Introduction

Green spaces in cities, such as parks, allotments, cemeteries, and wastelands, are important components of green infrastructure (EC – European Commission 2013) and provide habitats for numerous animal and plant species (Cornelis and Hermy 2004; Angold et al. 2006). Over the last decades, many studies have shown high species richness and abundance in urban green spaces. This has particularly been the case for studies on birds (e.g. Husté and Boulinier 2007; Carbó-Ramírez and Zuria 2011) and plants (e.g. Fischer et al. 2013; Matthies et al. 2013), but also for other taxa such as mammals (e.g. Garden et al. 2007), butterflies (e.g. Snep et al. 2011), and carabid beetles (e.g. Knapp et al. 2008). These relationships have been found in several cities in Europe (e.g. Bräuniger et al. 2010; Fischer et al. 2013), Asia (e.g. Saito and Koike 2013; Koh and Sodhi 2004), and North America (e.g. Oliver et al. 2011; Turner et al. 2005). In order to clarify the relevance of urban green spaces for nature conservation it is, in addition to total species richness, valuable to assess the number of endangered and native species (e.g. Pyšek et al. 2004). The richness of plant and animal species in urban green spaces has often been studied in relation to alien species (McKinney 2006; La Sorte et al. 2014). For instance, Pyšek et al. (2004) found that green spaces support a large number of non-native plant species in the city of Plzen, Czech Republic. DeCandido (2004) documented a decline in plant species richness in a park in New York City from 1947 to 1994, which was partly due to increasing numbers of exotics. High numbers of non-native species have also been documented for birds (e.g. Shwartz et al. 2008; Platt and Lill 2006) and selected invertebrates (e.g. Tonietto et al. 2011; Vilisics and Hornung 2009). Therefore, it has often been argued that high species numbers in urban green spaces are caused by high numbers of alien species (e.g. McKinney and Lockwood 2001). Nevertheless, a variety of other studies found evidence that also native species (Wania et al. 2006; Kühn et al. 2004) and endangered species (Meffert and Dziock 2012; Matthies et al. 2015) reach high species numbers in urban green spaces. These findings consistently show that different species groups within taxonomic groups contribute to the species richness in urban green spaces (cf. McKinney 2008). Urban green spaces can be seen as habitat islands within the urban matrix. A key finding of the urban habitat island approach is that green spaces show patterns of species-area relationships as observed in island biogeography (e.g. MacGregor-Fors et al. 2011). The species-area effect explains how larger areas usually facilitate greater species richness (MacArthur and Wilson 1967). For urban green spaces, an increasing number of species with increasing patch areas has been shown for vascular plants (Angold et al. 2006; Li et al. 2006), birds (van Heezik et al. 2013; Carbó-Ramírez and Zuria 2011; Chamberlain et al. 2007; Husté et al. 2006), and mammals (Hodgkison et al. 2007). In contrast, no significant species-area relationships have been found for butterflies and carabid beetles in urban green spaces (Bräuninger et al. 2010; Lizée et al. 2016).