- Top of page
- Data and methods
- Discussion and conclusions
The flora of urban areas has fascinated botanists for many years. The urban flora has particularly attracted attention after major incidents of destruction, for example the 1666 Great Fire of London, when Sisymbrium irio L. suddenly became abundant (Kent 1975), and the bombing of Berlin (Scholz 1960) and London (Fitter 1945) during the Second World War. Plants of great beauty appear without human assistance in the wake of smaller-scale destruction, such as clearance of old buildings (Mabey 1973), creating ‘urban commons’, which have been celebrated by Gilbert (1992) and Mabey (1996). These urban plants live in a highly unnatural environment. Certainly it is a far cry from the natural home of Chamerion angustifolium (L.) Holub in the boreal forest to the rubble of a ruined house in London.
The attribute of naturalness is much used in the evaluation of sites for nature conservation (Usher 1986). In cities, naturalness has less obvious value. Indeed, ‘nativism’ (Peretti 1998) may be quite irrelevant to the communities that are of most interest to urban ecologists. Nevertheless, the question of how natural a community is cannot be ignored. Urban people are fascinated by wild nature, which is often seen as a part of an ideal countryside (Bunce 1994); naturalness continues to be a major criterion used in site assessment.
For these practical reasons and also because the study of human impacts is of great interest in itself, applied ecologists have devised several measures of unnaturalness or human impact. Hemeroby, the best-known of these, was developed from earlier more informal concepts by Jalas (1955), who proposed a four-point scale based largely on the degree of disturbance to the soil. Jalas's (1955) definitions were subsequently extended to a 10-point scale, which has been used to categorize both plants and places in central Europe (Kowarik 1990, 1999; Sukopp 1990; Grabherr et al. 1995, 1996). Hemeroby on the 10-point scale is a measure of human impact varying from 0 (ahemerobic or completely natural) to 9 (polyhemerobic, consisting of pioneer vegetation of railways, rubbish dumps and salted motorways). Most plants in the flora of the Berlin area have been assigned a hemeroby value (Kowarik in Lindacher 1995), with the intention that these values should be used in much the same way as the indicator values of Ellenberg (1979).
As part of a study of urban floras, we wished to categorize the urban flora of central England. Although there are certain plants that are well known to be urbanophiles, many plants of cities also occur in the wider countryside. Chamerion angustifolium and Urtica dioica L. are in no sense urban specialists, although they are both frequent in British cities. An analysis of species presence in 2-km squares in Britain (Roy, Hill & Rothery 1999) showed that there was a significant effect of urban cover but, at this scale, the urban flora was not picked out as definitely as we had expected. Indeed, Phragmites australis (Cav.) Trin. and Sagina procumbens L. were indicated as species that had relatively high frequency in urban areas although neither is an urban specialist.
In order to discriminate the urban specialists more clearly, it is necessary to use a smaller unit for recording. The quadrat scale is the most obvious one to use. Numerous quadrat surveys have been made, several of which are available electronically (see below). If there is a category of urban specialists, then they should grow in quadrats together. They would be expected to have high hemeroby values, and perhaps high values on other existing scales of urbanity or disturbance, such as that of Frank & Klotz (1990).
If hemeroby is to become a clearly defined concept, then values ought to be confirmed by measurement. This is often difficult. Several different scales have been proposed (Sukopp 1969). The degree of human impact can be judged in a general way but does not necessarily correspond to a simple index like the number of human visits in a year. For example, an arable field may be passed over by machinery only a few times a year but is intensively influenced. Many visitors, on the other hand, may walk to the top of a mountain, which still retains its semi-natural character.
One way to check the values of an ecological index is to find out whether they are similar to those of associated species in large-scale quadrat data. Ellenberg indicator values could in most cases be effectively confirmed in this way, and there were generally good reasons for discrepancies (Hill et al. 2000).
In this study, we set out to answer the following questions. How distinctive is the urban flora in central England? If we can define a measure of urbanity, does it relate clearly to other measures of disturbance? How internally consistent are existing measures of disturbance? Is it even possible to measure the degree of disturbance in a satisfactory way?
