Urbanization and homogenization – Comparing the floras of urban and rural areas in Germany
Introduction
Urbanization acts in many ways on existing biodiversity (Sukopp and Werner, 1983, Gilbert, 1989, Wittig, 1991, Collins et al., 2000, Pickett et al., 2001), e.g. by altering quality of air, water, and soil (Sukopp and Starfinger, 1999), temperature regime and rainfall patterns (Landsberg, 1981, Oke, 1982), habitat fragmentation and disturbance (Kowarik, 1995). Although urbanization results in native habitat destruction and is regarded as a major threat to biodiversity (Wilson, 1988, Thompson and Jones, 1999, McKinney, 2004a, Liu et al., 2003), cities are richer in plant species than surrounding areas (Walters, 1970, Haeupler, 1975, Klotz, 1990, Pyšek, 1993, Pyšek, 1998, Kowarik, 1995, Dobson et al., 2001, Blair, 2001, McKinney, 2002b, Araújo, 2003, Hope et al., 2003). This could in part be ascribed to the influx of alien species (McKinney, 2004b, McKinney, 2002b, Kühn et al., 2004b) both from intentional and unintentional introductions, but also due to natural factors as at least in some regions cities were built up in areas of natural heterogeneity which supports natural biodiversity (Kühn et al., 2004b).
Thus there are several contrasting processes that can be observed: (i) introduction of often ubiquitous alien plant species (i.e. biological invasion), (ii) decline or extirpation of native species, and (iii) a remaining high species richness of native species, based on those that are common. The increase in abundance and range expansion of common (alien and native) species and especially their translocation and the decrease in abundance and range contraction (or even extinction) of rare species will lead to biotic homogenization (Elton, 1958, McKinney, 2006), i.e. distinct biota will become more similar in species composition. However, ecological processes leading to biotic homogenization are not restricted to biological invasions but may result from other environmental modifications (McKinney, 2006, Olden and Poff, 2004a, Olden and Poff, 2003), such as urbanization. The degree of homogenization depends on taxonomic identities and of species in an assemblage as well as the rate and spatial pattern of species turnover (Olden and Poff, 2004b). In this account we do not focus on alien species as drivers of homogenization but on urbanization itself. There are some studies on homogenization for various taxa, e.g. vascular plants (Rejmánek, 2000, Weber and Pyšek, 2001, McKinney, 2004b, McKinney and Lockwood, 2001), snails (Cowie, 2001), freshwater faunas (Rahel, 2002) including fish (Rahel, 2000, Duncan and Lockwood, 2001), amphibians and mussels (Duncan and Lockwood, 2001), as well as marine biota (McKinney and Lockwood, 1999). However, little has been worked so far on the relationship between urbanization and homogenization prior to this special issue (e.g. Blair, 2001, on butterflies and birds).
In this paper, we focus on the spatial relationship between urbanization and taxonomic homogenization (sensu Olden et al., 2004) of local floras on a regional scale: we tested whether areas with high proportion of urban land cover are more similar to each other than areas with less or even little proportions of urban land cover with respect to vascular plant species classified into five not mutually exclusive groups: (i) all species, which comprise of (ii) only natives, (iii) all alien plant species which could be partitioned into those that immigrated (iv) prior to 1500 and (v) after 1500. In this way we examine alien species not as a driver of biotic homogenization but alien plant assemblages as a respondent to urbanization as well.
Section snippets
Data sources
Species composition in grid cells in Germany was taken from the database FLORKART maintained by the German Federal Agency for Nature Conservation. This is the largest and most comprehensive database on plant distribution in Germany as it incorporates several more regional databases and was collated from several mapping schemes involving thousands of volunteers and from literature reviews. Data entries are classified into three periods: pre-1950, 1950–1979, since 1980. Mapping started in the
Results
The average number of native species is higher in the “most urbanized” cells (mean native species richness 543.9 ± 79.5, N = 60) than in “less urbanized” cells (mean native species richness 524.6 ± 90.7, N = 1868) or “rural” cells (mean native species richness 507.6, N = 1099). The range of values of the two latter groups is higher due to a much larger sample size (Fig. 1A). Alien species provide similar results: They are more species rich in “most urbanized” cells (pre-1500 alien plant species: 112.8 ±
Discussion
This study revealed that urbanization has led to homogenization for native plant species and for pre-1500 alien plant species. Post-1500 alien plant species assemblages were more heterogeneous within “most urbanized” cells than in “less urbanized” or “rural” cells. This pattern could simply arise from a positive relationship between similarity indices and species richness. Wolda (1981) performed extensive simulations on similarity indices and found that there was a strong dependence of sample
Acknowledgements
We would like to thank Mike McKinney for inviting us to this special issue. The German Federal Agency for Nature Conservation, namely Hans Fink und Rudolf May, provided FLORKART and land use data. Herbert Sukopp pointed out that cities are usually located in areas below 300 m a.s.l. Mike McKinney, Petr Pyšek and two anonymous referees provided very valuable comments on the manuscript. Dan Michin improved our English.
This work was funded by the UFZ – Umweltforschungszentrum Leipzig-Halle through
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