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In The Heartland
An Under Recognized Resource
William Wordsworth in his Green Linnet said that birds, butterflies, and flowers all make one band of lovers (paraphrased). Plants are incapable of all but the slightest movements that limit their ability to engage in sexual reproduction. Many flowering plants must rely on other methods, such as pollinators, to move pollen, male gametes, from the anthers to the pistil, containing the female gametes.
Pollination is an ecological process linking plants and animals. The interdependence of plant reproduction and survivability of the pollinators requires that we consider these relationships when attention is given to a specie of plant or pollinator. A few plants have evolved to become independent of external influences on pollination. These include peas, beans, wheat, rye, and barley (6).
The pollinators are one of the most important resources responsible for the well being of humanity. For without the pollinators, plant and animal species would loose their “margin of life” (8) necessary to sustain their existence leading to a decline in the available food. Eighty percent of the species of our food plants worldwide depend on pollination by animals, most of which are insects (5).
Pollinators act as agents that fertilize plants by moving pollen from one plant to another. Many living pollinators are insects, but this group also includes birds and mammals. Livestock, wildlife, and even humans moving through a forest or a grassland can aid pollination by causing a transfer of pollen clinging to hair, fur, and clothing from one plant to another. Some pollinators such as bats and moths are seldom recognized as aiding in fertilizing the female plant parts as they perform their work in the dark of night.
Wind and water aids in moving pollen from one plant to another. Plants such as the conifers have flowers (cones) specifically designed to take advantage of wind born pollen. Migrating humming birds and Monarch butterflies move in north - south direction in North America servicing seasonal needs of flowering plants.
Flowering plants and insects have evolved simultaneously. Some plants require a specific insect for pollination such as the cocoa tree. Although the cocoa tree produces a flower that woes many insects, only a single tiny fly can negotiate the intricate barricades inherent in the flower to provide successful pollination of the plants (3). Many plants depend on a single pollinator to survive. A tree on the island of Mauritius became extinct after the loss of the Dodo bird. It was found that this tree required that its seed pass through the gut of the Dodo bird to complete the fertilization and initiate the germination process (5).
Plants have developed specific colors, perfumes, and even offered rewards to encourage pollinators to visit their reproductive accessories. Not all flowering plants produce the nectar so often associated with the production of honey. Some plants such as the orchids trick male bees and wasps to their flowers by emitting a female scent. The males who emerge a few days ahead of the females engage the flowers repeatedly in an attempt to satisfy their purpose while moving pollen packets from one orchid flower to another (3).
Some tropical plants flower in colors that attract specific humming birds with the appropriate length beak. A majority of bird-pollinated plants express red colored flowers. Studies have determined that red is a visual attractant for birds and is supported by the fruits in the temperate zones that provide food for birds (2). Certain desert plants such as century plants used for tequila production have limited pollinators such as bats.
Many conditions influence the chances for pollination to occur. The earliest known insect pollinators were beetles (3). The symbiotic evolution of plants and pollinators has yielded some interesting partnerships. Although the act of pollination appears to be entirely accidental, the pollinators perform their tasks impeccably and unconsciously.
Not all plants require animal pollination. Many grasses including corn have evolved to depend entirely on the wind or by self-pollination in closed flowers. Pastures used for hay often have several varieties of grasses and legumes. Most forage legumes require cross-fertilization and some such as red, alsike, and white clovers are even self sterile (1). Wind provides little benefit to these legumes that require that pollinating insects be present. A lack of pollinators results in little or no seed production to sustain adequate stands of legumes in most pastures.
Alfalfa is one of the most studied pollinated plants and has an atypical plant and animal relationship. More than 100 known pollinators exist for alfalfa and this plant depends on insects for survival. In a one acre seed producing plot, as many as five colonies of honeybees are required to assure efficient pollination for a seed crop. Studies at the Legume Seed Research laboratory in Logan, Utah revealed that alfalfa plots without bees and no insect control produced only 5 pounds of seed per acre. Fields where insects were controlled, but without bees produced 15 pounds of seed per acre. Those with bees and no insect control produced 31 pounds of seed per acre (6).
Cutting hay before bloom becomes a management decision one must make in favor of high quality hay or perpetuation of the native pollinators. A little used management tool is to leave strips of the pasture uncut to provide habitat for native pollinators. Incorporating native bunch grasses in borders surrounding hay fields to provide suitable nesting can also enhance native pollinator populations.
Weather can affect plant pollination. Regional droughts caused by deforestation have resulted in low flowering plant populations that support local pollinating bees and wasps (5). Temperature extremes affect domesticated honeybee colonies sometimes causing a loss of the entire population in a region. Bumblebees can literally starve to death during long rainy periods that cause them to remain confined in their nests.
The factor most affecting the current health of the pollinator populations is loss of habitat. Expansion and fencerow-to-fencerow agricultural operations have reduced portions of the biosphere that the pollinators depend on for lodging and food. The uses of herbicides, pesticides, and chemical fertilizers all have a harmful affect on the pollinators. Insect dependent bird populations have been displaced due to loss of diversity of pollinating insects caused by habitat loss.
Rodent populations even have an affect on some pollinators. Bumblebees usually select abandoned rodent nests for their domicile. A determination should be made to determine the practicability of encouraging or controlling some species of rodents (6).
