More Info

Saturday, November 8, 2008

Pollinator decline

The term Pollinator decline refers to the reduction in abundance of pollinators in many ecosystems worldwide during the end of the twentieth century.

Pollinators participate in sexual reproduction of many plants, by ensuring cross-pollination, essential for some species, or a major factor in ensuring genetic diversity for others. Since plants are the primary food source for animals, the reduction of one of the primary pollination agents, or even their possible disappearance, has raised concern, and the conservation of pollinators has become part of biodiversity conservation efforts.

Observation of pollinator decline

As plantings have grown larger, the need for concentrated pollinators at bloom time has grown. At the same time populations of many pollinators has been declining, and this decline has become a major environmental issue today. Pollination management seeks to protect, enhance, and augment agricultural pollination.

For example, feral honey bee populations in the US have dropped about 90% in the past 50 years, except for the Southwest where they have been replaced by Africanized bees. At the same time managed honey bee colonies have dropped by about two thirds. On the other hand, this has been offset by a natural increase in native pollinator populations in parts of the US, where such had been partially displaced by the invasive honey bees imported from Europe.

Monoculture needs very high populations at bloom, but can make the area quite barren, or even toxic when the bloom is done.

The study of pollinator decline is also interesting some scientists, as bees have the potential to become a keystone indicator species of environmental degradation. Any changes in their abundance and diversity will influence the abundance and diversity of the prevailing plant species. This is a mutual dependency as bees rely on a steady nectar source and pollen source throughout the year to build up their hive.


The value of bee pollination in human nutrition and food for wildlife is immense and difficult to quantify.

It is commonly said that about one third of human nutrition is due to bee pollination. This includes the majority of fruits, many vegetables (or their seed crop) and secondary effects from legumes such as alfalfa and clover fed to livestock. In the United States, only about 30% of crops utilize honeybees for their pollination, and even among those some of the bee usage is superfluous, native pollinators actually doing the work.

In 2000 Drs. Roger Morse and Nicholas Calderone of Cornell University, attempted to quantify the effects of just one pollinator, the Western honey bee, on only US food crops. Their calculations came up with a figure of US $14.6 billion in food crop value.

There has not been sufficient study to quantify the effects of pollinator decline on wild plants and wild life that depends on them for feed. Some plants on the endangered species list are endangered because they have lost their normal, native pollinators because of displacement by invasive honey bees. It is important to recognize that honey bees are not native to the Western Hemisphere, so any loss of honey bees does not represent a threat to native plants; the role of honey bees in the Western Hemisphere is almost exclusively agricultural. To the extent that honey bees compete with native bee species, a decrease in the honey bee population may be beneficial to native plants and pollinators.

Increasing public awareness

The steady increase in beekeeper migration (for pollination service on agricultural crops) has masked the issue of pollinator decline from much public awareness, however sudden blocks to such migration could have catastrophic results on the global food supply.

Possible explanations for pollinator decline

Pesticide misuse

It is a label violation to apply most insecticides on crops during bloom, or to allow the pesticide to drift to blooming weeds that bees are visiting. Yet such applications are frequently done, with little enforcement of the bee protection directions. Pesticide misuse has driven beekeepers out of business, but can affect native wild bees even more, because they have no human to move or protect them.

Bumblebee populations are in jeopardy in cotton-growing areas, since they are dosed repeatedly when pesticide applicators apply insecticides on blooming cotton fields while the bees are foraging.

Widespread aerial applications for mosquitoes, med-flies, grasshoppers, gypsy moths and other insects leave no islands of safety where wild insect pollinators can reproduce and repopulate. One such program can reduce or endanger pollinator populations for several years.

Many homeowners feel that dandelions and clover are weeds, that lawns should only be grass, and that they should be highly treated with pesticides. This makes a hostile environment for bees, butterflies and other pollinators.

Rapid transfer of parasites and diseases of pollinator species around the world

Increased international commerce within modern times has moved diseases such as American foulbrood and chalkbrood, and parasites such as varroa mites, acarina mites, and the small African hive beetle to new areas of the world, causing much loss of bees in the areas where they do not have much resistance to these pests. Imported fire ants have decimated ground nesting bees in wide areas of the southern US.

Loss of habitat and forage

The push to remove hedgerows and other "unproductive" land in some farm areas removes habitat and homes for wild bees. Large tractor mounted rotary mowers may make farms and roadsides look neater, but they remove bee habitat at the same time. Old crops such as sweet clover and buckwheat, which were very good for bees have been disappearing. Urban and suburban development pave or build over former areas of pollinator habitat.

Clearcut logging, especially when mixed forests are replaced by uniform age pine planting, causes serious loss of pollinators, by removing hardwood bloom that feeds bees early in the season, and by removing hollow trees used by feral honey bees, and dead stubs used by many solitary bees.

