1. Proboscis length has been proposed as a key dimension of plant pollination niches, but this niche space has not previously been explored at regional and global scales for any pollination system. Hawkmoths are ideal organisms for exploring pollinator niches as they are important pollinators in most of the biodiverse regions of the earth and vary greatly in proboscis length, with some species having the longest proboscides of all insects. 2. Using datasets for nine biogeographical regions spanning the Old and New World, we ask whether it is possible to identify distinct hawkmoth pollination niches based on the frequency distribution of proboscis length, and whether these niches are reflected in the depths of flowers that are pollinated by hawkmoths. We also investigate the levels of specialization in hawkmoth pollination systems at the regional and community level using data from interaction network studies. 3. We found that most regional hawkmoth assemblages have bimodal or multimodal distributions of proboscis length, and that these are matched by similar distributions of floral tube lengths. Hawkmoths, particularly those with longer proboscides, are polyphagous and at the network level show foraging specialization equivalent to or less than that of bees and hummingbirds. In the case of plants, shorter-tubed flowers are usually visited by numerous hawkmoth species, while those that are longer-tubed tend to exclude shorter-proboscid hawkmoths and thus become ecologically specialized on longer-proboscid hawkmoth species. Longer-tubed flowers tend to have greater nectar rewards and this promotes short-term constancy by long-proboscid hawkmoths. 4. Our results show that pollinator proboscis length is a key niche axis for plants and can account for patterns of evolution in functional traits such as floral tube length and nectar volume. We also highlight a paradoxical trend for nectar resource niche breadth to increase according to proboscis length of pollinators, while pollinator niche breadth decreases according to the tube length of flowers.

The concept of coevolution was first developed by Darwin, who used it to explain how pollinators and food-rewarding flowers involved in specialized mutualisms could, over time, develop long tongues and deep tubes, respectively. He famously predicted that Angraecum sesquipedale, a long-spurred Malagasy orchid, must be pollinated by a hawkmoth with an exceptionally long tongue. Darwin’s idea of a coevolutionary “race” was championed by contemporary naturalists, including Alfred Wallace, and a hawkmoth fitting the expected tongue-length profile was eventually discovered in Madagascar during the early twentieth century. However, strong empirical support for the mechanism behind Darwin’s coevolutionary model has been forthcoming only in the past two decades. It is now established that selection often strongly favors plants with floral tubes that exceed the length of their pollinator’s tongues. There is also evidence that pollinators gain an energetic benefit from having tongues that enable them to consume most or all of the nectar in deep tubular flowers. Alternative explanations for the evolution of long pollinator tongues, such as evasion of predators that use flowers as ambush sites, are considered much less compelling and lack quantitative support. Another important advance in coevolution research has been the development of approaches that explicitly predict a geographical mosaic of coevolution. The expectation that coevolution can lead to geographical diversification and trait covariation among strongly interacting organisms is strongly supported by studies of long-proboscid fly and oil-bee pollination systems in South Africa. Macro- and microevolutionary studies of pollination systems suggest that coevolution can operate alongside other one-sided evolutionary processes, such as shifts, in shaping plant and pollinator traits.

<h4>Background and aims</h4>Unrelated organisms that share similar niches often exhibit patterns of convergent evolution in functional traits. Based on bimodal distributions of hawkmoth tongue lengths and tubular white flowers in Africa, this study hypothesized that long-tongued hawkmoths comprise a pollination niche (ecological opportunity) that is distinct from that of shorter-tongued hawkmoths.<h4>Methods</h4>Field observations, light trapping, camera surveillance and pollen load analysis were used to identify pollinators of plant species with very long-tubed (>8 cm) flowers. The nectar properties and spectral reflectance of these flowers were also measured. The frequency distributions of proboscis length for all captured hawkmoths and floral tube length for a representative sample of night-blooming plant species were determined. The geographical distributions of both native and introduced plant species with very long floral tubes were mapped.<h4>Key results</h4>The convolvulus hawkmoth Agrius convolvuli is identified as the most important pollinator of African plants with very long-tubed flowers. Plants pollinated by this hawkmoth species tend to have a very long (approx. 10 cm) and narrow flower tube or spur, white flowers and large volumes of dilute nectar. It is estimated that >70 grassland and savanna plant species in Africa belong to the Agrius pollination guild. In South Africa, at least 23 native species have very long floral tubes, and pollination by A. convolvuli or, rarely, by the closely related hawkmoth Coelonia fulvinotata, has been confirmed for 11 of these species. The guild is strikingly absent from the species-rich Cape floral region and now includes at least four non-native invasive species with long-tubed flowers that are pre-adapted for pollination by A. convolvuli.<h4>Conclusions</h4>This study highlights the value of a niche perspective on pollination, which provides a framework for making predictions about the ecological importance of keystone pollinators, and for understanding patterns of convergent evolution and the role of floral traits in plant colonization.

Geographic differences in floral traits may reflect geographic differences in effective pollinator assemblages. Independent local adaptation to pollinator assemblages in multiple regions would be expected to cause parallel floral trait evolution, although sufficient evidence for this is still lacking. Knowing the intraspecific evolutionary history of floral traits will reveal events that occur in the early stages of trait diversification. In this study, we investigated the relationship between flower spur length and pollinator size in 16 populations of <i>Aquilegia buergeriana</i> var. <i>buergeriana</i> distributed in four mountain regions in the Japanese Alps. We also examined the genetic relationship between yellow- and red-flowered individuals, to see if color differences caused genetic differentiation by pollinator isolation. Genetic relationships among 16 populations were analyzed based on genome-wide single-nucleotide polymorphisms. Even among populations within the same mountain region, pollinator size varied widely, and the average spur length of <i>A. buergeriana</i> var. <i>buergeriana</i> in each population was strongly related to the average visitor size of that population. Genetic relatedness between populations was not related to the similarity of spur length between populations; rather, it was related to the geographic proximity of populations in each mountain region. Our results indicate that spur length in each population evolved independently of the population genetic structure but in parallel in response to local flower visitor size in different mountain regions. Further, yellow- and red-flowered individuals of <i>A. buergeriana</i> var. <i>buergeriana</i> were not genetically differentiated. Unlike other <i>Aquilegia</i> species in Europe and America visited by hummingbirds and hawkmoths, the Japanese <i>Aquilegia</i> species is consistently visited by bumblebees. As a result, genetic isolation by flower color may not have occurred.