- Top of page
- Data and methods
- Discussion and conclusions
Of the 20 most urban species (Table 3) none was unequivocally a British native, although the eight European species that were first found before 1680 are now classified as archaeophytes, presumed to have been established in Britain before 1500 (Preston, Pearman & Dines 2002). The great majority of the most urban species were in fact relatively uncommon in our sample; only Artemisia absinthium and Senecio squalidus were present in more than 0·5% of the quadrats. While this pattern in part reflected the mainly rural position of the sample quadrats, for which the mean urban land cover was 13%, it also emphasized the fact that most urban specialists are relatively rare. The proportion of annual associates varied widely from Acer platanoides, Aster novi-belgii, Foeniculum vulgare, Lupinus × regalis and Solidago canadensis, which had fewer annual associates than the average (12%) for all quadrats, to Anthemis cotula, Apera interrupta, Conyza canadensis, Diplotaxis tenuifolia, Lactuca serriola, Reseda luteola, Senecio squalidus and Vulpia myuros, which had more than twice the average proportion of annual associates.
Table 3. The 20 most urban species, ordered by the proportion of urban land cover in 1-km squares where they occurred. None of these species is native to Britain; those marked (a) are archaeophytes, introduced before 1500
|Name||Longevity (a-annual, b = biennial, p = perennial)||Number of samples||U : mean urban land cover (%) ||A : mean proportion of annual associates (%) ||Continent of origin||Date of first British record in wild||Date of introduction to Great Britain for known garden plants|
|Buddleja davidii||p|| 40||75||19||Asia||1927||1890s|
|Lactuca serriola (a) ||b|| 67||74||31||Europe||1632||–|
|Melilotus albus||a, b|| 41||64||22||Europe||1822||–|
|Melilotus officinalis||b|| 95||59||21||Europe||1848||–|
|Lupinus × regalis||p|| 28||58|| 9||North America||1955||1937|
|Conyza canadensis||a|| 75||58||32||North America||1690||–|
|Solidago canadensis s.l. ||p|| 104||56||10||North America||1888||1648|
|Aster novi-belgii||p|| 58||56||11||North America||1860||1710|
|Artemisia absinthium (a) ||p|| 171||56||21||Europe||1551||1200|
|Oenothera glazioviana||b|| 16||54||21||North America||1860||1778|
|Senecio squalidus||a, b, p|| 585||50||31||Europe||1792||1620s|
|Apera interrupta||a|| 23||49||46||Europe||1848||–|
|Diplotaxis tenuifolia (a) ||p|| 24||48||27||Europe||1597||–|
|Acer platanoides||p|| 26||47|| 8||Europe||1905||1683|
|Saponaria officinalis (a) ||p|| 25||46||14||Europe||1597||Medieval|
|Calystegia pulchra||p|| 23||46||13||Asia||1884||1823|
|Vulpia myuros (a) ||a|| 114||45||28||Europe||1633||–|
|Foeniculum vulgare (a) ||p|| 16||44|| 8||Europe||1677||Roman|
|Reseda luteola (a) ||b|| 106||44||35||Europe||1570||Iron Age|
|Anthemis cotula (a) ||a|| 38||44||37||Europe||1523||–|
|All samples|| ||26 710||13||12|| || || |
By contrast, all but three of the 20 most common species in highly urbanized 1-km squares had no particular association with urban land but were merely common in both town and country (Table 4). Artemisia vulgaris and Senecio squalidus were strongly concentrated in urban areas and Chamerion angustifolium was nearly twice as common there as in the countryside at large. In the other direction, Agrostis capillaris, Dactylis glomerata, Poa trivialis, Rubus fruticosus and Urtica dioica were less frequent in urban areas than in the countryside at large but were still present in at least 14% of quadrats located in highly urbanized 1-km squares. Much the same pattern was shown by the mean cover of urban land in 1-km squares containing each species. By this criterion, Agrostis capillaris and Urtica dioica were the least urban of the common species; Artemisia vulgaris and Senecio squalidus were the most urban.
