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European weeds world-wide most successful in metropoles of the northern hemisphere Norbert Müller (Erfurt-Germany) University of Applied Sciences Erfurt, Leipzigerstr. 77, 99084 Erfurt, [email protected] A recently completed study of the most frequent vascular plants in metropolitan cities of the northern hemisphere proved that European weeds are the most successful plants in the colonisation of urban habitats. The two most successful tree species are Black Locust from North America and Tree-of-Heaven from China. Looking at typical urban habitats such as sidewalks, gardens, lawns, walls, wastelands and urban forests in European cities we always can find so common weeds such as White Goosefoot (Chenopodium album), Winter Grass (Poa annua) or Narrow-leaved Ribwort (Plantago lanceolata). A recently compiled world-wide study in metropoles of the northern hemisphere has shown that these weeds are also most successful in North American and Asian cities. The aim of this study was a summarisation of the 50 most frequent plants of the following cities: Berlin, Rome, Yokohama (Tokyo agglomeration), New York, San Francisco and Los Angeles. Fig. 1: Locations of the examined metropoles (map from www.mapquest.com 2003) © Norbert Müller 2004 Native (indigenous) plant species dominate in every city in Europe, while alien species are dominant in American cities with up to 80%. Plants from European origin form, therefore, the main part of urban vegetation in the American cities studied. Fig. 2: Origin of the 50 most frequent plants in metropoles of the northern hemisphere 100% neogen 90% N-America 80% S-America 70% Africa 60% Australia 50% Europe 40% Eurasia 30% Asia 20% Cosmopol. 10% native 0% Berlin Rom Yokohama New York S. Francisco Los Angeles When comparing all cities the most frequent weeds (Top Ten) are mainly European weeds except Conyza canadensis from North America. Summarising the 20 most frequent plants in all cities there are only two trees - Black Locust (Robinia pseudoacacia) from North America and Tree-of-Heaven (Ailanthus altissima) from China. © Norbert Müller 2004 2 Tab. 1: The Top Ten weeds in metropoles of the Northern Hemisphere with natural habitat, origin and life-form Weeds – Common Name Scientific name White Goosefoot Horseweed Fleabane Bermuda Grass Narrow-leaved Ribwort Broadleaf Ribwort Winter Grass Prostrate Knotweed Chickweed White Clover Common Dandelion Natural Habitat Origin Lifeform annual Chenopodium album L. none - neogene Europe annual Conyza canadensis Cron. riparian habitats N-America Cynodon dactylon Pers. none - neogene Cosmopolitan perennial Europe perennial Plantago lanceolata L. grasslands none - neogene Europe perennial Plantago major L. none - neogene Europe annual Poa annua L. annual Polygonum aviculare L. none - neogene Europe annual Stellaria media (L.) Villars none - neogene Europe Europe perennial Trifolium repens L. grasslands none - neogene Europe perennial Taraxacum officinale L. How can we explain the world-wide success of European weeds in the colonisation of urban habitats? Firstly we have to consider that in general almost all European plants can be regarded as invaders, if the last 10 thousand years in which they had to colonise the European landscape after the glacier period are used as a point of departure. Secondly all European plants have a long history of co-evolution with humans or even developed as a result of human impact. Regarding the natural habitats (that means the original habitat in the natural vegetation) of the urban plants it is evident that almost all weeds have no natural habitat. It is hypothesised that urban plants have developed since the last 6000 years of European agriculture by introgression and selection from native species (neogene species). Due to their long co-evolution with man in the European landscape they are well adapted to human disturbances. It is suspected that some species even developed only in urban areas such as Plantago major and Polygonum aviculare. Some Botanists also argued that Rome as the city with the longest world-wide urban tradition is the centre of evolution of some ruderal species. Finally the question is how to evaluate alien plants in urban areas regarding the Convention on Biological Diversity. Alien plants do increase the biodiversity in urban areas in comparison with the surrounding landscape, because of the propagation of alien species and the evolution of new species. This view emphasizes the positive aspect of alien species in urban areas. The long tradition of agricultural landscapes in Europe lead to a continuous increase in biodiversity over the last 6000 years under human influence. Due to man-made habitats such as grasslands and arable fields several new species from the Mediterranean and near East found new habitats after deliberate or undeliberate introductions (so called archaeophytes) such as Capsella bursa pastoris or Papaver rhoeas. Due to human impact a variety of new species and ecotypes developed by differentiation as a consequence of selective and introgressive hybridisation. Therefore the world-wide fast development and increase of urban-industrial areas with specific urban habitats especially since the industrial revolution can be compared in the same way, as an increase of biodiversity due to human impact. © Norbert Müller 2004 3 On the other hand the propagation of invasive species is one of the main threats to biodiversity and therefor the Convention aims also to control those species which are a danger for biodiversity. Urban areas are one of the main sources of alien plant invasions more attention should be given in future on potential invasive species in these areas. Potential invaders should already be eradicated in urban areas and not commercially available from nurseries. Only indigenous species should be used in future for restoration and ornamental purposes outside urban areas. Lit.: MÜLLER, N. 2003: Woher stammen die erfolgreichsten Stadtpflanzen – ein Vergleich von Großstädten der alten und neuen Welt. – Nürtinger Hochschulschriften 20: 107 – 130 Fig. 3 : Narrow-leaved Ribwort is worldwide one of the most frequent plants in metropoles of the northern hemisphere © Norbert Müller 2004 4