Abstract
‘Pollination syndromes’ involving floral nectar have eluded satisfactory evolutionary explanation. For example, floral nectars for vertebrate-pollinated plants average low sugar concentrations, while such animals prefer high concentrations, perplexing pollination biologists and arousing recent controversy. Such relationships should result from evolutionary games, with plants and pollinators adopting Evolutionarily Stable Strategies, and nectar manipulating rather than attracting pollinators. Plant potential to manipulate pollinators depends on relationships between neighbouring flowers within plants, for all nectar attributes, but this has not been investigated. We measured nectar volume, concentration and sugar composition for open flowers on naturally-growing Blandfordia grandiflora plants, presenting classic bird-pollinated plant syndrome. To evaluate potential pollinator manipulation through nectar, we analysed relationships between neighbouring flowers for nectar volume, concentration, proportion sucrose, log(fructose/glucose), and sugar weight. To evaluate potential attraction of repeat-visits to flowers or plants through nectar, we compared attributes between successive days. Nearby flowers were positively correlated for all attributes, except log(fructose/glucose) as fructose≈glucose. Most relationships between nectar attributes for flowers and plants on successive days were non-significant. Nectar-feeding pollinators should therefore decide whether to visit another flower on a plant, based on all attributes of nectar just-obtained, enabling plants to manipulate pollinators through adjusting nectar. Plants are unlikely to attract repeat pollinator-visits through nectar production. Floral nectar evolution is conceptually straightforward but empirically challenging. A mutant plant deviating from the population in attributes of nectar-production per flower would manipulate, rather than attract, nectar-feeding pollinators, altering pollen transfer, hence reproduction. However, links between floral nectar and plant fitness present empirical difficulties.
Highlights
Correlations between attributes of plants and their pollinators, known as ‘pollination syndromes’, have been detected for all aspects of floral nectar, but these have so far eluded satisfactory evolutionary explanation[8,9,10]
Because birds might respond to amounts of sugar encountered, and not separately to nectar volume and sugar concentration, we considered possible relationships between flowers and plants in terms of weight of sugar per nectar sample, which is the product of nectar volume and sugar concentration
Nectar attributes for nearby flowers on the same plant were highly and positively correlated for nectar volume, sugar concentration, proportion sucrose, and weight of sugar, but not significantly correlated for log F/G (Supplementary Material, Table S1; Figs. 2–4)
Summary
Correlations between attributes of plants and their pollinators, known as ‘pollination syndromes’, have been detected for all aspects of floral nectar (volume, concentration, composition), but these have so far eluded satisfactory evolutionary explanation[8,9,10]. The rates of nectar volume and energy production per flower are positively correlated with pollinator body size, leading some to suggest that this is expected because large pollinators require more energy than smaller pollinators[1,3,4]. This premise would either require that plants evolve to benefit pollinators, which is not the way evolution generally works[1,3,4], or would require that relatively large pollinators respond more strongly to higher-than-average encountered nectar volumes than smaller pollinators, which is neither expected nor observed[1]. It is even more difficult to explain observed negative correlations between nectar sugar concentration and body size of pollinators, separately for vertebrate and invertebrate pollinators[1,2,3,4,15,16], especially as pollinators of all sizes prefer relatively concentrated nectar[8,17]
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