The effects of biological diversity on marine ecosystem functions

The biological diversity found back in natural systems is an important determinant of the functioning of ecosystems and their stability in changing environments. However, biodiversity can be defined at various hierarchical levels, going from the within-species to the community and landscape level. In addition, biodiversity at a fixed hierarchical level can affect ecosystem functions and their stability through various mechanisms. I will illustrate this using a theoretical framework, and translate this framework to a simple trait-based model for competitive communities. Next, I will turn to practical examples of how trait diversity at various hierarchical levels can affect marine ecosystem functions and discuss some challenges for future research, including the up-scaling to complex multi-trophic systems (‘food webs’). Lastly, I will present a new collaborative research project that will start end 2018, where we plan to study the effect of intraspecific trait diversity on the resistance of species diversity and ecosystem productivity to multiple stressors

Contrasting indirect effects of an ant host on prey–predator interactions of symbiotic arthropods

Indirect interactions occur when a species affects another species by altering the density (density-mediated interactions) or influencing traits (trait-mediated interactions) of a third species. We studied variation in these two types of indirect interactions in a network of red wood ants and symbiotic arthropods living in their nests. We tested whether the ant workers indirectly affected survival of a symbiotic prey species (Cyphoderus albinus) by changing the density and/or traits of three symbiotic predators, i.e., Mastigusa arietina, Thyreosthenius biovatus and Stenus aterrimus, provoking, respectively, low, medium and high ant aggression. An ant nest is highly heterogeneous in ant worker density and the number of aggressive interactions towards symbionts increases with worker density. We, therefore, hypothesized that varying ant density could indirectly impact prey-predator interactions of the associated symbiont community. Ants caused trait-mediated indirect effects in all three prey-predator interactions, by affecting the prey capture rate of the symbiotic predators at different worker densities. Prey capture rate of the highly and moderately aggressed spider predators M. arietina and T. biovatus decreased with ant density, whereas the prey capture rate of the weakly aggressed beetle predator S. aterrimus increased. Ants also induced density-mediated indirect interactions as high worker densities decreased the survival rate of the two predatory spider species. These results demonstrate for the first time that a host can indirectly mediate the trophic interactions between associated symbionts. In addition, we show that a single host can induce opposing indirect effects depending on its degree of aggression towards the symbionts.status: publishe

Effects of a herbicide and copper mixture on the quality of marine plankton

Pesticides and metals are often used in agriculture and are therefore often simultaneously discharged to nearby estuarine and marine areas. The effects of this organic-inorganic chemical mixture on food quality of aquatic organisms are currently unknown. In this study we test if a mixture of copper (inorganic) and the herbicide Primextra® Gold TZ (organic) affects the quality of the diatom Thalassiosira weissflogii and the copepod Acartia tonsa - two key species that fuel the local food-web. We quantified quality (i.e. energy content as food for the next trophic level) in terms of fatty acids, proteins and thiobarbituric acid reacting substances. We found non-additive effects (positive and negative) of the metal-herbicide mixture on the diatom and copepod species. In general, nutritionally important biochemical parameters of Acartia tonsa were most sensitive to the chemical stressors