Plants and Pollinators: An Overview
100 Plants to Save the Bees (Xerces Society) The initiation is simple: plant more flowers.
When we observe animals pollinating nearly 90 percent of the plant species found on earth, we are witnessing a process more than 250 million years in the making. Sexual reproduction among plants, from a botanical standpoint, is nothing more than the transfer of pollen grains from a flower’s male anthers to a flower’s female stigmas, enabling fertilization. Once transferred, pollen grains germinate, grow pollen tubes into the plant’s ovaries, and deliver gametes to produce seed and endosperm.
In very primitive plants, this process was carried out by wind or water. Between 245 million and 200 million years ago, however, the first flowering plants arose, with the earliest fossil records containing relatives of today’s magnolias and water lilies. During this prehistoric time frame, flowering plants evolved two major reproductive adaptations: exposed male stamens that bear small, nutrient-rich pollen grains; and enclosed female carpels that protect ovules. These adaptations accelerated plant reproduction (and pollinator diversity), leading to diverse and dominant communities of flowering plants that almost 100 million years ago had spread across the globe.
Plants Meet Pollinators
Beetles, flies, and wasps are thought to be the first pollinators, accidentally spreading pollen while feeding on flowers. This set the stage for more complex plant-pollinator relationships to evolve, including prehistoric flowering plants that first attracted passive pollinators by providing sugary nectar, protein-packed -pollen, fragrant resins, and vitamin-rich fats.
Flowers then responded to particular pollinators, co-evolving with them to provide diverse bloom times, colors, scents, shapes, sizes, and rewards, and improving their reproductive efficiency. For example, flattened, large, scented, off-white flowers with accessible pollen, such as magnolia, attracted beetles, while tubular, large, scented, white flowers that bloom at night attracted moths.
Meanwhile, flowers also developed a variety of strategies to avoid self-fertilization and encourage genetic diversity:
- physical distance between (male) anthers and (female) stigmas
- male and female flower structures that are fertile at different times
- separate male and female plants
Enter the Bees
The widespread distribution of diverse flowering plants 100 million years ago coincided with the appearance of intentional pollinators: bees. Bees are believed to have co-evolved with flowers from predatory wasps. In general, both bees and wasps consume sugars as adults and proteins as larvae. Herbivorous bee larvae eat pollen as their protein source, however, while wasp larvae are typically carnivorous.
Pollen is essential for the reproduction of both bees and flowers, so the two groups have co-evolved for mutual success. Adult bees evolved behavioral and physiological adaptations to gather and transport pollen more efficiently, such as:
Buzz-pollination. Flight muscles can create sound vibrations that dislodge pollen from flowers.
Floral constancy. An individual pollinator may specialize in foraging one flower type.
Pollen-collecting hairs. The “pollen basket” and other specialized hairs on a bee’s body carry pollen back to the colony.
Although most bees are pollen generalists, capable of foraging on many plant species, many are specialists that forage on only a small group of specific flowers.
What Makes a Good Pollinator Plant?
A flower’s color, odor, shape, size, timing, and reward (nectar or pollen) can increase or decrease the number of visits by specific pollinators. Some examples of how plants “reach out” to bees and others:
Ultraviolet invitations. Bees can see ultraviolet light but not red light; thus, flowers in the ultraviolet range attract more bee visits, while red-hued flowers reduce them.
Color phases. Many flowers signal pollinators by changing color at different stages of development, attracting pollinators when they need them most, thus increasing the efficiency of the pollinators they depend upon.
Nectar Guides. Contrasting patterns of flower shades, tints, and tones further direct pollinators toward floral rewards such as nectar or pollen, much like the nighttime runway lights of an airport.
Fragrance. Minty or sweet, musky or ethereal, pungent or putrid, floral odors result from variations in chemical compounds. Fragrance can attract particular pollinators over long distances, varying in concentration and intensity according to species, flower age, and site conditions.
Flower shape, size, and timing work together with color and odor to regulate pollinator visits. Abundant and diverse shapes and sizes, symmetrical or asymmetrical forms, arrangements on stems or branches in simple or complex groups, maturing at different rates: these variations can make it easier or harder for visitors to reach nectar and pollen.
For example, shallow, clustered flowers with landing platforms (such as sunflowers) have easily accessible floral rewards and attract many short-tongued pollinators such as sweat bees, beetles, and flies. In contrast, deep or tubular flowers without landing platforms often have hidden floral rewards accessible only by long-tongued or strong pollinators. A classic example of this latter flower type is bottle or closed gentian (Gentiana spp.), whose flowers remain closed and depend for pollination on bumble bees, which pry the petals apart and climb right inside.
Finally, many plants bloom according to a distinct seasonal rhythm — their phenology — which may be closely timed with the life cycle of specific pollinators. Others, meanwhile, bloom continuously or irregularly during the growing season, attracting many different types of pollinators. These rhythms can invite or exclude different pollinators depending upon the season or even the hour.
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