March 17, 1998
KALAMAZOO -- The rising level of carbon dioxide, one of the main greenhouse gases linked to global warming, could begin having an even more direct impact on the quality of the world's food systems, a Western Michigan University researcher reports.
Research conducted by Dr. David N. Karowe, associate professor of biological sciences, shows that just the presence of an increased level of carbon dioxide in the atmosphere impacts the growth of food plants and the life cycles of two levels of insects that rely on those plants for their existence.
His work, funded primarily by the National Science Foundation and the U.S. Department of Agriculture, hints at long-term impact on the quality and quantity of food. And, unlike the global warming issue in which scientists and lay persons debate whether global temperature increase is due to human influence, Karowe says there is no debate about whether the carbon dioxide level is increasing in the atmosphere.
"It's been increasing since the beginning of the industrial age and is now increasing exponentially," he says. The level of carbon dioxide in the atmosphere has increased by about 20 percent since the start of the industrial age -- roughly 150 years ago -- but is expected to double in the next 60 to 75 years, given current energy use and government regulations and treaties.
That's important because carbon dioxide is part of every food web, Karowe points out. Photosynthesis, which converts atmospheric carbon dioxide into carbohydrates, occurs in all green plants and provides the food that fuels nearly all ecosystems on Earth. He's trying to determine what happens to those food plants if the level of carbon dioxide in the atmosphere doubles.
Working with plants in the cabbage family, Karowe's studies have involved growing the plants at field locations equipped to deliver twice the current level of carbon dioxide in the atmosphere. With more carbon to draw from but the same amount of nitrogen available, he says, the nitrogen in the plant is diluted by increased carbon levels. As a result, the plant grows larger, but the nutritional value of its leaves decreases.
Karowe found that caterpillars of the cabbage butterfly, which feed on the cabbage plant and lay eggs on its leaves, eat 40 to 60 percent more when plants are grown under elevated carbon dioxide, presumably to compensate for the lower nutrient value. Still, they grow to a size 10 to 15 percent smaller than normal. They also digest the leaves less efficiently.
Because of volatile chemicals emitted by both the damaged plant and the undigested food in the feces of the caterpillar, a second level of insects, a wasp that lays its eggs in the caterpillar, finds it two to three times easier to locate the caterpillar. Since the wasp larvae kill their host, the caterpillar's survival rate decreases. But the wasp larvae produced also suffer, growing only to a size about 15 percent smaller than normal.
"In this particular system, changes in plant quality appear to result in increased mortality for the caterpillar as well as diminished size for the wasp," he notes.
Karowe says the fact that he can document the impact on growth and survival of three levels in the food web, called trophic levels, is what sets his work apart. Other researchers have documented changes in two levels of a food web and most food webs support four or five trophic levels. What the changes to each level of the food web will ultimately mean is still unclear, however.
"Ecosystems will likely be affected in quite noticeable ways," he says, "but what those ways are we still don't know. One basic question is whether any of the trophic levels will adapt and change along with the changes in the atmosphere and plants."
Karowe says the cabbage plant family is typical of a large number of food plants in the way it uses carbon dioxide. He also is examining plants that interact with nitrogen-fixing bacteria in the soil. Legumes are typical of that type of plant. He reports initial studies show a less clear-cut impact of high carbon dioxide levels on legumes and on caterpillars of the suffer butterfly, which feed on legumes.
Karowe notes that his research is only related to global warming by the fact that he is studying one of the major components contributing to global warming. Global warming is an indirect result of increased levels of carbon dioxide and other gases such as methane and chlorofluorocarbons.
"If global warming happens as much and as quickly as predicted, " Karowe says, "the direct results of increased carbon dioxide may seem minimal in comparison to the indirect impact of warming. But the carbon dioxide level by itself, I believe, could have a strong impact on the quality and quantity of food."
Karowe has presented his work at several professional conferences, including the Gordon Research Conference on plant-herbivore interactions held recently in Ventura, Calif. Gordon Research Conferences are a series of international meetings convened on highly specialized topics to discuss the research frontiers of the biological, chemical and physical sciences. The meetings, limited in size, are designed to foster the exchange of ideas and information on the most advanced aspects of the participating researchers' fields.
Media contact: Cheryl Roland; firstname.lastname@example.org
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