What draws pollinators to flowers is more complex than it seems. Ill.: netclipart.com
We humans tend to think that the beauty and scent of flowers were created just for us. We selfishly fill our gardens with blooms and we even bring bouquets of cut flowers indoors. In fact, however, if plants spend so much energy producing colorful, scented flowers, it’s not to please people, but rather to seduce pollinators, especially insects, but also birds and even mammals.
From Wind to Insects
The pollination of land plants used to be primarily carried out by wind. This is called anemophily and it first appeared on the earth about 350 million years ago.
Even today, anemophily remains the preferred pollination method for conifers, grasses and many shade trees, but it’s also very costly in terms of energy and resources. Wind-pollinated plants must produce a phenomenal amount of pollen, because the wind is simply not a trustworthy pollen distributor: it can be absent, too strong, blow in the wrong direction, etc. Anemophilous plants have to produce millions of pollen grains in the hope that at least a few will reach a fertile flower of the same species on another plant (the goal of pollination). It does work—if it didn’t, the species would go extinct!—, but so much energy is required that some trees need years to recuperate from a single year of prolific pollination.
But pollen is rich in carbohydrates and minerals and way back in time, insects, especially beetles (small hard-shelled insects that were the main arthropod group of the time) began to visit the flowers in search of this resource. These insects were therefore harmful to plants, stealing their precious pollen. But sometimes, too, they did accidentally carry pollen from one blooming plant to another. Thus, evolution eventually pushed plants to stop fighting insect pollen predation and instead to use beetles as pollen carriers.
The Origin of Attractive Flowers
The first plants with flowers designed to attract pollinating insects probably arose about 125 million years ago. These first blooms looked much like today’s magnolias or water lilies. Basically, they developed colorful leaves that surrounded the pollen-producing sex organs, showing just where insects needed to go for food, like a neon sign outside a diner. Eventually, those colored leaves evolved into the organs we call petals today … and thus attractive flowers were borne. Pollination by insects (entomophily), was no longer purely an accident, but planned.
From these early stages of insect pollination, the population of flowering plants (Angiosperms) exploded. There are some 250, 000 species of them today, found all over the world wherever plants grow and more than 80% of all flowers are pollinated by insects. And as they evolved, competition became tougher, so the flowers developed more and more specific attractions. More colorful petals, sugar-rich nectars, stickier pollen, intoxicating scents, etc.
And insects in turn evolved to be more efficient pollinators, notably developing hairier bodies to carry pollen, and became better at finding flowers and at harvesting pollen, with elongated proboscises designed to better sip up nectar, for example.
Many flowers also become more and more selective, looking for pollinators who would be faithful to them. While a generalist pollinator may pick up pollen from a daisy, if it delivers it to a dandelion, pollination will fail. So, it may be better to offer a flower so attractive to a certain insect that it will specifically visit that flower over and over in the future.
Over millennia, this growing specificity has sometimes led to exclusive symbioses (mutualism). Yuccas, for example, can only be pollinated by yucca moths (Tegeticula spp.). Yuccas are entirely dependent on the yucca moth for their propagation; the yucca moth, which not only pollinates yucca flowers, but feeds on yuccas, totally depends on yuccas for its survival. In the orchid family, in particular, such strict symbioses are legion.
From this race to produce more and more attractive flowers emerged the thousands of shapes, colors and fragrances of flowers we know today and also well over 20,000 different pollinating insects. Bees, butterflies, hoverflies, wasps and many other insects have evolved specifically to be flower pollinators and most simply can’t live without plant pollen or nectar.
Colors That Attract Insects
Insects see in color, but in a different range of colors from that of the human eye. For them, for example, ultraviolet rays, which we don’t see at all, seem very intense and attractive. On the other hand, they can’t see red. When you see an insect visiting a flower that looks red, you can be sure that there are ultraviolet marks that are invisible to us.
You’ve probably also noticed that the petals of many flowers—petunias, irises, foxgloves, etc.—are richly marked with stripes or spots of a contrasting color. These are called nectar guides: they direct the pollinators towards the part of the flower where the nectar is concentrated. But even a flower that appears to be just one color to our eyes often has nectar guides. If we can’t see them, it’s because they are ultraviolet in color. We’d need to use a device with a special filter to be able to see them.
