The Structure Of Tri-trophic Food Web Of "Plant-tephritid Fly-Parasitoid" In An Alpine Meadow In Hongyuan County,Sichuan Province,China:the Plant Effect On Wasp Species Diversity

Food webs are constituted by prey and predator species that are trophically linked within community.Food webs not only influence material turnover rate and energy flow efficiency of ecosystems,but also play an important role in maintaining community species diversity.The "plant-herbivore-parasitoid" food web is widespread in nature and involves more than half of the multicellular organisms.Previous studies have mostly addressed the two trophic levels of "plant-herbivore" and "herbivore-parasitoid".However,there is growing evidence showing that plants may significantly influence the behavior and growth of parasitoids,and vice versa.Therefore,these two trophic-level networks of "plant-herbivore" and "herbivore-parasitoid" should be integrated to understand the impact of food web structure on species diversity.In this study I determined the topology of the "Compositae plant-fly-parasitoid"food web in an alpine meadow to understand the plant effect on parasitoid species diversity.I,together with other lab members,harvested mature capitula of Compositae plants for three years in Hongyuan County,eastern Tibetan Plateau.I collected pre-dispersal seed predators(mostly tephritid flies)and parasitoid wasp from the plant capitula once they were infested.I identified the pupariums(left by the parasitoids)to species using DNA barcoding technology.Accordingly,I constructed a plant-fly-parasitoid food web made up of 29 species of plants,19 species of flies and 22 species of parasitoids.In order to investigate the influence of plants on parasitoid species diversity,I also constructed a population-based "fly-parasitoid" food web.I calculated nestedness and modularity for both species-based and population-based food webs,so that the plant influence on the resource partitioning of parasitoids could be analyzed.The results show that,network connectence is 70%lower,nestedness is 67%lower,but modularity is 211%higher in the population-based "fly-parasitoid" network than the species-based "fly-parasitoid" network.Moreover,in the population-based network the number of modules is 8,larger than that of species-based network(4);the degree of specialization in the parasitoid wasps is 230%higher in the population-based network.Both the richness of plants and flies have significant effects on the richness of parasitoids(P<0.001).The explanation of plant richness(R2=0.760)is higher than that of flies(R2=0.693).Further analysis reveals that the extinction of plants may cause severer secondary extinction of the parasitoids than the extinction of flies.This study reveals that the population-based network is much less nested but more modular than species-based network,such that plants have an important influence on the structure of "herbivore-parasitoid" network and the species diversity of parasitoid wasps.The results also indicate that plants can facilitate species coexistence among parasitoids by promoting the resource partitioning of parasitoids,which is in turn associated with the low nestedness and and higher modularity of the population-based"herbivore-parasitoid" network.