| Under the background of global environmental change,it is of great significance to study the top-down effect on plant growth in the coastal wetland.As an important part of coastal wetland ecosystem,macrobenthic animals play an important role in regulating the growth of vegetation.The Phragmites australis population growing in the tidal wetland,as the dominant plant species in the high marsh of the Yellow River Delta,has a unique genetic and important ecological function.However,the Phragmites australis population degraded these years because of the rapid environmental changes,and thus it needs to be protected.In this study,the P.australis population of the high marsh in the Yellow River Delta was selected as the study site.The dominant crab species,density,morphological trait and feeding behavior in this study site were investigated by the fence method and field observation.The food source and feeding habit of Helice tientsinensis were studied by laboratory feeding test and stable isotope source analysis.The effects of H.tientsinensis herbivory on the growth and reproduction of P.australis were studied by using simulating consumed degree and field in-situ experiment.The main results are as follows:(1)The dominant crab species in the P.australis population of the high marsh in the Yellow River Delta was H.tientsinensis,and the density of H.tientsinensis was 5.5±1.5 ind/m2,the morphological trait of male and female crabs were significantly different(P<0.001).which had the behavior of climbing P.australis to feed on the leaves at night.(2)H.tientsinensis has obvious feeding preference in indoor conditions(food sources:fresh leaves,dead leaves,roots,sediment),and prefers to feed on P.australis leaves(fresh leaves,dead leaves).In the wild environment(food sources:fresh leaves,dead leaves,suspended solids,sediment,Assiminea latericea and Perinereis aibuhitensis),the A.latericea is the main food,but the fresh and dead leaves of the P.australis were also important food sources.The proportion of fresh leaves[may:(6.4±0.2)%;July:(5.8±0.2)%;September:(12.5±0.3)%]and dead leaves[may:(12.4±0.2)%;July:(15.5±0.3)%;September:(15.1±0.3)%]had significantly temporal heterogeneity.(3)In the experiment of simulating consumed degree,the individuals P.australis was significantly reduced by feeding treatment(CK:7.90±2.38 individuals;moderate consumption:5.5012.17 individuals;severe consumption:5.90±1.85 individuals;P=0.041).However,the leaf length and breadth showed compensatory increase,and the leaf length(CK:12.46±3.15 cm;moderate consumption:14.36±4.09 cm;severe consumption:12.50±3.46 cm;P=0.008)and width(CK:1.07±0.25 cm;moderate consumption:1.26±0.30 cm;severe consumption:1.11±0.27 cm;P=0.013)treated with moderate consunption were significantly higher than that of CK.The treatment had no significant effect on the basal diameter and biomass(P>0.05).(4)In the field environment,the H.tientsinensis herbivory had significant control effect on the growth and reproduction of P.australis.H.tientsinensis herbivory significantly reduced the biomass(no cage:3.75±1.18 g;crab cage:11.82±3.54 g;P<0.001),plant number(no cage:4.60±0.89;crab cage:27.20±9.88;P<0.001),and height(no cage:68.14±12.66 cm;crab cage:84.38±187.93 cm;P=0.016)of P.australis.Meanwhile,under the background of environmental changes(warming and drying trend),H.tientsinensis herbivory would enhance the degradation of P.australis population(2018:5.33±0.33 g;2019:2.38±0.43 g).H.tientsinensis herbivory had no significant effect on the leaf length and breadth(P>0.05).(5)H.tientsinensis herbivory resulted in the significant increase of flavonoids(contrast:2.87±0.22 mg/g-1;severe:4.17±0.26 mg/g-1;P=0.005)and tannin(contrast:153.38±8.50 mg/g-1;severe:217.14±16.14 mg/g-1;P=0.006).However,these secondary metabolites produced by P.australis did not assist it resist the herbivory of H.tientsinensis.(6)The disturbance from H.tientsinensis behaviors may affect the soil total carbon content(P<0.001),which may have an indirect effect on the growth of P.australis.There was no significant effect of H.tientsinensis behavior on soil conductivity,pH,total nitrogen,total sulfur and total phosphorus(P>0.05).The results of this study showed that the main benthic animal species in the P.australis vegetation of the high marsh in the Yellow River Delta was H.tientsinensis,which inhibited the growth and reproduction traits and population density of P.australis by consuming its leaves.Under the background of warming and drying trend,the crab herbivory degraded the P.australis population in the high marsh of the Yellow River Delta.Therefore,this paper revealed the important role of top-down effect on the dynamic of P.australis population in the high marsh of the Yellow River Delta,and provided a theoretical basis for the protection of the common reed population in the coastal wetland of the Yellow River Delta.In the future,the effect of top-down effect on P.australis population in coastal wetland of the Yellow River Delta should be recognized in the protection of the common reed populations. |
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