Floral richness drives flower–insect networks in city gardens and allotments

Urban–rural studies have largely found a negative effect of urbanisation on pollinator diversity. However, within cities different types of green spaces may vary in their environmental characteristics, thus influencing their quality as pollinator habitat. Private green spaces, such as residential gardens, can contribute to a large proportion of land in the urban mosaic. Yet, comparisons in pollinator habitat between green space types are still lacking.

Here, we compare nine gardens and eight allotments (defined as land privately leased by individuals and typically used to cultivate crop plants) in Glasgow, Scotland, to assess which environmental drivers influence flower visitor diversity, community dissimilarity and plant–flower visitor network structure.

Our study revealed that gardens and allotments supported similar flower–visitor richness, showing they are equitable habitats for pollinators within cities. This is likely because no differences in floral diversity were found between gardens and allotments. In fact, floral richness was the main driver of flower visits within spaces, while the area of surrounding green space influenced flower visits by amplifying the effects of floral richness. Thus, the landscape context may influence flower visits, yet pollinator diversity can still be supported by providing a higher floral richness at sites in dense urban areas.

Native plants received more visits than non‐native plants, although flower visitor richness to non‐native plants increased later in the season as non‐native plant availability rose. This indicates that while native plantings should be prioritised, non‐native plants may still provide supplementary forage in September.

Floral richness was also associated with changes to plant–flower visitor network structure. Most notably, specialisation increased with floral richness, highlighting more selective interactions between plants and pollinators with increased resource availability.

Overall, our study indicates that gardens and allotments support pollinators equally, and greater floral richness in these sites can help increase pollinator diversity. Private green spaces therefore have an important role in supporting pollinators within cities, and these habitats should be considered in urban planning.