Volume 177, Issue 1, 1 January 2015, Pages 141-150
a
Naturalis Biodiversity Center, PO Box 9517, Leiden, Netherlands
b Leiden University, PO Box 9517, Leiden, Netherlands
c School of Life Science, University of KwaZulu-Natal, Private Bag X 01, Scottsville, South Africa
b Leiden University, PO Box 9517, Leiden, Netherlands
c School of Life Science, University of KwaZulu-Natal, Private Bag X 01, Scottsville, South Africa
Abstract
The bird pollination
syndrome is characterized by red, unscented flowers with dilute nectar
in long nectar tubes. However, the extent to which plants with such
traits actually depend on birds for seed production is seldom determined
experimentally, and traits such as colour and scent production are
often assessed only subjectively. We documented bird pollination and
quantified floral traits in the critically endangered Satyrium
rhodanthum (Orchidaceae) from mistbelt grasslands in the summer-rainfall
region of South Africa. Direct
observations and motion trigger camera footage revealed amethyst
sunbirds as the only pollinators, despite the presence of other
potential pollinators. Experimental exclusion of sunbirds significantly
reduced pollination and fruit set to near zero. Pollination success in
naturally pollinated plants was close to 100% in one year, and fruit set
varied from 23 to 64% in other years. Pollen transfer efficiency was
5.8%, which is lower than in related insect-pollinated species, probably
due to a tendency of birds to wipe pollinaria from their beak. Flowers
of S.rhodanthum only reflect light in the red range of the spectrum, and
they produce only a few aliphatic and monoterpene scent compounds at
comparatively low emission rates. Nectar volume and sugar concentration
varied between 2.7 and 3.7μL and 23.7 and 25.9%, respectively. We
conclude that S.rhodanthum is highly specialized for pollination by
sunbirds. Colour, scent and nectar characteristics differ from
insect-pollinated Satyrium species and are consistent with those
expected for bird-pollinated flowers, and may contribute to lack of
visitation by other potential long-tongued pollinators. Habitat loss
probably underlies the critically endangered conservation status of
S.rhodanthum, but the specialization for pollination by a single bird
species means that reproduction in this orchid
is vulnerable to losses in surrounding communities of plants that
subsidize the energetic requirements of sunbirds. © 2014 The Linnean
Society of London.
Author keywords
Bird vision; Exclusion experiment; Floral scent; Flower colour; Pollen transfer efficiency; Pollination syndrome; Sunbird