Complex population dynamics in microbial systems

Abstract

The study of spatial and temporal population dynamics has a long history in ecology, going back to the beginning of the 1900´s. Both intrinsic and extrinsic mechanisms are involved in determining the temporal and spatial occurrences of populations and species. Different dynamic patterns result from the strength and the interplay of the two mechanisms. The fact that in-trinsic driven population dynamics are woven together with extrinsic, often stochastic dynamics makes analyses of intrinsic mechanisms difficult and led to a controversial discussion about the relevance in nature. However, there is a gap between results from mathematical modelling showing the occurrence and meaning of intrinsically driven dynamics, and empirical proves. Recently, laboratory experiments under clearly defined and controlled conditions were shown to be a suitable tool to study intrinsic, deterministic population dynam-ics. Deterministic chaos is one type of dynamic behaviour exhibited by a change in one or more intrinsic parameters beside extinction, damped oscil-lations, and stable limit cycle. Most discussed is the relevance of chaotic be-haviour in population dynamics, due to the fact that empirical evidence is lim-ited to a simple one-species system. Furthermore, chaotic fluctuations are thought to lead to extinction of a population, because chaotic dynamics can obtain very small population sizes, even more vulnerable when mixed together with stochastic events. The question, if chaos occurs in the real world and under which circumstances chaos may be found in nature, is still open. Clearly defined laboratory experiments were established to analyse intrinsically driven dynamics in a multi-species system. Different dynamic behaviours were found in chemostat experiments with a two-prey-one-predator system of a bacterivorous ciliate as the predator and two bacteria strains as the prey organisms. The different population dynamics - extinction, damped oscillations, stable limit cycles and chaos - were triggered by a change in the dilution rate of the chemostat system and verified by calculations of the corresponding Lyapunov exponents. Therewith, chaos was shown in an experimental three-species system for the first time. The different dynamics in the microbial food web revealed a surprisingly short transition (4-7 days) to a different dynamic behaviour when the dilution rate as the control parameter was changed. All dynamics persisted in experiments when different local populations with different dynamics (chemostats with different dilution rates) were coupled. Experiments showed that the dynamic behaviours of the coupled populations were only triggered by the demographic parameter � in this case the dilution rate - and reacted independent of the constant inflow of organisms from populations with different dynamics. Here, we were able to shed more light on the question about the relevance of chaos in the real world. In conclusion spatio-temporal chaos might be more common in nature than generally assumed. Microbial communities with fast reproduction rates might be favoured candidates to show chaos and other complex dynamics in nature. Intrinsically driven dynamics might be persistent when perturbated by a constant fluctuating inflow of organisms and might lead to the establishment of chaos in habitats with constant flows (e.g. aquatic organisms in rivers and oceanic currents, and water drainage to groundwater). The fast transition to a different dynamic behaviour after a change in a control parameter shows how distinct intrinsic driven processes might be. A reason why chaotic dynamics in nature are not observed might be due too the large sampling intervals in most field studies