Study On Complex Behaviour Of Community Based On Dynamical Systems Theory

With global warming and the increasing extinction of species,it is imperative to study and protect species diversity.On this basis,ecologists advocate a deeper understanding of the core problem of "species diversity-system stability-structural complexity".However,up to now,topics such as "the relationship between species diversity and system stability","the general mechanism of species coexistence" and "population chaos and extinction in natural communities" have been widely discussed by ecologists.Unfortunately,there is still no consensus on the mechanisms involved.In this thesis,the complex dynamic behaviour of communities(i.e.Ficus racemosa,plankton community and Gadus morhua)was explored by combining the phase-space reconstruction technique of experimental/field data and dynamic system theory.The thesis mainly includes:theoretical analyses of species coexistence and diversity-stability debate,empirical study on community chaos,and eco-evolutionary chaos.Specific research results are listed as follows:1.With fig species and their associated fig wasp community as model system,this research brings community structure into the ‘diversity-stability’ debate by establishing the fig-fig wasp food web model.The simulations of the model indicated that wasp diversity was promoted by population density of the parasitoids of the pollinating wasps,and that top-down control from the parasitoids regulated community diversity and stability.Moreover,only moderate parasitoidism on the pollinators resulted in the regular/chaotic coexistence,while population chaos would maintain wasp diversity in the fig-fig wasp system.Increases in the wasp diversity could increase community stability when biodiversity of fig wasps was low.2.This research established an omnivorous food web model based on the network structure of two natural ecosystems(plankton community and fig-fig wasp system).It analyzed the changes of both food web structure and stability under the different resource levels and predation preference of the generalist predator.The results of model analyses showed that weak predation strength can promote stable coexistence,and an integration of omnivory,increased competition,top-down control and bottom-up control can promote species diversity and food web stability.3.This research combined field data with a mathematical model to explore the population fluctuations and the maintenance of stability in the fig-fig wasp system.First,a discrete,seasonally explicit host-mutualist-exploiter model was estabolished and Levenberg-Marquardt optimization algorithm was adopted to fit fig-wasp field data.The model parameterized with field data predicted that population chaos could be a result of interactions between host regulation and environmental variation.Model simulations showed that the host decreased the strength in the ratio of extra reward to host sanction(RS)to maintain its higher population density when the external environment variation(e.g.,seasonal temperature)is small.4.This research simulated plankton system via combining both seasonal temperature and a food web model with prey preference,and further used the Markov chain Monte Carlo(MCMC)algorithms to estimate model parameters.Theoretical predictions showed both internal factor(intraspecific competition coefficient of calanoid copepods)and external factor(seasonal temperature)are the key factors,which can produce population chaos.Meanwhile,phase space reconstruction method showed Lyapunov exponents of empirical data are positive.Both theoretical predictions and empirical data showed the plankton community presents the chaotic dynamics.5.With Gadus morhua as model system,this research combined analyses of empirical data and an eco-evolutionary model to uncover the chaotic dynamics of body length in a fish population(northeast Atlantic cod: Gadus morhua).Consistent with chaotic attractors,both the largest Lyapunov exponent(LE)of empirical data was positive and approximately matched the LE of the theoretical model calculation,thus suggesting the potential for eco-evolutionary chaos in this fish population.The research also found that the autocorrelation function(ACF)of both empirical data and eco-evolutionary model showed a similar lag of approximately 7 years.The main innovations of this thesis include: first of all,the structure of a food web shared by different communities was proposed,and general mechanisms of multi-species coexistence in the food web were analyzed.Moreover,the chaotic dynamics of fig-fig wasp system was confirmed by combining theoretical model and field data,and the mechanism of population chaos was given.Finally,the eco-evolutionary chaos of body length in a fish population(Gadus morhua)was revealed.This thesis will re-examine the underlying driving forces behind the complex dynamics(e.g.chaos)in ecological communities and the maintenance conditions for species diversity and species coexistence,so as to provide theoretical support for biodiversity conservation.