A Handsome Native Plant: Columnea polyantha and its Pollination Strategy
Anyone who ever cared for African Violets already knows the family of flowering plants called Gesneriaceae. There are no species whatsoever in that family native to the United States or Canada, as it is a largely tropical family, but here in Costa Rica there are 27 genera and 140 species. At Finca Cantaros on the Pacific highland side of the Talamanca Mountains, we are fortunate to have the prolonged wet conditions that allow at least two native Gesneriads to grow wild: Drymonia macrantha and Columnea polyantha. I find both extremely attractive, but Columnea polyantha is the subject of today’s post because I just learned something new about it recently: its principal pollinator.
The seeds of this plant are excreted onto tree limbs, into small indentations of the trunk bark, or onto climbing vines by birds that have eaten the fruit of C. polyantha. The seedlings then grow as epiphytes, their roots taking advantage of the trees without being harmful parasites. Noting the shape and color of the flower, you have probably guessed that C. polyantha is pollinated by hummingbirds. But which one(s)?
C. polyantha is hirsute all over except for the main stem. During the flowering season (February to early June here) the dark red hairs of the upper side (lamina) of its leaves attract hummingbirds’ attention as they fly over in the forest understory. The birds descend to investigate and find bright yellow tubular flowers extending pertly under the leaves. The little red accents on the flowers’ entrance lobes entice them further to drink the nectar within. However, as often as I have passed this plant during the four to five months it is blooming, I’ve never managed to arrive when a hummingbird is feeding. With 18 species of hummingbirds on the Finca Cantaros bird list, 12 of which are reasonably common, this bad luck mystified me. The plant seems to shout out for hummingbirds, and I was getting more curious daily.
Enter doctoral students Jessica Anne Greer, Adam Hadley and their adviser, Matthew Betts, Ph.D of the Forest Landscape Ecology Lab at Oregon State University. At the nearby Wilson Botanical Garden and at other sites over the past three years during our region’s dry season, the team has literally counted pollen grains of different plant species collected from the beaks of hummingbirds captured temporarily in mist nets. While the team was using our property for a control study this field season, looking at visitation of hummingbirds to a wild heliconia common here, I asked whether they happened to know which hummingbirds visit C. polyantha. Several weeks later, Jessica had time to study the data for three field seasons at other sites, 2012-2014, and she generously gave me a report.
Of the 22 species of hummingbirds they captured and released over three years, only 3 species carried pollen from C. polyantha. Both the Violet Sabrewing and the Green-crowned Brilliant carried a very small amount of pollen (averaging 2 grains per bird), while the Green Hermit carried a much higher pollen load (averaging 105 grains per bird). They recorded one individual Green Hermit with 1564 grains of C. polyantha pollen! It seems clear that the Green Hermit has the highest potential to pollinate based simply on the number of pollen grains carried. It has a very long beak, compared to the other two hummingbirds: it seems better suited to reach the nectar deep within the long tubular flowers. Of 33 individual birds carrying this pollen, 29 were Green Hermits.
Mystery solved. We can now say with some assurance that Green Hermits are the C. polyantha’s best friend–its main pollinator. Next year I will make myself a blind and simply sit long enough to witness an actual pollination event at this strikingly beautiful plant.
Guest photographer and friend Andrew Russell of Heredia, Costa Rica, generously granted permission to use his photographs of the Green Hermit, Violet Sabrewing and Green-crowned Brilliant.
Reference: Gesneriads of Costa Rica by Ricardo Kriebel Haehner. Instituto Nacional de Biodiversidad (INBio), Costa Rica, 2006, pp. 11, 148.