USGCRP logo & link to home

Updated 12 October, 2003
The Ecological Effects of Global Warming on North American Birds and Butterflies
Horizontal blue line.
USGCRP Seminar, 22 October 1996

What is the evidence for changes in migration patterns, movements, and extinctions of North American birds and butterflies? What is the relationship between climate change (warming) and the changes described above? What are the ecological and societal consequences of these changes? What are the ecological and practical implications of land use, urbanization, and development in the face of the changes described above?


Dr. H. Ronald Pulliam
Science Advisor to the Secretary of the Interior, the Honorable Bruce Babbitt, Washington, DC


Dr. Terry L. Root
Associate Professor, School of Natural Resources and Environment, University of Michigan, Ann Arbor, MI
Dr. Camille Parmesan
National Center for Ecological Analysis and Synthesis, University of California at Santa Barbara, CA


Global warming would be expected to cause the normal range for any particular species to shift both northward and upward in elevation. This prediction comes from the fact that such range shifts are well-documented for climate changes that occurred during the many glacial (ice-age) and inter-glacial (warm) periods of the Pleistocene (i.e., the last few million years). For example, when the Earth was warming up after the last ice age peaked around 20,000 years ago, scientists have been able to see (in the fossil record, in pollen samples from ancient lake bottoms, etc.) that species that were mobile enough to move northward and upward in elevation did just that. Species that could not keep pace with the climate changes became extinct, either because their mobility was insufficient to keep up with the rate of habitat change, or because suitable habitat was not available in more northern areas. Both global temperature analyses and continental U.S climate analyses indicate that modest but significant warming has occurred during this century, thus it is essential to look at how animal and plant species and ecosystems are responding.

We are constantly learning more about how our planetıs ecological puzzle fits together. History has shown just how complex natural communities are and how interdependent plants and animals are. For example, eradicating the wolf populations in the lower 48 States has indirectly contributed to the decline of dabbling ducks, a consequence that the U.S. Government has spent much effort trying to reverse. The elimination of wolves in the lower 48 States prompted an increase in coyote populations, which mobilized eradication efforts causing declines in the coyote populations, which in turn resulted in surprisingly large populations of red foxes, which are effective predators of dabbling ducks. Accounting for such complexities of nature is essential to generating more rigorous forecasts about the possible consequences (e.g., decline in dabbling ducks) that various actions (e.g., eradication of the wolf) may create. Such examples make clear that understanding how changes in fluctuations in climate have influenced plants and animals over past decades and centuries can help provide society with an understanding of the possible ecological consequences of predicted global warming over coming decades and centuries. Such scientific information will provide managers and decisionmakers with a firmer basis for making important policy decisions.

The Case of North American Birds

Using the National Audubon Societyıs Christmas Bird Count data, Dr. Terry Root has been able to show that, over North America, the northern limits of a majority of bird species are strongly associated with various climatic variables (e.g., winter temperature). Further analyses have revealed that both the ranges and the abundances of birds shift, on an annual basis, in concert with temperature. Indeed, in another study, Dr. Root and colleagues have shown that a significant number of migrating birds are arriving in northern Michigan up to 21 days earlier now than they did in 1960. Apparently, many bird species can and do respond to changing climatic conditions. These birds, however, having ranges limited by vegetation, will probably not be able to shift their ranges with the changing climate, at least not until the vegetation itself shifts. Consequently, natural communities of birds could be torn apart, thereby possibly resulting in a disruption of Natureıs ecological checks and balances until or unless new ones can be established.

The Case of Butterflies in Western North America

Because butterflies are very sedentary, each population has evolved very specific adaptations to its local habitat. These insects only just manage to fit their life cycle into the 8­12 week window of time when their host plants are edible. This window can occur from March to August, depending on the exact locality. Any long-term climate change is likely to affect the length of this window, and either cause the population to do really well (if, for example, caterpillars can have a longer growth period) or go extinct (if, for example, caterpillars have an abbreviated growth period). As a consequence, studies on sedentary butterfly species indicate that, in general, butterflies are very sensitive to climate changes.

Dr. Parmesanıs work on the biogeography of Edithıs Checkerspot has shown that the level of population extinctions is four times as high at the far southern end of its range (in Mexico) than at the far northern end of its range (in Canada). Extinctions are also about two and one-half times as great at lower elevations as compared to populations above 8,000 ft. This skewed pattern of extinctions is effectively shifting the range of the butterfly both northward and upward in elevation.

Although her work on butterflies incorporates the most complete data set to document this type of range shift, other (smaller) studies from around the world are also indicating northward movement of speciesı ranges. If studies on the ranges of dissimilar species over many geographic regions continue to reveal similar trends and patterns, then a conservative conclusion would be that the currently detected levels of global and regional warming are affecting the ability of organisms to live in their traditional habitats. Based on the response of butterflies to past changes and fluctuatuations in climate, Dr. Parmesan and colleagues predict that those species that cannot move to new habitats to keep pace with climate change will die out. These changes will be exacerbated because the existence of new habitat for butterflies to move into is likely to be much more severely limited than during historical climate shifts because potential habitats are being rapidly taken over by humans, particularly by urbanization and changes in the natural landscape for agricultural purposes.

Biography of Dr. Terry L. Root

Dr. Terry L. Root is an Associate Professor in the School of Natural Resources and Environment at the University of Michigan and an intermittent employee of the U.S. Geological Surveyıs recently created Biological Resources Division. Dr. Rootıs work has focused on large-scale ecological questions addressing factors shaping the ranges and densities of animals, primarily birds. Because climate was found by Dr. Root to be such an important factor in shaping the ranges and densities of birds, she has incorporated her findings in helping to forecast the possible consequences global warming may have on animal communities. In 1990, she received the prestigious Presidential Young Investigator Award from the National Science Foundation. In 1992, she was one of 10 individuals selected worldwide as a Pew Scholar in Conservation and the Environment. Both honors underscored Dr. Rootıs work in basic research, her application of that work to complex real-world problems, her ability to work at the cutting edge of research requiring interdisciplinary effort, and her outreach to decisionmakers and the general public. She received her bachelorıs degree in Mathematics and Statistics from the University of New Mexico, after which she worked as a scientific programmer at Bell Laboratory and on NASAıs Voyager Project. Upon returning to school, she obtained her masterıs degree in Biology from the University of Colorado in 1982, and her Ph.D. in biology from Princeton University in 1987. After serving a short post-doctoral fellowship at the Museum of Zoology at the University of Michigan, she accepted an Assistant Professorship at the School of Natural Resources and Environment.

Biography of Dr. Camille Parmesan

Dr. Camille Parmesan is currently a post-doctoral fellow with the National Center for Ecological Analysis and Synthesis at the University of California at Santa Barbara. Her research has focused on multiple aspects of population biology, including the ecology, evolution, and behaviors of insect/plant interactions. Dr. Parmesan has been investigating Edithıs Checkerspot butterfly since 1982. Since that time she has continued to work, in varying capacities, on both laboratory and field projects centered around the population biology of this butterfly. She has also been the recipient of a National Science Foundation fellowship as well as a NASA Global Change fellowship. Dr. Parmesan received her bachelorıs degree in Zoology from the University of Texas at Austin in 1984, and later received her Ph.D. in Biological Sciences from the University of Texas at Austin in 1995.



US CCSP  logo & link to home USGCRP logo & link to home
US Climate Change Science Program / US Global Change Research Program, Suite 250, 1717 Pennsylvania Ave, NW, Washington, DC 20006. Tel: +1 202 223 6262. Fax: +1 202 223 3065. Email: Web: Webmaster: