The Structure,Formation Mechanism,and Ecological Function Of "Compositae-Tephritidae" Bipartite Foodweb In An Alpine Meadow

Food webs,as an integration of interlinked trophic relationships,are a produc t of the evolution and adaptation of organisms and ecological systems and also an important factor affecting the structure and function of ecosystems.In recent years,the advances in computer technology and network analysis have greatly enhanced the level of food web research.One important finding is that nestedness is a fundamental feature of both mutural and antagonistic bipartite networks,in which the food spectrum of specialists is a subset of generalists.The formation of this nested structure is mostly attributed to two processes,namely the "neutral" process emphasizing the importance of species relative abundance and the "niche" process emphasizing the function of species traits.However,there are few studies exploring the relative contribution of the"neutral" and "niche" processes to the nestedness.For this reason,I studied the structure,formation mechanisms,and ecological consequence of the"Compositaeplant-Tephritidfly" bipartite in an alpine meadow.On the eastern part of the Tibetan Plateau,the Compositae species dominate most of summer-grazed pastures in the Zoige Plateau alpine meadows,and their fruits are often consumed by flies fromTephritidae,thereby resulting in a bipartite network between Compositaeplant-tephritid fly.Plant capitulum density,capitulum size(depth and diameter),number and size of seeds within capitula,and seed loss had been recorded for all Compositae species in six different habitats,and I identified the fly species by both morphology and DNA barcodings.Then,three different matrixes had been developed to characterize plant-fly bipartite network:1)a qualitative binary matrix that describes whether a plant species interacted with a fly species at the species level;2)quantitative frequency matrix of interactions between plant and fly species;and 3)quantitative proportion matrix reflecting the seed damage level in different Compositae speciesThe following main results had been obtained after data analyses.After analyzing the bipartite consisting of 19 species of Compositae(35,337 capitula in total)and 23 species of fruit fly(6253 individuals in total;emerged from the capitula),I found that the linkage density of the overall bipartite network of"Compositaeplant-Tephritidfly" is 7.71,the connectance is 0.35,and the average number of links per species is 3.64.The nestedness coefficient(nestedness temperature)of the binomial network is 22.64,which is significantly smaller than the confidence interval of 5%-95%predicted by the random model[56.21-71.49],indicating that the overall network is nestedly structured.Despite the large difference in the species richness and species pair relationships,the plant-fly network presented nested in each habitat.Capitulum size and species relative abundance are the two major factors affecting the frequency of interactions between plant and fly species(number of fly individuals per capitulum).Capitulum size and the abundance explained17%and 35%of the variation in the frequency,respectively.The number of fly species was positively correlated with capitulum size but not relative abundance,indicating that capitulum size(niche process)was the primary factor determining how many fly species were associated with a plant species.Results of GLMs show that capitulum size and abundance accounted for 30%and 15%of the variation in the number of fly species(associated with a plant species),respectively.Thus,both neutral and niche processes contributed to the formation of the bipartite,in which niche process determines whether an interaction can occur between plant and fly species and neutral process determines the interaction strength.The proportion of seed loss was positively correlated with capitulum size and relative abundance across species.Capitulum size and abundance explained 38%and 33%variation in the seed loss,respectively.Because the capitulum size positively correlated to plant size and seed size,and because species with larger plants and larger seeds were generally more competitive,our results indicated that more abundant and more competitive species suffered more seed damage in the meadow community.I therefore suggest that the structure of ""Compositaeplant-Tephritidfly""network may maintain species diversity by suppressing these dominant superior.In conclusion,our results show that 1)the bipartite network of"Compositaeplant-Tephritidfly" is nestedly structured;2)this structure is formed due to both neutrality and ecology processes;and 3)this structure suppresses dominant species to facilitate species existence.More efforts should be paid to determine food web structure in high resolution and experimental verification of the formation mechanisms are in need.