Cotula coronopifolia : invasive or just another alien species?

Sammanfattning: It is known that alien species can become invasive in a new environment and in new geographic areas. The recent and rapid expansion of Cotula coronopifolia (cotula)on the island of Öland in Sweden has raised questions about its potential invasiveness. The lack of information about the species ecology and behavior lead to this study which focuses on cotula’s history of expansion, dispersal mechanisms and potential vectors, expansion speed and habitat preferences, and if there is any spatial competition between cotula and native species. A literature review, a field study and a couple of floating experiments indicates that the main dispersal mechanism of cotula is its good floating ability regarding both plants and seeds. The present population in Sweden is most likely a result of secondary spread by water across the Baltic Sea around 2002. A mapping survey done over two seasons, 2018 and 2019, of two populations on the island of Öland and a demographic study showed a high dispersal potential, with 2.4 million seeds produced per m2, and an expansion speed of, at least, 380 m per year. The field study, in combination with the mapping survey, showed that cotula has a broader niche in terms of habitat preferences than the literature indicated. I also found that the adult plants were able to endure much more frost and ice than previously known. A survey of 290 vegetation plots (0.0625 m2) with cotula present showed that there is a negative correlation between the cover of cotula and the native species and also with species number. This indicates spatial competition between cotula and the native species. The present populations will likely continue to grow and spread since there is plenty of uncolonized habitat available along the coasts of the Baltic Sea. Hence, there is a need for an action plan to prevent further establishment and a readiness for rapid eradication once the species reaches new areas.