Characteristics And Interspecific Associations Of Communities Invaded By Alternanthera Philoxeroides In The Plain Lake Area

Alternanthera philoxeroides has been invading China for nearly 90 years and has caused serious ecological disasters in the warm and humid areas of China,especially in the middle and lower reaches of the Yangtze River plain.It is an invasive weed with a wide invasive range and a high degree of invasion in China.To effectively predict and evaluate the level of A.philoxeroides invasion and provide reference for biological invasion control and native biodiversity protection,we investigated 55 communities invaded by A.philoxeroides in Jianli,Hubei Province.The composition of the invasive community was analyzed ecologically through a pre-survey and a field plant sample survey.We explored the capacity and similarities and differences in resource used by main species based on Shannon niche breadth,Schoener niche similarity and Pianka niche overlap of main species.Invasion response of native communities to A.philoxeroides analysed by alpha-diversity indices and and we explored the interspecific associations between the community species usingχ² test,Ochiai index（I_O）,Dice index（I_D）and the variance ratio（R_v）and contribution law method.Results showed that:（1）There were 84 species in 74 genera from 36 families in the communities invaded by A.philoxeroides.In terms of the composition of families and genera,the overall picture shows an outstanding diversity of a few families and genera,with a larger proportion of some families containing a few species and genus.Of these,Asteraceae and Gramineae are by far the dominant plants in the invaded community,with 15 species in 12 genera and 13 species in12 genera each.In terms of life type composition,herbaceous plants accounted for 92.86%of the total,with annual and perennial herbs accounting for 44.87%and 47.44%respectively.（2）The importance value and niche breadth of A.philoxeroides was the largest among the84 species that we studied,the dominant accompanying species were Cynodon dactylon,Echinochloa crus-galli,Digitaria sanguinalis,Erigeron canadensis.A.philoxeroides had the highest niche width which overlapped greatly with companion species（?O??_i?_k=0.283）,while the values between other dominant species were not high(?O??_i?_k=0.156,?C??_ik?=0.208).（3）The species diversity of the invaded communities in the study area was from high to low in the Four Lakes main channel>Yangtze River Mainstream>Dongjing River,and the terrestrial community diversity was much higher than that of the aquatic community.A significant negative correlation（r<0,P<0.05）was found between the A.philoxeroides importance value and the alpha-diversity index of the invaded community,indicating that the increase in the importance of A.philoxeroides populations significantly reduced community diversity,especially in aquatic communities.We also found that certain population densities of A.philoxeroides increased community diversity,but beyond a certain threshold it would form monodominant communities resulting in fewer companion species and reduced community diversity.（4）The variance ratio and W tests showed that the overall interspecific associations were significant positive association among the main species,but community stability analysis showed that the coordinate（30,70）between the inverse accumulative percentage of species and their accumulative relative frequency was far from the stability point（20,80）,which indicated that the invaded communities was in an unstable state.The results of the interspecific associations showed that 85%of the species pairs were not significantly associated,most of the dominant species were not closely related to each other,but the positive to negative association ratio to reach the significance level was 3.5,indicating that the strong interspecific role of a few dominant species influenced the overall stability and successional process of the community.There were negative correlations between A.philoxeroides and 14 main species,which was related to its invasiveness.There was a close positive correlation and high niche overlap between the A.philoxeroides and E.crus-galli,indicating that the two plants had similar resource needs and coordinated symbiosis,which may be one of the reasons for the rapid invasion of the A.philoxeroides into farmland.Combining the results of interspecific associations with the ecological habits of the plants,the 16 main species in the invaded community were divided into three ecological species groups using the boundaries of negative associations between groups and positive associations within groups as far as possible.In view of the above findings,it is recommended that monitoring and control of A.philoxeroides in the plain lakes should be increased.For terrestrial communities,biological control measures such as plant substitution can be considered for species with high niche overlap and strong negative associations with A.philoxeroides such as C.dactylon and Polygonum hydropiper,while species with low niche overlap such as Cayratia japonica and s Eclipta prostrata can be used to prevent vicious interspecific competition.For aquatic communities,physical removal should be stepped up as well as manual removal and drying or burning of the entire A.philoxeroides plant to prevent secondary invasion and,if necessary,the introduction of natural enemies for control.