Challenging the sustainability of urban beekeeping using evidence from Swiss cities

Urban beekeeping is booming, heightening awareness of pollinator importance but also raising concerns that its fast growth might exceed existing resources and negatively impact urban biodiversity. To evaluate the magnitude of urban beekeeping growth and its sustainability, we analysed data on beehives and available resources in 14 Swiss cities in 2012–2018 and modelled the sustainability of urban beekeeping under different scenarios of available floral resources and existing carrying capacities. We found large increases in hives numbers across all cities from an average 6.48 hives per km2 (3139 hives in total) in 2012 to an average 10.14 hives per km2 (9370 in total) in 2018 and observed that available resources are insufficient to maintain present densities of beehives, which currently are unsustainable.

Author Correction: Pollinator importance networks illustrate the crucial value of bees in a highly speciose plant community

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Pollinator importance networks illustrate the crucial value of bees in a highly speciose plant community

Accurate predictions of pollination service delivery require a comprehensive understanding of the interactions between plants and flower visitors. To improve measurements of pollinator performance underlying such predictions, we surveyed visitation frequency, pollinator effectiveness (pollen deposition ability) and pollinator importance (the product of visitation frequency and effectiveness) of flower visitors in a diverse Mediterranean flower meadow. With these data we constructed the largest pollinator importance network to date and compared it with the corresponding visitation network to estimate the specialisation of the community with greater precision. Visitation frequencies at the community level were positively correlated with the amount of pollen deposited during individual visits, though rarely correlated at lower taxonomic resolution. Bees had the highest levels of pollinator effectiveness, with Apis, Andrena, Lasioglossum and Osmiini bees being the most effective visitors to a number of plant species. Bomblyiid flies were the most effective non-bee flower visitors. Predictions of community specialisation (H2′) were higher in the pollinator importance network than the visitation network, mirroring previous studies. Our results increase confidence in existing measures of pollinator redundancy at the community level using visitation data, while also providing detailed information on interaction quality at the plant species level.

Constructing more informative plant–pollinator networks: visitation and pollen deposition networks in a heathland plant community

Interaction networks are widely used as tools to understand plant–pollinator communities, and to examine potential threats to plant diversity and food security if the ecosystem service provided by pollinating animals declines. However, most networks to date are based on recording visits to flowers, rather than recording clearly defined effective pollination events. Here we provide the first networks that explicitly incorporate measures of pollinator effectiveness (PE) from pollen deposition on stigmas per visit, and pollinator importance (PI) as the product of PE and visit frequency. These more informative networks, here produced for a low diversity heathland habitat, reveal that plant–pollinator interactions are more specialized than shown in most previous studies. At the studied site, the specialization index was lower for the visitation network than the PE network, which was in turn lower than for the PI network. Our study shows that collecting PE data is feasible for community-level studies in low diversity communities and that including information about PE can change the structure of interaction networks. This could have important consequences for our understanding of threats to pollination systems.

Pollination biology of Heracleum sphondylium L. (Apiaceae): the advantages of being white and compact

Two questions were addressed in the present study: (1) What are the main pollinators of the two subspecies of <em>H. sphondylium</em>?, and (2) Do the studied plants share the pollinators' set or are they attractive for different groups of insects? The survey showed that among 40 insect taxa visiting both subspecies of <em>H. sphondylium</em> approx. only 53% carried significant pollen loads. However, the Pollinator Importance Coefficient (IC) calculated for each insect group, and based on observation of insects' abundance, within-umbel activity and pollen load revealed that only two taxa in case of <em>H. s</em>. ssp. <em>sibiricum</em> (<em>Thricops nigrifrons</em>, <em>Eriozona syrphoides</em>) and four in case of <em>H. s</em>. ssp. <em>sphondylium</em> (<em>T. nigrifrons</em>,<em>E. syrphoides</em>, <em>Meliscaeva cinctella</em> and <em>Arge ustulata</em>) were truly important pollinators. Although both subspecies were visited by similar insects, <em>H. s.</em> ssp. <em>sphondylium</em>, with its characteristic compact and white umbels, was visited more frequently by <em>Diptera</em> and <em>Hymenoptera</em>, while yellow-greenish loose umbels of <em>H. s</em>. ssp. <em>sibiricum</em> were preferred by <em>Coleoptera</em>. This paper indicates that the concept of faithful pollinators may also apply to a broader spectrum of <em>Apiaceae</em>, usually considered primitive in terms of pollination strategies, and suggests possible ways of differentiation in two closely related taxa.

Effects of climate on pollination networks in the West Indies

We studied the effect of climate on the plant-pollinator communities in the West Indies. We constructed plots of 200 m × 5 m in two distinct habitats on the islands of Dominica, Grenada and Puerto Rico (total of six plots) and recorded visitors to all plant species in flower. In total we recorded 447 interactions among 144 plants and 226 pollinator species. Specifically we describe how rainfall and temperature affect proportional richness and importance of the different pollinator functional groups. We used three measures of pollinator importance: number of interactions, number of plant species visited and betweenness centrality. Overall rainfall explained most of the variation in pollinator richness and relative importance. Bird pollination tended to increase with rainfall, although not significantly, whereas insects were significantly negatively affected by rainfall. However, the response among insect groups was more complex; bees were strongly negatively affected by rainfall, whereas dipterans showed similar trends to birds. Bird, bee and dipteran variation along the climate gradient can be largely explained by their physiological capabilities to respond to rainfall and temperature, but the effect of climate on other insect pollinator groups was more obscure. This study contributes to the understanding of how climate may affect neotropical plant-pollinator communities.