INVASION, DISPERSAL AND ECOLOGY OF THE SOUTH-AFRICAN NEOPHYTE SENECIOINAEQUIDENS IN THE NETHERLANDS - FROM WOOL ALIEN TO RAILWAY AND ROAD ALIEN

Senecio inaequidens, a perennial pioneer plant, invaded The Netherland s at Tilburg with sheep's wool from South Africa in 1939, where it fai led to establish a permanent population. In 1942 a new invasion from a n expanding invader population at Liege occurred at Eijsden, establish ing a new dispersal centre in southern Limburg, The Netherlands. In th e 1980s a further dispersal throughout The Netherlands resulted in add itional dispersal centres at the railway stations at Amsterdam and Hen gelo.,The dispersal along new railway lines in the Amsterdam region co nfirmed the importance of trains for occasional long-distance dispersa l events because in general the majority of the anemochorous achenes r emained in the vicinity of the parent plants. Expansion dynamics and e cology was studied in a new established population at the railway stat ion in Hoofddorp. The species has a high self-fertility. Adaptation to the Atlantic climate of The Netherlands was accompanied by the select ion for flowering earliness from August to May and prolongation of the flowering period to end of December. Achene mass mostly declined from high values in July to 70%:lower ones in :December. Low dormancy of e arly summer achenes allowed establishment of a new generation in the s ame year; late autumn achenes had a high dormancy and germinated in ne xt spring. Dormant achenes persisted for two winter periods and surviv ed frost at -15 degrees C. Caterpillars of Tyria jacobaeae were record ed as leaf herbivores on S. inaequidens, but had nearly no impact on a plant's performance. The extinction or survival of the various founde r populations as wool aliens in Europe is discussed in relation to fou nder effects, adaptation to winter temperature in western Europe, prol ongation of the flowering period and the importance of modern traffic means and climatic changes for its rapid expansion after 1985.