Abstract
Geographic variation in the reproductive traits of animal‐pollinated plants can be shaped by spatially variable selection imposed by differences in the local pollination environment. We investigated this process in Babiana ringens (Iridaceae), an enigmatic species from the Western Cape region of South Africa. B. ringens has evolved a specialized perch facilitating cross‐pollination by sunbirds and displays striking geographic variation in perch size and floral traits. Here, we investigate whether this variation can be explained by geographic differences in the pollinator communities. We measured floral and inflorescence traits, and abiotic variables (N, P, C, and rainfall) and made observations of sunbirds in populations spanning the range of B. ringens. In each population, we recorded sunbird species identity and measured visitation rates, interfloral pollen transfer, and whether the seed set of flowers was pollen limited. To evaluate whether competition from co‐occurring sunbird‐pollinated species might reduce visitation, we quantified nectar rewards in B. ringens and of other co‐flowering bird‐pollinated species in local communities in which populations occurred. Variation in abiotic variables was not associated with geographical variation of traits in B. ringens. Malachite sunbirds were the dominant visitor (97% of visits) and populations with larger‐sized traits exhibited higher visitation rates, more between‐flower pollen transfer and set more seed. No sunbirds were observed in four populations, all with smaller‐sized traits. Sunbird visitation to B. ringens was not associated with local sunbird activity in communities, but sunbird visitation was negatively associated with the amount of B. ringens sugar relative to the availability of alternative nectar sources. Our study provides evidence that B. ringens populations with larger floral traits are visited more frequently by sunbirds, and we propose that visitation rates to B. ringens may be influenced, in part, by competition with other sunbird‐pollinated species.
Highlights
Much of the striking floral variation within and among angiosperms has been attributed to evolutionary responses to variation in the pollinator environment (Johnson, 2010; Kay & Sargent, 2009)
We established the patterns of variation and covariation among floral and inflorescence traits in a sample of B. ringens populations across the geographical range of the species and asked the following specific questions: (a) Are there associations between B. ringens trait variation and abiotic factors among populations? (b) Is geographic variation in reproductive traits associated with visitation by different sunbird species? (c) Is geographical varia‐ tion in sunbird visitation rates and reproductive traits associated with variation in seed set, pollen limitation, and pollen transfer? (d) Is sunbird activity and relative sugar availability in local bird‐pol‐ linated communities positively associated with visitation rates to B. ringens?
Geographic variation in floral and inflorescence traits of B. ringens is associated with variation in the visitation rates of pollinating sun‐ birds
Summary
Much of the striking floral variation within and among angiosperms has been attributed to evolutionary responses to variation in the pollinator environment (Johnson, 2010; Kay & Sargent, 2009). The species possesses striking geographical variation in floral and inflorescence traits (Figure 2) which may be associated with contrasting pollination environments across the Western Cape region (de Waal, Anderson, & Barrett, 2012). The average floral‐tube length of B. ringens populations varies from approximately 25–45 mm (de Waal, Anderson, et al, 2012), spanning the floral‐tube length of both sunbird pollination guilds Both long‐ and short‐billed sunbirds have been observed visiting B. ringens, and one of the goals of this study was to evaluate the hypothesis that variation in floral and inflorescence traits of this species may be shaped by spatially variable selection imposed by the local pollina‐ tion environment. We established the patterns of variation and covariation among floral and inflorescence traits in a sample of B. ringens populations across the geographical range of the species and asked the following specific questions: (a) Are there associations between B. ringens trait variation and abiotic factors among populations? (b) Is geographic variation in reproductive traits associated with visitation by different sunbird species? (c) Is geographical varia‐ tion in sunbird visitation rates and reproductive traits associated with variation in seed set, pollen limitation, and pollen transfer? (d) Is sunbird activity and relative sugar availability in local bird‐pol‐ linated communities positively associated with visitation rates to B. ringens?
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