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Urban reserves, like other protected areas, aim to preserve species richness but conservation efforts in these protected areas are complicated by high proportions of alien species. We examined which environmental factors determine alien species presence in 48 city reserves of Prague, Czech Republic. We distinguished between archaeophytes, i.e. alien species introduced since the beginning of Neolithic agriculture up to 1500 A. D., and neophytes, i.e. modern invaders introduced after that date, with the former group separately analysed for endangered archaeophytes (listed as C1 and C2 categories on national red list). Archaeophytes responded positively to the presence of arable land that was in place at the time of the reserve establishment, and to a low altitudinal range. In addition to soil properties, neophytes responded to recent human activities with the current proportion of built-up area in reserves serving as a proxy. Endangered archaeophytes, with the same affinity for past arable land as other archaeophytes, were also supported by the presence of current shrubland in the reserve. This suggests that for endangered archaeophytes it may have been difficult to adapt to changing agricultural practices, and shrublands might act as a refugium for them. Forty-six of the 155 neophytes recorded in the reserves are classified as invasive. The reserves thus harbour 67% of the 69 invasive neophytes recorded in the country, and particularly worrisome is that many of the most invasive species are shrubs and trees, a life form that is known to account for widespread invasions with high impacts. Our results thus strongly suggest that in Prague nature reserves there is a high potential for future invasions.
Alien plants, archaeophyte, Czech Republic, nature reserve, neophyte, plant invasions, red list, urban
Urbanization is the most dramatic form of natural habitat destruction making cities a rather hostile environment for natural wildlife. Conserving the native biodiversity in urbanized areas is therefore particularly challenging because remnants of natural habitats in urban areas are restricted to small and isolated patches. These often harbour fragmented populations of native plants and animals that face risks associated with small population sizes and pressures from heavily altered urban environments (
However, overall species diversity in entire cities has been intensively studied (
However, as in other protected areas, the conservation focus in nature reserves in urban areas is on the diversity of native species. Urban areas are where these “two diversities” come into the sharp conflict that results from the mismatch between human efforts to protect natural biodiversity and their activities that create ideal environments for alien species invasions. This matrix of urban development and nature reserves is therefore an appropriate testing ground to explore resistance patterns of natural vegetation against penetration by alien plants. It has been shown for other environments that nature reserves and protected areas possess some resistance against invasions (
This study analyses patterns of species richness of alien vascular plants in the city of Prague, Czech Republic. Based on the same data set as in
The city of Prague, Czech Republic, contains 88 nature reserves in an area of 496 km2 (
For each reserve, the total number of vascular plant species was recorded; only naturally occurring species were considered; planted shrubs and trees were excluded. The total number of plant species in all reserves was 1309 (about a half of the Czech flora;
Species were classified into native and alien, with archaeophytes and neophytes distinguished among the latter group (e.g.
Since some archaeophytes appear on national red lists despite their alien origin, (
The numbers of neophytes, archaeophytes and endangered archaeophytes recorded in reserves were used to calculate their proportions among total numbers of species, used in statistical analyses.
Explanatory variablesExplanatory variables describing factors that were assumed to affect the patterns of alien species occurrence in the reserves reflect geography (no. 1–7 below), habitat characteristics (8–9), substrate (10–12) and urbanization (13–16), and included: 1. reserve area (ha); 2. degree of reserve isolation (categorical: isolated, > 1 000 m from the closest reserve; clustered, < 300 m from the closest reserve; neighbouring, adjoining other reserves); 3. reserve age, expressed as years since the establishment; 4. aspect (north to north-east; plain; south-east and west; south and south-west; valley with all aspects present); 5. mid altitude, i.e., the mid value between minimum and maximum altitude; 6. altitudinal range, i.e. the difference between maximum and minimum altitude; 7. presence or absence of railway; 8. past habitat, reflecting the proportional representation of the following habitat types at the time of reserve establishment, with each type treated as an independent variable: forest, arable land, pasture, grassland, orchards, shrubland (including rocky outcrops) and built-up area; 9. present habitat, referring to the current state, using the same classification; 10. soil type (categorical variable with following levels: alluvial; acid; calcareous; neutral; acid and alluvial; acid and neutral; acid and calcareous; acid, neutral and calcareous); 11. presence or absence of bare rock; 12. presence or absence of a quarry; 13. minimum distance to natural habitat; 14. minimum distance to built-up area; 15. built-up perimeter, i.e. length of perimeter formed by built-up area; 16. natural perimeter, i.e. length of perimeter formed by other than built-up area.
