Invasion, dispersal and ecology of the South African neophyte Senecio inaequidens in the Netherlands: from wool alien to railway and road alien.
Senecio inaequidens, a perennial pioneer plant, invaded the Netherlands at Tilburg with sheep's wool from South Africa in 1939. It failed to establish a permanent population, but in 1942 a new invasion from an expanding invader population at Liège occurred at Eijsden, establishing a new dispersal centre in the Netherlands at southern Limburg. In the 1980s, further dispersal throughout the Netherlands resulted in additional dispersal centres at the railway stations at Amsterdam and Hengelo. Dispersal along new railway lines in the Amsterdam region confirmed the importance of trains for occasional long-distance dispersal events because, in general, the majority of the anemochorous achenes remained in the vicinity of the parent plants. Expansion dynamics and ecology was studied in a new established population at the railway station in Hoofddorp. The species has high self-fertility. Adaptation to the Atlantic climate of the Netherlands was accompanied by the selection for flowering early from August to May and prolongation of the flowering period to the end of December. Achene mass mostly declined from high values in July to 70% lower ones in December. Low dormancy of early summer achenes allowed establishment of a new generation in the same year; late autumn achenes had high dormancy and germinated the following spring. Dormant achenes persisted for two winter periods and survived frost at -15°C. Caterpillars of Tyria jacobaeae were recorded as leaf herbivores on S. inaequidens, but had nearly no impact on the plant's performance. The extinction or survival of the various founder populations as wool aliens in Europe is discussed in relation to founder effects, adaptation to winter temperature in western Europe, prolongation of the flowering period and the importance of modern traffic means and climatic changes for its rapid expansion after 1985.