Broader phenology of pollinator activity and higher plant reproductive success in an urban habitat compared to a rural one

Abstract

Urban habitat characteristics create environmental filtering of pollinator communities. They also impact pollinating insect phenology through the presence of an urban heat island and the year-round availability of floral resources provided by ornamental plants.Here, we monitored the phenology and composition of pollinating insect communities visiting replicates of an experimental plant assemblage comprising two species, with contrasting floral traits: Sinapis alba and Lotus corniculatus, whose flowering periods were artificially extended. Plant assemblage replicates were set up over two consecutive years in two different habitats: rural and densely urbanized, within the same biogeographical region (Ile-de-France region, France).The phenology of pollination activity, recorded from the beginning (early March) to the end (early November) of the season, differed between these two habitats. Several pollinator morphogroups (small wild bees, bumblebees, honeybees) were significantly more active on our plant sets in the urban habitat compared to the rural one, especially in early spring and autumn. This resulted in different overall reproductive success of the plant assemblage between the two habitats. Over the course of the season, reproductive success of S. alba was always significantly higher in the urban habitat, while reproductive success of L. corniculatus was significantly higher in the urban habitat only during early flowering.These findings suggest different phenological adaptations to the urban habitat for different groups of pollinators. Overall, results indicate that the broadened activity period of pollinating insects recorded in the urban environment could enhance the pollination function and the reproductive success of plant communities in cities.

Keywords: Lotus corniculatus; Sinapis alba; flowering phenology manipulation; phenology; plant reproductive success; plant–pollinator interactions; pollinator assemblage composition; urban–rural gradient.

FIGURE 1

Distribution of rural (dots) and urban (triangles) sites (SEF: Station d’Ecologie Forestière of Fontainebleau‐Avon; CEREEP A and B: CEREEP‐Ecotron Ile‐de‐France; SU: Pierre et Marie Curie Campus of Sorbonne Université; JDP: Jardin des Plantes; CIUP: Cité Internationale Universitaire of Paris). Colors represent areas dominated by agricultural landscape (yellow), by seminatural habitats (green), or by impervious zones (gray). Water‐covered surfaces are represented in blue (source: European Environment Agency, Corine Land Cover, 2018)

FIGURE 2

Detail of the flowers of each focal plant species: Sinapis alba (Brassicaceae, left) and Lotus corniculatus (Fabaceae, right). © Alexis Orion (CC BY 4.0), originals can be retrieved at www.inaturalist.org/photos/56174372 and www.inaturalist.org/photos/71587817. The photographs have been cropped and the scale bars added

FIGURE 3

Pollinator visitation rates (mean number of visits per 5‐min session) per morphogroup (stacked) and flowering period, for the two plant species: S. alba (upper two graphs) and L. corniculatus (lower two graphs)

FIGURE 4

Predicted pollinator visitation rates (number of visits per 5‐min session) on Sinapis alba, per floral round, for all pollinators combined (a) and major pollinator morphogroups: (b) small wild bees, (c) syrphid flies, (d) honeybees, (e) large solitary bees, (f) bumblebees. Bars represent estimated marginal means ± SE (green = rural; blue = urban). Stars represent significance levels from Tukey's post hoc tests

FIGURE 5

Predicted pollinator visitation rates (number of visits per 5‐min session) on Lotus corniculatus, per floral round, for all pollinators combined (a) and major pollinator morphogroups: (b) bumblebees, (c) large solitary bees, (d) butterflies. Bars represent estimated marginal means ± SE (green = rural; blue = urban). Stars represent significance levels from Tukey's post hoc tests

FIGURE 6

Reproductive success estimators, per flowering period, and for the two plant species: (a) and (b), respectively, fruit set and seed set rates of S. alba; (c and d), respectively, fruit set and seed set rates of L. corniculatus. Bars represent mean value ± SE. Stars represent significance levels from Tukey's post hoc tests