The variables are the same as in
The response variables were proportional representations of species numbers of archaeophytes (mean 11.8%, range 2.4–21.5%), neophytes (mean 6.0%, range 2.0–17.4%) and endangered archaeophytes (mean 0.2%, range 0–1.3%) within all wild-growing species of vascular plants in each reserve. To prevent these proportions from species-poor reserves having undue influence, the proportions were weighted by the total numbers of species in each reserve (e.g.
Using trees, the data are successively split along coordinate axes of the predictors, represented by the environmental characteristics, so that at any node, the split is selected that maximally distinguishes the response variable, represented by the proportional representation of the species, in the left and the right branches (
Five-fold cross-validation was used to obtain estimate of regression accuracy for each tree, and the best tree, having the smallest cross-validated mean absolute error, was chosen for interpretation. The quality of the best tree was expressed as R2 value (
The absolute numbers of archaeophytes, neophytes and endangered archaeophytes closely correlated with their proportional representation in the species pool of each reserve (Spearman’s rank: archaeophytes rs = 0.76; neophytes rs = 0.70; endangered archaeophytes rs = 0.99). However, it cannot be a priori excluded that the alien species respond very differently than native species to the predictors; if so, it may not be appropriate to weigh the proportions of alien species by the total number of wild growing species in each reserve, as it could change some of the conclusions presented. To verify that the results on proportions are generic, all analyses were repeated using numbers of alien species as the response variable. Comparing to previous analyses on proportions, there were no changes in conclusions, and thus only the results on proportions are presented.
ResultsThe most important factors affecting the proportion of archaeophytes among all species in a reserve were mainly the presence of arable land before the reserve was established, but also its altitudinal characteristics (Figure 1A): archaeophytes were more abundant when the reserve had a low altitudinal range (Figure 2). The proportion of neophytes consistently increased with the proportion of present built-up area and depended on soil type: neophytes were more represented on alluvial and neutral to calcareous soils than in reserves with acidic soils (Figure 1B and 3).
As with all archaeophytes, the proportion of endangered archaeophytes among all species in a reserve positively depended on the past presence of arable land (Figure 1C and 4A), but there was also an important effect of shrubland. The proportion of endangered archaeophytes abruptly declined in reserves with less than 30% of currently present shrubland (Figure 4B).
Rank of importance of the individual predictor variables from boosted regression trees for archaeophytes A neophytes B and endangered archaeophytes C Variable importance is scaled to have values between 0 and 100. Results for the best trees with R2 = 0.71 A R2 = 0.69 B and R2 = 0.54 C White bars are predictors in which large values means positive effect, black barks in which large values mean negative effect, and grey bars are predictors with effect varying equivocally.
Partial dependence plot of representation of archaeophytes on altitudinal range. The partial dependence describes positive and negative dependences of the representation of archaeophytes on altitudinal range, averaging out the effects of the other predictor variables in the model.
Bivariate partial dependence plots of representation of neophytes on present built-up area and soil type. Otherwise as in Figure 2.
Partial dependence plots of representation of endangered archaeophytes on past arable land (A) and current shrubland (B). Otherwise as in Figure 2.
Prague nature reserves are important sanctuaries for native plants because they harbour approximately half of the native flora in the Czech Republic (
Our study shows that the numbers of alien species in urban nature reserves can be predicted by relatively few factors. We used a number of variables that reflected site geography, land-use history and connectivity, and propagule pressure, but only five of them were needed to explain from 54 to 71% of the overall variability. That habitats were the most important factor for archaeophytes corresponds well to the recent results of studies on regional determinants of plant invasions in the Czech Republic and Europe that show habitat identity to play a decisive role, more important than propagule pressure and climate (
The results also reflect that the two groups differ in long-term dynamics (
Neophyte introductions, however, continue at an accelerating rate in Europe (
Though it is is questionable whether species of alien origin should be a part of red lists, these species are perceived by botanists as elements of local nature, especially when they are rare, and many of them are typical of traditional cultural landscapes in Europe and considered to be species of cultural and historical importance (
Endangered archaeophytes, in addition to the same affinity for past arable land as other archaeophytes, are supported by the presence of current shrubland in a reserve. This seems to indicate that there are species among this group for which it might have been difficult in the past to adapt to changing agricultural practices and that new technologies might have negatively impacted their population dynamics (
Our results suppport previously raised concerns about studies that analyse patterns of regional plant invasions and lump all aliens regardless of the time of immigration. It has been repeatedly shown that archaeophytes and neophytes are ecologically distinct groups that differ in habitat affinities, historical dynamics, pollination patterns and response to climate (
In total, 15 threatened archaeophytes were recorded in nature reserves studied, eight of them considered critically endangered (C1 category): Conringia orientalis (occurring in 3 reserves), Erysimum repandum, Marrubium vulgare, Torilis arvensis (2), Adonis flammea, Misopates orontium, Polycnemum arvense and Polycnemum majus (1). Additional seven species belong to the endangered (C2) category: Adonis aestivalis (5), Veronica triloba (3), Anthriscus caucalis, Stachys annua, Veronica agrestis (2), Geranium molle and Sclerochloa dura (1).