Table 4. Most common species in quadrats located in 1-km squares with more than 40% cover of urban land (urban-and-vicinity quadrats); all species are British natives except for those marked (a) and (n), which are archaeophytes and neophytes, respectively
|Name||Total number of occurrences in all quadrats||Number of occurrences in urban-and-vicinity quadrats||W : proportion (%) of records in urban-and-vicinity quadrats ||U : proportion (%) of urban land near quadrats where species present |
|Agrostis stolonifera|| 7 483||1069||14·3||16|
|Holcus lanatus|| 8 732|| 959||11·0||13|
|Arrhenatherum elatius|| 6 962|| 827||11·9||15|
|Festuca rubra|| 6 918|| 714||10·3||13|
|Dactylis glomerata|| 7 677|| 651|| 8·5||12|
|Cirsium arvense|| 4 527|| 604||13·3||16|
|Plantago lanceolata|| 4 337|| 598||13·8||15|
|Lolium perenne|| 5 232|| 597||11·4||14|
|Taraxacum officinale|| 4 714|| 577||12·2||14|
|Rubus fruticosus|| 6 723|| 567|| 8·4||12|
|Trifolium repens|| 4 863|| 503||10·3||12|
|Poa pratensis|| 4 226|| 496||11·7||14|
|Agrostis capillaris|| 5 945|| 464|| 7·8|| 9|
|Elytrigia repens|| 4 546|| 463||10·2||15|
|Poa trivialis|| 5 979|| 456|| 7·6||12|
|Poa annua|| 4 009|| 437||10·9||13|
|Chamerion angustifolium|| 2 198|| 419||19·1||20|
|Artemisia vulgaris (a) || 796|| 392||49·2||41|
|Urtica dioica|| 6 484|| 370|| 5·7||10|
|Senecio squalidus (n) || 585|| 366||62·6||50|
|All quadrats||26 710||2595|| 9·7||13|
Although many urban plants grow in disturbed habitats, there was no very marked tendency for the characteristic plants of urban areas to be associated with annuals (Fig. 2). Indeed, the characteristically urban plants, i.e. those with at least 40% mean urban cover near to where they were found, had an intermediate proportion of associated annuals (A), mostly in the range 10–40%. Above this range, the great majority of species were annuals that occur frequently if not always on arable land. Two of the three perennials for which A > 40%, Brassica oleracea and Solanum tuberosum, are grown as annual field crops. The third perennial, Potentilla argentea, was more surprising. In fact, although it was found in open communities with many annuals, its most frequent associates were Agrostis capillaris, Plantago lanceolata, Poa pratensis and Veronica arvensis, only one of which is an annual. Three species, Apera interrupta, Sisymbrium altissimum and Sisymbrium orientale, had A > 40% and U > 40%.
Figure 2. Mean urban land cover in 1-km squares where species occurred in relation to the proportion of associated annuals in quadrats. Longevity classes are distinguished by symbols: circle, annual; triangle, biennial; dot, perennial.
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When species with high numbers of annual associates were compared with urban species, their mean Ellenberg values were generally similar (Table 5). The plants with numerous annual associates were almost exclusively arable weeds. They and urban species had a mean signature that indicated drier, less acid and more fertile sites than the average for all species. The largest difference from the overall average was in the nitrogen values, which are broadly an indication of fertility. In terms of Grime's (1979, 2001) established strategies, the annual arable species were almost entirely ruderals. Of the species characterized by high cover of nearby urban land, only three, Artemisia absinthium, Saponaria officinalis and Senecio squalidus, had been assigned values for their established strategy. As a result, the mean was not significantly different from that of the species that were frequent in urbanized 1-km squares. The proportion of associated annuals was, however, significantly higher, 0·22 as opposed to 0·15, suggesting that the mean ruderality value would have been genuinely higher if more species had been scored for it.
