Bumblebees and butterflies in green structure elements in Malmö, Sweden

The hypothesis that green structure elements that were designed to support biodiversity have higher abundances and a higher species richness of bumblebees and butterflies was not confirmed. There are serval reason that can explain this fact. The objects established within the BiodiverCity project had often a more species rich design, but this design was not oriented towards supporting pollinator diversity. The chosen plant material was in some cases (e.g. inner yards) not in particular attractive for pollinators. There was also no specific focus on native species, which are important for the urban fauna (Berthon et al. 2021). It can be concluded that a general approach to create more species rich green structure elements does not necessarily support bumblebees and butterflies, if the design is not directed towards these species groups. Kirk et al. (2021) emphasises the need to consider biodiversity aspects at the start of a project for being successful. This applies also for projects aimed to support pollinators. Besides a lack of consideration or knowledge about habitat requirements of bumblebees and butterflies, other factors influenced this outcome. Butterflies occur in low abundances in many green structure elements, but are more abundant in more semi-natural habitat types (grasslands, sown wildflower meadows or verges; more below). Still, certain conventional green infrastructure types can be very attractive for bumblebees as flowerbeds with flowering plants attractive for this species group (for example lavender). Thus, there were both attractive (e.g. flowerbeds) and unattractive (e.g. road verges, lawns) conventional green structure elements and attractive (e.g. sown wildflower areas) and unattractive (e.g. inner yards) green structure elements designed for biodiversity. These outweighed each other to a certain degree. Finally, the number of green structure elements designed for biodiversity was limited and heterogeneous, which influenced the results of this study.

The species numbers recorded for bumblebees and butterflies in this study were low. The species observed represented a sub-set of mostly very common species or species found in an urban context in other comparable studies in Sweden (e.g. Ahrné et al. 2009; Öckinger et al. 2009; Gunnarsson and Federsel 2014, Johansson et al. 2018; Aguilera et al. 2019; Persson et al. 2020). Regarding bumblebees, it was striking in particular that otherwise comparatively common species, such as B. hortorum, B. pasquorum and B. pratorum, were not found. One reason for this low bumblebee species richness was the type of green structure investigated in this study, where the focus was on a variety of typical green structure elements in city areas with a higher density of buildings and population. This excluded certain habitats not present in the target areas, such as larger parks, many urban semi-natural habitat types known to be species-rich (such as ruderal areas) and certain types of gardens (allotment gardens, private gardens of detached houses). Most other studies in Sweden investigating bumblebees, or more broadly pollinators, in urban areas have either included gardens/allotment gardens (Gunnarsson and Federsel 2014) or focused in particular on these (Ahrné et al. 2009; Persson et al. 2020). Gardens have been shown to support higher species numbers of bumblebees than green structure elements other than flowerbeds (Gunnarsson and Federsel 2014). However, Persson et al. (2020), who studied 14 rural and 39 urban gardens in and around Malmö (Sweden), did not observe B. pasquorum in any of the urban gardens, and observed B. pratorum in only one and B. hortorum in only three of the 39 urban gardens. In general, bumblebee diversity in that study was lower in urban gardens than in rural gardens (Persson et al. 2020).

Another reason for the low bumblebee species numbers observed in the present study was the comparatively high degree of urbanisation in the study areas. Areas with a high degree of urbanisation have been shown to support fewer bumblebee species (e.g. Bates et al. 2011; Persson et al. 2020). Comparisons with studies carried out in rural areas in the Malmö region (Scania) indicated that the species pool of the region as regards bumblebees is much larger. Söderman et al. (2016) observed 17 different bumblebee species and Carrié et al. (2018) 21 different species. The bumblebees species found in the present study thus represent around one-third of the regional species pool in Scania (Carrié et al. 2018).

While bumblebee species numbers were low, abundances in certain habitats were high. Abundance of nectar resources was the most important factor affecting individual numbers of bumblebees, followed by the diversity of flowering plant species available. This confirms previous findings in similar studies (e.g. Ahrné et al. 2009; Gunnarsson and Federsel 2014; Foster et al. 2017). That means that flowerbeds with few, but attractive nectar sources, such as lavender and sage, and sown habitats rich in several flowering species can both attract a number of bumblebees, even in very urban areas. This was also seen for the biodiverse green roofs included in this study, which offered abundant nectar sources at least during parts of the season. Even road verges in inner-city parts were used for foraging by bumblebees, when the sward height allowed clover species to flower.

Butterfly species numbers observed in this study were very low. Ten species were found (Pieris napi and P. rapae were both present, but could not always be separated in the field and were therefore considered together). The species observed represented a sub-set of the most common species found by Öckinger et al. (2009) and Aguilera et al. (2019), who investigated parks, ruderal sites and other green spaces in Malmö. The most notable finding was the absence of common grassland species such as Coenonympha pamphilus. While bumblebees were attracted by a high cover of flowering vegetation, even if this was provided by few plants species, this was not the case for butterflies in this study. Butterfly species numbers and abundances were positively affected by flowering species diversity, as previous studies have shown (e.g. Nagase et al. 2019) and by the size of the green structure element (Soga and Koike 2012; Sing et al. 2016). The size effect was not identified for bumblebees. This means that is more difficult to attract butterflies to small areas, even if attractive nectar resources are provided by a limited number of plant species. The low species richness for butterflies in the urban areas studied was expected (see e.g. Blair and Launer 1997; Matsumoto 2015; Ramírez-Restrepo and MacGregor-Fors 2017), but an unexpected finding was that even very common species were absent. The reason for the extreme low diversity of butterflies, as for bumblebees, was the high degree of urbanisation, which resulted in a limited number of urban habitat types in urban green structures of comparable small size. The high intensity of management of certain habitats studied, for example most road verges, further reduced the availability of crucial resources. High management intensity has been shown to reduce butterfly diversity in urban areas (Aguilera et al. 2019). Two other factors probably also affected butterfly species numbers negatively. One was the almost total lack of areas with high grass vegetation, which is important for several common grassland butterflies. The other was lack or low availability of larval host plants or larval habitat. One butterfly species found, Zygaena filipendulae, is classified as nearly threatened in Sweden (SLU 2020), although the species is commonly found in Malmö (Öckinger et al. 2009; Aguilera et al. 2019). The butterfly species found in this study represented about one fifth (Hammarstedt 1996) to on eighth (County Administration Board in Scania 2015) of the regional (Scania) species pool.

