Interactions And Mechanisms Among Herbivores In An Alpine Meadow Of Qinghai-tibetan Plateau

Species interaction is one of the hotpots in ecology for its key roles in modifying ecosystems' biodiversity and stability.Herbivores are the basis of natural food chain and act as key stone species and ecosystem engineers.Herbivores have profound impacts on the structure,function and process of ecosystems.In a natural ecosystem,many kinds of herbivores are co-existed.However,the current researches on herbivores interactions mainly forcused on the unidirectional effects between those herbivores with similar body size or closely related,whereas the ecological roles of some herbivorous insects were ignored.Additionally,few konwlogy of herbivores interaction could be abtained in Qinghai-Tibetan Plateau,one of most important research area of the nature science worldwide.Here,the interactions and underline mechanisms among multiple herbivores(include two large herbivores-yak and Tibetan sheep,a burrowing rodent-pika and a herbivorous insect-caterpillar)were studied by using the sets of combining controlled field experiments in the alpine meadow of Qinghai-Tibetan Plateau.The species interactions between the two large herbivores,burrowing rodent,and herbivorous insect were respectively analyzed,and the potential mechanisms were also exposed.The study has practical significance for the biodiversity conservation of natural ecosystems and grassland managements.The main conclusions of the study are as follows:1)Yak and Tibetan sheep exerted a competitive effect.Both yak and Tibetan sheep significantly reduced their intake bites because of the declining of corresponding food resources.The diet preferences between yak and Tibetan sheep was significantly different,yak preferred the dominat plant—Kobresia pygmaea,while Tibetan sheep mainly eaten forbs such as Aster flaccidus,Saussure stella and Patentilla nivea.The biomass of K.pygmaea and forbs(favorited by Tibetan sheep)were decreased because of the grazing by the herds.There was a significant positive correlation between intake bite of yak and the aboveground biomass of K.pygmaea,and a significant positive correlation between the intake bite of Tibetan sheep and the biomass of forbs.Therefore,the decreasing food resources could be the reasonable mechanism by which yak and Tibetan sheep competed with each other.Here,we proposed a competitive interaction between herbivores with different food requirement in low productive ecosystems.In the grazing management of the grassland,the interaction between livestock should be considered.2)Pikas and yak/ Tibetan sheep posed competitive interactions between them.Both yak and Tibetan sheep decreased the density of pikas,while pikas decreased intake bite and feeding time of yaks,but increased those of Tibetan sheep.Pikas had large diet recipe,which almost eaten every plant species of the grassland and preferred sedge and forbs.The competitive relationship between large herbivores and small burrowing herbivores was caused by the decline of food resources.In addition,plateau pika could deal with the adverse effects of large herbivores by changing its forage characteristics,mainly by eating different functional plant groups.Our study showed that small herbivores like pika could affect the behaviour of large herbivores such as yak and Tibetan sheep.These resluts suggests that the testing bidirectional effects between herbivores can accurately and completely clarify the competition and facilitation in terrestrial animal communities.3)Caterpillars posed a competitive interaction with yak but a positive interaction with Tibetan sheep.Large herbivores consistently increased the density of caterpillars.The caterpillars,in turn,decreased yak's but increased Tibetan sheep's foraging time and intake bites.Caterpillars mainly eaten the dominant plant K.pygmaea,same with yak but differ from Tibetan sheep.The caterpillars' preference for K.pygmaea significantly reduced food resources of yak,while promoted food resources(multiple forbs)of Tibetan sheep.The caterpillars,in turn,facilitated yak but againsted Tibetan sheep by differently changing available food resources of the two large herbivores.In our study site,the caterpillars specialized on two broad-leaf forbs-Lamiophlomis rotate and Gentiana straminea which provide shelter(i.e.dwelling,oviposition,and pupation)for caterpillars.The vertebrate herbivores' non-preference for L.rotata and G.straminea,the caterpillars' preferred habitat,increased the densities of the two plant species,thus favoring the caterpillars.Diet preferences of herbivores modified the habitat and food resources,thereby causing a diet-mediated competition between yak and caterpillars,and facilitation between sheep and caterpillars.Our study indicates that two different mechanisms(the changes in habitat and food availability)induced by herbivore jointly determined competitive and facilitative interactions between distantly related herbivore species.4)There was a positive interaction between pikas and caterpillars.The results showed that at the peak of its population in July,pikas significantly increased the density of grassland caterpillars.Similarly,caterpillars benefited pikas in June and July when its density was higher.The interaction between the two herbivores was density-dependent,both of which appeared at the peak of the population density.The positive effect of pikas on caterpillars was due to the improvement of habitat conditions for caterpillars(increasing the density of L.rotata and G.straminea).Feeding by caterpillars could induce the compensatory growth of sedge and some forbs thus increased the N content of them.Consequently the caterpillars positively affected pikas by providing higher quality available food resources for pikas.Our results suggested that the key stone species like pika had profound effect on the biodiversity of the grassland ecosystem.Additionally,in alpine meadow of the Qinghai-Tibetan Plateau,the outbreak of pikas might induce pest jeoperdose of the grassland,and this situation should be paid more attention in the conversation of the grassland.Based on the above results,our study could provide insights into the complex interactions and diverse potential mechanisms among multi-herbivors in alpine meadow ecosystems.In our study area,the interactions among herbivores were diverse.Yak,Tibetan sheep and pikas,and yak between caterpillars showed competive interactions,while Tibetan sheep between caterpillars and pikas between caterpillars had positive interactions.The mechanisims by which the herbivores interacted with each other were the changes in available food resources and habitat conditions.The study could complement and deepen the interspecific interaction theory,indirect interaction net web,food web theory,and enrich the connotation of “ecosystem engineer”.Our study confirms that there will be a mutually beneficial relationship among a variety of herbivores at the same nutritional level.In particular,the reciprocity between herbivores is more common among distantly related herbivores.In the study of indirect interaction of herbivores,examining the unidirectional effects between herbivores should be paid more attention in the future.In this study,we found that small herbivores and even those invertebrate insects had indirect effects on ungulate and burrowing mammals.Our study found that herbivores affect other trophic species through “bottom-up” effect or “top-down” effect.The study has certain practical significances for the grassland management and utilization.The interaction between herbivores should be fully recognized in grassland grazing,and the species and combinations of grazing livestock should be determined.In the prevention and control of grassland rodent pests,the mutually beneficial relationship between different species should be taken into account.