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Pollinators (Excerpt from Sustaining Life)

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There are thought to be more than 100,000 different pollinator species on Earth. Declines in their numbers, reaching 70 percent in some places, have been reported in every continent except Antarctica. The consequences of such steep declines in pollinators for the world’s food supply are potentially enormous.

While the majority of the world’s staple crops (wheat, rice, maize, potatoes, yams, and cassavas) are either wind or self-pollinated or are propagated vegetatively (e.g., by stolons [aerial shoots from a plant that produce new root systems and new off shoots] or by rhizomes), many other important agricultural species rely on pollinators. For instance, more than 80 percent of the 264 species grown as crops in the European Union are dependent on insect pollination. In addition, the yield of tomatoes, sunflowers, olives, grapes, and soybeans—all major crops—is optimized by regular pollination. Fruit trees and legumes may be particularly hard hit by a loss of pollinators, especially since they are grown intensively.

In late 2006 in the United States, honeybees began dying in great numbers along the East Coast of the United States, as well as in Texas, California, and other states. A total of 24 states were affected, and losses of up to 70% of hives were reported. The disease has been named colony collapse disorder (CCD) and is causing great alarm among beekeepers and farmers who grow such crops as alfalfa, almonds, apples, oranges, peaches, blueberries, and cranberries which are all heavily dependent on honeybee pollination. Billions of dollars in agricultural losses are expected from the declines. As of early 2007, the cause of CCD had not been determined, and there was concern among some researchers that certain pesticides, such as the insect neurotoxin imidacloprid, banned in France because of suspected honeybee toxicity, might be involved. A study published in Science has since demonstrated a strong correlation between the presence of a virus, called Israeli Acute Paralysis Virus, or IAPV, and CCD hives. IAPV was first detected in Israel, where infected bees became paralyzed and died outside of the hives (a characteristic of CCD). The virus may have been carried into the United States by infected bees, perhaps from Australia, and resulted in the epidemic of CCD. But it is still not clear whether IAPV is the cause of CCD, either by itself or in combination with other factors.

When compared to wind-pollinated plants, or plants that are pollinated by a broad range of organisms, plants that have specific animals pollinating them, such as figs that are pollinated only by fig wasps, have the lowest risk of pollen being wasted during transport. These same plants, however, also have the highest risk of pollination failure if their pollinators are lost. For this reason, a decline in biodiversity may have cascading effects on species survival, because it may disrupt these close-knit, highly efficient, co-evolved relationships. Just as a high diversity of pollinators may help increase the diversity of plants, a high diversity of plants supports more pollinators. A recent study in the United Kingdom and the Netherlands, for example, shows a marked parallel decline in bee species in the two countries in recent decades and in the plant species that depend on them to reproduce.

In agricultural regions, crops may be isolated from the habitats that support the pollinators they depend on to be productive. Experiments on isolated “islands” of radish and mustard plants, which were set up in an agricultural landscape at varying distances from a species-rich grassland, showed that increasing isolation resulted in fewer bee visits per hour to the radish and mustard islands, and also in reductions in the diversity of the visitors. In addition, the development of fruits and seeds declined with increasing isolation from the grasslands. In another study, the amount of woody border had a significant positive effect on the overall diversity of insect families in agricultural fields.

Aaron Bernstein, MD, MPH

Aaron Bernstein is the Interim Director of The Center for Climate, Health, and the Global Environment, a pediatrician at Boston Children’s Hospital, and an Assistant Professor of Pediatrics.

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