Food quantity and quality effects on the expression of inducible defences in Daphnia


This is my PhD-project to achieve a doctoral degree of science (Dr. rer. nat.).

Trophic interactions are a key element to understand food web processes and ecosystem functioning. Nutrients are transferred from primary producers to higher trophic levels, while at the same time predation is one of the main drivers for natural selection. In response to predation, many aquatic prey species have evolved defences that reduce the predation impact. However, defence against one predator could render the prey more vulnerable to other predators. Hence, phenotypic plasticity in defensive traits in prey is a widespread mechanism to cope with a frequently changing predator spectrum. This plasticity may imply changing demands in resources, e.g. for changes in morphology or life history (e.g. number and size of offspring). Therefore, the composition of the phytoplankton community, and thereby the nutrient availability for grazers, may lead to nutrient limitations that potentially also constrain the expression of defences. To get a more comprehensive understanding of how prey suffers from insufficient nutrient supply, we will study the impact of nutrient availability on the expression of defences in cladocerans of the genus Daphnia. Daphnia are unselective filter-feeders, which play a crucial role for the nutrient transfer from primary producers to higher trophic level in lentic freshwater ecosystems and provide a well-established model system in research on predator-prey interactions and nutritional ecology. This project, combining adaptive plasticity and trophic interactions under nutritional constraints, will advance our understanding of food web processes in freshwater ecosystems.

In this project, we will explore the impact of food quantity and quality on the expression of inducible defences using established predator-prey systems. It is in cooperation with the working group of Dr. D. Martin-Creuzburg from the Limnological Institute of the University of Konstanz and the working group of animal ecology of Prof. C. Laforsch of the University of Bayreuth. We will evaluate how defence expression and resource allocation in Daphnia is influences by the availability of carbon and essential lipids, and how the different defensive traits are affected by supplying single nutrients and different combinations of nutrients. Furthermore, we will assess the demand of Daphnia for total carbon as well as for essential lipids during simulated predator exposure (i.e. by using kairomones) in nutrient concentration gradient experiments to determine direct costs for the expression of defences. By exploring these changes and assessing nutritional costs for inducible defences, we will be able to gain an improved mechanistic understanding of how inducible defences in Daphnia are affected by the phytoplankton community and how nutrient limitations may constrain the expression of defences. This will contribute to our understanding of trophic interactions, population dynamics and food web processes.