Urban green spaces – the effects of patch size and distance to the urban edge on vascular plant and bird species diversity

Green spaces in cities provide habitat to numerous vascular plant and bird species (Knapp et al. 2008, Kühn et al. 2004, Meffert & Dziock 2012, Melles et al. 2003) and may fulfill basic nature conservation functions (Niemelä 1999). However, little is known about patterns of species diversity and key factors that influence species numbers for different statuses, like overall, native or endangered species. Therefore, the aim of our study was to test for the effects of the size of green spaces and their distance to the urban edge on species numbers of vascular plants and birds. In particular, the following hypotheses were tested for urban green spaces in Hannover, Germany.

i) Total, native and endangered species numbers of vascular plants as well as birds increase with increasing patch size.

ii) Total, native and endangered species numbers of vascular plants as well as birds decrease with increasing distance to the urban edge.

We surveyed 32 green spaces identified by a stratified random selection, taking patch size and distance to the urban edge into account. Patch size varied from 0.7 ha to 72.3 ha, and distance to the urban edge from 190 m to 2872 m. The selected green spaces included forests, parks, cemeteries, allotments and fallow lands. A survey of vascular plants (self-established as well as planted) was conducted from June to August 2011 and March to May 2012. Birds were surveyed using line transects during the breeding season from March to June 2012. Correlation and regression analysis was conducted to detect the relationship between species numbers and patch size as well as distance to the urban edge.

A total of 1372 vascular plant species, including 577 native and 109 endangered species were found. In addition we identified a total of 83 bird species, including 79 native and 23 endangered species. Vascular plant and bird species numbers are not correlated to the distance of green spaces to the urban edge. In contrast, the total numbers as well as the number of native and endangered species of vascular plants and birds are significantly positively correlated to the size of urban green spaces (Table 1). This relationship can be best described by a logarithmic function (e.g. Figure 1 and Figure 2).

Our results suggest that the distance to the urban edge has little impact on the number of plant and bird species in green spaces. In contrast, size of green spaces in cities is a relevant factor determining vascular plant and bird species diversity.  Therefore, in growing cities planners should focus on the conservation of large green spaces to maintain high species numbers. Accordingly, in newly planned as well as shrinking cities it is necessary to concentrate on the establishment of large green patches in order to enhance species diversity.