Table 5. Mean attributes of species in categories defined by annuality ( A ), urban land cover ( U ) and frequency in urban-and-vicinity quadrats ( V ); means are arithmetic means except for plant heights, which are medians. Values in parentheses are standard errors
|Ecological attribute||Species with annuality > 0·5||Species with high urban land cover (Table 2)||Species most frequent in urban squares (Table 3)||All species|
|Ellenberg indicator value|
|Light||7·2 (0·13)||7·6 (0·28)||7·0 (0·15)||6·8 (0·04)|
|Moisture||4·4 (0·10)||4·6 (0·18)||5·2 (0·14)||6·0 (0·07)|
|Soil reaction||6·6 (0·13)||6·9 (0·15)||6·4 (0·18)||6·1 (0·05)|
|Nitrogen||6·1 (0·21)||5·8 (0·29)||6·0 (0·23)||4·8 (0·06)|
|Salt||0·2 (0·14)||0·4 (0·22)||0·4 (0·15)||0·2 (0·03)|
|Number of species with values||29||19||20||830|
|Mean (SE) established strategy|
|Competitive ability||0·8 (0·31)||3·6 (0·99)||5·1 (0·67)||3·0 (0·12)|
|Stress tolerance||0·0 (0·00)||1·1 (0·55)||1·6 (0·35)||3·5 (0·14)|
|Ruderality||9·2 (0·31)||5·3 (0·71)||3·3 (0·60)||3·5 (0·13)|
|Number of species with values||15||3||19||488|
|Mean (SE) values of other attributes|
|Plant height (cm)||60 (9)||150 (25)||95 (14)||66 (1)|
|Number of species with values||30||20||20||726|
|Hemeroby (HK)||6·8 (0·12)||6·8 (0·48)||5·2 (0·56)||3·9 (0·08)|
|Number of species with values||21||6||8||466|
|Associations with annual plants (A)||0·59 (0·01)||0·22 (0·02)||0·15 (0·01)||0·15 (0·00)|
|Total number of species||30||20||20||902|
Species whose associates were mostly annuals and species found in places with high urban land cover both had a mean score for hemeroby of 6·8 (Table 5). This was significantly higher than the mean for frequent species in highly urban areas, but for the urban specialist category the significance was only borderline at 5%. In comparison with the average for all species, it therefore appeared that the urban specialists were more ruderal and hemerobic, and that the frequent species were more competitive and of intermediate hemeroby. These patterns are clearly apparent in Fig. 3. This shows that the highly urban species included a few highly hemerobic species, notably Buddleja davidii (several urban specialists did not have a value for hemeroby). From a British perspective, some of the highly hemerobic species towards the bottom left of the diagram are surprising, for example Ajuga reptans, Digitalis purpurea and Veronica serpyllifolia, which have a hemeroby value of 6 but are not confined to situations with high human impact.
Figure 3. Hemeroby in relation to the proportion of annuals and percentage urban land cover near where species were found; large dots signify species with high hemeroby (6–9), small dots denote the remainder.
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The pattern shown by highly ruderal species was somewhat more concentrated but the ruderals (in the sense of Grime 1979, 2001) were nevertheless spread widely along the annuality axis (Fig. 4). Few urban specialists were highly ruderal; only Senecio squalidus and Sisymbrium altissimum fell into this category. Highly ruderal species with very few annual associates were Cardamine flexuosa, Cardamine pratense, Geranium robertianum, Rhinanthus minor and Senecio aquaticus.
Figure 4. Ruderality in relation to the proportion of annuals and percentage urban land cover near where species were found; large dots signify species with high ruderality (i.e. 6·6–10·0, equivalent to R, R/CR, R/SR and R/CSR), small dots denote less ruderal species.
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When levels of urbanity were plotted against annuality and xenicity, the nature of the urban specialists was clear (Fig. 5). They had medium levels of annuality. Relative to their annuality, the proportion of associated neophytes was high. Arable species had a medium to high number of neophyte associates. Both cities and arable fields experience high human impact. However, in the British samples studied, arable specialists had in general higher xenicity than urban specialists.
Figure 5. Urbanity in relation to the proportions of annuals and neophytes; species are represented by differing symbols according to their urbanity.
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The internal consistency of the scales was measured by correlating species scores with mean associated values. For annuality the correlation was 0·954; for xenicity it was 0·888; for urban cover it was 0·815. The relation (not shown here) between A and MA(A) was curvilinear over only a small part of its range, as was the relation between X and MA(X). The relation between U and MA(U) was so curvilinear that MA(U) was shaped more like U3 than U. This was because much of southern Britain is urbanized, so that for U < 25%, U often did not measure whether the species occurred in cities but merely whether it occurred in southern Britain where urban land cover is greater. To eliminate the influence of these irrelevant species, a new variable:
was defined and used to derive a new associated-value variable MA(U25). This variable had a slightly higher correlation, 0·821, with U and lacked the tendency shown by MA(U) to give relatively high values to agrestal and aquatic plants.
