Thesis defence by Jürg Spaak

Reinterpretation of niche and fitness differences improves our understanding of species coexistence

Species coexistence is the indefinite co-occurrence of two or more species in the same location at the same time. Modern coexistence theory explains this by niche and fitness differences, which measure how similar the resource requirements of species are and how different their intrinsic fitness is. Species coexist if their niche differences overcome fitness differences, which promote and hamper coexistence respectively.

In my PhD thesis, I first propose an intuitive a and broadly applicable definition of niche and fitness differences to subsequently apply it to various community types that were so far out of the reach of modern coexistence theory (Chapter 2). I then compare this new approach with the traditional approach to measure niche and fitness differences. Specifically, I investigate the effect of higher order and non-linear interactions on niche and fitness differences, their effect on the outcome of species interactions and how we interpret their coexistence. Most importantly, I show that the omission of higher order interactions, when computing niche and fitness differences, leads to qualitatively wrong predictions concerning whether and why species coexist (Chapter 3). In chapter 4, I show that species richness increases fitness differences, but not niche differences. This is an important step from simple two-species communities towards the higher complexity of natural communities. MCT and biodiversity ecosystem function are closely related, as they both use niche differences to explain their results. Nonetheless, there are very few collaborations between the two fields. In chapter 5, I investigate the effects of trait richness on coexistence, niche and fitness differences, and ecosystem function to close the gap between coexistence theory and biodiversity ecosystem functioning. In chapter 6, I extend the focus of MCT towards the entire community. Most importantly, to communities with facilitative species interactions or species that depend on the presence of other species.

Jürg will present the results of his PhD thesis, conducted in the Research Unit of Environmental and Evolutionary Biology (URBE) under the supervision of Frederik De Laender. The defense will be held online at the following link: https://us02web.zoom.us/j/81120991137