Influence Of Ants On Plant Diversity In Different Degraded Songnen Grassland

The Songnen grassland is one of the most important grazing and mowing pastures in China.However,the grasslands were seriously degraded as a result of overgrazing and unlimited reclamation of human beings.The most distinct consequence of degradation is saline-alkalized,which affect the relationship between the plant diversity and soil animal diversity by changing the competitive relationship of the plants and affecting the survival environment of soil animals.Ants,one of the important soil animals in the Songnen grassland,can directly and indirectly affect plant diversity in the multi-trophic level.Ants can change seed bank diversity by transporting and collecting seeds,and affect soil physicochemical properties by building mounds.Likewise,it can also form a stable symbiosis with specific soil animals.Based on the recent studies,there exists the direct and indirect role between plants and soil animals in the grassland ecosystem together.Leading in the mutualism and indirect effect may change the classical ecological paradigm in which the species hostile relationship such as predation and competition is vital.Therefore,under the different degraded grassland,studies on the relationship between the ant diversity and plant diversity as well as the key factors infusing on diversity at the multi-trophic level,are essential to understand the dynamic mechanism of interaction between ants and plants under the condition of global change.The data have an important significance in improving and enriching the mutual benefit theory and food web theory.The experiment was conducted in Northeast Songnen grassland.According to the size of saline-alkaline patches and the different species of plants,grassland was divided into the light degraded and the heavy degraded site.In each experimental site,the block was divided into 120 plots.we randomly selected ten mounds of Lasius alienus and ten mounds of Lasius flavus respectively as sampling point and selected one point apart from one meter of the mound as control sampling point.From June 2014 to November 2015,we investigated ant diversity,the distribution pattern of the mounds,plant biomass and diversity,the number of insects in the field,root architecture and nutrients of major species and soil seed bank in the lab.The main results are present as follows:(1)The Songnen grassland degradation changed spatial distribution pattern of the mounds,which changed from homogenous to aggregation,reduced the density of the mounds and increased the relative area of L.flavus mounds,thus it changed the ant diversity.Moreover,a portion of the mounds were destroyed and another portion were expanded which can help ants adapt to the external environment on the degraded grassland.Therefore grassland degradation increased the average area and the height of the mounds(2)Regardless grassland degradation degree,the soil physicochemical characteristics was changed by the mounds of both L.flavus and L.alienus.Compared with the surrounding soil,soil moisture was decreased by 26.2% and 23.5% in L.alienus and L.flavus mounds respectively,the pH was decreased by 9.21% and 6.32% respectively,the EC was decreased by 19.5% and 23.5% respectively,while soil available phosphorus was increased by 83.5% and 118.3% respectively.(3)Leymus chinensis,Calamagrostis epigejos,Artemisia scoparia,Kalimeris integrifolia,Setaria viridis,Artemisia anethifolia were the main species in the experimental site.By analyzing the nutrient components of the six common species roots,we found that root water content of A.scoparia and A.anethifolia was relatively lower than those of other species,and there were no significant differences in the soluble sugar,acid detergent fiber(ADF)and protein as compared with those in the other species.In addition,the root length and superficial area of A.scoparia were obviously higher than other species,and the thick root(diameter greater than 1 mm)length of A.scoparia was 4 times longer than other species.Besides,the mounds could increase the root length of L.chinensis,C.epigejos and A.scoparia,but there was no effect on K.integrifolia and A.anethifolia.Through the survey,we observed that there were a lot of aphids and mealy bugs parasticing on the root of A.scoparia and A.anethifolia,which were more than those in L.chinensis,C.epigejo and K.integrifolia in the mounds.Different from our expectations,ants selecting host plants for symbiotic insects is not based on the nutrition ingredients,but much dependent on the root architecture.(4)The mounds increased the number and species richness of soil seed bank,and L.flavus had more effect on soil seed bank than L.alienus.In the light degraded grassland,the number of the mounds of both L.flavus and L.alienus was 2 times and 3 times more than those in surrounding soil.In the heavy degraded grassland,both the number of seeds of L.flavus and L.alienus were 2 times more than those in the surrounding soil.Grassland heavy degradation increased the quantity of L.alienus mounds and the numbers of A.scoparia seeds in the surrounding soil,but there was no significant influence on the number of A.scoparia seeds in L.flavus mounds.(5)The mounds of L.flavus and L.alienus didn'tchange the species richness,Pielou's index and Shannon-Wiener index of plant community,which indicated that building mounds didn't significantly influence the plant diversity.These results were different from our expectation.But L.flavus could select the habitat with high plant abundance and diversity to build mounds,which can help L.flavus to choose the suitable host plant for aphid and mealy bug.L.flavus and L.alienus altered the relative density of plant community,and increased the relative density of composite family.Grassland heavy degradation increased the relative density of composite family in the habitat of L.alienus.However,L.flavus was not affected by the degree of grassland degradation.Additionally,both of L.alienus and L.flavus increased the aboveground biomass of plants.In the habit of L.alienus,the mounds increased the aboveground biomass of Poaceae and Asteraceae.In the habitat of L.flavus,the mounds significantly increased the aboveground biomass of Asteraceae.The effect of ants on plant height is different between different plants,the mounds can increase the height of annual or biennial plants(S.viridis and A.scoparia),but had no effects on perennial grass(L.chinensis and C.epigejos).In summary,the behavior of ants has an important ecological significance on plant diversity in Songnen grassland.L.alienus and L.flavus can indirectly affect the plant communities by changing the physical and chemical properties of soil.L.flavus provides a suitable habitat for A.scoparia to grow.A.scoparia provide available nutrition for aphids and mealy bugs.Aphids and mealybugs provide honeydew for ants.The symbiotic relationship between ants,insects and A.scoparia has a direct impact on the plant community diversity.Heavy grassland degradation increased the mound relative area of L.flavus,and indirectly changed the diversity of plants.Although L.alienus had less impact on plants than L.flavus,but it has an important effect on the seed bank and plant diversity by predation on insect and foraging seeds.Moreover,the mounds provide a stable soil for plants,which relieve the effects of grassland degradation on plant diversity and biomass.Although the mounds occupied a small area of grassland which could not play a decisive role in the overall plant diversity,they could change the local microenvironment and increase the heterogeneity of grassland.Therefore it has a positive effect on the protection of grassland diversity.