The degree of urbanisation reduces wild bee and butterfly diversity and alters the patterns of flower-visitation in urban dry grasslands

Abstract

Insect-provided pollination services are increasingly threatened due to alarming declines in insect pollinator populations. One of the main threats to insect pollinators and consequently pollination is urbanisation. Here, we investigate the effects of local habitat quality (patch size, flowering plant richness, bare soil cover, vegetation structure), degree of urbanisation (impervious surfaces) and 3D connectivity on bee, hoverfly and butterfly flower visitors and plant-flower visitor networks in flower-rich urban dry grasslands. Overall, the degree of urbanisation and the quality of the local habitat influenced the flowering plant and pollinator communities. Although flowering plant abundance increased with urbanisation, bee species richness and butterfly species richness decreased with increasing impervious surfaces. Flowering plant richness and ground nesting resource availability were positively related to bee richness and local vegetation structure boosted hoverfly and butterfly visitation rates. In terms of plant–pollinator interactions, insect pollinators visited a lower proportion of the available flowering plants in more urbanised areas and network modularity and specialisation increased with patch size. Our findings show that urban dry grasslands are valuable habitats for species-rich pollinator communities and further highlight the importance of minimizing the intensity of urbanisation and the potential of local management practices to support insect biodiversity in cities.

The Impact of Urbanization on Taxonomic Diversity and Functional Similarity among Butterfly Communities in Waterfront Green Spaces

“…However, it is worth noting that we found a positive effect of urbanization on butterfly abundance, which differs from the majority of studies demonstrating a negative impact of urbanization on butterfly abundance [5,48,49]. Urbanization likely increases the abundance of flowering plants, providing a richer nectar source to support individual butterflies [50]. However, most of these urban flowering plants are exotic ornamental species that not all butterflies can feed on, which forces some butterfly species to stay away from the city, especially some of the large specialist butterflies [26].…”

Impacts of land use change on native plant-butterfly interaction networks from central Mexico

“…This pattern has been attributed to the richness of alien plants in urban sites, which attracts generalist butterflies ( Lopezaraiza-Mikel et al, 2007 ). However, despite the increase in the number of plants with which a given butterfly species interacts in the network (number of links) in the urban site, there is a decrease in butterfly species richness associated with impervious surfaces ( Herrmann, Buchholz & Theodorou, 2023 ). Our study is consistent with these findings, since despite the high richness of alien plants in the urban site, a smaller number of butterfly species were observed compared to the native forest.…”

Adapting to changing methodology in a long-term experiment

“…In 2021, we chose five additional taxa to record on the sticky cards to provide additional statistical power to our comparison of the old and new card types. Aside from Orthoptera, which are herbivores (Joern 1979) commonly captured on these traps, we selected these taxa because they are important beneficial insects either providing biocontrol (Chalcidoidea, Ichneumonoidea, Syrphidae) or pollination services (Lepidoptera, Syrphidae) (Bonet 2009;Herrmann, Buchholz, and Theodorou 2023;Skevington et al 2019). Given that these taxa are very commonly captured and known to be sensitive to environmental conditions (Benthall et al 2022;Eckberg et al 2015;Holthouse, Spears, and Alston 2021;Toennisson, Klein, and Burrack 2019), they provided an additional, more sensitive test beyond that which would be provided by simply examining the core taxa monitored at the site.…”