By investigating the love affairs of nearly 3,000 orchid species, scientists are unraveling new insights into the astonishingly diverse world of plant pollination.
The research, published in the Botanical Journal of the Linnean Society, is based on an extensive global database of orchid pollination. With records on more than 2,900 orchid species, the database includes a multitude of ingenious and specialized pollination strategies across various global regions.
The study sheds light on some of the outrageously deceptive tactics used by orchids to attract pollinators, and which types of pollinators are taking the bait.
The information in the database is meticulously organized by habitat, geographical distribution, and taxonomy, which allows scientists to identify the distinct patterns of reproductive biology in the Orchidaceae family.
Study co-author Dr. Ryan Phillips emphasized the significance of this comprehensive database: “From these data, we identify general patterns and knowledge gaps limiting our understanding of orchid biology at the global level.”
The study of Orchidaceae is not new to the world of biology. Charles Darwin himself recognized the intriguing nature of these plants and used them to study evolution.
Darwin was particularly fascinated by the peculiarities of the flowers, positing that their intricate structure was an evolutionary adaptation aimed at enhancing the transfer of pollen between plants and thereby boosting the fitness of their offspring.
“Because of the unusual floral traits and often unconventional pollination attraction strategies, orchids have been at the forefront of understanding floral adaptations to pollinators,” explained Dr. Phillips. Darwin hypothesized that the Madagascan orchid Angraecum sesquipedale, with its extraordinary 40 cm long nectar spur, would be pollinated by a moth equipped with a proboscis of matching length.
Led by Dr. James Ackerman of the University of Puerto Rico, the team discovered that over 75 percent of orchid species are dependent on pollinators for reproduction.
The experts also found that nearly half of the studied orchids don’t reward their visiting pollinators. Instead, these plants deploy cunning tactics of deceit to attract their pollinators. This special dependence on a single pollinator species was found not only in orchids native to the rainforests of Costa Rica or the montane grasslands of South Africa but also in those that used deceptive tactics.
Dr. Noushka Reiter, a co-author of the study, highlighted the potential vulnerability of these orchids to human-induced threats such as climate change. “Specializing on one pollinator species leaves many orchids particularly vulnerable. With the loss of pollinators, we would also lose these pollinator-dependent orchid species.”
According to the researchers, orchid pollination strategies may hold even more surprises than a science fiction novel. Australia, for instance, is the global hub for sexual mimicry in pollination, a tactic involving deceiving a range of insects from wasps to bees to gnats.
In South Africa, some orchids imitate carrion, while on Reunion Island they mirror rainforest fruits, and in Brazil, they mimic the smell of aphids. These elaborate ruses aim at tricking their pollinators. Meanwhile, in the American tropics, hundreds of Orchidaceae species produce fragrances that certain bees collect for their courtship rituals.
One striking example is the sexually deceptive Caladenia barbarella, an Australian orchid named after the infamous comic book character known for her amorous exploits. This naming choice aptly reflects the plant’s deceptive pollination technique.
Dr. Phillips emphasized that the hallmark of the orchid family is the high proportion of species that employ deceit to draw pollinators. They exploit the sensory abilities of pollinators through chemical, visual, or tactile stimuli, usually in combination.
The study identifies two major types of orchid deceit. One is food deception, where the orchid mimics a particular food to lure in a pollinator. The other is sexual deception, where male pollinators are tricked into visiting flowers that send out signals comparable to a receptive female insect.
“The floral signals can be so persuasive that insects attempt copulation and may even ejaculate,” said Dr. Phillips. “I’ve even had the wasps fly in through the car window at the traffic lights and start making love to the orchid specimens on the front seat.”
These deceptive strategies are not as rare as one might think. Around 20 genera worldwide, encompassing hundreds of Orchidaceae species, are known to use these tactics.
A third type of deception, known as brood-site deception, has been observed less frequently in orchids. In this method, the flower mimics a desirable place for female insects to lay their eggs, such as mushrooms, dung, or carrion.
The research provides a host of intriguing findings. For example, the study of orchid diversity in Australasia and Africa is relatively comprehensive, with 15% and 20% coverage, respectively. However, the species in Temperate Asia, Tropical Asia, and South America remain largely unexplored.
The database also reveals that approximately 76% of orchid species are completely reliant on pollinators for reproduction, and around 55% have a single known pollinator species.
While 54% of orchid species provide some reward to their pollinators, deception was recorded in 46% of the species.
Food deception was the most common deceitful tactic, used by 60% of the deceptive species, followed by sexual deception, present in 20 orchid genera.
Wasps and bees were identified as the most common pollinators, with flies and mosquitoes not far behind. However, the researchers caution that this impressive database, covering over 2,900 species, still accounts for less than 10% of the orchid family.
“Despite containing over 2,900 species, our database covers less than 10% of the family. While they are centers of orchid diversity, the tropical regions of Africa, Southern America, and Asia, are significantly under-represented in orchid pollination studies, especially among epiphytic orchids,” Dr. Phillips noted.
He believes the study of orchid pollination provides tremendous opportunities to unearth new and bizarre pollination strategies, and to better understand the adaptations flowering plants have developed to attract pollinators.
Aside from its scientific significance, this research also has crucial implications for conservation, given that many orchid species depend on one primary pollinator species for their survival.