16 May 2023

The degree of urbanisation reduces wild bees and butterfly diversity

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German research confirms that the degree of overbuilding and the quality of the local habitat influence the activity of bees and other insects. Interactions with plants decrease. Yet urban meadows and pastures, with the right actions, can be precious contexts for pollinators. Starting from a suitable height of the vegetation…

by Emanuele Isonio

 

For three months between May and August 2021, they monitored the movements of bees and other pollinating insects within the city of Berlin and in the neighboring areas of the federal state of Brandenburg. The German capital is in fact one of the largest metropolitan regions in Germany, with an area of almost 900 square kilometers and a population of 3.6 million inhabitants. They selected 56 study sites within the CityScape Lab Berlin research platform, created to study the effects of urbanization on biodiversity: urban pastures (characterized exclusively by spontaneous and wild vegetation) are in fact an essential component of city green spaces. And they represent optimal habitats for wild bees, butterflies and good foraging grounds for hoverflies.

I grafici indicano nell'ordine (dall'alto a sinistra) le relazioni tra: diversità delle api selvatiche e superfici impermeabili; diversità delle api selvatiche e ricchezza delle piante da fiore; diversità delle farfalle e superfici impervie; la visita di farfalle e l'altezza della vegetazione; visite della sirfide e altezza della vegetazione; visite delle api ai fiori e superfici impermeabili. Le linee tracciate mostrano la relazione prevista e le aree ombreggiate indicano gli intervalli di confidenza al 95%. FONTE: Herrmann, Buchholz, Theodorou "The degree of urbanisation reduces wild bee and butterfly diversity and alters the patterns of flower-visitation in urban dry grasslands", 2023

Relationships between (a) Shannon diversity of wild bees and impervious surfaces (1000 m); (b)
Shannon diversity of wild bees and flowering plant richness; (c) Shannon diversity of butterflies and impervious
surfaces (100 m); (d) butterfly flower visits and vegetation height; (e) hoverfly flower visits and vegetation
height; (f) honey bee flower visits and impervious surfaces (100 m). Plotted lines show the predicted relationship
and shaded areas indicate the 95% confidence intervals.

Three research centers in the field

Thanks to this analysis, three researchers from the Federal Research Center for Cultivated Plants “Julius Kühn”, the University of Münster and the University of Halle-Wittenberg have come to a conclusion: the urban “densification”, the extension of waterproofed, the management style of city green areas have over time represented a negative factor for the life of bees and other pollinators. Not only that: they have reduced their diversity, their interaction with the plant world and the way insects visit flowers. A discovery that is useful not only for denouncing the problems associated with wild overbuilding but also for identifying those actions which, on the contrary, can improve the role of green areas – wild and otherwise – in safeguarding pollinating animal species.

“Our findings – the study points out – show that urban grasslands are valuable habitats for pollinator communities and further underscore both the importance of minimizing urbanization intensity and development potential and management practices have in supporting insect biodiversity in cities”.

A danger to ecosystem services

The research obviously starts from the consideration that the species and number of insects are increasingly threatened due to the alarming decline in their populations. A constant decrease that endangers the provision of ecosystem services provided by bees and other insects. Obviously starting with pollination. “Pollinating insects – underline the researchers – play a key role in almost all terrestrial ecosystems. They are in fact responsible for the reproduction of most wild flowering plants and global food crops. The loss, fragmentation and degradation of their habitats damage their lives and, consequently, the services they can guarantee us. “Processes closely linked – the research recalls – to both intensive agriculture and urbanisation”.

api insetti impollinatori servizi ecosistemici vantaggi agricoltura

Bees and pollinating insects in general play a crucial role in our food security and ecosystems. Here, 5 of the “services” guaranteed by their presence. SOURCE: FAO Archive.

Moreover, the study shows that on the one hand urban areas are associated with innumerable environmental stress factors: heat islands, air, noise, light, water pollution and, last but not least, pollution of the underlying soil. All negative aspects overall for the diversity of insects. But, at the same time, cities have structurally different habitats within them. And this can be a stimulus to biodiversity, for example the high availability of flowers and plants in one’s parks, gardens and green areas can offer new nesting opportunities. In the study, the German researchers found 166 pollinator species and 67 plant species. Of the interactions between them, 54% were performed by wild bees, 22% by honey bees, 13% by hoverflies, and 10% by butterflies.

Good bee-saving practices

The point is precisely this: to be able to reduce stress factors, instead enhancing the urban aspects that can help protect pollinators. Also because each of them has its own specificity.

In general, even with increasing urban density, “the availability of local plant resources and vegetation structure still have strong effects on overall pollinator activity, open diversity, and hoverfly and butterfly visit rates. This leads to a series of best practices that city center administrators can apply. Real local management practices capable of supporting the conservation of insect biodiversity in cities.

Firstly, by improving for example the richness of flowering plants, by establishing species-rich flower strips of sufficient size. Also very useful is the reduction in the frequency of mowing the grass, “tolerating high herbaceous vegetation on urban green soil” (with all due respect to lovers of English lawns…). Finally, in the research there is an invitation to “create open areas of sandy soil“. An action that can in fact provide ideal grounds for bees that nest on the ground.