Last spring’s alert that bee populations in this country were falling sounded alarms for environmentalists everywhere. But what if this is just nature’s way of making room for a better pollinator?

That question underlines Maria Clara Castellano’s doctoral work, presented November 26 at U of T’s Ramsay Wright Zoological Laboratories. She has discovered that tiny hummingbirds pollinate certain flowers much more efficiently than bees, even those flowers exclusively pollinated by bees in nature. By manipulating certain features of Penstemons, colourful hood-shaped flowers, Castellano has demonstrated that some flowers can, over time through evolution, become more attractive to one type of pollinator than another. According to Castellano, this “evolutionary shift” in which pollinator a flower attracts occurs often in nature.

Castellano modified a species of Penstemon that is pollinated only by bees, P. strictus, into a shape that would better accommodate hummingbirds. Using fishing line to allow the flower to droop, she created one resembling a hummingbird-friendly flower. She also removed a lower protruding flap, or “bee landing-pad,” from the flower. Conversely, Castellano artificially introduced bees to another species, P. barbatus, which are pollinated exclusively by hummingbirds. She found that hummingbirds pollinate both species of flowers more efficiently than bees, and concluded that “hummingbird” flowers have evolved repeatedly from “bee” flowers among Penstemons.

Professor James Thomson, Chair of U of T’s Zoology Department and Castellano’s doctoral supervisor explains: “What Maria Clara really showed was that hummingbirds are surprisingly good pollinators of any Penstemons they choose to visit, even for those species that are adapted to be pollinated by bees. This is rather surprising, and it probably explains why there have been frequent evolutionary shifts from bee to bird pollination in this genus.” He adds that this research serves to increase the body of knowledge of how nature works. It also supports the notion that pollinators exist in a mutual relationship with flowers that have adapted to encourage visits by those animals.

Castellano reasons that hummingbirds are more efficient pollinators because unlike bees, they don’t eat pollen or cause it to fall uselessly to the ground. They simply feed on a flower’s nectar and fly away, with pollen clinging to their feathers.

So why aren’t all Penstemon flowers pollinated by hummingbirds? Some species of this flower simply do not produce enough nectar to attract hummingbirds, unless the birds’ usual food supply is severely restricted. Hummingbirds expend a lot of energy when they fly, and require a food source high in sugars and energy, like nectar. They can’t afford to feed on less bountiful flowers.

Professor Thomson’s basic message is this: “In our gardens and elsewhere, we should try to preserve both birds and bees, and other pollinating animals. Some of these animals are probably getting rarer due to human modification of habitats.”