| Schistosomiasis is the second largest tropical parasitic disease in the world after malaria.The endemic schistosomiasis areas in China are distributed in Hunan,Hubei,Anhui,Jiangxi and Yunnan provinces.Oncomelania hupensis(Snail)is the only intermediate host of schistosomiasis japonicum,so snail control is an important measure to control the spread of schistosomiasis.So far,the snail area in China have been 3.63 billion m2.More than 95%of snail area have distributed in the beaches of Yangtze river.Forestry schistosomiasis prevention project has effectively reduced snail area and decreased snail density by afforestation on the beaches in China.However,the research on mechanism of snail control by afforestation have not made significant progress,which has greatly restricted the development of Forestry schistosomiasis prevention project.The former studies have indicated that afforestation on beaches can change the factors of water,solar radiation and temperature in order to limit snail survival.But the key factors that inhibit snail population and the mechanism of affecting snail distribution are still unclear so far.The previous researches mainly focus on the influence of abiotic factors on snail by afforestation.But there were few studies on the effect of biotic factors on snail distribution,especially on the interaction between biotic and abiotic factors on the distribution of snail population.Based on it,this research will pay attention on the snail population,herbaceous communities,soil and algae.The sample area have been set on 5 types of beaches which were Sedge,Reed and 3-year old,5-year old and 8-year old artificial poplar forest lands.The research content included snail survey,herbaceous community survey and the analysis on soil physio-chemical characterisitics and algae.The purpose of this study is to elucidate the mechanism between different biological factors and non-biological factors on the snail population distribution and the key factors reflecting the effect of snail control by afforestation,and going to reveal the mechanism of snail control by afforestation.The research innovation will provide the important basis for scientific evaluating the quality and benefit of Forestry schistosomiasis prevention project.The main results and conclusions are as follows:(1)The snail density on the different beaches of Sedge,Reed and artificial poplar forest land were respectively 12.74±7.57 one/m2,3.89±2.75 one/m2 and 2.12±1.25 one/m2.There was no snail survival in the artificial popular forest with the age of 5-year and 8-year old.The results indicated that Sedge beach was the most suitable area for snail breeding.After afforestation,snail density has significantly decreased.(2)Herbaceous plants were the important conditions for the snails survival.There was no snail on the beaches without herbaceous communities.After afforestation,the snail density decreased significantly with the growth of forest age.Herbaceous community structure also has changed significantly meanwhile.The study has shown that the dominance and Niche breadth of Leonurus japonicus,Cnidium monnieri(Linn.)Cuss,and Alternanthera philoxeroides all increased with age of forest as well.The coverage and density of understory herbaceous communities decreased significantly(p<0.05),ware respectively as follows:Artificial poplar forest(68.5±14.1%)<Reed(90.6±10.1%)<Sedge(94.8±0.3%)and Artificial poplar forest(90.2±22.4 individual/m2)<Reed(90.6±10.1 individual/m2)<Sedge(94.8±0.3%individual/m2).With the growth of forest age,the coverage and density of understory herbaceous community decreased significantly(p<0.05),which was shown as follows:3a>5a>8a.The stability of understory herbaceous community increased significantly,which were as follows:Artificial poplar forest>Reed>Sedge.The community similarity analysis showed that the highest similarity coefficient of herbaceous community was 0.45 between Sedge and Reed.And the herb communities under 5a and 8a artificial poplar forest have the highest similarity with the similarity coefficient of 0.55.Snail is suitable for breeding in natural grassland habitats with similar community structure such as Sedge and Reed.With high similarity,the herb communities under 5a and 8a artificial poplar forest were non-snail habitats which has significant snail control effect.(3)Algae is an important food source for snail survival which can be ingested by swallowing soil.In the beachland,soil algal mainly consist of Chlorophyta,Cyanophyta and Bacillariophyta.As the dominant species,the content of Chlorophyta has accounted for more than 80%and significantly higher than others.The content of three kinds of algae in unit mass dry soil always were as follows:Chlorophyta>Cyanophyta>Bacillariophyta.The quantities of all soil algal have greatly decrease after afforestation.Especially there was no Bacillariophyta in the soil of 5-year and 8-year age forest.So the soil condition under artificial forest is disadvantage of soil algae survival expecially Bacillariophyta.In the digestive system of snail,the most quantities of algae were Chlorophyta and the least were Bacillariophyta.Bacillariophyta quantities in the digestive system of snail was the highest in Reed beach but the lowest in poplar forest.Combined with the analysis of snail density.Both snail and Bacillariophyta have not been detected in the forest land of 5-year and 8-year age,which indicated that the snail distribution was closely related with Bacillariophyta in soil.(4)Snail population mainly lives on the ground surface and in the 0-10cm soil layer.The soil is the essential material base for the snail breeding,which provides water and food for the snail.After afforestation,the contents of nutrient elements in the 0-10cm soil layer were significantly reduced including Soil water content,Soil organic matter,Total nitrogen,Total phosphorus,Total potassium and Hydrolytic nitrogen.The soil pH value of 0-10cm under the forest increased significantly as follows:Artificial poplar forest(7.54±0.20)<Reed(7.06±0.04)<Sedge(6.42±0.27).And the soil pH value also increased with the growth of forest age.There has been the highest content of Zn element is the highest and the lowest content of Cu and Mn elements in the 0-10cm soil layer.Among different forest ages,there has been the highest content of Zn,Cu and Mn elements in the 0-10cm soil layer of 5-year old forest,which were as follows:5a>8a>3a.The results indicated that the physic-ochemical properties of the surface soil in which snail breeding have changed significantly after afforestation.(5)The interaction analysis have shown that the key factors affecting snail density included Soil pH,Bacillariophyta,Coverage of herbaceous community,Density of herbaceous community and Dominance of molluscicidal plants.There are the direct effects of Bacillariophyt,Density of herbaceous community and Dominance of molluscicidal plants on snail density were respectively(0.690*),(0.526**)and(-0.225*).Soil pH and Coverage of herbaceous community had indirect effects on snail density with the path coefficients of(-0.516*)and(0.219*),respectively.Coverage of herbaceous community had a significant positive effect on snail density indirectly through direct effect on Bacillariophyt(0.3 18***).Soil pH had a significant negative effect onsnail density indirectly through the direct effect on Bacillariophyt(-0.566***)and Dominance of molluscicidal plants(0.558***).All the key factors together explained the spatial variation of snail density of 79.9%.After afforestation,Coverage and Density of herbaceous communities were significantly reduced(p<0.05),resulting in a significant decrease in the content of Bacillariophyt and the number of snail population(p<0.05).The increase of Dominance of molluscicidal plants,such as Leonurus japonicus,Cnidium monnieri(Linn.)Cuss,and Alternanthera philoxeroides,had a significant negative effect on the quantities of Bacillariophyt and snail density.The snail distribution was affected by both biological and non-biological factors.In conclusion,Bacillariophyt is the most important biological element reflecting the snail distribution.After afforestation,the interaction of such key factors as the Coverage and Density of herbaceous communities,Soil pH and Dominance of molluscicidal plants together led to a significant decrease of snail density,among which the effect intensity of biological factors was significantly greater than that of non-biological factors. |
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