Dynasym

Most concepts in community ecology and food web theory assume that the form of interactions between species is fixed and that the effects of the interaction on the partners, whether positive or negative, do not change. Increasingly, however, studies show that these effects are not always fixed, but that the costs and benefits of the interaction for both partners vary with conditions and can influence the net effect of the interaction and thus the form of the interaction (hereafter referred to as the symbiotic form). Most processes that determine the effects of species interactions are density-dependent: population growth slows with increasing density due to self-limitation, or the feeding rate of predators reaches a maximum despite increasing prey density. Changes in the net effect, both magnitude and sign, of these density-dependent responses may result from a reduction in the benefits and/or an increase in the costs of the interaction. This may ultimately lead to a change in the form of the symbiosis (e.g. a switch from a mutualistic to a predator-prey interaction). However, the changes in the density-dependent costs and benefits of the numerical and functional responses, and the associated possibility of a change in the form of the symbiosis, are currently poorly understood. As these responses are crucial for population dynamics, the study of changes in symbiosis form resulting from changes in costs and benefits has implications for the application of basic concepts currently used to predict and analyse population dynamics. An important aspect that has received little attention to date is the fact that density-dependent symbiosis forms determine population dynamics, which in turn determine symbiosis forms. Thus, there is the potential for a density-symbiosis feedback loop with population dynamics that we cannot predict with current theory based on fixed interactions. In this project, we will use different ciliate-algal systems to experimentally determine the density-dependent costs and benefits of interactions and the form of symbiosis. After determining how this density dependence is modified by resource and energy availability, we will experimentally test the role of shifting costs, benefits, and symbiosis form on population dynamics, both in experiments and in modelling analyses. Finally, we will directly test the density-symbiosis feedback and compare population dynamics in experiments with and without a shift in symbiosis form.