Background and aims Since its emergence in the mid‐20th century, invasion biology has matured into a productive research field addressing questions of fundamental and applied importance. Not only has the number of empirical studies increased through time, but also has the number of competing, overlapping and, in some cases, contradictory hypotheses about biological invasions. To make these contradictions and redundancies explicit, and to gain insight into the field’s current theoretical structure, we developed and applied a Delphi approach to create a consensus network of 39 existing invasion hypotheses. Results The resulting network was analysed with a link‐clustering algorithm that revealed five concept clusters (resource availability, biotic interaction, propagule, trait and Darwin’s clusters) representing complementary areas in the theory of invasion biology. The network also displays hypotheses that link two or more clusters, called connecting hypotheses , which are important in determining network structure. The network indicates hypotheses that are logically linked either positively (77 connections of support) or negatively (that is, they contradict each other; 6 connections). Significance The network visually synthesizes how invasion biology’s predominant hypotheses are conceptually related to each other, and thus, reveals an emergent structure – a conceptual map – that can serve as a navigation tool for scholars, practitioners and students, both inside and outside of the field of invasion biology, and guide the development of a more coherent foundation of theory. Additionally, the outlined approach can be more widely applied to create a conceptual map for the larger fields of ecology and biogeography.

Global change has complex eco-evolutionary consequences for organisms and ecosystems, but related concepts (e.g., novel ecosystems) do not cover their full range. Here we propose an umbrella concept of “ecological novelty” comprising (1) a site-specific and (2) an organism-centered, eco-evolutionary perspective. Under this umbrella, complementary options for studying and communicating effects of global change on organisms, ecosystems, and landscapes can be included in a toolbox. This allows researchers to address ecological novelty from different perspectives, e.g., by defining it based on (a) categorical or continuous measures, (b) reference conditions related to sites or organisms, and (c) types of human activities. We suggest striving for a descriptive, non-normative usage of the term “ecological novelty” in science. Normative evaluations and decisions about conservation policies or management are important, but require additional societal processes and engagement with multiple stakeholders.

Plant-soil feedbacks (PSFs) have become a commonly invoked mechanism of plant coexistence and abundance. Yet, most PSF experiments have been performed in greenhouse conditions. To test whether or not greenhouse-measured PSF values are of similar magnitude and positively correlated with field-measured PSFs, we compared PSF values from five different studies that measured PSF values in both greenhouse and field conditions. For 36 plant species, greenhouse-measured PSF values were larger than and not positively correlated with field-measured PSF values. Similarly, these 36 species produced 269 soil-specific PSF values, and for each site there was no positive correlation between these greenhouse-and field-measured PSF values. While PSFs were observed in both greenhouse and field conditions, results provided no support at the soil, site or species level that a positive correlation exists between greenhouse-and field-measured PSF. Further, greenhouse-measured PSF appear to overestimate field-measured PSF. Although from five studies, results strongly suggest that field experiments are needed to understand the role of PSFs in plant communities in natural settings.

Summary1. Plant-soil feedback (PSF) may affect above-ground higher trophic levels in glasshouse experiments, but evidence from field studies on the relevance of these multitrophic interactions for plant performance is lacking. Therefore, we examined whether PSF effects of several native and invasive plant species occur also in the field and influence plant damage by above-ground herbivores. 2. Root zone soil from an abandoned urban field was used as inocula for the PSF experiment. First, we grew eight urban grassland plant species (five natives and three invasive species) separately in a glasshouse, with soil biota communities conditioned by the respective species itself ('home soil') or by a mixture of all other species ('foreign soil'). After 13 weeks, one cohort of the plants was placed on an urban field in Berlin to assess damage by naturally colonizing herbivores, while another cohort of the plants stayed in the glasshouse. 3. We observed that the extent of the PSF effects differed between the field and glasshouse cohorts of plants. While we found positive PSF responses for five of the eight plant species in the glasshouse, we found no PSF effects in the field. Further, there was no trend that invasive or native species differed in the direction or extent of PSF responses. Concerning the leaf damage by herbivores of the field plants, we found no evidence that the soil history (home vs. foreign soil) affected the effects of above-ground herbivores on the plants. 4. Synthesis. We conclude that PSF effects are more likely to be found under glasshouse conditions. In the field, PSFs seem to play a minor role for the selected urban grassland species. More generally, our study highlights the need to focus on PSFs under natural conditions and in natural communities (including higher trophic levels), which is often overlooked in PSF research.