Monsonís Nomdedeu, Mar (2010) Influence of temperature on the complex dynamic behaviour of a microbial food web. PhD thesis, Universität zu Köln.
Ecosystems are set to extrinsic drivers like climate parameters. These are known to show non-linear dynamics and potential chaotic behaviour. One of the most important drivers is temperature; it influences a large variety of ecological processes (e.g. growth rate and other metabolic rates). On the other hand, populations show density dependent, intrinsic, non-linear dynamics including complex, irregular patterns. The interactions between extrinsic and intrinsic dynamic behaviour are difficult to determine in natural ecosystems and have been discussed in literature. The assessment of the consequences derived from climate change represents a great challenge for ecologist. Deeper knowledge on the mechanisms driving temperature effects on natural food webs is needed. In this work I investigated a well defined simplified microbial food web consisting of two prey bacteria (Pedobacter sp. and Acinetobacter johnsonii) and one predator ciliate (Tetrahymena pyriformis). This simple food web permits the study of intrinsic dynamics as well as the influence of extrinsic disturbances. The experimental setup developed by my colleagues and me, consisted of chemostats where parameters like the flow rate were computer controlled, so external noise was reduced to the minimum. Experimental parameters could be determined with great precision, and therefore the dynamic behaviour showed by the experiments is considered to be purely intrinsic. A mathematical model based on experimental data was developed with the aim of analyzing the temperature scenario investigated experimentally. The model included temperature dependent growth rate functions that were fitted to experimental data. Although the model did not capture the whole complexity of the food web, reflected some qualitative temperature effects observed experimentally. The bacterium Acinetobacter johnsonii showed grazing-resistant morphologies. I hypothesised that this morphological plasticity was responsible for part of the irregular fluctuations of the abundances observed in the chemostat experiments. In cooperation with David Heckman I developed a mathematical model with the aim of testing this hypothesis at a theoretical level. Numerical analysis showed a stabilization of the food web represented by two characteristics: the possibility of coexistence for a wider range of external parameters, and the absence of chaotic fluctuations. I was able to show for the first time experimentally, that changes on extrinsic temperatures may shift population dynamics to different attractors depending on the specific temperature response of populations. I analyzed the impact of a temperature increase from 20°C to 25°C. The results presented here suggest that the ecological responses to temperature can affect the dynamic behaviour in food webs.
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