Master Thesis

Distribution patterns of invasive plant species on the island of La Palma

Daniel Mederer (02/2021-08/2021)

Support: Carl Beierkuhnlein, Anna Walentowitz

Oceanic islands are places of high endemic species richness that are severely threatened by the ongoing introduction of invasive plants. The distribution of these species, however, is generally assessed by an island-level approach, which occludes major internal differences due to the diversity of ecosystems found there. Here, I analyze the distribution of invasive and non-native plant species on the high-elevation island of La Palma (Canary Islands, Spain) at the ecosystem level. Furthermore, I investigate correlations between seven plant functional traits and species invasiveness. For this purpose, I illustrate general patterns with a series of graphics and conduct logistic regressions for each ecosystem. I find that forest ecosystems and the alpine zone are still largely free from invasive species. Invasive species numbers decrease with increasing elevation and consist mainly of species with broad environmental tolerances, which supports the Directional Ecological Filtering Hypothesis. Open ecosystems have higher numbers of invasive species, and succulent scrub is the ecosystem with the highest proportion of invasive species overall. Both greater anthropogenic influence on these ecosystems and higher light availability are possible explanations for this finding, but a firm conclusion requires further analysis. In general, invasive species on La Palma are still concentrated around cultural land and settlements, with the majority of invasives being used as ornamental plants. I therefore recommend stronger restrictions for the cultivation of alien species on La Palma. This is particularly relevant as the total number of invasive species has increased significantly in the 11 years since the last estimate and because conditions are expected to deteriorate further as Climate Change continues.