Abundance of butterflies was considerably lower (by around 33%) than that of bumblebees. This is not always the case, since Haaland and Gyllin (2010) found more butterflies than bumblebees in peri-urban and rural green structures using the same survey methodology for both species groups in the same areas. Thus, abundances of these two groups in relation to each other are context-dependent. Bumblebees appeared to be more often able to use resources in parts of the city where butterflies were mostly absent, for example intensively managed road verges, lawns and inner yards, but butterflies also used green roofs and flowerbeds to a lower degree.

Planted flowers and shrubs and wild flowers (self-seeded, sown or planted, e.g. on green roofs) were used as a food resource by both butterflies and bumblebees, but to differing degrees. Bumblebees visited planted herbs such as lavender, sage and sea holly more often than butterflies. A notable observation was high use of thistles by butterflies. Previous studies have shown use of both non-native and native plants by butterflies and bumblebees (e.g. Garbuzow and Ratnieks 2013; Rollings and Goulson 2019). Some have even found higher use or higher abundances of butterflies in relation to exotic plants (Shapiro 2002; Bergerot et al. 2010; Majewska et al. 2018). The latter was not confirmed in the present study.

The results obtained in this study show that the scope to support rich bumblebee and butterfly fauna in inner city areas may be limited, even when providing habitats with higher plant diversity than usual. The vision of a dense green city, which offers high liveability and at the same time supports biodiversity and related ecosystem services may be an ideal formulated in the context of densification, but might be difficult to achieve regarding biodiversity. Dense city areas are often characterised by a lack of habitats that can potentially support biodiversity, such as grasslands, ruderal areas, large parks, private gardens and allotment gardens (see e.g. Tappert et al. 2018). This was also the case in the areas investigated here. Potential for green space is particularly limited in compact city areas, but ‘biodiversity in cities needs space’ according to Beninde et al. (2015). Urban green spaces, especially less formal spaces, are subject to constant changes in size and/or management, which can reduce insect diversity (see e.g. New 2018). In this context, it should be mentioned that the sown wildflower patch where B. subterraneous was observed has since been partly converted to a bicycle parking area. The left-over grassland in Västra hamn, one of the sites with most butterfly species, will soon be incorporated into an adjacent park, with a more formal design and partly planted with trees (though high grassland vegetation will be kept in some parts).

While common bumblebee species were seen to be attracted by favourable habitat patches, sometimes even in large numbers, in the very urbanised areas studied, these were more difficult for butterflies to access. Thus, there seems to be a limit to how far quantity of green space can be replaced by quality with regard to biodiversity. Green space in very urbanised areas also tends to receive more intensive management, which has been shown to reduce biodiversity (e.g. Aguilera et al. 2019). Providing attractive green space for urban residents and biodiversity is challenging (Aronson et al. 2017), especially in highly urbanised areas where green space is scarce. Finally, the focus on adult nectar resources when enhancement of insect habitats is considered in an urban context might lead to too little attention being paid to other resources needed to complete the insect life cycle, as host plants or overwintering habitats.

This does not mean conditions for bumblebees and butterflies cannot be improved in densely built-up areas. Even if the number of species that can survive in these areas is limited, the aim should be to create conditions so that these species can survive in the city. Measures known to be beneficial, such as providing flowerbeds with attractive nectar sources for these insect groups, should continue to be offered or enhanced. Sowing wildflowers has been shown to increase insect numbers in urban settings (e.g. Blackmore and Goulson 2014; Mody et al. 2020) and this measure could be applied to a much greater extent. Green roofs can be a habitat for particular life cycle stages of certain insect species (Benvenuti 2014), especially for generalists (Williams et al. 2014; Hofmann and Renner 2018). Since arthropod diversity on green roofs has been shown to increase with higher connectivity (Braaker et al. 2017), biodiversity can probably be enhanced by a greater number of green roofs designed for this purpose. A reduction in management intensity for some existing green space elements (e.g. road verges, lawns) would benefit biodiversity (Aguilera et al. 2019). Left-over green space or informal urban green space (Rupprecht et al. 2015) is scarce in dense city areas, but could support higher biodiversity when present (Rupprecht et al. 2015). Finally, to enhance insect diversity in urban areas, it is important to base green structure design partly on expert knowledge in the field of urban insect ecology. Some approaches may fail to fulfil their potential because of lack of knowledge (e.g. regarding choice of plant material, lack of consideration of host plants or generally other life-cycle stages). Thus, better collaboration between landscape architects and ecologists might enable greater success in supporting and enhancing insect diversity in cities.