On the other hand, 46 of the 155 neophytes recorded in reserves (Appendix 2) are classified as invasive (
We thank two anonymous reviewers for their comments, Laura Meyerson for editing our English, and Jan Pergl, Ivan Ostrý and Zuzana Sixtová for technical assistance. The study was funded by grants no. 206/09/0563 from the Czech Science Foundation, and long-term research plans no. AV0Z60050516 (from the Academy of Sciences of the Czech Republic), and no. MSM0021620828, and project no. LC06073 (both from the Ministry of Education, Youth and Sports of the Czech Republic). Petr Pyšek acknowledges support from by the Praemium Academiae award (AV0Z60050516) from the Academy of Sciences of the Czech Republic.
Appendix 1. Numbers of species in Prague nature reserves and characteristics of reserves. Ttotal includes both native and alien species. Only characteristics of the reserves that affect the proportion of archaeophytes and neophytes sre shown (see Methods for details). Soil types present in a reserve: al – alluvial; ac – acid; ca – calcareous; n – neutral.
Reserve | Species numbers | Reserve characteristics | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Total | Archaeophytes | Neophytes | Archaeophytes endangered | Age (yrs) | Area (ha) | Altitudinal Range (m) | Past arable (%) | Present shrubland (%) | Present built-up (%) | Soil type | |
1. Baba | 368 | 67 | 29 | 1 | 21 | 7.3 | 80 | 0.00 | 97.68 | 0.00 | ac |
2. Barrandovské skály | 160 | 14 | 14 | 1 | 21 | 11.6 | 100 | 0.00 | 51.86 | 0.05 | ca |
3. Bažantnice v Satalicích | 158 | 24 | 17 | 0 | 52 | 15.8 | 10 | 0.00 | 0.00 | 0.00 | ac |
4. Bohnické údolí | 202 | 12 | 10 | 0 | 21 | 4.6 | 60 | 0.00 | 0.00 | 0.00 | ac |
5. Podhoří | 363 | 57 | 30 | 2 | 21 | 8.4 | 75 | 0.00 | 57.86 | 0.00 | ac |
6. Zámky | 379 | 64 | 23 | 1 | 21 | 5.2 | 60 | 0.00 | 51.05 | 0.00 | ac |
7. Branické skály | 160 | 21 | 13 | 0 | 35 | 9.1 | 50 | 0.00 | 48.02 | 0.00 | ca |
8. V Hrobech | 187 | 24 | 6 | 0 | 15 | 1.3 | 10 | 0.00 | 98.46 | 0.00 | ac |
9. Šance | 225 | 9 | 6 | 0 | 21 | 116.8 | 177 | 0.00 | 0.43 | 0.00 | ac |
10. Čimické údolí | 341 | 49 | 23 | 1 | 35 | 11.2 | 30 | 9.17 | 8.90 | 0.00 | ac |
11. Dalejský profil | 280 | 46 | 18 | 1 | 21 | 22.8 | 50 | 0.00 | 19.32 | 0.00 | ac-ca |
12. Divoká Šárka | 683 | 104 | 47 | 4 | 39 | 25.4 | 105 | 2.56 | 32.07 | 1.58 | n |
13. Dolní Šárka | 248 | 29 | 12 | 1 | 21 | 6.2 | 70 | 0.00 | 65.69 | 0.00 | n |
14. Homolka | 192 | 11 | 4 | 0 | 21 | 13.5 | 60 | 0.00 | 33.90 | 0.00 | ca |
15. Hrnčířské louky | 221 | 15 | 10 | 0 | 15 | 29.3 | 20 | 0.17 | 44.27 | 0.00 | ac-al |
16. Chuchelský háj | 271 | 21 | 11 | 0 | 21 | 19.8 | 90 | 0.00 | 0.00 | 0.00 | n-ca |
