Analysis Of The Mechanism Of Species-Area Relationship Formation Based On Microbial Microcosm Experiments

Background: The Species-Area Relationship(SAR)describes the positive correlation between species richness and habitat area and is considered one of the few "laws" in ecology.In theory,SAR is the basis of island biogeography equilibrium theory,metacommunity theory and ecological community theory.In practice,SAR has become a key tool for regional biodiversity assessment,extinction rate prediction and protected area delineation.SAR is,therefore,a cornerstone of many ecological theories and biodiversity conservation practices.However,the mechanism of SAR occurrence is still unclear.There are many hypotheses about the formation of SAR,each emphasizing the role of area per se,habitat heterogeneity,passive sampling,resources,and other factors.However,an increase in habitat area in natural ecosystems implies an increase in resources and habitat heterogeneity,making the interconnection between SAR formation hypotheses indistinguishable and making it impossible to determine each hypothesis’ s independent roles directly.Purpose: In essence,SAR is determined by three key processes that determine biodiversity: speciation,dispersal and extinction,and each hypothesis ultimately shapes SAR through these three processes.This study intends to independently test the role of each SAR formation hypothesis from the perspective of these three processes.We will first investigate the factors that interfere with the study of the SAR mechanism and then establish a microcosmic system that excludes these interfering mechanisms to analyze the existing SAR formation hypotheses individually.Methods:(1)Plant leaves were selected as the island models,27 leaves of different sizes from three garden plants were collected and monitored for leaf area and surface microorganisms to investigate the interference factors of microbial SAR.(2)The microcosm system was constructed by using different sizes of sterile culture bottles and well-mixed pao cai soup.The microbial diversity dynamics in the microcosm were continuously monitored for 60 days.The SAR was constructed by replacing the area with the microcosm volume.The system excluded the effects of habitat heterogeneity,dispersal,and resources and independently explored the role of extinction in shaping SAR.(3)Two microcosmic systems were constructed using sterile culture bottles,dry matter(dried radish)and pao cai microorganisms.In one group,the dry matter mass increased with increasing volume.Moreover,the dry matter mass decreased with increasing volume in the other group.A total of 157 samples were collected at 11-time points for 30 days of continuous monitoring.The system was used to exclude the effects of habitat heterogeneity and dispersal,and to investigate whether the increase in total resources could shape SAR by changing the extinction rate.(4)An open microcosm system was constructed using a mixture of sterilized pao cai soup and beakers with different opening sizes.To construct a passive sampling model for the passive collection of species from regional species pools and explore SAR under the extinction and dispersal trade-offs.In addition,the sterile filter paper was exposed to air for 12 hours to simulate the passive sampling process from the regional species pool.Factors such as habitat heterogeneity and extinction were excluded to independently verify the role of dispersal in shaping SAR.A total of 15 samples were collected from the two systems.(5)Spatially heterogeneous gradient microcosm systems were constructed with closed opaque culture bottles and microbial communities of pao cai.Microbial diversity in each microcosm was monitored continuously at 11-time points over 30 days,and a total of154 samples were collected.The effects of dispersal and resources were excluded to independently explore the role of habitat heterogeneity in shaping SAR through extinction.The bacterial 16 S r RNA gene V3 and V4 variable regions and the fungal ITS gene ITS1-1F variable region were sequenced using the Illumina Nova Seq platform to obtain microbial community data.Amplicon sequence variant(ASV)was used instead of OTU for microbial diversity assessment.SAR curves were obtained by fitting the data using logit and semi-logit models.Results:(1)After removing the interference of pure sampling effects,SAR of foliar microorganisms was found in the deciduous tree Ficus altissima but not in the evergreen trees Magnolia grandiflora and Eriobotrya japonica.When the results of the three species were combined and analyzed,the SAR of foliar microorganisms was also not found.Beta diversity analysis indicated that the microbial community composition varied considerably among the leaves of the three trees,while the similarity of microbial communities among the leaves of Ficus altissima was higher within the same species.(2)Extinction was able to shape SAR independently,but the occurrence of SAR was discontinuous in time.During the 60-day monitoring period,significant SAR was observed only on days 3-5,7,and 22-30.The stage of SAR occurrence was consistent with the dynamic trend of extinction rate.SAR occurred only during the stable period of community development.(3)More resources did not shape higher microbial population densities.No negative correlations were found between extinction rates and resources or volume in the microcosm system.No significant SARs were found in the experimental groups where either total resources were positively or negatively correlated with microcosm volume,and resources did not change the structure and complexity of microbial interaction networks.(4)The passive sampling hypothesis of SAR was experimentally confirmed by the fact that the microcosmic system with both dispersal and extinction processes did not produce SAR and that the larger filter paper did receive more migrating species after removing the effects of extinction processes.(5)Habitat heterogeneity did not show a constant positive correlation with species richness but showed increasing,decreasing and single-peaked patterns at different time points.Conclusions:(1)Sampling effects and the historical background of biodiversity in the study area interfere with the formation of the microbial SAR.(2)SAR is temporally discontinuous,and the temporal dynamics of extinction shape the temporal discontinuity pattern of SAR.(3)Resources in the microcosmic system of this study do not shape SAR by regulating extinction rates.If the resource hypothesis is indeed valid,dispersal should be its primary mechanism.(4)Dispersal shapes SAR with a high slope by increasing the number of species(especially rare species)and increasing the rate of species replacement among habitats.These two processes are the mechanisms of action by which dispersal shapes biodiversity patterns.(5)Spatial heterogeneity and microbial diversity emerged as differently correlated at different time points,with the habitat heterogeneity hypothesis having the potential to shape SAR at points in time when a positive correlation between heterogeneity and diversity was observed.