Nectar corridors

Migratory pollinators require a continuous supply of nectar sources to gain their energy requirements for the migration. In some areas development or agriculture has disrupted and broken up these traditional corridors, and the pollinators have to find alternative routes or discontinue migration. A good example is the endangered lesser long-nosed bat (Leptonycteris curasoae) which was formerly the main pollinator of a number of cactus species in southwestern United States. Its numbers have severely declined, in part due to disruption of the nectar corridors that it formerly followed. Other migratory pollinators include monarch butterflies and some hummingbirds.

Hive destruction

Bees are often viewed negatively by homeowners and other property owners. A search for "carpenter bees" on the Internet primarily yields information on removal rather than information regarding bees in a positive light. Recent hysteria regarding killer bees has contributed to these views. Beekeepers find increased vandalism of their hives, more difficulty in finding locations for bee yards, and more people inclined to sue the local beekeeper if they are stung, even if it is by a yellow jacket.

Light pollution

Increasing use of outside artificial lights, which interfere with the navigational ability of many moth species, and is suspected of interference with migratory birds may also impact pollination. Moths are important pollinators of night blooming flowers and moth disorientation may reduce or eliminate the plants ability to reproduce, thus leading to long term ecological effects. This is a new field and this environmental issue needs further study.

Threat by invasive honey bees

Many native pollinators decline in population when faced with competition from invasive honey bees. For example, the western honey bee is invasive in the United States, the wild population comprised entirely of feral bees escaped from European bee colonies imported to fertilize non-native, old-world crops. Where colony collapse disorder reduced invasive honey bee populations in the US, native pollinators sometimes have made recoveries, restored to their natural niche by the loss.

Air pollution

Researchers at the University of Virginia have discovered that air pollution from automobiles and power plants has been inhibiting the ability of pollinators such as bees and butterflies to find the fragrances of flowers. Pollutants such as ozone, hydroxyl, and nitrate radicals bond quickly with volatile scent molecules of flowers, which consequently travel shorter distances intact. There results a vicious cycle in which pollinators travel increasingly longer distances to find flowers providing them nectar, and flowers receive inadequate pollination to reproduce and diversify.

Solutions to pollinator decline

The decline of pollinators is compensated to some extent by beekeepers becoming migratory, following the bloom northward in the spring from southern wintering locations. Migration may be for traditional honey crops, but increasingly is for contract pollination to supply the needs for growers of crops that require it.

Conservation and restoration efforts

Efforts are being made to sustain pollinator diversity in agro- and natural eco-systems by some environmental groups. Prairie restoration, establishment of wildlife preserves, and encouragement of diverse wildlife landscaping rather than monoculture lawns, are examples of ways to help pollinators.

Use of alternative pollinators

Honey bees are usually the most widely chosen insects in most managed pollination situations. However they are not the most efficient pollinators of some flowers. Alternative pollinators, such as for example, leafcutter and alkali bees in alfalfa pollination and bumblebees in greenhouses for tomatoes are used to augment and in some cases replace honey bees. A wide variety of other bees can be found in the environment that are specialist pollinators (some only using one plant species). However, most of these alternative insects' value as pollinators and their relationships with plants are as yet little known.

In the US, some think that other pollinators will in time replace the lost honey bees, blamed on introduced acarine and varroa mites, but general pollinator decline was already happening before these entered the picture. Only in a few areas are wild populations of pollinators building up; in most areas they are declining as quickly as honey bees.

Furthermore pollinators cannot be exchanged on a one-for-one basis. They are not all equal. Some are generalists, some are specialists. Some are brawny; some are feeble. Some have long tongues; some short. Some work at colder temperatures than others. Bees may deliberately collect pollen, but have different collection techniques, which can greatly affect their efficiency as pollinators.

Flowers are frequently specifically adapted to one pollinator, or a small group of pollinators because of floral structure, color, odor, nectar guides, etc. Proposed alternative pollinators may not be physically capable of accomplishing pollination, or they may not be attracted to the flower of that plant species, or they may rob nectar by cutting sepals, thus avoiding pollination. Understanding the pollination needs of a species is vital to understanding of a plant species, yet this is often poorly understood. In horticulture it is critical to the economic success of the grower, and crops have sometimes been abandoned from general use in an area because of lack of understanding of pollinator needs.


Graham_Cliff said...

"Light pollution......This is a new field and this environmental issue needs further study."

Not so - Colin Henshaw warned in 1994 that light pollution or light at night (the 24 hour day) would kill off insects, reduce pollinator numbers and reduce plant biodiversity. Read the original letter now archived at Harvard here -
You can read more here -
The emerging reality is frightening. What do you think?

Graham_Cliff said...

The full URL for Colin Henshaw's JBAA 1994 Harvard archived "missive" is here -