Co-evolution and a Tale of Two Lobelias

Two of the most striking plants in bloom at the end of Summer on our farm are red and blue lobelias, in the bluebell family. Both tend to be found in damp, disturbed areas where there is reduced competition with grasses, and have spectacular flowers. But the “rest of the story” is that the ecology of pollination of these related flowers is quite different.

Look at the bright red cardinal flower ( Lobelia cardinalis) from the side and notice the long nectar tube and the position of the stigma at the end of a long, arching style. This flower seems to be designed for a hummingbird. Compare this with some other flowers specialized for hummingbirds, the red bee-balm and the fire pink. Notice the similarities and how the structure is most suitable for a small, hovering bird.

In contrast the blue flowers of the great lobelia ( Lobelia siphilitica ) have a very different structure and you can see from this photo that it is visited by bumblebees.

Now why would flowers “choose” to be so specialized that they are predominantly pollinated by only one species of bird in the Eastern US? Wouldn’t this limit their chances of being pollinated? One might think so but this tightly linked co-evolution of an animal and plant is not that unusual. So it must provide benefits which might include a greater likelihood of pollination and less competition with other species. Indeed hummingbirds set up territories (called trap-lines) whereby they regularly visit their nectar sources and defend them against other hummers. Such fidelity could increase pollination success. However if hummingbirds declined in numbers, this could spell a lot of trouble for the flowers.

So the natural process of evolution involves a lot of “cost benefit analyses” carried out in the arena of fang and claw. It is interesting that there are “cheaters” that steal the nectar without pollinating the plants. I have included a photo from our Florida yard of a female orchard oriole piercing the base of a Cape honeysuckle to steal the nectar. This is a flower originally from South Africa where it is specialized for pollination by sunbirds. There are actually a group of birds in tropical America that have short sharp bills and specialize in this method of flower piercing. So the measure of success in the natural world is reproduction, apparently without altruism. Of course if there were too many nectar-stealers and not enough pollinators, these flowers would disappear.

Bill Dunson, Englewood, FL & Galax, VA
wdunson@comcast.net