by
Science Editor
Raymond J. Boyd
Bureau of Land Management
Service Center
Denver Federal Center, Bldg. 50
Denver, CO 80225
Text by
B.D. Keeland
National Biological Service
Southern Science Center
700 Cajundome Blvd.
Lafayette, LA 70506
|
|
by Science Editor Raymond J. Boyd Bureau of Land Management Service Center Denver Federal Center, Bldg. 50 Denver, CO 80225 Text by B.D. Keeland National Biological Service Southern Science Center 700 Cajundome Blvd. Lafayette, LA 70506 |
|
Terrestrial Ecosystems | ||
| Overview | ||
| Whereas several other sections in this volume cover individual species or locations, the articles in this section address the integration of individual species into communities and ecosystems (see glossary). Terrestrial ecosystems include a rich variety of community types and cover a range extending from nearly aquatic wetlands along our coasts and myriad rivers, lakes, and streams, to mountain tops and arid, desert locations. The diversity of these ecosystems offers both challenge and opportunity. The challenge stems from the sheer number of potential ecosystems to be analyzed. Grossman and Goodin (this section) discuss 371 imperiled and critically imperiled communities, and state that this number represents only 10%-15% of all terrestrial communities. This implies a minimum of 2,500-3,500 individual terrestrial community types. Obviously, a single report cannot hope to address more than a few of these many terrestrial communities and ecosystems. | ||
| Discussions of biological diversity have traditionally revolved around the protection of individual species. More recently, we have begun to realize that protection of community or ecosystem diversity is equally important. Patchwork conversions of natural landscapes for agriculture, silviculture, and development result in a fragmentation that leaves small remnant areas of natural ecosystems (Burgess and Sharpe 1981). As these natural patches become smaller and more isolated, their ability to maintain healthy populations of many plant and animal species is reduced (Harris 1984). As individual species are lost from each fragment, the community changes and both species and ecosystem diversity are reduced. Thus, large numbers of natural ecosystems are now in danger. | ||
| Kendall (this section) discusses one such imperiled ecosystem. The whitebark pine (Pinus albicaulis) ecosystem of the western mountains is endangered because of the combined effects of an introduced disease and fire suppression. The effects of introduced diseases on natural species and ecosystems have been well documented. Several species, such as the American chestnut (Castanea dentata), have been virtually eliminated and other species have been greatly reduced by introduced diseases. The effects on ecosystems where these species were previously found have been dramatic (Shugart and West 1977). | ||
| Alteration of natural fire regimes has played a major role in the reshaping of natural ecosystems. In many systems a reduction in fire frequency can lead to invasion by fire-intolerant species and eventual loss of the original ecosystem. This is shown by Henderson and Epstein (this section) in their discussion of how fire supression and other factors caused tremendous losses of oak savannas throughout the Midwest. In other systems, an increase in fire frequency can also lead to changes in ecosystem structure and function. Although we now realize that fire is a natural and necessary part of many ecosystems, it was not until after the devastating fires of Yellowstone National Park that the general public was alerted to the benefits of such fires (Elfring 1989). An effective fire-suppression program can allow accumulation of vast amounts of detritus (dead organic material such as leaves, branches, and stems). If this material is not consumed periodically by small fires burning along the forest floor, it will accumulate to the point of providing raw materials for an exceptionally intense fire that can burn tree crowns and destroy the existing forest. Ferry et al. (this section) discuss four fire-adapted ecosystems that have been affected by modified fire regimes and conclude, "Managers must balance the suppression program with a program of prescribed fire applied on a landscape scale if we are to meet our stewardship responsibilities." | ||
| Numerous variables in addition to disease and fire affect our natural resources. These variables include pollution (Peterson, this section; Nash et al., this section), conversions to other uses, harvesting activities such as logging, and global climate change. Cole (this section) demonstrates that over the past 5,000 years change has been a natural part of our terrestrial ecosystems. Within a given ecosystem some species decline in importance while others increase over time, resulting in a change in the overall character of the ecosystem. A key feature to stand out in the 5,000-year chronology developed by Cole is that current rates of change are about 10 times higher than presettlement rates. Human intervention in one form or another is now the principal agent of change. Darr (this section) provides a review of U.S. Forest Service data and discusses changes being brought about by forestry-management practices. At a reduced spatial scale, Keeland et al. (this section) discuss changes within the forested wetlands of the southeastern United States. Forested wetlands have been especially reduced and fragmented as a result of land-use conversions, predominantly to agricultural activities. | ||
| A common thread here, as in all sections in this report, is that if unchecked, human activities will continue to result in an upset balance of species interactions, alteration of ecosystems, and extensive habitat loss. | ||
| References | |
|---|---|
|
Burgess, R.L., and D.M. Sharpe, eds. 1981. Forest island dynamics in man-dominated landscapes. Springer-Verlag, New York. 310 pp. Elfring, C. 1989. Yellowstone: fire storm over fire management. BioScience 39(10):667-672. |
Harris, L.D. 1984. The fragmented forest. The University of Chicago Press, Chicago. 211 pp. Shugart, H.H., and D.C. West. 1977. Development of an Appalachian deciduous forest succession model and its application to assessment of the impact of the chestnut blight. Journal of Environmental Management 5:161-179. |