Fifty years of invasion ecology - the legacy of Charles Elton

The publication, in 1958, of Charles Elton's book The ecology of invasions by animals and plants launched the systematic study of biological invasions. Invasion ecology has grown to become an important multi-disciplinary subfield of ecology with growing links to many other disciplines. This paper examines the citation history of Elton's book using the Web of Science. We also examine Elton's influence in shaping the current research agenda in invasion ecology, for which we use the 28 papers in a special issue of Diversity and Distributions (Volume 14: 2) as a representative sample. After 50 years, Elton's book remains the most cited single source in the field (> 1500 citations), and is cited more often every year (> 100 times) than any other invasion-related publication, including influential papers in journals. Most citations to Elton's book refer to particular topics/concepts covered in the book, rather than citing it as a general reference about invasions. The shift in the distribution of topics/concepts cited with reference to Elton over time follows the same trend as for biogeography and ecology in general (increasing emphasis on analytical studies, multi-scale analyses, multi-disciplinary studies, etc.). Some topics emphasized by Elton are still the focus of current research (dispersal and spread of invasive organisms, impact on biodiversity, role of disturbance and enemy release) but several prominent themes in modern studies were not addressed by Elton. The emergence of new themes can be attributed to a general change in approach and emphasis underpinning research questions in conservation biogeography and applied ecology over the last half century (risk analysis, multi-scale comparisons, propagule pressure, experimental approaches) and to the recent emergence and increasing availability of large data sets on the distribution of introduced species and to the emergence of key technologies (e.g. geographic information systems, modelling techniques, including niche-based modelling, and molecular methods). Half a century after its publication, Charles Elton's book on invasions remains influential, but massive changes in the status of invasions and other environmental issues worldwide, together with advances in technology, are reshaping the game rules and priorities of invasion ecology.