Abstract
ABSTRACTAim Although urban areas only occupy c. 2.8% of the earth's land surface, urbanization threatens biodiversity as areas of high human population density often coincide with high biodiversity. Therefore, nature conservation should concentrate on both remote areas and densely populated regions. Protecting rare plant species in rural and urban areas can contribute to the protection of biodiversity. We therefore need to understand why species are rare. Studies on causes of rarity often concentrate on either plant traits or extrinsic threats (such as habitat fragmentation or nitrogen enrichment). However, there are only a few studies that combine traits and extrinsic threats, although such analyses might clarify causes of rarity. We assessed how the affinity of vascular plant species to urban land use (‘urbanity’) interacts with plant traits in determining species frequency.Location Germany, resolution c. 12 km × 11 km.Methods Species with a low frequency may be rare because they occur in rare habitats or because of other reasons, although their habitat is frequent. Therefore, we calculated the frequency of species corrected for habitat frequency, i.e. relative species frequency. We explained relative species frequency by the interactions of species traits and species affinity to urban land use using generalized linear models. Simultaneous autoregressive error models controlled for phylogenetic relationships of species.Results Relative species frequency depends on species affinity to urban land use, independent of the different interactions between traits and urbanity used as predictors. The higher the urbanity the higher is species frequency. Urbanity interacts with species preferences towards temperature and soil acidity. Moreover, dispersal, nitrogen preferences and origin explain relative species frequency, amongst others.Main conclusions Many rare species, especially those preferring cool or acidic habitats might already have disappeared from urban areas. Analyses that combine species traits and environmental effects can explain the causes of rarity and help to derive better conservation strategies.
Highlights
Urban areas only cover around 2.8% of the earth’s land surface (Millennium Ecosystem Assessment, 2005) but more than 50% of the total world population lives in urban areas (United Nations, 2008)
Relative species frequency depends on species affinity to urban land use, independent of the different interactions between traits and urbanity used as predictors
Urban areas have a strong impact on biodiversity, as areas of dense human settlement and high biodiversity often coincide (e.g. Araújo, 2003), i.e. urban areas are often richer in plant species than their rural surroundings (e.g. Hope et al, 2003; Kühn et al, 2004a)
Summary
Urban areas only cover around 2.8% of the earth’s land surface (Millennium Ecosystem Assessment, 2005) but more than 50% of the total world population lives in urban areas (United Nations, 2008). Araújo, 2003), i.e. urban areas are often richer in plant species than their rural surroundings Urban areas contain many rare plant species (Kühn et al, 2004a; Schwartz et al, 2006). The high urban species richness is resulting partly from the fact that urban areas are very heterogeneous and often include habitat types that are rare or absent in the surrounding area (Sukopp & Starfinger, 1999; Godefroid & Koedam, 2007). We need strategies for nature conservation in urban areas that sustain a high species diversity including rare and endangered species (Schwartz et al, 2006). We need to know why species are rare and how urban land use influences rarity
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