The 20 most urban plants according to the original variables U and W, and the 20 with most urban associates, as measured by MA(U) and MA(U25) were, with only six exceptions, species that are indicative of the phytosociological classes Artemisietea vulgaris, Stellarietea mediae and Galio–Urticeteae (Table 6). The exceptions were Centranthus ruber, normally a plant of walls, Cerastium tomentosum and Symphoricarpos albus, which are garden plants usually found close to where they were planted, Vulpia myuros, which in central Europe grows in dry grasslands of the Koelerio–Corynephoretea, Cichorium intybus, found on waysides in Molinio–Arrhenatheretea grassland, and Acer platanoides, a large tree whose seedlings and saplings are found in urban areas near to where it is planted.
Table 6. Species that either had high urbanity, as measured by mean urban cover or frequency in highly urban 1-km squares, or which were strongly associated with species having high urbanity, together with their main phytosociological class. The Stellarietea have been divided into two orders, Polygono-Chenopodietalia (P) and Sisymbrietalia (S). The urban code specifies the presence (1) or absence (0) of the species in the list of 20 most urban plants according to the urban variables urbanity ( U ), fidelity to highly urban squares ( W ), associated urbanity ( MA ( U )) and associated urbanity above the 25% threshold ( MA ( U25 ))
|Species||Number of occurrences||Urban code||Class|
|(a) Urban specialists with many urban associates|
|Apera interrupta|| 23||1111||Stellarietea mediae (P) |
|Artemisia absinthium||171||1111||Artemisietea vulgaris|
|Buddleja davidii|| 40||1111||Galio-Urticetea|
|Conyza canadensis|| 75||1111||Stellarietea mediae|
|Diplotaxis tenuifolia|| 24||1111||Artemisietea vulgaris|
|Foeniculum vulgare|| 16||1001||Artemisietea vulgaris|
|Lactuca serriola|| 67||1111||Stellarietea mediae (S) |
|Melilotus albus|| 41||1111||Artemisietea vulgaris|
|Melilotus officinalis|| 95||1111||Artemisietea vulgaris|
|Oenothera glazioviana|| 16||1111||Artemisietea vulgaris|
|Reseda luteola||106||1011||Artemisietea vulgaris|
|Senecio squalidus||585||1111||Artemisietea vulgaris|
|(b) Urban specialists with fewer urban associates|
|Acer platanoides|| 26||1100||Querco-Fagetea|
|Anthemis cotula|| 38||1100||Stellarietea mediae (P) |
|Aster novi-belgii|| 58||1100||Galio-Urticetea|
|Calystegia pulchra|| 23||1100||Galio-Urticetea|
|Centranthus ruber|| 16||0100||Asplenietea trichomanis|
|Cerastium tomentosum|| 15||0100||(garden throw-out)|
|Lupinus × regalis|| 28||1100||Artemisietea vulgaris|
|Saponaria officinalis|| 25||1000||Galio-Urticetea|
|Solidago canadensis s.l. ||104||1100||Galio-Urticetea|
|Symphoricarpos albus|| 10||0100||(usually planted)|
|(c) Less urban species with many urban associates|
|Cichorium intybus|| 12||0010||Molinio-Arrhenatheretea|
|Crepis vesicaria|| 33||0001||Stellarietea mediae (S) |
|Diplotaxis muralis|| 28||0011||Stellarietea mediae (S) |
|Lepidium campestre|| 11||0010||Stellarietea mediae (S) |
|Melilotus altissimus|| 46||0011||Artemisietea vulgaris|
|Oenotheria biennis|| 22||0011||Artemisietea vulgaris|
|Papaver dubium||116||0010||Stellarietea mediae (P) |
|Sisymbrium altissimum|| 35||0011||Stellarietea mediae (S) |
|Sisymbrium orientale|| 38||0011||Stellarietea mediae (S) |
|Tanacetum vulgare|| 96||0001||Galio-Urticetea|
The major classes Artemisietea vulgaris (order Onopordietalia acanthii) and Galio–Urticeteae (order Lamio albi–Chenopodietalia) correspond, respectively, to plants that are characteristic of rather open habitats and plants that form dense, often clonal, vegetation. The assignment of Buddleja davidii, which colonizes open habitats but later forms thickets, to Galio–Urticeteae follows Mucina (1993). Plants corresponding to the Stellarietea mediae can be divided into two orders, Polygono–Chenopodietalia (basically agrestal) and Sisymbrietalia (annual-dominated communities of waysides and waste places). Of the normally agrestal species, Anthemis cotula, Apera interrupta and Papaver dubium occupied mainly or entirely non-agrestal habitats in our sample.