Dr. Karen Oberhauser’s Study Indicates Low Monarch Population Summer, 2004 Link to Student Worksheet “There is a disturbing indication that the population is going down,” says Dr. Karen Oberhauser, upon reviewing data collected this summer in the Upper Midwest. During the breeding season, hundreds of volunteers participate in a study she coordinates called the “Monarch Larva Monitoring Project.” The volunteers count the number of milkweed plants that bear monarch eggs and caterpillars. In the Upper Midwest this year, monarchs were present on fewer than 5% of the milkweed plants monitored. This compares to monarch presence on a high of over 25% of the milkweed plants counted in 1997, and an annual average of 13%. For the third year in a row, the population is well below average. Why the Low Numbers in the Upper Midwest? Dr. Oberhauser says she is concerned about the data because she can only speculate about the causes for the decline. Scientists do not yet understand how and why monarch populations vary in time and space. “It’s kind of a complicated problem because there’s not one smoking gun,” Dr. Oberhauser says. Many factors can affect the size of the population. Research has not yet shown clear links, but some possible causes for the reduced numbers are: In the wintering habitat: Winter Storms? Lethal storms at the Mexican sanctuaries in the winter of 2002, and again in 2004, killed as much as 85% of the population. This has meant fewer monarchs returning to breed in the spring. Deforestation? Deforestation in Mexico has increased dramatically in recent years. The monarchs need the forest for protection against freezing. In the breeding habitat: Herbicide Use? There may be fewer milkweed plants available to monarchs due to recent changes in agricultural crop practices. More herbicides are now in use because crops have been genetically-modified to tolerate herbicides. Weeds (including milkweed) cannot grow in herbicide-treated fields, but the crops can. This change has been rapid. The use of herbicide-tolerant soybeans grew from 50 percent of fields planted in 2000 to 85 percent in 2003 according to Dr. Oberhauser. Cool Summer Temperatures? This summer’s cool temperatures may have contributed to lower numbers. Several monarch generations live and die during the summer months. The monarch population builds with each successive generation. However, it takes longer for monarchs to develop in cool temperatures. Thus, this summer’s low temperatures may have resulted in fewer monarch generations, so fewer monarchs. July August Predator Levels? Predators always play a significant role in monarch mortality. A host of factors affects the size of predator populations. This means that scientists need more information about monarch-predator populations before the impact of predators can be measured. (Cool temperatures, changes in farming practices, and other factors that affect monarch populations could affect predator populations, too.) Other? What other factors can you imagine that might have resulted in fewer monarchs? Copyright 2004 Journey North. All Rights Reserved. Please send all questions, comments, and suggestions to our feedback form