The first steps towards unifying concepts in invasion ecology were made one hundred years ago: revisiting the work of the Swiss botanist Albert Thellung

Aim Biological invasions are a major threat to biodiversity, and The ecology of invasions by animals and plants by Charles Elton (1958) is often recognized as the starting point for modern invasion research. Yet there were predecessors in invasion research whose contribution to the development of ideas and concepts in this field is often underestimated. To contribute to a balanced perception of pioneers in invasion research, we retrace the work of the Swiss botanist Albert Thellung (1881-1928) whose main work, La flore adventice de Montpellier, appeared 100 years ago, in 1912, and illustrate how his ideas contributed to the current state of the art in the fields of invasion science and biogeography. Location None. Methods We discuss conceptual approaches in the invasion-related work by Albert Thellung. Results Thellung's early work covered topics that are still central to widely used invasion frameworks. He promoted concepts to classify alien species (degree of naturalization, introduction pathways and time period of introduction) and adopted these systematically at a regional scale in his alien flora of Montpellier, comprising 800 non-native species. He introduced an exact population-based definition of naturalization, with links to environmental barriers, and elaborated the first assessment of pathway efficiency by relating introduction modes to naturalization. With conceptual papers and a first review of human-mediated plant introductions, Thellung stimulated further research on plant invasions as well as modern terminological frameworks for alien plants. Main conclusions Albert Thellung was an outstanding member of the group of pre-Eltonian invasion scientists. He opened up focussed research in the field of alien plants in Europe, and his theoretical approaches were a powerful step towards unifying concepts in invasion ecology.