The importance of sunbirds as pollinators of the southern African plants Alberta magna and Streptocarpus dunnii.
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Date
2023
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Abstract
Bird pollination is important for plant reproduction and is found in about 65 flowering plant
families. I studied putative bird pollination systems of two southern African plant species,
Alberta magna (Rubiaceae), which grows above cliffs and Streptocarpus dunnii
(Gesneriaceae), which occurs among boulders in open grassland. A special emphasis of this
study was to test the utility of close-focusing motion-activated camera traps for documenting
bird pollination of plant species that are rare and occur in habitats that present challenges for
conventional bird pollination studies. The aims of this project were to 1) quantify floral traits
for comparison with other bird-pollinated species; 2) establish the breeding systems,
including reliance on pollinators for reproduction; 3) identify floral visitors and measure their
contribution to reproductive success; and 4) determine if seed production is pollen limited.
Floral traits of S. dunnii, including large volumes of dilute nectar and long tube lengths, also
aligned with an ornithophilous pollination syndrome. In naturally-pollinated plants, overall
fecundity was high, with 80% of flowers developing fruits with large numbers of seeds.
Malachite sunbirds (Nectarinia famosa) were the only legitimate visitor to flowers of S.
dunnii. Greater double-collared sunbirds (Cinnyris afer), which have shorter bills than
malachite sunbirds, occasionally rob flowers of nectar. Streptocarpus dunnii was found to be
fully self-compatible but does not self-autonomously. Selective exclusion experiments
showed that the species is almost entirely reliant on birds for seed production. The species did
not experience pollen limitation, indicating that sunbirds are effective pollinators. Measured
floral traits of A. magna, including large volumes of dilute nectar and long tube lengths, were
consistent with an ornithophilous pollination syndrome. Motion triggered cameras and
observations showed that southern double-collared sunbirds (Cinnyris chalybeus) and olive
sunbirds (Cyanomitra olivacea) were the most common visitors to A. magna, while amethyst
sunbirds (Chalcomitra amethystina) were occasional visitors. Selective exclusion
experiments showed that bird visitors are important for reproductive success which was
found not to be pollen-limited. The low levels of fecundity in A. magna indicate that there
may be other potential factors that limit seed production in this species and the degree of self-compatibility
in this species was not firmly established. I conclude that both species are
specialized for pollination by sunbirds and that close-focusing camera traps are effective for
documenting bird pollination systems of rare plant species which occur in habitats that are
challenging for conventional human observations.
Description
Masters Degree. University of KwaZulu-Natal, Pietermaritzburg.