How flower colour signals allure bees and hummingbirds: a community‐level test of the bee avoidance hypothesis

Abstract

Summary Colour signals are the main floral trait for plant–pollinator communication. Owing to visual specificities, flower visitors exert different selective pressures on flower colour signals of plant communities. Although they evolved to attract pollinators, matching their visual sensitivity and colour preferences, floral signals may also evolve to avoid less efficient pollinators and antagonistic flower visitors. We evaluated evidence for the bee avoidance hypothesis in a Neotropical community pollinated mainly by bees and hummingbirds, the campo rupestre. We analysed flower reflectance spectra, compared colour variables of bee‐pollinated flowers (bee‐flowers; 244 species) and hummingbird‐pollinated flowers (hummingbird‐flowers; 39 species), and looked for evidence of bee sensorial exclusion in hummingbird‐flowers. Flowers were equally contrasting for hummingbirds. Hummingbird‐flowers were less conspicuous to bees, reflecting mainly long wavelengths and avoiding red‐blind visitors. Bee‐flowers reflected more short wavelengths, were more conspicuous to bees (higher contrasts and spectral purity) than hummingbird‐flowers and displayed floral guides more frequently, favouring flower attractiveness, discrimination and handling by bees. Along with no phylogenetic signal, the differences in colour signal strategies between bee‐ and hummingbird‐flowers are the first evidence of the bee avoidance hypothesis at a community level and reinforce the role of pollinators as a selective pressure driving flower colour diversity.

Assessment of floral colour signals at a community through the eyes of the birds and bees

“…The research shows that whilst there was no phylogenetic signal impacting upon how plants evolved their colour signals, the community‐level study in Brazil observed evidence of a significant shift in flower signals for hummingbird‐pollinated flowers towards longer wavelength reddish signals, whereas bee‐pollinated flowers more typically had short wavelength rich signals. This community‐level study provides the first detailed insights into how flower colour may potentially evolve free of phylogenetic constraints within a particular region, and thus helps explain how larger continental effects on flower colour evolution may eventuate (Shrestha et al ., ; Camargo et al .). The very interesting and highly novel finding is that whilst for the tetrachromatic colour vision of hummingbirds for either ‘bee’ or ‘bird’ visited flowers were salient and could thus be easily detected by birds against a foliage background, in the colour model for bees, the bird‐pollinated flowers were often close to the achromatic centre of colour space and thus would often be poorly detected by bees.…”

“…New research by Camargo et al . in this issue of New Phytologist (pp. 1112–1122) shows that flower colouration in a community is complex, requiring a balance of presenting salient signals to attract some flower visitors, whilst minimizing how attention might be captured by other visitors that might disrupt plant reproduction.

‘This community‐level study provides the first detailed insights into how flower colour may potentially evolve free of phylogenetic constraints within a particular region …’

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“…Camargo et al . studied the floral colour signals of 283 species of flowering plants in Serra do Cipó National Park and its buffer zone, Morro da Pedreira (an environmentally protected area, located in Minas Gerais, south‐eastern Brazil) (Fig.…”

“…Theories on pollination syndromes have typically relied on human colour assessments of flower colour and considered optimal flower colour discrimination or preference by pollinators as an evolutionary driver, but an alternative or complementary driver of flower colour evolution could be chromatic contrast that flowers have relative to natural backgrounds for different flower visitors (Bukovac et al ., ). The Camargo et al . study thus evaluated plants in the rupestrian grassland, using careful spectrophotometer and photographic measurements of flowers and their foliage backgrounds, and then employed appropriate colorimetric models for bees or birds to understand how flower traits may have evolved into optimal signals.…”

“…(a) The campos rupestres landscape in southeast Brazil, where Camargo et al . (in this issue of New Phytologist , pp.…”