17. Jabloňka | 158 | 14 | 17 | 0 | 35 | 1.3 | 70 | 0.00 | 100.00 | 0.00 | ac-n |
18. Jenerálka | 357 | 44 | 10 | 2 | 35 | 1.5 | 30 | 0.00 | 35.76 | 0.00 | ac |
19. Zlatnice | 188 | 12 | 13 | 0 | 35 | 3.3 | 50 | 0.00 | 0.00 | 0.00 | ac-n |
20. Nad mlýnem | 266 | 32 | 11 | 0 | 35 | 4.0 | 40 | 15.66 | 8.84 | 0.00 | ac |
21. Klapice | 254 | 6 | 6 | 0 | 15 | 16.2 | 106 | 0.00 | 11.63 | 0.00 | ca |
22. Královská obora | 362 | 54 | 63 | 0 | 15 | 104.5 | 42 | 0.00 | 26.78 | 3.10 | al |
23. Lochkovský profil | 205 | 16 | 4 | 1 | 15 | 39.1 | 95 | 0.00 | 29.49 | 0.00 | ca |
24. Meandry Botiče | 117 | 16 | 11 | 0 | 35 | 6.7 | 15 | 0.00 | 93.58 | 0.60 | ac-al |
25. Milíčovský les a rybníky | 435 | 33 | 17 | 0 | 15 | 93.3 | 30 | 0.00 | 10.07 | 0.20 | ac-al |
26. Modřanská rokle | 333 | 54 | 29 | 0 | 15 | 124.9 | 80 | 0.00 | 3.13 | 0.09 | ac |
27. Okrouhlík | 137 | 23 | 9 | 0 | 21 | 0.6 | 20 | 0.00 | 0.00 | 0.00 | ac |
28. Pitkovická stráň | 173 | 14 | 4 | 0 | 34 | 0.5 | 20 | 0.00 | 3.92 | 0.00 | ac |
29. Počernický rybník | 251 | 38 | 21 | 0 | 15 | 41.8 | 5 | 0.00 | 69.18 | 0.00 | ac |
30. Podbabské skály | 264 | 51 | 19 | 1 | 21 | 0.8 | 30 | 0.00 | 35.71 | 0.00 | ac |
31. Cholupická bažantnice | 264 | 51 | 19 | 1 | 21 | 13.8 | 12 | 0.00 | 0.00 | 0.00 | ac |
32. Obora v Uhříněvsi | 326 | 52 | 24 | 0 | 21 | 34.9 | 22 | 0.00 | 4.33 | 0.34 | ac-al |
33. Prokopské údolí | 606 | 84 | 29 | 8 | 25 | 101.5 | 110 | 0.78 | 39.33 | 0.38 | ac-ca |
34. Radotinské údolí | 552 | 64 | 22 | 3 | 50 | 103.3 | 80 | 6.90 | 3.79 | 0.00 | ca |
35. Rohožník a lom v Dubči | 332 | 48 | 18 | 0 | 15 | 3.5 | 20 | 0.00 | 8.70 | 0.00 | n |
36. Tiché údolí a Roztocký háj | 381 | 26 | 15 | 0 | 52 | 114.2 | 105 | 0.46 | 5.12 | 0.00 | n-ca |
37. Sedlecké skály | 211 | 22 | 5 | 0 | 21 | 7.5 | 70 | 0.00 | 37.33 | 0.00 | ac |
38. Slavičí údolí | 263 | 16 | 7 | 0 | 15 | 38.3 | 105 | 0.00 | 0.00 | 0.00 | ac-ca |
39. Trojská | 195 | 42 | 15 | 0 | 21 | 1.3 | 35 | 0.00 | 100.00 | 0.00 | ac |
40. Údolí Kunratického potoka | 283 | 21 | 12 | 0 | 15 | 152.0 | 80 | 0.00 | 10.55 | 0.13 | ac |
41. Vizerka | 350 | 33 | 12 | 0 | 15 | 3.1 | 40 | 0.00 | 25.89 | 0.00 | ac |
42. Havránka | 401 | 59 | 34 | 0 | 21 | 4.2 | 50 | 0.00 | 58.10 | 0.00 | ac-n |
43. Zmrzlík | 335 | 46 | 15 | 0 | 15 | 16.4 | 80 | 30.46 | 10.28 | 0.01 | ac-ca |
44. Klánovický les | 435 | 46 | 35 | 0 | 21 | 225.5 | 20 | 0.00 | 0.00 | 0.00 | ac |
45. Obora Hvězda | 460 | 55 | 29 | 1 | 15 | 84.2 | 50 | 0.00 | 9.40 | 0.77 | ac |
46. Staňkovka | 171 | 7 | 5 | 0 | 15 | 44.5 | 160 | 0.00 | 3.76 | 0.00 | ac |
47. Vinořský park | 202 | 27 | 23 | 0 | 21 | 34.1 | 20 | 0.00 | 0.00 | 0.00 | ac |
