Aims The neutral theory of biodiversity provides a powerful framework for modeling macroecological patterns and interpreting species assemblages.However,there remain several unsolved problems,including the effect of r...Aims The neutral theory of biodiversity provides a powerful framework for modeling macroecological patterns and interpreting species assemblages.However,there remain several unsolved problems,including the effect of relaxing the assumption of strict neutrality to allow for empirically observed variation in vital rates and the‘problem of time’—empirically measured coexistence times are much shorter than the prediction of the strictly neutral drift model.Here,we develop a nearly neutral model that allows for differential birth and death rates of species.This model provides an approach to study species coexistence away from strict neutrality.Methods Based on Moran’s neutral model,which assumes all species in a community have the same competitive ability and have identical birth and death rates,we developed a model that includes birth–death trade-off but excludes speciation.This model describes a wide range of asymmetry from strictly neutral to nearly neutral to far from neutral and is useful for analyzing the effect of drift on species coexistence.Specifically,we analyzed the effects of the birth–death trade-off on the time and probability of species coexistence and quantified the loss of biodiversity(as measured by Simpson’s diversity)due to drift by varying species birth and death rates.Important Findings We found(i)a birth–death trade-off operating as an equalizing force driven by demographic stochasticity promotes the coexistence of nearly neutral species.Species near demographic trade-offs(i.e.fitness equivalence)can coexist even longer than that predicted by the strictly neutral model;(ii)the effect of birth rates on species coexistence is very similar to that of death rates,but their compensatory effects are not completely symmetric;(iii)ecological drift over time produces a march to fixation.Trade-off-based neutral communities lose diversity more slowly than the strictly neutral community,while non-neutral communities lose diversity much more rapidly;and(iv)nearly neutral systems have substan展开更多
基于Easley、Hvidkjaer和O’Hara的序贯交易模型与PIN(Probability of Inform ation-based Trading,基于信息的交易比率)指标对我国股市知情交易情况进行的实证分析研究结果表明:(1)我国股市信息不对称程度较高;(2)由于知情交易者利用...基于Easley、Hvidkjaer和O’Hara的序贯交易模型与PIN(Probability of Inform ation-based Trading,基于信息的交易比率)指标对我国股市知情交易情况进行的实证分析研究结果表明:(1)我国股市信息不对称程度较高;(2)由于知情交易者利用坏消息的能力有限且流动性交易水平较高,我国股市知情交易比率并不太高;(3)知情交易比率与后续期间股票收益率的负相关性,表明我国股市中市场操控型知情交易比较严重。因此,我们认为应进一步完善上市公司的信息披露制度,降低投资者之间的信息不对称程度,同时确保流动性投资者参与股市的积极性;在引入做空机制时应慎重考虑和综合权衡,避免不适当地增加流动性投资者所承担的逆向选择风险水平,降低股市的流动性供给和风险分散功能;证券市场监管部门应进一步加强对异常交易活动的监控,加大对市场操纵行为的打击力度,以确保我国证券市场的健康发展。展开更多
基金National Natural Science Foundation of China(D.Y.Z.and K.L.)Ministry of Science and Technology of China(D.Y.Z.and K.L.)+2 种基金Geomatics for Informed Decisions Network of Canada(F.H.)Natural Sciences and Engineering Research Council of Canada(F.L.)University of Alberta International(the China-UofA Joint Research Lab program).
文摘Aims The neutral theory of biodiversity provides a powerful framework for modeling macroecological patterns and interpreting species assemblages.However,there remain several unsolved problems,including the effect of relaxing the assumption of strict neutrality to allow for empirically observed variation in vital rates and the‘problem of time’—empirically measured coexistence times are much shorter than the prediction of the strictly neutral drift model.Here,we develop a nearly neutral model that allows for differential birth and death rates of species.This model provides an approach to study species coexistence away from strict neutrality.Methods Based on Moran’s neutral model,which assumes all species in a community have the same competitive ability and have identical birth and death rates,we developed a model that includes birth–death trade-off but excludes speciation.This model describes a wide range of asymmetry from strictly neutral to nearly neutral to far from neutral and is useful for analyzing the effect of drift on species coexistence.Specifically,we analyzed the effects of the birth–death trade-off on the time and probability of species coexistence and quantified the loss of biodiversity(as measured by Simpson’s diversity)due to drift by varying species birth and death rates.Important Findings We found(i)a birth–death trade-off operating as an equalizing force driven by demographic stochasticity promotes the coexistence of nearly neutral species.Species near demographic trade-offs(i.e.fitness equivalence)can coexist even longer than that predicted by the strictly neutral model;(ii)the effect of birth rates on species coexistence is very similar to that of death rates,but their compensatory effects are not completely symmetric;(iii)ecological drift over time produces a march to fixation.Trade-off-based neutral communities lose diversity more slowly than the strictly neutral community,while non-neutral communities lose diversity much more rapidly;and(iv)nearly neutral systems have substan
文摘基于Easley、Hvidkjaer和O’Hara的序贯交易模型与PIN(Probability of Inform ation-based Trading,基于信息的交易比率)指标对我国股市知情交易情况进行的实证分析研究结果表明:(1)我国股市信息不对称程度较高;(2)由于知情交易者利用坏消息的能力有限且流动性交易水平较高,我国股市知情交易比率并不太高;(3)知情交易比率与后续期间股票收益率的负相关性,表明我国股市中市场操控型知情交易比较严重。因此,我们认为应进一步完善上市公司的信息披露制度,降低投资者之间的信息不对称程度,同时确保流动性投资者参与股市的积极性;在引入做空机制时应慎重考虑和综合权衡,避免不适当地增加流动性投资者所承担的逆向选择风险水平,降低股市的流动性供给和风险分散功能;证券市场监管部门应进一步加强对异常交易活动的监控,加大对市场操纵行为的打击力度,以确保我国证券市场的健康发展。
