Download European weeds world-wide most successful in metropoles of the

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