Four years with Cotula coronopifolia  : Monitoring and climate suitability modelling

Cotula coronopifolia is a short-lived plant that originates from South Africa and Namibia. It has been present in Europe since the 18th century. This study includes a monitoring inventory of two C. coronopifolia populations on Öland, Sweden, which has taken place continuously four consecutive years (2018-2021). It also includes a climate suitability model projected over Europe.  The monitoring inventory gives a picture of the population’s yearly dynamic in terms of spread, expansion and densities. The population density has decreased during these four years and the expansion speed has shifted from over 370 m/year between 2018 and 2019 to a contraction of -20 m/year thereafter. The decreasing densities seem to be due to lower grazing intensities in the last two years. It is also possible that C. coronopifolia have occupied most of the possible habitat available at these two locations.  The model was made to visualize the potential geographic range of C. coronopifolia based on contemporary climate variables such as temperature and precipitation. Predicted suitable habitat was extensive in the coastal regions of western Europe but surprisingly low in the eastern parts of Europe. The most important variable in the model turned out to be “the annual range between maximum temperature and minimum temperature” and that range is generally smaller in coastal climates. That, together with the fact that most of the occurrence records comes from the western parts of Europe (which may reflect the introduction history of the species rather than the climate and/or a geographic reporting bias), should be considered as alternative explanations for the difference in suitability between eastern and western Europe. However, the model seems to give a plausible picture over the contemporary potential distribution range in Sweden where the reporting effort is generally high. This study shows that C. coronopifolia is a species suitable for modelling. Future, refined models should include dispersal mechanisms, propagule pressure and future climate scenarios to predict plausible population spread.