive weed species increased,and several alien invasive plant species become crop weeds.Thus the challenge of alien plant invasions in Anhui Province may possibly become more serious.Geranium caroliniamum and Alternanthera philoxeroides showed the greatest increases of infestation,followed by Erigeron annuus,Conyza bonariensis,Conyza canadensis,Conyza sumatre,nsis,Coronopus didymus and Daucus carota.Bidens frondosa and Sonchus asper frequently spread into croplands.Recently,alien crop weeds were mainly with attributes of introduced into China after 1840,not wind-pollination,with seeds lighter than 2.5mg and not wind-dispersed,non-perennial and broadleaved.Alien weed species originating from America continent showed significant higher richness per site than those originating from Eurasia,but not abundance.In the past 20 years,richness and abundance of alien invasive plants belonging to all groups increased significantly,except for graminoid.Moreover,the increases in richness and dominance values of alien weed species from groups of native to America continent,introduced into China after 1840,non-perennial,broadleaved and lighter seed all significantly higher than their opposite groups,respectively.Compared with lands with dry crop rotation,alien crop weeds in lands with rice-wheat(oil rape)rotation were more likely to originate from America continent,not wind-pollination,with lighter seeds and broadleaved.Moreover,compared with oilseed rape field,alien crop weeds in wheat lands were more likely to be introduced into China before 1840,from Eurasia,wind-pollination,with heavier seeds and grass of sedge.The increases in the amount of chemical fertilizer applied,population density,traffic frequency and agricultural machinery showed positively influences on the increase in richness of alien weed species in surveyed croplands,and increases in the amount of herbicide applied and annual mean temperature showed negative influences.Nevertheless,as to the increase in total dominance value(summarizing coverage,abundance and height)of overall alien crop weeds in the surveyed croplands,the increase in the amount of herbicide applied showed positive influence,while increases in mean temperature of the hottest month and traffic frequency showed negative influences.Together all the results implied that the increase in alien weed invasions during the past 20 years were mainly attributed to increasing applications of chemical fertilizer and herbicides.Case studies with Geranium caroliniamum and Alternanthera philoxeroides:G.caroliniamum was the most serious alien crop weed species in surveyed arable lands with frequency of 90.97%in recent dataset.Also,this species spread the most rapid in the past 20 years,and changes in population density and annual precipitation showed significant and positive correlations with changes in abundance of this species.Geranium caroliniamum in lands with rice-wheat(oil rape)rotation was more frequent than in lands with dry crop rotation and more abundant in lands with rice-dry crop rotation and located in the areas with mean July temperature lower than 28.7℃.Alternanthera philoxeroides was the most serious perennial alien crop weed species in the surveyed croplands.Change in annual mean temperature significantly and positively correlated with increasing abundance of A.philoxeroides.Annual mean temperature,traffic frequency,mean January temerature and crop rotation significantly affected frequency of this species,while mean temperatures of January and July,annual precipitation and crop rotation significantly affected its abundance.Alternanthera philox.eroides was more serious in areas with higher temperature;however,its occurrence was low in areas with hot summer.Alternanthera philoxeroides seldom occur in areas with mean July temperature higher than 28℃,mean January temperature lower than 3.4℃ and annual precipitation lower than 695mm.A case study with the invasions of Solidago canadensis:Plant communities in Solidago canadensis invaded habitats shared similarities with those in its native ranges(clearly implied on Bray-Curtis similarity),such as which were characterized by S.canadensis being dominant,significantly lower species richness,a-diversity and β-diversity,as well as a decrease in the correlation coefficient between geographic distance and floristic similarities.Solidago canadensis dominated in various habitats in China,where more than 20 different,dominant species should have occurred.Serious invasive species can quickly remodel and homogenize diverse communities by dominating them.