Perfumes for All Tastes
Most insects are also very attracted to sweetly scented flowers and therefore many flowers use scent as a calling card, investing a lot of energy into perfume production. It just so happens that humans tend to like the same scents as most insects, so much so that one of the first things any person does when coming in contact with a flower is to bend down and sniff it.
Pollinating birds, including hummingbirds, have no little to no interest in scents, and hummingbird-specific flowers, such as the cardinal flower (Lobelia cardinalis), are generally odorless.
Some insects, on the other hand, have radically different olfactory preferences. Carrion flies, for example, are attracted by the smell of putrefaction and some flowers thus emit foul odors in order to attract them. Such is the case of the giant carrion flower (Stapelia gigantea), a succulent plant somewhat like a cactus whose flower not only smells of spoiled meat, but whose color and texture are reminiscent of a rotting animal carcass. When your carrion flower blooms, it’s best to move it outside!
Other foul-smelling flowers that attract carrion flies include the various corpse flowers (Amorphophallus spp.), including the famous titan arum (Amorphophallus titanum), renowned as having the world’s largest inflorescence (composite flower), and the rafflesia (Rafflesia spp.), an Asian parasitic plant that produces the largest individual flower. Apparently, flowers need to be pretty big to stink bad!
Some flowers are not active to insects active during the day, such as bees, hoverflies, wasps and butterflies, but rather moths. Many of these flowers remain closed during the day to protect their precious pollen and nectar from diurnal insects, then open at dusk. They are almost always very fragrant … but at night only.
The smell of some night-bloomers can attract moths from afar: up to 11 km. The pollinator then has to follow the scent, but may have difficulty locating the flower when it gets nearby. That’s why so many night-blooming flowers are white: it’s the most visible color in the darkness. Under the light of the moon, white flowers shine like a lantern, at least to the eyes of nocturnal insects, and tell the insect that followed the scent trail exactly where to go.
Among the plants with nocturnal scents that we grow in our gardens are nicotianas, moonflower (Ipomaea alba) and jasmine.
Red is for the Birds
Birds, unlike insects, can see red. In the New World (the only place hummingbirds are found), plants with red flowers, especially if they are also rich in nectar, often use these tiny long-beaked birds as their exclusive pollinators. Again, coevolution has occurred: the flowers began to produce more nectar to please the hummingbirds, but deep in the bottom of the flower, away from marauding insects, then the bill of hummingbirds lengthened to better reach the nectar.
In Africa, Asia and Australia, other birds perform the same role and look for the same color: red. If you want to attract hummingbirds or other nectar-feeding birds to your garden, consider climbing honeysuckles, monardas, columbines and cardinal flowers.
Curiously, there are no pollinating birds in Europe and therefore no native red flowers, with one exception. You do find corn poppies (Papaver rhoeas) with bright red flowers there, often in vast numbers … so how does that work? It turns out the red part of the flower leaves local pollinators totally indifferent. It’s the ultraviolet marks in the flower’s center, that seem black to us, that bees and other pollinators go for.
Unless you live in the tropics or near-tropics, you are unlikely to run into pollinating bats, but they do exist and in fact are known to pollinate more than 500 plant species.
Bats prefer large white or pale bell-shaped flowers with a sweet-musty smell that produce abundant nectar. They visit at night, of course … and take advantage of their flower visitations to fill up on any insects also present in the flowers.
There are other mammals that pollinate flowers, especially in Australia where many flowers have adopted a very nectar-rich inflorescence rather like a bottlebrush that facilitates pollination by small nocturnal marsupials.
Now that you know that flowers are as useful to insects and birds as they are attractive to people, try to plant more in your gardens. It’s not asphalt and turf that will attract pretty butterflies and birds!
In a way, some flowers have been forced into a pseudosymbiosis with people. Some have been so extensively bred that they are sterile and unable to produce viable seed; but if they produce flowers or fruit or vegetable (or whatever they were breed for) that are sufficiently appealing, people perpetuate them vegetatively.