Anthropogenic Dispersal of Ragworts

By Fleeces and Iron Horses: Anthropogenic Dispersal of Ragworts Stephen A Harris Department of Plant Sciences, University of Oxford, South Parks Road, Oxford, OX1 3RB, UK. E-mail: [email protected]. April 2004 Opprobrium has been heaped upon the abilities of ragworts (genus Senecio; Asteraceae) to disperse and colonize since the earliest derivations of their common names. One origin of the common name groundsel (Senecio vulgaris) is from the Anglo-Saxon meaning ‘ground-eater’; a reference to its ability to colonize newly cultivated land. Common ragwort (Senecio jacobaea) is characterized in its native and introduced ranges as a livestock killer; a reference to the highly toxic pyrrolizidine alkaloids it contains. However, it would be unreasonable to think that all ragworts, some 1,250 of them, are worthy of such epithets. Although the genus is successful worldwide, only those that have attracted attention, as major weeds, will be considered here. Ragwort fruits are ideally suited for wind-dispersal; they are small, dry and have long silky hairs attached to them. Indeed, Senecio means ‘old man’ and is a reference to the fruits’ white hairs. Under natural conditions, common ragwort fruits can be dispersed up to 14 m by wind and may even be dispersed by water. Furthermore, mature ragwort fruits survive for a long time and therefore take advantage of the opportunities offered by man, his animals and transportation systems for long-distance dispersal; ragworts are, as Joseph Dalton Hooker described weeds, ideal "tramps of our flora". The wool-trade has provided tremendous opportunities for the long-distance transport of plants, especially between South America, South Africa, Australia and Europe. Small fruits can easily get trapped in fleeces or they may be contaminated with fruit-containing faeces. Fleece cleaning is a harsh process but for those species that have fruits that can survive it, the areas around wool-processing factories can readily be exploited. Montpellier (France) has been a site of wool-processing since at least 1700, and has a particularly rich and diverse flora of wool aliens. One of the most detailed and comprehensive accounts of wool aliens in the UK recorded 348 species as aliens around the wool-processing areas of Galashiels (Scotland). However, the wool factories are not ‘botanic gardens of exotics’, since most species do not survive for more than a few generations. However, some species will be successful and spread, and may eventually become problematic weeds. Narrow- leaved ragwort (Senecio inaequidens), a South African native, made its first European appearances around wool-processing factories in Germany (Hanover in 1889). In fact, all five primary sites of narrow-leaved ragwort introduction in Europe are associated with the wool trade: Mazamet (France); Calais (France); Verona (Italy); Lüttich (Belgium) and Bremen (Germany). Transportation systems are effective means of ragwort fruit dispersal, whether they are attached to the vehicles themselves, carried in their slip-streams or part of their cargos. The Oxford botanist George Claridge Druce expressed his belief in the importance of trains for the dispersal of Oxford ragwort (Senecio squalidus) fruits thus: "the vortex of air following the express train carries the fruits in its wake. I have seen them enter a railway-carriage window near Oxford and remain suspended in the air in the compartment until they found an exit at Tilehurst [a railway station some 40 km from Oxford]". In fact, Druce was so convinced of the efficacy of the rail network that he proposed that the arrival of the railways in Oxford in 1844 was directly responsible for the successful spread of Oxford ragwort throughout the UK. However, there is direct evidence that the spread of narrow-leaved ragwort in Germany and the Netherlands is associated with the road and rail networks; fruits are attached to vehicles, caught in the slip-stream of vehicles and transported by soil during building works. Not only do roads and railways provide a means of dispersal, they also provide ideal habitats for colonization by annual ragworts. As has been seen with the wool aliens, ships are important for inter-continental dispersal of ragwort fruits. Fireweed (Senecio madagascariensis) appears to have been introduced to Australia in ship ballast from the South African Cape. Whilst the earliest records (1672) of groundsel into North America suggest that European colonists may have transported it in their ships, along with others, such as the ‘whiteman’s footprint’ (Plantago major). Once established in their new homes, ragworts are transported as part of normal agricultural practice. Thus, fruits of groundsel are found in contaminated crop seeds, on farm vehicles, in hay and even in manure from animals fed on groundsel- contaminated hay. These factors, together with regular soil disturbance in agricultural areas, create ideal conditions for the spread of groundsel and other ragworts. Although dispersal of ragworts as part of the horticultural trade is unlikely, there are anecdotal accounts of ‘enterprising’ UK farmers selling common ragwort as cut flowers to ignorant townsfolk and groundsel is offered as part of wild flower seed mixtures. Ragworts have harnessed the opportunities offered by man to scatter their fruits; to paraphrase Genesis, ragworts have been fruitful and multiplied and replenished the earth and subdued it. Man’s domesticated livestock, his migration and colonization pathways and transportation networks have all been harnessed to increase the dispersal of this fascinating and adaptable group of plants. Back to Top Back to Contents page .

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