Orchids are well known to be the masters of floral deception, with
up to one third of all species completely lacking floral rewards such as nectar
or harvestable pollen (Internicola and Harder, 2012). Generalised food deception
(GFD) is the most common mechanism utilized by orchids to attract pollinators (Peter and Johnson, 2013), and operates by using a
pollinator's instinctive expectations of a rewarding flower, and copying it.
Orchids use sweet scents, bright contrasting colours and petal patterns which
trigger powerful food seeking behaviors in pollinators, to allure them into
their flowers (Peter and Johnson, 2013).
Internicola and Harder (2012) describe how although deceptive orchid
species pose very little competition to rewarding species, they have developed adaptations for successful reproduction;
- Flowering early in the season, which avoids competition with rewarding species and exploits naive pollinators (pollinators which are young and inexperienced).
- Flowering for extended periods, which maximizes the chance of pollination when faced with infrequent visits and experienced pollinators.
Although orchids can reproduce successfully when employing GFD,
they are very restricted. For example a deceptive orchid species can only exist
as a few individuals over a large area to reduce the risk of associative
learning (the ability a pollinator to remember, and avoid reward lacking species). After investigating
associative learning between Calypso
bulbosa and bumble-bees, Internicola and Harder (2012) discovered that
while associative learning had severe consequences for most deceptive orchids,
it had a limited effect on deceptive species that were specifically mimicking a
co-occuring rewarding species. It is assumed that overall, the bumble-bees
associated that particular morphology of flower with a reward; after visiting a rewarding species after a deceptive species, the bumble-bee was left with no
negative associations. This provides evidence supporting that the use of GFD in
orchids is far more successful in mimics than general deceptionists, due to positive associative learning,
resulting in continued visitation from pollinators.
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| Calypso bulbosa flowers, photographed by Magnus Manske |
Fascinating. If only a few individual orchid mimics can occur within a larger group of rewarding plants, what influence does this have on the overall fitness of the orchids? What about cross-pollination between mimicking orchids and the plants around them?
ReplyDeleteThe overall fitness of the orchids has been hypothesised by Jersakova et al. (2006) to be a combination of
ReplyDelete- a relocation of resources; the energy saved from not providing rewards goes into an extended flowering period and more robust seed production,
- and a higher lever of cross-pollination; due to pollinators visiting fewer flowers on the deceptive plants, limiting self pollination and producing more diverse progeny.
From what has been studied (Peter and Johnson, 2013), orchids tend to mimic different species, so cross-pollination between the 'real' flower and the 'deceptive' flower is not a problem. Many orchids have also developed pollenaria that sticks firmly to inaccessible parts of pollinators, to prevent it from falling off and being wasted on the wrong flower.