Invasive behaviour of Lactuca serriola (Asteraceae) in the Netherlands: Spatial distribution and ecological amplitude

Species invasions have been a central theme in ecology over the past decades, with a focus on invasions of non-indigenous European species in the New World. However, within Europe, native species may also become invasive. Such species rapidly increase their geographic range and may at the same time broaden their ecological amplitude. We report on Lactuca serriola L., which has shown a rapid range expansion in many parts of Europe. We reconstructed its geographical spread through The Netherlands; furthermore, we tested for changes in its ecological amplitude. Before 1950, L. serriola was rare in The Netherlands, recorded from only 80 grid-cells (of 5 x 5 km(2)). Subsequently, its distribution increased from 219 grid-cells (1980), to 546 cells (990) and 998 grid cells (2000). It currently occupies at least 60% of The Netherlands. Analyses of relevees demonstrate, using detrended correspondence analysis (DCA), that the variance in the assemblage of species co-occurring with L. serriola increased significantly since 1940, indicating that the species currently occurs in a broader range of vegetation types. Correlating averages of Ellenberg's indicator values per vegetation relevee with time, we found correlations for continentality (+), acidity (-) and (soil) humidity (+). Moreover, the proportion of phanerophytes in the relevees increased over time. These findings suggest that in addition to its original ruderal. habitat, L. serriola also currently occurs, in more closed vegetation types. Based on analyses of two other co-occurring Asteraceae species, we showed that these patterns were specific for L. serriola only. We conclude that L. serriola has broadened its ecological amplitude and discuss four mutually non-exclusive explanations for the recent invasiveness of L. serriola: (i) Effects of a changed environment: global. warming and ruderalisation; (ii) metapopulation dynamics and increased diaspore pressure; (iii) microevolution; IN) genetic reinforcement due to hybridisation with conspecific (crop) species.