The Importance of Pollinators

Plants are the most prolific and important primary producers on Earth.  Through photosynthesis, they capture energy from the sun and convert it into organic molecules such as sugars.  These energy rich molecules are the fuel for Life, and plants supply the majority of these compounds for the rest of the living organisms on Earth. Without plants, most other organisms, including us, would not be able to survive.

As with all living things, plants must reproduce. Many plants can reproduce vegetatively.  For instance, many aquatic plants can grow shoots that break-off and are carried by currents to new locations where they establish and grow, but this form of reproduction only produces clones that are identical to the mother plant. If environmental conditions change, these clones may not be adapted to survive the new conditions. To ensure  survival of their offspring in a world with constantly changing environments, most plants also reproduce sexually to instil diversity into their progeny.  Sexual reproduction is the process by which an organism combines its genes with another related organism to form offspring that share traits of both parents.  The “shuffling” of genes between the two parent plants, increases diversity among their offspring which increases the likelihood that some will survive even if conditions change.

The earliest plants, such as algae, mosses, and ferns, relied mainly on water and wind to carry sperm cells from one plant to another. When the gametes (sex cells) combined, they would form a developing embryo that would have to have suitable growing conditions to develop and survive.  One group of plants evolved seeds, which are able to survive for long periods of time until proper growing conditions occur.  These early seed producing plants are the Gymnosperms and include the conifers, cycads, and Ginkgo.  They were the first plants to produce pollen, but, like the ferns and mosses, they still relied on wind to transfer pollen. This left them at the mercy of the climate.

About 140 million years ago, a new group of plants evolved.  These plants produce seeds like the gymnosperms, but the ovary where the seeds develop is protected in a carpel housed in a specialized structure called the flower.  Flowers are basically specialized sex organs which help to protect and control the conditions around the developing seeds. The development of the flower drove speciation among the angiosperms, allowing them to diversify and explore other methods of transferring pollen.  Some angiosperms developed traits that attracted insects and other animals to visit the flowers, and those plants gained a competitive advantage over the plants that relied on wind and water pollination.

Not long after the evolution of the first flowers, during a period called the Cretaceous, angiosperms started to diversify rapidly, so much so, that they quickly became the most species-rich group of plants on the planet. Today, there are over 300,000 species of flowering plants, which constitute about 90% of the species of plants on Earth. Likewise, insects and other pollinating animals underwent a period of rapid speciation.  Coevolution between flowers and pollinators has resulted in the dazzling assortment of flower shapes, colors and smells that we see today and an amazing array of specialized adaptations among the pollinators.  Diversification also has occurred among several groups of vertebrates that visit flowers.  Mammals, birds, and reptiles that feed on flower nectar also coevolved with plants.

The selective force driving flower-pollinator diversification is a concept called “pollinator fidelity.”  Pollinator fidelity is a measure of how likely a pollinator will visit flowers of the same species of plants.   Plants with adaptations that enhance pollinator fidelity are more likely to have their pollen delivered to the correct species of flower than plants that cannot ensure fidelity. Some plants ensure pollinator fidelity by specializing their flowers to serve a small set of pollinator species. Other flowers are more general but use other strategies to ensure pollinator fidelity  This leads to the development of “pollination syndromes,” the coupling of certain flower adaptations with particular pollinator behaviors.

POLLINATION SYNDROMES

- Tubular flowers, especially red ones, tend to be pollinated by hummingbirds. Their anthers and pistils are situated to contact the head feathers of the bird which prevents most insects from pollinating them. One dramatic example of hummingbird specialization is displayed by Heliconia tortuosa, a common forest flower in Central America.  It is pollinated by only two species of hummingbirds (the Green Hermit and Violet Sabrewing) which have specialized bills.  Although other hummingbirds will visit this particular Heliconia, the plant only responds when visited by one of these two hummingbirds because they are the only species with bills that can access the nectaries. 

- Some orchids perform sexual deception to get pollinated.  Their flowers deceive male wasps by mimicking the shape, color and smell (pheromones) of female wasps.  The male wasp pollinates the flower by trying to mate with it. This type of pollination leads to highly specialized flowers and is one of the many reasons that orchids are the most diverse family of plants.

- Other plants with smaller, more generalized flowers ensure pollinator fidelity by synchronizing their flowering or producing large composite flowers.  Increasing the abundance of flowers of a particular species increases the likelihood that pollinators will visit related flowers just by shear statistics. Many of these plants (like Asteraceae and Apiaceae) display their flowers prominently on the tops of the plants to make them highly visible.

- Some plants produce large volumes of nectar to attract pollinators like honeybees and other social insects which are extremely efficient at locating the most productive flowers. Many social bees, especially honeybees and bumblebees are able to communicate the location of these nectar sources to their nest mates.

- Some flowers limit which pollinators can access the reproductive parts by mechanical means.  Nightshades have pollen that is trapped deep in the flower and can only be released by insects that can vibrate the flower vigorously, such as bumblebees, a phenomena called “buzz pollination.” Flowers like the white turtlehead (Chelone glabra) remain closed and can only be accessed by the strongest of insect pollinators such as the carpenter bees.

- Other flowers are extremely fragrant to attract particular insects from long distances. The earliest insect pollinated plants, the ancestors of today’s magnolias and waterlilies produced large sticky-sweet smelling flowers to attract beetles that feed on fermenting and decaying plant matter.  Many of the Cactus which are pollinated by bats or moths at night produce very aromatic flowers. Taking smell to the other extreme, corpse flowers emit the odor of rotting flesh to attract flies and beetles that consume dead animals.

Evolution of flowering plants has led to adaptations that exploit a wide range of animal behaviors. In turn, the pollinators have responded by developing specialized relationships with particular groups of plants, and pollinator fidelity has created reproductive isolation that drives speciation.

Pollinator conservation is absolutely critical to biodiversity on Earth and ecological function in most terrestrial ecosystems.  Because most species of plants have a mutually beneficial relationship with their pollinators, survival of one depends on the other. Plants are the primary food sources for most ecosystems.  They support insects and wildlife that feed on them and shelter in their cover. They form the organics that support soil microbes and invertebrates that consume the decaying vegetation and build living soil for other plants to use.  Plants alter microclimates and affect hydrology. Pollinators are food for an array of predators some of which actually seek particular insects to eat and plants to patrol.  Also, many pollinators are predators or parasites of other insects, keeping pest populations in check. The decline of pollinators or their host plants would cause broad reaching changes to ecosystems, degrading conditions and negatively impacting species throughout the system.  So, it is critical that humans account for pollinators and their host plants when altering land uses and vegetative cover.