The sexual deception of orchids - Anne Gaskett
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Sexual deception by orchids was discovered independently by two amateur orchid researchers: Edith Coleman, an Australian school teacher (published in 1928) and Maurice-Alexandre Pouyanne, a French judge in Algiers (published in 1917). Before this, the pollination of these types of orchids remained a mystery. In Charles Darwin’s 1862 book about orchid pollination he discussed the pollination of Ophrys bee orchids, “I have never seen one visited by any insect. Robert Brown imagined that the flowers resembled bees in order to deter their visits, but this seems extremely improbable”. "Mr. Price has frequently witnessed attacks made upon the Bee Orchis by a bee… What this sentence means I cannot conjecture.” Both Coleman and Pouyanne made many careful and excellent observations and experiments, and concluded that the orchids attracted insects with airborne scents that were undetectable to humans, and were only visited by male, not female, insects, as they attempted to mate with (not attack!) the flowers.
Sexual deception is only found in orchids, with two known exceptions: one Iris from Europe and one daisy from South Africa. Other orchid tricks, such as mimicking an insect’s egg-laying site, are found in a range of other plants including the largest flowers in the world, the magnificent Rafflesia and Titan arum.
Why does sexual deception nearly always evolve just in orchids? All plants rely on relationships and networks to survive, but orchids seem to have a particularly wide range of unusual features and relationships with other species. Orchid seeds need fungal mycorrhizae in the soil to germinate, and orchid flowers often need specialist pollinators. In turn, both the fungi and the pollinators have symbiotic relationships with other species such as host trees (whose roots are essential for the fungi), and host caterpillars (most sexually deceived pollinators are parasitoid wasps, which lay their eggs in the larvae of other insect species). Maybe biodiversity allows complex networks and interactions – and these drive orchids’ own incredible diversity? Orchids are certainly successful, they grow almost everywhere all around the world, even on sub-antarctic islands.
These relationships benefit orchids, but might not be so good for their pollinators. Deception by orchids generally just steals an insect’s time and energy, but Cryptostylis Tongue orchids are so convincing they fool their pollinator into wasting his sperm. Like most insects tricked by sexual deception, he is a parasitoid wasp – these usually have limited sperm and can mate only a few times in their life. Wasting sperm on an orchid could be a big problem.
As for most plants and animals, the top threats to orchids are habitat loss, climate change and introduced species. Orchids are also at great risk from people - 40% of IUCN Redlist orchids with threat data are at risk from tourism and recreation, including residential and commercial development, poaching or collecting orchids from the wild, and by human recreational activities. All orchid species are covered by CITES (the Convention on International Trade in Endangered Species of Wild Flora and Fauna), which prevents orchids being moved from one country to another without specific permission. Other species covered by CITES include all primates, cetaceans, and cacti, plus individual species such as rhino, tigers, pandas, and birds of paradise.
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