Virtual issue: Plant-pollinator interactions

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This virtual issue of Functional Ecology gathers 10 papers appearing in the journal during the past two years that address plant-pollinator relationships. The compilation is intended to coincide with the special feature Plant-pollinator interactions from flower to landscape.

A major theme of this Virtual Issue is the role of floral traits in the evolution of plant-pollinator relationships and hence much of angiosperm and insect diversity. To humans, the most obvious floral trait is colour, and Bergamo et al. (2016) use white and pink colour morphs of Costus arabicus flowers, differing only in red reflectance, to examine two hypotheses for the red colour of hummingbird-pollinated flowers. Because bees prefer the white flowers and hummingbirds show no preference, they conclude that red colouration is less important for attracting hummingbirds than for excluding bees. Artificial flowers are widely used to investigate responses to floral traits, and different shapes can be made with a 3D printer. Using this method, Campos et al. (2015) show that the hawkmoth Manduca sexta feeds more easily from trumpet-shaped flowers than from flat disc-shaped ones, showing the importance of corolla curvature as a mechanical guide. Pollinators drive selection of floral traits, but this selection could be affected by limited resources. Using supplemental hand-pollination and nutrient addition, Sletvold et al. (2016) show that selection on the spur length of orchid flowers is pollinator-mediated and does not change with increased resources in the form of essential minerals and water. Variation in floral traits influences foraging decisions, and one of the factors contributing to variation in quality of nectar rewards is microbial colonisation. Schaeffer et al. (2016) manipulate the presence of a cosmopolitan nectar yeast, Metschnikowia reukaufii, in nectar and show that bumblebees Bombus impatiens visit more yeast-inoculated flowers and forage for longer on them; however, reproduction of the bumblebees is unaffected. Yeast may act as an honest signal indicating the availability of nectar.

Defence chemicals occur in the nectar of many plant species, and are another source of variation in nectar quality. Lucas-Barbosa et al. (2016) look at potential trade-offs for pollinators and herbivores of Brassica nigra plants. Both odours and visual cues change after pollination and herbivory and affect the behaviour of flower visitors, with butterflies using different cues for oviposition and nectar foraging. Tiedeken et al. (2015) examine the effects of grayanotoxins in the nectar of invasive Rhododendron ponticum, finding different effects on the survival and behaviour of three native bee species, with bumblebees being most tolerant. Interactions between pollinators and invasive plants are further explored by Razanajatovo & van Kleunen (2016). They compare three groups of plants – native, invasive, and naturalised but non-invasive – in a breeding system experiment and find that pollen limitation, perhaps surprisingly, does not seem to play a major role in the spread of introduced plants.

The remaining papers deal with other aspects of anthropogenic change. Insecticides pose major risks to pollinators, and the neonicotinoids are especially controversial. Stanley & Raine (2016) expose bumblebee colonies to thiamethoxam at a sublethal level and look at the responses of foragers to morphologically complex flowers in a flight arena. Bees treated with pesticide collect more pollen but take longer to learn flower handling; this has implications for colony provisioning. In another study on bumblebees and anthropogenic change, Holland & Bourke (2015) maintain laboratory colonies at different temperatures and find little effect on lifespan, but more queens are produced in colonies at higher temperatures. The responses of social insect pollinators to climate change must be considered at both individual and colony levels. Finally, we conclude with the review by Harrison & Winfree (2015) which formed part of a previous special feature on urban environments. This paper draws together the evidence for responses of plant-pollinator interactions to habitat fragmentation, introduced plant species, urban warming and environmental contaminants. Urban landscapes provide valuable study systems for studying the effects, often combined, of these drivers of global change.

Flower colour and visitation rates of Costus arabicus support the ‘bee avoidance’ hypothesis for red-reflecting hummingbird-pollinated flowers
Pedro J. Bergamo, Andre R. Rech, Vinicius L. G. Brito 

Shape matters: corolla curvature improves nectar discovery in the hawkmoth Manduca sexta 
Eric O. Campos, Harvey D. Bradshaw Jr, Thomas L. Daniel

Urban drivers of plant-pollinator interactions
Tina Harrison, Rachael Winfree

Colony and individual life-history responses to temperature in a social insect pollinator
Jacob G. Holland, Andrew F. G. Bourke

Visual and odour cues: plant responses to pollination and herbivory affect the behaviour of flower visitors
Dani Lucas-Barbosa, Pulu Sun, Anouk Hakman, Teris A. van Beek, Joop J. A. van Loon, Marcel Dicke

Non-invasive naturalized alien plants were not more pollen-limited than invasive aliens and natives in a common garden
Mialy Razanajatovo, Mark van Kleunen

Consequences of a nectar yeast for pollinator preference and performance 
Robert N. Schaeffer, Yu Zhu Mei, Jonathan Andicoechea, Jessamyn S. Manson, Rebecca E. Irwin

Resourceā€ and pollinatorā€mediated selection on floral traits
Nina Sletvold, Matthew Tye, Jon Agren

Chronic exposure to a neonicotinoid pesticide alters the interactions between bumblebees and wild plants
Dara A. Stanley, Nigel E. Raine

Nectar chemistry modulates the impact of an invasive plant on native pollinators
Erin Jo Tiedeken, Paul A. Egan, Philip C. Stevenson, Geraldine A. Write, Mark J. F. Broke, Eileen F. Power, Iain Farrell, Sharon M. Matthews, Jane C. Stout

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