Orchids have a variety of tools for getting pollinated. They can provide specialist foods for pollinators. They can provide scents to aid bees in courtship. Sometimes, they just mimic a female insect, and lure a male for pollination that way. Research published in Ecological Entomology shows that some Dracula orchids also use reproduction to lure flies. But they offer more than sex.
Location of the study site.
To find out what the orchids were offering, Tobias Policha and colleagues climbed the Andes of Ecuador. On the mountain slopes, about a mile above sea level, the team found the Dracula orchids. This might seem like extreme altitude, but it’s a matter of perspective, as Tobias Policha explained: “1250m to 2200m is actually a pretty modest elevation for the Andes. Quito, the capital city, sits at 2,850m! Most of our work occurred below 2,000m.
The Andes are ‘young’ mountains. This youth means that they are VERY steep. Further, the orchids were in peak condition during the rainy season, Policha said. And this rain made things tougher. “It does rain a lot (~3m/yr!), and the mud is certainly something to reckon with. It takes a few hours of hiking through sometimes knee-deep mud to get to the research station, and then additional hiking to reach the study sites.”
Previous fieldwork had found the orchids were not pollinated by fungus gnats, as suspected. Instead they used drosophilids, flies mainly in the genera Zygothrica and Hirtodrosophila. These flies, like fungus gnats, use mushrooms as breeding sites, but also as brood sites. So the authors decided to test if the orchids were mimicking brood sites too. If they were, then the flies would be leaving eggs on the plants to raise their larvae.
It was a good plan, hindered slightly by a problem peculiar to Ecuador. Ecuador is a biodiversity hotspot. It’s all very well saying ‘fly’ or ‘mushroom’, but which orchid, which flies and which mushrooms?
Policha said that the orchids were not so much of a problem. “The field station has been visited over the years by several orchidologists (Lorena Endara, Luis Baquero, Carlyle Luer, Stig Dalstrom), so there is a decent plant list. Besides that, Dracula are quite distinctive. There have been a handful of species described from the reserve (including D. lafluerii), so not EVERYTHING is known!
“Identifying the mushrooms and flies took somewhat longer! In both cases, we relied on both morphology and DNA, and our team included world experts in both taxa.
“We could often get to family or genus pretty reliably for mushrooms in the field. And we had the benefit of learning the ‘myco-flora’ over several field seasons. Bryn Dentinger will be working on identifying and describing new species of fungi from our study.”
While orchids and fungi will sit still to be classified, Policha could not say the same of the insects. “The flies were where things got dicey! The rain also did NOT help with this. We would spend hours sitting in the rain watching the movements of small “fruit-flies” (at least relatives of Drosophila!). They would sometimes come and go so quickly as to seem to disappear, and they would frequently visit in “mixed-flocks” of sometimes dozens of species! We could not reliably ID these insects in the field.
“Adding to the difficulty was the fact that ~1/3 of the species are new to science and have not been described. This was in part why we used the DNA barcoding approach to identify at least unique groups of flies, even when there was not a name to put on them. David Grimaldi (AMNH) will be working on describing the new species from our work.”
The team were also puzzled by how the orchid was mimicking the mushrooms. Were they replacing the fungi by providing a viable brood site for the flies? Or were they merely mimicking the mushrooms, and doing nothing to raise the next generation of pollinators?
What the authors found was that they were right in some respects. Many of the flies that visited the mushrooms in a location, also came to the Dracula orchids. However, some seemed to specialize in either flowers or mushrooms.
Scientists have found some yeasts growing on Dracula orchids, and in the guts of insect visitors. This match shows that the flies can find a meal on the flowers. Tobias Policha and colleagues also saw the flies performing courtship behaviour on the flowers.
Zygothrica spp. on Dracula lafleurii. Video by Jacky Poon.
While a Dracula orchid is a great place to meet other flies, it’s not necessarily a good place to raise children. The authors write: “…there was no overlap between species emerging from mushrooms and those emerging from Dracula.” Yet some fly species do emerge from Dracula orchids. In the conclusion of the paper, the authors add: “…one might predict that given the resources available in the flowers, some fly species would evolve to exploit Dracula orchids exclusively, and this seems to be the case; many of our visitors appear to be flower specialists that do not visit mushrooms and vice versa.”
This variety of results caused a bit of difficulty in explaining how the orchids were behaving. There are a few ways mimicry can work. A well-known example is the flies that adopt black and yellow stripes to look like wasps or bees, despite having no sting. This is Batesian mimicry. Another form of mimicry is Müllerian mimicry. This mimicry is where organisms converge on a similar look because it helps solve the same problem. Policha said that the way bees and wasps both use yellow and black stripes to warn predators of their stings is Müllerian mimicry in action. However, it’s not clear what the orchids are doing.
“We were surprised. I think that we originally assumed that it would be a Batesian system of deception, but after watching the organisms for so long, we realized that they were actually fulfilling important aspects of their natural history on the orchids. It was really in response to these observations that we initiated the rearing study to confirm that they were not achieving the ultimate fitness benefit (reproduction) from the orchid substrates like they were from the mushrooms.”
Yet some flies could have young hatch successfully on some orchids, so the mimicry is not purely deceptive. Policha is currently thinking about the orchids’ strategy, including whether the orchids are swapping from one approach to another. “How this story fits into mimicry theory is still not totally clear. Whether or not we are at a transition state in terms of the evolution from deception to convergence will ultimately depend on what the flies will be able to do on the orchids.
“In terms of thinking about selection pressures, Batesian and Müllerian mimicry exert opposite types of frequency-dependent selection (negative and positive, respectively). In a certain way, the orchids are benefiting from the potential positive frequency-dependent selection of a Müllerian ring by masking the ultimately deceptive aspects of fitness loss through offering a place to shelter, feed, and mate.”
The findings show that for orchids mimicry isn’t all about deception. While Dracula orchids are mimicking mushrooms, they’re also able to provide rewards for some partners. It’s perhaps no surprise that, in a place where you find such a diversity of life, you should also find a diversities of tactics dealing with other organisms.