Lisa Palmer, YALE ENVIRONMENT 360
Specimens of goldenrod sewn into archival paper folders are stacked floor to ceiling inside metal cabinets at the Smithsonian National Museum of Natural History. The collection, housed in the herbarium, dates back to 1842 and is among five million historical records of plants from around the world cataloged there. Researchers turned to this collection of goldenrod — a widely distributed perennial plant that blooms across North America from summer to late fall — to study concentrations of protein in goldenrod pollen because it is a key late-season food source for bees.
The newer samples look much like the older generations. But scientists testing the pollen content from goldenrod collected between 1842 and 2014, when atmospheric concentrations of carbon dioxide rose from about 280 parts per million to 398 ppm, found the most recent pollen samples contained 30 percent less protein. The greatest drop in protein occurred from 1960 to 2014, when the amount of carbon dioxide in the atmosphere rose dramatically. A field experiment in the same study that exposed goldenrod to CO2 levels ranging from 280 to 500 ppm showed similar protein decreases.
More than 100 previous studies have shown that elevated levels of atmospheric carbon dioxide decrease the nutritional value of plants, such as wheat and rice. But the goldenrod study, published last month, was the first to examine the effects of rising CO2 on the diet of bees, and its conclusions were unsettling: The adverse impact of rising CO2 concentrations on the protein levels in pollen may be playing a role in the global die-off of bee populations by undermining bee nutrition and reproductive success.
“Pollen is becoming junk food for bees,” says Lewis Ziska, a plant physiologist at the U.S. Department of Agriculture’s (USDA) Research Service in Maryland and lead author of the study. The study itself concluded that the decline of plant proteins in the face of soaring carbon dioxide concentrations provides an “urgent and compelling case” for CO2 sensitivity in pollen and other plant components.
Elevated CO2 levels affect plant physiology by enabling the plant’s starchier parts to grow faster and bigger, since atmospheric carbon dioxide is a building block for plant sugars. For goldenrod, this growth essentially dilutes the plant’s total protein, From 2006 to 2011, losses from managed honeybee colonies averaged 33 percent per year in the U.S. rather than concentrating it in the grain, which makes a starchier pollen.
“I knew there was work done on insects about how rising CO2 would reduce the protein content of leaves, and so insects will need to eat more leaves to get the same amount of protein,” says Ziska. “But until now, we didn’t know about how CO2 affects protein content in pollen.” The study is a synthesis of the knowledge about what is happening to bees and how CO2 impacts the quality of plants, and it brings those two disparate ideas together.
A number of new and accumulating pressures are threatening bee populations. From 2006 to 2011, annual losses from managed honeybee colonies averaged 33 percent per year in the United States, according to the USDA. Beekeepers have had to replace 50 percent of their colonies in recent years. Factors such as mite outbreaks and the use of neonicotinoid pesticides have been implicated in so-called “colony collapse disorder.”