Mechanism Of Population Dynamics For The Paradox Of Enrichment

Mathematical models of population dynamics describe the relationships among species or between species and the environment. They can be employed to describe, predict and even adjust the growing process and developing trends of species in nature by researchers. They have been extensively applied in theoretical ecology, economics, resources development and management, environmental assessment and management and so on.Interaction between predator and prey is one of the basic relationships among species and has received a great deal of attention recently. Rosenzweig (1971) predicted that enriching a simple predator-prey system leads to the destruction of its stable equilibrium and further enrichment will result in the extinction of species through mathematical investigation. This phenomenon is called "the paradox of enrichment" and has been confirmed and tested by several experiments. In contrast, there are numerous observations which did not support this proposal. During the past decades, a number of mechanisms were proposed to resolve the paradox of enrichment through mathematical model building and experimental test.The present thesis explored the mechanism of population dynamics for the paradox of enrichment and tried to answer the following questions:(1) how the predator-harvesting affects the respective dynamics of the prey-dependent and ratio-dependent prey-predator ecosystem with three different interval harvesting rates;(2) whether the presence of "unpalatable prey" helps dampen the oscillations of the predator-prey system through weak interaction with the predator? Combining controlled experiment and mathematical model building, we got the following results:1Through mathematical model simulation we found that interval predator harvesting has a different impact on the population dynamics of prey-predator system with prey-dependent and ratio-dependent functional response, respectively. It was shown that intermediate predator harvesting effort (20%per three days) could prevent mutual possible extinction and high harvesting effort (40%per three days) would cause the predator to extinction and the prey grew to its carrying capacity. However, the system with low interval harvesting strategy (10%per three days) showed large-amplitude cycle oscillations in the prey-dependent model case and converged to stable equilibrium in the ratio-dependent model case.2Using experimental controlled method we found that in both poor and high nutrition treatments, the density of Daphnia magna approached to0mg/L when fed only with Chlorella sp. at the end of experiment. While, Daphnia magna persisted during the experiment when fed with the combination of Chlorella sp. and Micocystis aeruginosa469. The density at the end of the experiment increased to0.25mg/L which is significantly higher than0. We concluded that the presence of Micocystis aeruginosa469could help stabilize the population dynamics of the prey-predator (algae-Daphnia) system.However, natural ecosystem is much more complex than models and laboratory systems. Various mechanisms may combine to lead to stability in natural systems. Therefore, knowing the mechanism of ecological system is an important component of ecological research. Furthermore, a better understanding of prey-predator system could provide key information for the suitable harvesting strategy.