Abstracts:  NVTB  PhD-day  2002

Nowadays, many species exist in small, fragmented populations where gene flow through migration is restricted, and local extinction and recolonization are frequent events. The patters of genetic variation in such metapopulations and the consequences for population persistence are poorly understood. We study how the interaction of genetic and demographic factors affect the ultimate genetic structure by systematically comparing results from experimental metapopulations with predictions derived from individual-based simulation models. We used ten Drosophila melanogaster metapopulations with different migration regimes and without migration. Each setup was replicated five times, and we monitored allele frequencies over 20 generations. The results are in line with standard theory qualitatively, but not quantitatively. These deviations between experimental results and standard theory are most likely caused by the Drosophila mating system, the stepping stone migration pattern and the timing of migration. Standard population genetic models assume random fusion of gametes and migration from a migrant pool with an equal migration rate $m$ between infinitely many demes, while in our experiments Drosophila has a polygynous mating system, and migration follows a unidirectional stepping stone pattern.

Simulations taking these factors into account are in much better agreement with experimental results than the predictions based on standard theory. A third important factor is the moment of migration, that may also cause large deviations from standard theory. We conclude that even in our simple Drosophila model system we find systematic deviations from standard theory, indicating that details such as mating system and migration pattern are very important. We expect even larger effects in our current experiments that include local extinction and recolonization. Thus, extreme caution is required when standard theory is applied to natural systems, which are much more complex than standardized experiments.

A difference in dominance rank is an often-used cue to resolve conflicts between two animals without escalated fights. On the group level, adherence to a dominance convention efficiently reduces the costs associated with conflicts, but from an individuals point of view, it is difficult to explain why a low ranking individual should accept its subordinate status. This is especially true if, as suggested by several authors, dominance not necessarily reflects differences in fighting ability but rather result from arbitrary historical asymmetries. According to this idea, rank differentiation emerges from behavioural strategies, referred to as winner and loser effects, in which winners of previous conflicts are more likely to win the current conflict, whereas the loser of the previous conflicts are less likely to do so. In order to investigate whether dominance, based on such winner and loser effects, can be evolutionarily stable, we analyse a game theoretical model. The model focuses on an extreme case in which there are no differences in fighting ability between individuals at all. The only asymmetries that may arise between individuals are generated by the outcome of previous conflicts. By means of numerical analysis, we find several behavioural strategies that utilize these asymmetries for conventional conflict resolution. One of these strategies gives rise to evolutionarily stable dominance relations generated by winner and loser effects.

No abstract available.

In the last decades, the question as to whether which nutrient ultimately limits growth of copepod populations has been a dominant in marine zoology. It is generally believed that nitrogen is the limiting factor. While recent theory predicts that organisms should use their limiting factors at maximum efficiency (according to Liebig's law), copepods have been shown to excrete significant amounts of ammonia, even under nitrogen starved conditions. This phenomenon troubles the concept of limitation, and calls for improvements on current theory.

In this study, we use the synthesizing unit concept for letting copepods cope with multiple limiting substrates. Synthesizing units can be used to merge substrates into products. Firstly, we outline the dynamics of the synthesizing units involved in the processes of maintenance and growth. Secondly, we analyze the effect of food composition and food density on copepod dynamics.

While multiple nutrient limitation is an important factor in predicting zooplankton densities, the theoretical study on the dynamics of food web models focuses around effects one potentially limiting nutrient. At a later stage, we will incorporate the copepod model into a food chain model with phytoplankton and nutrients.

We investigate whether coexistence of plants is possible given the condition that there is only one limiting resource, namely light. Light is in principle a homogeneous resource, which means that there is no heterogeneity in light supply in the horizontal plane. Within the vegetation however, the plants themselves determine how much light is present. With their leaf area the plants intercept light and create a vertical light gradient. Allocation pattern and competitive interactions create the shape of a plant. This shape, in particular the height and the leaf area, determines how much light a plant intercepts. Our main question is whether plants with different allocation patterns are able to coexist. We use a mechanistic model to simulate competition between plants. In our model, plants possess different investment patterns in height. As biomass investments are drawn from the available pool of carbohydrates, investments in height cannot be used for an investment in leaf area, and vice versa. In this way leaf area is affected by the investment pattern in height. First 1 discuss differences in plant fitness between species pairs within a season and explain whether plants should be similar or dissimilar in order to have a more or less equal fitness. Also 1 discuss possibilities of coexistence between these species pairs when frequency is included and show how very distinct patterns of coexistence emerge. Finally 1 discuss possibilities of stable coexistence between multiple species with help of game theory.