A city is a highly complex, anthropogenically constructed system – an urban ecosystem. Researchers that study this system come from very different academic fields, bringing with them their own methods and research questions. From the perspective of (biological) urban ecology, this thesis first takes a step back, and focuses on knowledge production in general academia (chapter 1). The concept knowledge in the dark, or short: dark knowledge, describes the gap between potential and actual knowledge. In chapter 1, several potential reasons for dark knowledge in general are discussed. Focusing on the acasemic system, these are for example loss of academic freedom, research and publication biases, a lack of reproducibility, financial interests and barriers in understanding each other among disciplines and different areas of society. We also discuss potential solutions. One important aspect is rethinking and improving research synthesis and finding ways to bridge language and information barriers both within and beyond the academic system. Chapters 2, 3 and 4 then take up a main theme from chapter 1: research synthesis, and within the setting of urban ecology show how different approaches to synthesis can help bridge communication between researchers within and beyond one discipline (biological urban ecology), identify biases and knowledge gaps, and visualize and summarize available knowledge. The chapters proceed from a very broad perspective on urban ecology to the topic of urban biotic homogenization, and then a very specific aspect within urban biodiversity research: the influence of mowing of urban lawns on arthropods, which is one specific cause of biotic homogenization in cities. In Chapter 2, together with a group of urban ecologists predominantly based in Berlin, I collected 62 research hypotheses from urban ecology. In a second step, my co-authors and I present a first map of these hypotheses in a structured, bipartite network. As urban ecology is a multi-disciplinary field that is of high interest to urban planners and administrations, knowledge transfer between different stakeholders is particularly important. The network we propose consists of four distinct clusters, into which the hypotheses we previously identified can be grouped: (i) Urban species traits & evolution, (ii) Urban communities, (iii) Urban habitats and (iv) Urban ecosystems. This work is intended to grow, and as an invitation to researchers, practitioners and others interested in urban ecology to contribute to collecting additional hypotheses, jointly fill the network (or rather the underlying Wikidata project) with empirical data. Chapter 2 is thus intended as a first step towards an open and community curated knowledge base for urban ecology. Chapter 3 focuses on one of the hypotheses from our network: urban biotic homogenization (UBH). Urbanization, which is restructuring ecosystems at an unprecedented pace, is hypothesized to cause the homogenization of urban species communities. This idea has also been applied to other biodiversity levels like genetic diversity, behavioural diversity, functional diversity, and the like. There is, however, good reason to also formulate a hypothesis predicting the opposite effect: biotic diversification, that predicts species communities (and other levels of biodiversity) to become biologically more diverse because of ongoing urbanization. In chapter 3, I disentangle the different connotations, scales and “auxiliary hypotheses”, i.e., hypotheses that often unspokenly accompany a tested research hypothesis, which have been applied in the research literature on urban biotic homogenization and diversification. Applying the hierarchy-of-hypotheses approach, I systematically map and structure the comprehensive body of literature on UBH, comprising 225 individual tests of the hypothesis from 145 publications. Interestingly, UBH is generally used with two very different connotations in relation to scale (i.e., homogenization across cities versus within cities). There are several strong research biases, for example in relation to taxonomic focus, scale, and study systems. We visualize support and biases in an evidence gap map and provide a bibliographic network of the field. Chapter 4 is a meta-analysis of the impact of reduced mowing frequencies on the abundance and diversity of arthropods on urban grassland sites. It is based on 46 datasets on arthropod abundance and 23 datasets on taxa richness, respectively. As in chapter 3, we report severe geographical biases. While we find a medium positive effect (effect size: g = 0.54) of reduced mowing on arthropod abundance, the effect that reduced mowing has on urban arthropod taxa richness is larger (g = 1.25). Some functional groups benefit more from reduced mowing, especially winged insects, and perceived non-pest species. In the final, General Discussion, I try to connect several points that can be traced to all four chapters and discuss them in the context of urban ecology. These are: knowledge gaps and biases, with a brief discussion of how the concept of dark knowledge can (and should) be relevant to researchers from urban ecology, and research and knowledge synthesis. I finish my thesis by reflecting on how these are important in the context of urban ecological knowledge in the Anthropocene, and how they should be extended in the face of planetary crisis.
