Study On The Stabilization Effect Of Weak Interspecific Interactions On Alpine Meadow Community

Alpine meadow ecosystem is one of the most important natural ecosystems on the Qinghai-Tibet plateau.Under the background of global change and loss of species diversity caused by human activities,the Qinghai-Tibet plateau experiences increasing degradation.Thus,it is urgent to study the stability maintenance mechanism of alpine meadow communities on the Qinghai-Tibet plateau.According to empirical studies,positive interactions in communities may be as common as negative interactions in competition,and also play an important role in community construction.Moreover,previous studies on the verification of temporal stability mechanism usually test the positive interaction by taking promotion effect and weak interaction together.The effect of weak interaction is far from been fully tested.Although weak interaction has been tested in a few low-diversity aquatic microcosms,few confirmatory studies on terrestrial natural communities can be found.Presumably,weak interactions may be more common than facilitationsince any ecosystem with a generalized predation relationship should exist.In grazing grassland ecosystem,the increase of grazing pressure of animals on plants directly affects the productivity and population dynamics of dominant species and weaken their competitiveness.The competitive releasing effect caused by this can improve the diversity and population dominance of the previously disadvantaged species.Since grazing attenuates the resource competition among planting species,the weak interaction should be an important mechanism to maintain the temporal stability of grazing grassland population and community.However,few relevant research can be found at present.In grassland ecosystem,fertilization is also an important disturbance factor.An study on the effects of grazing and soil nitrogen addition on the weak interaction between species was conducted.The role of weak interaction in maintaining the temporal stability of population and community was tested in this study.The result of this study can provide a direct evidence for explaining the formation mechanism of the alpine meadow dominated by Kobresia humilis which is a deviation climax community caused by grazing on the Qinghai-Tibet plateau.An 11-year experiment(2007～2017)with simulated grazing(clipping)and fertilization treatments was conducted in the alpine meadow.Coverage data of species and communities was measured.The data was analyzed to detect changes of the weak interactions between species under different experimental treatments.The weak interactions were then compared with two important mechanisms of community temporal stability-the effects of portfolio effect and asynchronous effect.The purpose of the study was to determine the role of the weak interaction mechanism in maintaining the temporal stability of alpine meadow community(ICVcom)and population temporal stability(ICVpop)under the complex influence of disturbance and nitrogen addition.The research aims to solve the following problems:(1)Confirm that the increase of consumer downward pressure(clipping intensity)will strengthen the positive effect of the weak interaction among species on the community’s temporal stability,and verify the hypothesis of the weak interaction;(2)To verify the supplementary hypothesis of the weak interaction hypothesis proposed by Jiang et al.(2009),that is,the relationship between species diversity and population stability depends on the complexity of the trophic level of the ecosystem,and the positive correlation between diversity and population stability is more likely to occur in the multi-trophic level community than in the single-trophic level community;(3)To explore whether the increased availability of soil resources can weaken the effect of weak interaction caused by consumer pressure.The effects of clipping,fertilization and their interaction on the main variables were analyzed by AN OVA.The correlation between the two variables was determined by Pearson correlation analysis in communities with different levels of clipping and fertilization.The types of relationships between these variables were further clarified by regressive analyses.The relative contributions of clipping,fertilization,species diversity,weak interaction,portfolio effect,asynchronous effect and population temporal stability to community temporal stability were revealed by constructing structural equation model(SEM).The main results are as follow:(1)Under the clipping scenarios,relative index of interaction(RII)increases,logarithmic response than(InRR)decreases with clipping strength increases.The intensity of interspecific competition in each clipping treatment showed a descending order as follow:no clipping>moderate clipping>heavy clipping,suggesting the weak interaction was mainly controlled by clipping intensity.This indicates that the presence of grazer turns the negative interaction between plant species from strong competition to weak interaction,and the stronger the graze intensity is,the stronger the weak interaction is.At the same time,the results showed that the effect of fertilization on the nature and intensity of plant interaction was not significant,and the interaction between fertilization and clipping treatments was not significant.Compared to RII,InRR showed better applicability and sensitivity in the measurement of weak interaction and the construction of structural equation model.(2)The species richness(SR)and community temporal stability(ICVcom)were significantly positively correlated in the compound community,the moderate clipping community on the fertilization gradient,and the different fertilization level communities on the clipping gradient.There was a significant positive correlation between SR and population temporal stability(ICVpop)in the composite community,the three cut horizontal communities on the fertilization gradient and the unfertilized and fertilized communities on the cut gradient.This relationship was related to the decrease of SR,ICVcom and ICVpop after fertilization and the increase of ICVpop after clipping,indicating that the increase of species diversity was related to the improvement of population stability and community stability.The relationship was affected by clipping disturbance and trophic enrichment.(3)The relationship between ICVcom and RII and lnRR in no-fertilization and fertilized communities on clipping gradient was significantly linearly positive and linearly negative.Clipping not only increased ICVcom,but also increased RII and decreased lnRR,indicating that the relationship between interspecific interactions and the temporal stability of community coverage was mainly affected by clipping.In other words,the increase of stability was related to the reduction of interspecific competition and the increase of interspecific weak interaction,which had a significant positive effect on the temporal stability of the community.Fertilization reduced ICVcom while had no significant effect on RII and lnRR,indicating that fertilization did not affect ICVcom by changing RII and lnRR.(4)In the no-fertilization community,the SR-ICVpop relationship changed from the negative correlation at the no-clipping temporal to the positive correlation at the moderate clipping temporal,while in the fertilization community,the SR-ICVpop relationship was not significantly positive correlation at the three clipping level communities.On the one hand,it indicates that the temporal stability of diversity and population can be changed from the negative correlation of single trophic level(no clipping)to the positive correlation of multi-trophic level(clipping).Predation improves the temporal stability of populations and communities by reducing competitive interactions among species,confirming the hypothesis proposed by Jiang et al.(2009).On the other hand,it also indicates that the negative SR-ICVpop relationship in single trophic level is concealed by trophic rich aggregation.The prediction of this hypothesis has certain limitations,which may be limited to the environment without trophic enrichment.(5)The mechanism affecting the temporal stability of community coverage varies with the disturbance environment of the community.SEM results show that the maintenance of community temporal stability depends on two mechanisms,among which the most prevalent and effective mechanism is the asynchronous effect of species,which is usually driven by changes in clipping or fertilization levels.Another mechanism is the weak interaction,but its effectiveness is only reflected in the compound community and the clipping gradient of the unfertilized community.In the composite community,the effect of weak interaction(0.48)was less than that of species asynchrony(0.68),but in the unfertilized community,the effect(0.82)was much greater than that of species asynchrony(0.67).The results show that the weak interaction is one of the important mechanisms to maintain the stability of grassland ecosystem in heterogeneous environment,especially under the condition of no fertilization.Although the portfolio effect is ubiquitous,its relative contribution to the temporal stability of the community is not significant compared to other mechanisms.In summary,the weak interaction mechanism driven by grazing or clipping is one of the important mechanisms to maintain the temporal stability of the population and community of alpine meadow grazing ecosystem,which confirms the relevant theoretical hypothesis,but the accumulation of soil trophics weakens its effect.This study highlighted that under the background of species loss caused by elevated nitrogen deposition,maintaining suitable level of grazing intensity and effectively reducing exogenous nitrogen input is necessary.This would help to reduce the loss of species,and be beneficial to strengthen a variety of mechanisms,including weak interaction.It would in turn help to maintain the temporal stability of the alpine meadow ecosystem on the Qinghai-Tibet Plateau.