The Monarch butterflies have increased their ranges due to deforestation that has increased the available milkweed plants required for their food source. So not all native pollinators are in decline. Monarch butterflies also provide winter feed for migratory songbirds helping to increase their chances of survivability.
The herbicides used to eradicate weeds in crop fields destroy nectar-producing plants that pollinators rely on to fill gaps in their diets. Many crops are grown for seed production and there is a need to provide a level of purity in the seed. These monoculture practices reduce the likely hood of species diversity and species survival. Introgressive hybridization (5) begins to occur when plant diversity diminishes. The phenomenon results in related plant species producing undesirable hybrid plants caused by pollinators seeking adequate quantities of nectar for survival.
Chemical pesticides used on crop ravaging insects frequently eradicate beneficial insects such as predatory wasps and ground dwelling pollinators. Over use of natural pesticides such as Bacillus thuringiensis (BT) used to control worms and caterpillars has been shown to cause harm to pollinators when used continuously or in excess. There has been much controversy over the use of genetically modified pest resistant crops such as the corn containing the BT gene and its effect on the Monarch butterflies.
Many of the pollinating insects go through a larval stage. This stage may represent an undesirable effect on crops due to crop destruction or visual damage. Eradication of the larvae of certain moths reduces the benefits they offer as adults. The economic threshold of a pest population is the population level below which the cost of taking control action exceeds the losses caused by the pest and should be considered when choosing to use pesticides (7).
Much can be done to reduce pesticide impacts on the pollinators. A simple question can be employed when considering the use of pesticides. “Will this application fit into a general program calculated to control all important pests without presenting a hazard to health or seriously affecting beneficial parasites, predators, and pollinators?” (4). Establishing and maintaining hedgerows around agricultural fields and along roadways, ditch banks, and canals will provide shelter and food for pollinators. Leaving less desirable plants such as elderberry, sumac, and pithy or hollow stemmed plants in borders will increase the likelihood of retaining beneficial insects, especially the native pollinators.
Cultivation is often used to reduce pest pressure in cropland. Tilling can destroy the habitat for nuisance insects such as the European corn borer, grasshoppers, and crickets (9). Compaction resulting from fall tilling may also provide habitat for beneficial ground dwelling wasps (5).
One of the most familiar pollinators is the common honeybee. This bee was introduced to North America more than 375 years ago (5) and by today’s definition would be considered an invasive specie. The honeybee quickly inhabited most of North America. For as much as is known about the habits of the common honeybee, few studies have been done to show the detriment they have caused from competition with the native pollinators. Recounts from Native Americans and early settlers indicate that stingless bees were prevalent pollinators prior to the introduction of the common honeybee. These stingless bees produced less honey, but the honey from these bees was used as a pharmaceutical to cure many human ailments and would store for years in crude conditions without crystallizing or fermenting (5).
Honeybees have an important attribute in pollinating plants used for seed crops. The bees have a tendency to visit flowers of the same species on a foraging trip thereby insuring cross-fertilization and homogeny in plant species (3)
Common honeybees are by far the most important insect pollinator for agricultural crops, but they does have some shortcomings. Common honeybees seek out the richest sources of pollen and nectar. They will neglect less attractive plants for richer food sources that are blooming at the same time. Many of the native bees actually neglect cultivated crops preferring a narrow selection of native plants. Native bees are rarely found in cultivated areas unless sufficient uncultivated terrain exists nearby (6).
Pollination is one of the world’s most vital processes linking plants and animals and keeping the biosphere running in an endless cycle. Pollinators are a resource being compromised by expansion, agricultural pressures, pollution, and loss of habitat. It is worth noting that one of every three mouthfuls of food we swallow is prepared from plants pollinated by animals (5).
and orchards that sustain our food supply require that we preserve
nearby wildlands to assure adequate yields. The stability of the world’s
food, fiber, and beverage supply depend on the remaining wildlands
and the animals that inhabit them. We cannot afford to ignore the
importance of the wildlands and the pollinators who inhabit them.
Our margin of life depends on a small link in the system of plant
reproduction, the pollinators.
1. Anderson, Clinton P. et al, 1948. Yearbook of Agriculture: Grass. United States Government Printing Office, Washington, D.C.
2. Barth, Friedrich G., 1991. Insects and Flowers, The Biology of a Partnership. Princeton University Press, Princeton, New Jersey.
Berenbaum, May R., 1995. Bugs in the System, Insects and Their Impact
on Human Affairs. Addison-Wesley Publishing Company, Reading, Massachusetts.
5. Buchmann, Stephen L. and Nabhan, Gary Paul, 1996. The Forgotten Pollinators. Island Press, Washington, D.C.
6. Freeman, Orville L. et al, 1961. Yearbook of Agriculture: Seeds. United States Government Printing Office, Washington, D.C.
7. Madigan, Edward et al, 1991. Yearbook of Agriculture: Agriculture and the Environment. United States Government Printing Office, Washington, D.C.
8. Poirot, Eugene M. 1964. Our Margin of Life. Vantage Press, New York, N.Y.
9. Wallace, Henry A. et al, 1938. Yearbook of Agriculture: Soils and Men. United States Government Printing Office, Washington, D.C.
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