March 17, 2008
Bob Stewart is well known to lovers of the natural world. He was the Marin County naturalist for many years, leading hikes around the county. He worked at Point Reyes Bird Observatory on the breeding bird atlas project. He has written several books on the butterflies of Marin, California and Arizona. And he even has a trail at Bolinas Lagoon named after him.
When I’m walking through my garden in spring, I hardly ever notice any bees flying about. But when my plum tree is in bloom, wow, it’s a regular symphony of buzzing! And yes, by Fourth of July I’m rewarded with buckets of plums to eat. How does this all come about? What is the relationship between these plants and animals?
It’s called pollination ecology. The transfer of pollen from the male parts of a plant to the female parts of a plant is called pollination. This is a mutualistic relationship that benefits both the plant and the pollinator. First, the plant must attract the insect by colorful flowers or odors. Then, the plant must supply a reward, either nectar or pollen. And of course the plant benefits from cross pollination and fertilization. This allows the plant to reproduce and the species to survive, and provides us with much of the food we eat.Sixty five percent of flowering plants are pollinated by insects. Some plants are pollinated by other animals, including birds and bats. Some plants, e.g., oaks, poplars, walnuts, birch, gymnosperms and grasses are wind pollinated. And some plants are self-pollinated, in which case, cross pollination providing genetic diversity does not occur.Beetle pollinated flowersBeetles have a highly developed sense of smell. Flowers they are attracted to are often white or dull in color and have strong odors. Western skunk cabbage is pollinated by small beetles in the family Staphylinidae.Pollination by beesBees are the most important group of pollinators. One third of the food supply of the United States depends on bee pollination. For example, honeybees pollinate 90% of California’s almond crop. We need these guys! They feed on nectar and simultaneously carry away a load of pollen from flower to flower. Scientists have shown that bees can learn to recognize colors, odors, and shapes. The bee can see ultraviolet, but cannot see red (it appears as black to them).Moths and butterflies as pollinatorsBoth moths and butterflies drink nectar from flowers with their long slender proboscis. The pollen from one flower is deposited on the proboscis and transferred to the female part of the next flower. Butterflies can see colors, but their sense of smell is not well developed. They are daytime fliers and like to rest on the flower to feed. In contrast, moths are twilight- or night fliers. Colors are not easily distinguishable at night and many flowers are not open at night. Moths are attracted to flowers with pale colors and strong odors. They do not need a place on the flower to land as they move quickly from flower to flower.Some pollinators are host specific. This means that the insect relies exclusively on one plant species and the plant relies exclusively on the insect for pollination. The adult insect benefits from the nectar of the flower then carries pollen to another flower. In the yucca-yucca moth relationship, the yucca moth is the pollinator. It also lays its eggs in the yucca flowers and the larvae live in the developing ovary and eat yucca seeds. Figs and fig wasps are another example of a mutualistic relationship.Pollination by birdsThroughout most of the United States, hummingbirds are the only birds that pollinate flowers. (In the Southwest white winged doves pollinate the giant Saguaro cactus.) Hummingbirds are attracted to brightly colored flowers, especially red and orange. Since birds have a poor sense of smell, they are not attracted to flowers with strong scents. Hummingbirds hover while they feed on nectar. They brush the top of their heads against the pollen bearing stamens before moving to the next flower.Wind pollinated flowersMany trees, such as box elder, oak, poplar, walnut and birch, are wind pollinated. All gymnosperms, such as pines, spruce and fir, spread their pollen by the wind. Corn and other grasses (oats, wheat, and barley, for example) are also wind pollinated. Corn has an especially interesting means of pollination and fertilization. The top of the corn ear has the male staminate flowers in a tassel. The female flowers are along the ear of the corn. Pollination takes place at the top, the pollen grain travels down the silk to fertilize the female flower. And voilá, one kernel of corn is formed. Corn is an important crop plant. More than half of the world crop is raised in the United States!The pollen of plants that are wind pollinated is very fine and produced in great quantities. It must be light enough to be carried by the wind and copious enough to survive the great losses that occur.Bat pollinated flowersFound in Marin, the Pipevine swallowtail butterfly only feeds on California pipevine (Artistolochia californica). The sprawling vine with flowers shaped like a calabash pipe are pollinated by fungus gnats which are attracted to the flower odor.Asclepias curassavica, milkweed, is pollinated by large wasps. The nectar of milkweed is a food source for Monarch butterflies. The butterflies lay their eggs on milkweed and the newly hatched larvae feed on the leaves.In the Marin Headlands, trails were closed to protect the habitat of the rare Mission blue butterfly. The Mission blue butterfly (Icaricia icarioides missionensis) feeds on species of lupine, e.g. Lupinus albifrons, L. variicolor, L. formosus. The yellow bush lupine is pollinated primarily by honey bees and bumble bees.Lithophragma parviflorum, woodland star, and the moth Greya politella. The moth is both a pollinator and a consumer of the the plant seeds,In a mutualistic relationship the plant and pollinator benefit each other. The plant expends less energy in pollen production and instead produces showy flowers, nectar and/or odors. A flower that attracted specific pollinators on a regular basis has an advantage over flowers that attracted promiscuous pollinators. Less wasted pollen. It is an advantage to the pollinator to have its own private food source—less competition. The plant must supply a reward, nectar and pollen—food; an attractant—brightly colored flower or odor; and a means of transferring the pollen to the pollinator. The plant and pollinator have co-evolved together.www.universityofcalifornia.edu/news/artuicle/4463p. 374, Biology of Plants“How Plants are Pollinated” by Joan Elma Rahn