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The mysterious ways orchids affect their army of pollinators

New research undertaken in the Swiss Alps has demonstrated a little-known relationship between altitude, orchid scent, and the droves of pollinators that flit between flowers.

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    Pollinator on the inflorescence of a Gymnadenia orchid.
    Courtesy of Karin Gross/University of Zurich
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Which orchids receive visits from pollinators and which are ignored? It's all about scent, say scientists.

New research, published Wednesday in PLoS One, looks at orchid populations in the Swiss Alps, comparing those in lowland areas with those perched high in the mountains.

“Our study is the first that shows that regional differences in floral scent can be caused by different patterns of selection in these regions,” says lead author Karin Gross of the University of Zurich in an email interview with the Monitor.

“This is the sort of research that shows how speciation and evolution happens,” says Tom Mirenda, an orchid collection specialist for the Smithsonian Gardens, in a telephone interview with The Christian Science Monitor. “These studies basically show how natural selection works.”

Orchids, like many flowers, have something of a symbiotic relationship with their pollinators: the animals provide a vital service in transporting pollen between individual plants, facilitating reproduction, and in return, the flowers provide a nutritional reward.

Previous research has shown that flowers growing in distinct geographic regions will evolve different traits, related to pollinator preference. This latest study is one of the few to focus on scent.

To test their hypotheses, the researchers investigated myriad facets of the relationship between orchids and pollinators in these alpine valleys and mountains, including flower size, color, and scent, as well as examining the specific makeup of the pollinating armies.

“To understand evolutionary changes in flowering plants, it is important to consider floral scent," writes Dr. Gross. "Floral scent is an important signal to mediate plant-pollinator interactions.”

Interestingly, a scent's potency only affected pollinators’ flower choice in low-lying areas; up in the mountains, it made no difference.

This correlates with the general theory underlying speciation:

Individuals of a given plant species become geographically isolated, perhaps by a precocious puff of wind.

In this new home, certain traits may be more attractive to the local pollinators than they were in the motherland, so individual plants showcasing those traits are more likely to survive and produce offspring. Over time, the new plant population becomes sufficiently different from the original to constitute a fresh species.

“Of course, this happens very, very slowly – over millennia,” the Smithsonian's Mr. Mirenda tells the Monitor.

How does this apply to the orchids? Perhaps it was too cold, high in the mountains, for the lowland pollinators to survive. Orchid seeds carried from lower valleys to germinate up high would need to appeal to a different kind of pollinator if they wanted to propagate.

“Fragrances are generally one of the things that attract a pollinator, so a different scent might be attracting a different kind of pollinator,” explains Mirenda.

As global climate change transforms mountain climates, scientists look to studies like this one to anticipate the ecological impacts.

“The results of our study could help to make predictions in relation to global warming,” Gross tells the Monitor. “With global warming, it is expected that lowland pollinators move to higher elevations.”

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