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June, 2015

Plant Kingdom Cupids

by Valerie

Beautiful flowers, nutritious vegetables, lumber and medicines, not to mention the oxygen we breathe, are all products of living organisms: plants. They are everywhere, so ubiquitous that we often take them for granted. We burn, cut and eat them. We even walk on them. It is easy to forget that plants are living beings and, like animals, they need food, water, suitable habitat and the chance to produce the next generation. Unlike most animals, though, individual plants cannot simply move about to get these needs met.

While a seed will only germinate when conditions are right (therefore taking care of the habitat, food and water requirements), when it comes to finding a mate, being rooted to one spot is an obvious hindrance. One option is to simply clone offspring through vegetative reproduction, but the history of life on Earth has proven that mixing genetic material with other individuals is by far the more advantageous way to go. Hence the evolution of pollen and the wonderful diversity of delivery systems.

When asked the question "What is the world's most important pollinator?" most people immediately think of bees. Insect pollination is certainly important for the vast majority of plant types so that answer has its validity, but not all plants are of equal importance when it comes to feeding humanity's billions. Without the cereal crops - rice, wheat and corn, as well as hay to feed livestock, it would be quite a challenge to support our species' numbers on this finite planet. So, in terms of human survival, the correct answer is: the wind.

Wind pollination and the much rarer water pollination require no reward system to get the job done. In these pollination syndromes (anemophily and hydrophily), male flowers simply release copious quantities of pollen that float, either in the air or on the water, until a lucky few make contact with a female flower of the same species. The flowers of these plants have no need to draw attention to themselves and lure tiny "Love's Messengers" to move their sperm, so they are small and inconspicuous.

pollen covered flower scarab (Euphoria kernii) in prickly poppy blossom
Insect pollinated flowers, on the other hand, are often showy to an extreme. Their purpose is to be noticed. These plants have essentially trained animals to do the job of transferring pollen, poetically called love dust, from male to female through the use of incentives or deception. Animal pollinators service over 80% of angiosperm species.

Pollination can be accomplished by any animal that is capable of picking up the grains and moving them to another plant. While insects are often the most appropriate size and shape for this task, vertebrates can do it as well. In a few extreme instances, natural insect pollinators have been eliminated and humans are then set to work hand-pollinating a desired crop. This has actually happened with some apple and pear tree orchards in China. There are also natural vertebrate pollinators. In North America, bats visit the lofty large night-blooming flowers of saguaro cactus, as well as some species of agaves. Hummingbirds are a frequent sight in gardens, where they feed at numerous types of flowers. It is often possible to see the dusting of pollen on their foreheads as they hover around the blooms.

The concept of pollination syndromes, while easy to apply to plants using wind, water, and a few specialized animals, tends to break down when it comes to the complexity of actual ecosystem interactions. The same flowers that attract hummingbirds also lure bees and flies; butterflies, wasps and beetles feed side by side; and many plants have different pollinators depending on the season, weather, or even time of day. In spite of these caveats, pollination syndromes do help us understand some aspects of the evolution of plant and animal relationships.

Long-tubed red flowers rely mainly on hummingbirds and butterflies, white color and strong fragrance attract nocturnal moths, and beetle-pollinated blooms are frequently cup-shaped with many pollen stamens. These are very broad generalizations and only address outward appearance. The amount of nectar and pollen, the timing of peak readiness, and even the quality of the nectar are significant as well. Sometimes large numbers of inefficient pollinators come to a particular flower that might also attract a less common species that is far more valuable to the plant. The factors between energy and resource expenditure by any plant and its successful pollination are numerous and varied. The field of plant/animal interactions is rich in possibilities; even casual garden observations can add to the general understanding of what is happening.

When it comes to insect pollination, bees do, of course, top the list. Not only do they partake of nectar, but they also collect pollen to feed their young. Most bees have hairy bodies, with special hairs on some parts adapted to holding pollen. After bees, flies are the second most important major insect group. While flies do not collect pollen, many have fuzzy bodies and are at least as mobile as bees, visiting large numbers of blossoms. Some flies have specialized mouthparts so they can sip nectar through a tube, much like butterflies' proboscises. Flies are often also more cold-tolerant and fly at lower temperatures than many bees. Other insects that transport pollen include wasps, beetles, moths, butterflies and true bugs. Depending on the hairiness, feeding technique, mobility, size and number of any particular insect, it might or might not be a major pollinator for any particular plant.

Many pollination discussions focus on our food crops, which is understandable from the standpoint of human survival. However, a diverse ecosystem is more resilient and adaptable so the more plant species there are, the better for overall habitat health. In the big picture, we, as biological beings, are dependent on our environment as much as any other animal. The majority of plant species comprising any biome are usually small and not particularly distinctive. Many wildflowers, i.e. weeds, also require insect pollinators but they don't warrant in-depth study because they are "just" part of the overall environment. What insects pollinate these ignored components? How do they contribute to the ecology of an area? Where do they fit into the food web? What would be the consequences if they disappeared?

Winter in the southern states is mild enough that plants grow and even flower during the cooler weather. Many of these are small and hug the ground, where the warmth is adequate to create an agreeable microhabitat suitable for tiny blossoms to be pollinated by small flies, beetles or moths. There are some plants that produce their blossoms right at ground level, where they are almost invisible, except to the beetles, wasps and flies that pollinate them. The tiniest of flowers require equally small pollinators. Some bees and flies are only a millimeter or two in length.

The next time you see a flowering tree or wildflower patch buzzing with activity on a sunny day, take a closer look. Bees are often present, but they are not the only animals enjoying the feast while, at the same time, hauling pollen between plants. Many flies and beetles mimic bees and wasps, so it is quite a challenge to distinguish them, but then, what would our world be like without such engagingly complex diversity?

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