To analyse the effects of reduced green space management in urban areas on the biodiversity of insects, we compared intensely mowed lawns (mowed 12 times per year) with meadows under reduced maintenance (mowed only twice per year) in the city of Tübingen (Baden-Württemberg, Germany). Over the entire field season, 177 wild bee individuals representing 43 species were caught using sweep nets. Areas with reduced maintenance showed significantly higher total species numbers and biodiversity indices. Our research supports the initiative “Bunte Wiese (Colourful Meadow) - Species Diversity in Public Greenspaces” of the University of Tübingen, which is campaigning for the enhancement of species diversity in public urban greenland areas by reorganising intensive mowing into a “twice a year” programme.

Colourful meadow, conservation, grassland, meadow, mowing, urban ecology, urban green space, wild bees

Urbanisation is one of the major environmentally relevant phenomena of our time. The expansion of urban areas is rapidly increasing. In western Germany, the areas settled by humans has increased by about 140% in the past fifty years (Kompakt 2011, Russel 2005). 13.6% of the area of Germany is covered by settlements and infrastructure (Bundesamt 2016). Fragmentation and destruction of natural areas occurs as a result of these developments (BMU 2008). Even in the countryside, intense agricultural use leads to monocultures and the loss of biological diversity (Landschaften 2006). Green spaces usually have a wide range of flowering aspects and offer a large number of niches for animals (Briemle and Fink 1993). The loss of natural grassland endangers these functions (García 1992). The intensification of grasslands (fertilization, high mowing intensity, ploughing) has a negative influence on the diversity of flowering plants and the animals depending on these resources (Haufe et al. 2015, Schuch et al. 2012).

Urban areas can offer classic nature, providing a wide range of different small habitats with positive impacts on, for instance, flower-visiting insects (Matteson et al. 2013). This explains the current focus of conservationists on these easily introduced replacement biotopes (Bischoff 1996). According to Westrich (1989), half of the German wild bee species can be found in urban areas. For example, 258 bee species have been recorded in Stuttgart (Schwenninger 1999). Although urban natural spaces cannot provide the same functions, continuity, and habitat qualities as natural areas (Müller 2005), the protection of urban nature must form an important part of all biodiversity projects (Müller 2005).

We have investigated the influence of maintenance reduction on public grasslands in the city of Tübingen (Germany, Baden-Württemberg) on the occurrence of flower-visiting wild bees. We assume that the simple reduction of the lawn mowing frequency compared with an intense monthly mowing regime in cities can make an important contribution to the international efforts to reduce the loss of biodiversity (Ade et al. 2012, Hiller and Betz 2014, Kricke et al. 2014, Unterweger et al. 2012, Unterweger and Betz 2014).

If mowing events occur too frequently and at inappropriate times, they can cause significant harm on the biodiversity of both plants and insects (Landschaften 2006). In particular wild bees (Hymenoptera, Apoidea) are highly sensitive to mowing (Buri et al. 2014, Schweitzer et al. 2012). The fast and highly engineered mowing of a flowering meadow at a warm day can kill about 50% of an entire insect population (Hemmann et al. 1987, Oppermann and Claßen 1998) and, for example, kill up to 90,000 honeybees per hectare (Fluri et al. 2000). This is possible as highly engineered mowing entails huge machines, high mowing speeds mowing and a high ground coverage per hour.so the whole foraging ground of a bee population can be cut within a few hours. Mowing affects the life on a meadow by (1) the mowing process itself, (2) the preparation for loading (swathing) and (3) the loading process (Di Giulio et al. 2001). Moreover, (4) the change of the microclimate including humidity (Albrecht et al. 2010) after mowing also contributes to the loss of faunistic biomass on meadows.

Unimproved flower-rich grassland is one of the most important habitats for bumble bees. However, in western Europe, it has been largely lost to agriculture (Goulson et al. 2008). Its restoration can boost bumble bee populations (Carvell 2002, Carvell et al. 2007). In addition, the loss of food sources leads to a decline of wild bees as they are no longer able to feed their offspring efficiently (Buri et al. 2014). Albrecht et al. (2010) have shown the positive effect on wild bees resulting from ecological improvements in meadows (transformation into two times/year mown hay meadows) that support species richness and ecological functions. Even in private gardens, more nature and less care provide more biodiversity by offering nesting sites for bees (Lindemann-Matthies and Marty 2013).

