Proc Natl Acad Sci U S A 97(4): 1630-1633

Climate change is affecting altitudinal migrants and hibernating species

^{Department of Biology, University of Maryland, College Park, MD 20742;}^{Rocky Mountain Biological Laboratory, P.O. Box 519, Crested Butte, CO 81224;}^{Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS 66045-2106; and}^{Center for Population Biology, One Shields Boulevard, University of California, Davis, CA 95616}

^{To whom reprint requests should be addressed. E-mail:}ude.dmu.liamu@5id.

Edited by Harold M. Mooney, Stanford University, Stanford, CA, and approved December 17, 1999

Received 1999 Oct 8

Abstract

Calendar date of the beginning of the growing season at high altitude in the Colorado Rocky Mountains is variable but has not changed significantly over the past 25 years. This result differs from growing evidence from low altitudes that climate change is resulting in a longer growing season, earlier migrations, and earlier reproduction in a variety of taxa. At our study site, the beginning of the growing season is controlled by melting of the previous winter's snowpack. Despite a trend for warmer spring temperatures the average date of snowmelt has not changed, perhaps because of the trend for increased winter precipitation. This disjunction between phenology at low and high altitudes may create problems for species, such as many birds, that migrate over altitudinal gradients. We present data indicating that this already may be true for American robins, which are arriving 14 days earlier than they did in 1981; the interval between arrival date and the first date of bare ground has grown by 18 days. We also report evidence for an effect of climate change on hibernation behavior; yellow-bellied marmots are emerging 38 days earlier than 23 years ago, apparently in response to warmer spring air temperatures. Migrants and hibernators may experience problems as a consequence of these changes in phenology, which may be exacerbated if climate models are correct in their predictions of increased winter snowfall in our study area. The trends we report for earlier formation of permanent snowpack and for a longer period of snow cover also have implications for hibernating species.

Abstract

In high mountains, such as the Colorado Rocky Mountains, there is a short, but active, growing season during which resources are abundant and temperatures mild. In contrast, there is a very long winter when the ground is covered with deep snow and temperatures can reach −40°C. Animal species in this environment have evolved various responses to the onset of winter, during which snow may cover the ground for more than 7 months. One set of species has adopted hibernation as a way to conserve energy during the time when food is unavailable (e.g., marmots, ground squirrels, chipmunks), and another set migrates to more favorable climates at lower altitudes or lower latitudes (e.g., elk, deer, birds). Climate change may pose special challenges to some of these species if it results in changes in the length of the summer or winter seasons or if the cues used by migrants at different altitudes between their winter and summer grounds change their synchrony.

A growing body of evidence suggests that climate change is affecting the phenology (seasonal timing) of animal and plant activity at low altitudes. For example, long-term studies in England have documented the earlier arrival of migratory birds (1), earlier reproduction by amphibians (2) and birds (3, 4), earlier breaking of leaf buds (5), and changes in moth phenology (6). There are fewer published studies to date about similar changes in North America, but one report documents earlier egg laying by tree swallows (7), a second reports earlier reproduction by Mexican jays (8), and a third reports a variety of phenological changes for both plants and animals (9); all of these examples are from low altitudes. We report here that there is no evidence for a change in timing of the growing season at our high-altitude study site in the Rocky Mountains over the past 25 years. We suggest that the apparent growing disjunction between low- and high-altitude phenology will create problems for the many migratory species that reproduce at high altitudes during the summer. We also report a change in the phenology of a hibernating species that is correlated with changing air temperatures and hypothesize that this change, in opposition to the lack of response in the growing season, will create problems for it.

Acknowledgments

The paper benefited from comments by Manuel Morales, Doug Gill, Joan Maloof, Paul Callo, and three anonymous reviewers. Field research was supported by the RMBL, the National Science Foundation (IBN-98–14509), and Earthwatch and its research corps.

Acknowledgments

Abbreviation

RMBL	Rocky Mountain Biological Laboratory

Abbreviation

Footnotes

This paper was submitted directly (Track II) to the PNAS office.

Footnotes

References

1. Sparks T H. Int J Biometeorol. 1999;42:134–138.[PubMed]
2. Forchhammer M C, Post E, Stenseth N C. Nature (London) 1998;391:29–30.[PubMed]
3. Crick H Q P, Dudley C, Glue D E, Thomson D L. Nature (London) 1997;388:526.[PubMed]
4. Crick H Q P, Sparks T H. Nature (London) 1999;399:423.[PubMed]
5. Sparks T H, Carey P D, Combes J. London Naturalist. 1997;76:15–20.[PubMed]
6. Woiwod I P. J Insect Conservation. 1997;1:149–158.[PubMed]
7. Dunn P O, Winkler D W. Proc R Soc London Ser B. 1999;266:2487–2490.
8. Brown J L, Li S-H, Bhagabati N. Proc Natl Acad Sci USA. 1999;96:5565–5569.
9. Bradley N L, Leopold A C, Ross J, Huffaker W. Proc Natl Acad Sci USA. 1999;96:9701–9704.
10. Menzel A, Fabian P. Nature (London) 1999;397:659.[PubMed]
11. Karl T R, Jones P D, Knight R W, Kukla G, Plummer N, Razuvayev V, Gallo K P, Lindseay J, Charlson R J, Peterson T C. Bull Am Meteorol Soc. 1993;74:1007–1023.[PubMed]
12. Harvey L D D. Nature (London) 1995;377:15–16.[PubMed]
13. Armitage K B In: Biodiversity in Marmots. Le Berre M, Ramousse R, Le Guelte L, editors. Moscow/Lyon: International Marmot Network; 1996. pp. 223–226. [PubMed][Google Scholar]
14. French A R. Oecologia. 1990;82:93–96.[PubMed]
15. Michener G R In: The Biology of Ground-Dwelling Squirrels. Murie J O, Michener G R, editors. Lincoln: Univ. of Nebraska Press; 1984. pp. 81–107. [PubMed][Google Scholar]
16. Andersen D C, Armitage K B, Hoffman R S. Ecology. 1976;57:522–560.[PubMed]
17. Bibikow D I Die Murmeltiere der Welt. Magdeburg: Westarp Wissenschaften; 1996. [PubMed][Google Scholar]
18. Van Vuren D, Armitage K B. Can J Zool. 1991;69:1755–1758.[PubMed]
19. Giorgi F, Mearns L O, Shields C, McDaniel L. Climate Change. 1998;40:457–493.[PubMed]
20. Office of Policy, Planning and Evaluation. Climate Change and Colorado. United States Environmental Protection Agency; 1997. , EPA 230-F-97–008f. [PubMed]
21. Kattenberg A, Giorgi F, Grassl H, Meehl G A, Mitchell J F B, Stouffer R J, Tokioka T, Weaver A J, Wigley T M L In: Climate Change 1995: The Science of Climate Change. Houghton J T, Filho L G M, Callander B A, Harris N, Kattenberg A, Maskell K, editors. Cambridge: Cambridge Univ. Press; 1996. pp. 285–357. [PubMed][Google Scholar]