48. Xaverovský háj | 268 | 27 | 14 | 0 | 21 | 97.2 | 30 | 0.00 | 3.01 | 0.00 | ac |
Appendix 2. List of invasive neophytes recorded in Prague nature reserves. Species are ranked according to the decreasing number of reserves in which they were recorded (n = 48). Plant names according to
Species | Family | Life form | Number of reserves invaded |
---|---|---|---|
Impatiens parviflora | Balsaminaceae | Annual | 40 |
Robinia pseudacacia | Fabaceae | Tree | 38 |
Symphoricarpos albus | Caprifoliaceae | Shrub | 24 |
Mahonia aquifolium | Berberidaceae | Shrub | 21 |
Quercus rubra | Fagaceae | Tree | 18 |
Sisymbrium loeselii | Brassicaceae | Annual | 18 |
Solidago canadensis | Asteraceae | Perennial | 18 |
Conyza canadensis | Asteraceae | Annual | 17 |
Echinops sphaerocephalus | Asteraceae | Perennial | 17 |
Syringa vulgaris | Oleaceae | Shrub | 17 |
Epilobium ciliatum | Onagraceae | Perennial | 16 |
Geranium pyrenaicum | Geraniaceae | Perennial | 14 |
Solidago gigantea | Asteraceae | Perennial | 14 |
Bidens frondosa | Asteraceae | Annual | 13 |
Galeobdolon argentatum | Lamiaceae | Perennial | 13 |
Galinsoga parviflora | Asteraceae | Annual | 12 |
Galinsoga quadriradiata | Asteraceae | Annual | 12 |
Lycium barbarum | Solanaceae | Shrub | 11 |
Heracleum mantegazzianum | Apiaceae | Monocarpic | 10 |
Cytisus scoparius | Fabaceae | Shrub | 10 |
Veronica persica | Scrophulariaceae | Annual | 10 |
Parthenocissus quinquefolia | Vitaceae | Woody vine | 9 |
Amaranthus retroflexus | Amaranthaceae | Annual | 8 |
Juncus tenuis | Juncaceae | Perennial | 8 |
Matricaria discoidea | Asteraceae | Annual | 8 |
Pinus strobus | Pinaceae | Tree | 8 |
Reynoutria japonica | Polygonaceae | Perennial | 7 |
Bunias orientalis | Brassicaceae | Perennial | 5 |
Rumex thyrsiflorus | Polygonaceae | Perennial | 4 |
Aster novi-belgii agg. | Asteraceae | Perennial | 4 |
Digitalis purpurea | Scrophulariaceae | Monocarpic | 4 |
Populus ×canadensis | Salicaceae | Tree | 4 |
Virga strigosa | Dipsacaceae | Monocarpic | 4 |
Ailanthus altissima | Simaroubaceae | Tree | 3 |
Telekia speciosa | Asteraceae | Perennial | 3 |
Aster lanceolatus | Asteraceae | Perennial | 2 |
Elodea canadensis | Hydrocharitaceae | Aquatic | 2 |
Helianthus tuberosus | Asteraceae | Perennial | 2 |
Impatiens glandulifera | Balsaminaceae | Annual | 2 |
Oenothera biennis | Onagraceae | Monocarpic | 2 |
Rhus hirta | Anacardiaceae | Shrub | 2 |
Amorpha fruticosa | Fabaceae | Shrub | 1 |
Aster ×salignus | Asteraceae | Perennial | 1 |
Lupinus polyphyllus | Fabaceae | Perennial | 1 |
Sedum hispanicum | Crassulaceae | Perennial | 1 |
Veronica filiformis | Scrophulariaceae | Perennial | 1 |