In 2010, both students and employees of the University of Tübingen founded a pressure group to support national and international aims to protect biodiversity (Unterweger et al. 2012). This group called “Initiative Bunte Wiese” (“The colourful meadow initiative”) has been instrumental in persuading decision makers to improve the maintenance of inner urban green areas with respect to conservation issues. This improvement involves (1) the reduction of mowing events towards only twice a year, (2) the use of bar mowers and (3) the removal of the mown grass from the surface (Unterweger et al. 2013, Unterweger and Braun 2015). The results of this management reduction have been evaluated in several research projects. We have investigated grasshoppers (Hiller and Betz 2014), true bugs (Unterweger 2013), beetles (Ade et al. 2012) and butterflies (Kricke et al. 2014). All these investigations have revealed a significant positive impact of reduced grassland maintenance towards species diversity and the occurrence of rare or endangered species.

In the present work, we investigate the influence of a reduced mowing regime on the diversity of flower-visiting wild bees in urban green spaces. We hypothesize that (1) endangered species are strongly linked to areas with reduced maintenance and (2) the number of both species and individuals is significantly higher in these areas.

Our results indicate that a reduction in mowing intensity has impacts on the wild bee fauna at various levels. Figure 2 shows that the reduced mowing regime results in a significant increase of wild bee species diversity. Buri et al. (2014), Goulson et al. (2008) and others have found that the abundance and richness of wild bee species significantly increases in meadows in which refuges were left uncut. The positive effects of undisturbed urban green spaces on wild bees has also been pointed out in Kutschbach-Brohl et al. (2010). This has also been confirmed for other insect orders such as beetles (Ade et al. 2012), grasshoppers (Hiller and Betz 2014), butterflies (Kricke et al. 2014) and true-bugs (Unterweger and Betz 2014). One reason for the observed differences between the intensely and extensively mowed sites can be seen in the direct effect that a single mowing event can have by reducing the number of individuals of wild bees by up to 50% (Hemmann et al. 1987, Oppermann and Claßen 1998).

Even the number of individuals of more common species can rapidly decline in areas of high mowing intensity. As seen in Table 1, single captures of species (i.e. species that could be detected only once during the investigation period) were made 12 times on areas with reduced maintenance and only four times on lawns over the whole sampling time. Six endangered species were found on the research areas and exclusively occurred on study plots with reduced maintenance. This shows the potential that a reduction of mowing intensity in public green areas can have for the conservation and maintenance of sub-populations of endangered wild bee species (Klaus 2013).

The number of sampled species had its maximum in May and August (IV = 15, V = 28, VI = 15, VII = 13, VIII = 20, IX = 11), a finding that indicates that mowing before the end of May has the greatest effect on urban wild bee populations. The importance of unmown summer meadows follows from the second peak in species richness in August. Wild bees often show a high preference for certain plant species and will benefit from the long-term reduction of maintenance (Weiner et al. 2011). It seems that cutting once per year would support wild bees the most. Nevertheless two mowing events per year has less impact than monthly mowing on the abundance and diversity of entomophilous flowering plants, which is important for nectar feeding insects.

Our results further support the conclusions drawn from studies of the initative “Bunte Wiese Tübingen” on other insect orders such as Orthoptera, Hemiptera, Coleoptera and Lepidoptera showing that a reduction of the maintenance of public urban green spaces supports the diversity of insects. Such actions are relatively easy to achieve in accord with local policy makers and form an effective way of meeting the demands of the global aspiration to stop the loss of biodiversity. The reduction of maintenance and the establishment of natural (infrequently, rather than intensely, mowed) green spaces and waysides can have a significant impact on mitigating the biodiversity crisis, even in our highly populated and highly degraded areas (low abundance and diversity numbers reported here indicate reduced mowing can lessen the impacts of urbanization but does not cure them) and, at the same time, increase the awareness of ecological problems occurring in urban human populations.

The English of this manuscript was corrected by Dr. Theresa Jones. Hans Schwenninger (Stuttgart) verified the species identifications. We thank the editor and our anonymous reviewers for their helpful comments.