The complexity of a city from pollinators’ eyes

Cities are really complex entities. Under nature point of view, urbanization means fragmenting the green areas and altering the local climate as cities are more hot and rainy than the surroundings. It is a general belief that these cemented surfaces harm wildlife, but yet we do see some species around us every day, including bees, meaning that some organisms do get along with us. While the negative footprint of human societies is well know, it is less known how pressing it is and how to lift its weight.

To tackle this problem, some colleagues and I conducted a study on the relationship between pollinators and urbanization, the study is freely accessible here. Pollinators are essential for delivering pollen and carrying out the ecosystem service of pollinating plants, a process from which we benefit. Pollination depends on how many pollinators are there, how many flower resources are offered to them in the nectar and from how many plant species the pollen is carried on their bodies. These were the aspects we investigated in relationship with the urbanization of the landscape and the climate. In spite that pollinators declined with the distance among green areas and with the stably hot urban temperatures, we found that pollinators unexpectedly peaked at medium levels of cemented landscapes. This tells us that some features of urbanization are potentially positive especially when they are moderate: some disturbance may promote wild bees and syrphids by increasing the places where to feed and nest.

Another surprise came from the nectar, more rich in sugars in the city core than outside and correlating with the urban higher precipitations. It is suggesting that plants may actually be more productive in the city as they are more watered, and thus pollinators could find more resources available there than elsewhere, which is a happy promise for them. As pollinators carry pollen, we looked at how many plants were transported and unfortunately found less plants carried as pollen in the city core than outside, and these were predominantly exotic. The contribution of pollinators to wild plants reproduction may be reduced in the city core compared to outside.

Studies like this one are a first step forward for understanding how to ameliorate the current situation and improving the sustainability of urban landscapes. One example? If few plant species are carried by pollinators in the city core, then we have to increase flower diversity there so that pollinators will find more food and get dusted by more diverse pollen. Another one? If fragmented green areas are affecting pollinators, then designing cities with connected green areas or improving the hospitability of green roofs and balconies are likely ways to increase pollinator diversity. There is still lot to do for understanding how pollinators manage surviving in cities and for helping them out.

Api selvatiche urbane a Milano

La stagione primaverile porta con sé l'esplosione della vita. Non solo le fioriture, ma anche gli impollinatori tornano in azione.

Sono dr. Paolo Biella, e insieme al gruppo di ricerca di cui faccio parte (Zooplantlab, Università di Milano-Bicocca, Dip. di Biotecnologie e Bioscienze) durante la primavera corrente e la prossima estate studierò impollinatori del gruppo delle api. In particolare, mi concentrerò su api selvatiche che non producono né miele né colonia (che in realtà costituiscono la stragrande maggioranza della diversità di api). Le api che studierò hanno la particolarità di fare nidi in cavità di varie tipologie, solitamente in muri o nel legno.

Istallerò nidi per attrarre api solitarie, nidi che abbiamo preparato nei nostri laboratori con il paziente aiuto e l'immensa passione degli studenti di dottorato e laureandi. I nidi saranno posizionati in vari contesti della città di Milano, grazie all'appoggio di associazioni ed enti territoriali (si veda mappa di seguito). Raccoglieremo dati concernenti il tasso di nidificazione e la diversità di pollini raccolti da questi affascinanti organismi del gruppo delle api.

Questo studio ci permetterà di comprendere come il paesaggio urbano possa sostenere le popolazioni selvatiche di impollinatori, e ci consentirà di comprendere meglio quali parametri ambientali urbani favoriscono alti tassi di nidificazione. In altre parole, potremo capire come rendere più sostenibili i paesaggi urbani dalla prospettiva delle api (il video seguente che mostra api nidificanti è di A. Ferrari).

Mappa dei siti in cui sono stati collocati i nidi

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Le persone che hanno contribuito alla realizzazione dei nidi sono:

Federica Barreca

Luca Caprotti

Matteo Ferrara

Andrea Ferrari

Andrea Galimberti

Tiziano Galimberti

Rosanna Giacovelli

Alberto Mattia Nodari

Emiliano Pioltelli

Nicola Tommasi

Luca Tonietti

Hanno anche contribuito:

Fem2Ambiente

Vivaio Bicocca

Isola Pepe Verde

Orti di via Padova

Boscoincittà

Cascinet

Parco Nord Milano

Are generalist pollinated plants the most important out there?

Those who are familiar with how habitats are being altered by human or natural activities also know that usually these alterations tend to be more harmful to some species and less harmful to other ones. As pollinators rely a lot on flower resources (that is pollen and nectar), an habitat alteration could target specific types of plants with some effects on pollinators too. To facilitate our understanding of how pollinators respond to habitat alteration we run an experimental test in which we removed several highly generalist plants from the plant community.

We found that removing these key plants had strong effects on the interactions between plants and pollinators. Earlier on, I presented some of the effects on pollinators (here) including how some insects preferred to go away rather than using local alternative flowers, and those who shifted flowers followed preferential flower types features. However, the pollinator responses also negatively impacted the ways pollinators interact with plants and the rate by which pollinators shifted to new plants. You will find all results at this link, or write me for more details.

Here below a “lay” summary presenting the paper in non-technical way, a requirement from the journal which I am not sure if was used eventually.

Field experiments reveal the alteration of plant and pollinator interactions when the generalist plants are removed

Paolo Biella, University of Milano-Bicocca, and colleagues

Species interact with each other in complex ways. Within these tangled systems, some species cover more prominent roles than others: in the case of plants and pollinators, some plants are highly abundant or provide very attractive floral resources to pollinators and thus they are highly visited. In this study we tested the effects, on pollinator richness and on the species interactions, generated by the experimental removal of these important plants from the system. In particular, we planned to remove 4 plant species from natural communities, one plant species at a time, based on the number of pollinators they had previously received. We discovered some important aspects of how a pollinator guild depends on the scenario set by the plant assemblage. We found that knocking out the most visited plants triggered the disappearance of many pollinators, and that these local extinctions were more in number than what expected from computer-based simulation models. On top of that, we discovered that the framework set by the interacting species changed towards more fragile states when the most visited plants were removed, which contrasts previous expectations that these multi-species systems are stable against species loss. Furthermore, the data showed that novel interactions were established after the important plant were lost. In particular, those generalist pollinators which visited both the plants targeted of removal and the non-target plants tended to establish novel links more often than the other pollinators. Additional interesting results from the experiment regard the emergence of opportunistic interactions established in a random fashion, and structural unpredictability of the network of species interactions when plants were removed. These latter aspects are fundamental because reveal that the framework of interactions set between plants and pollinators changed in their inner dynamics and rules of assembly. Overall, these findings supported the idea that generalist, highly visited, flowers play a key role in sustaining local pollinator guilds, and that otherwise the framework of interactions is perturbed. Nevertheless, our study indicates that some pollinators can find alternative resources by establishing novel links, a reorganizational ability that possibly buffers against even more dramatic effects of generalist plants loss.