Rare Terrestrial Ecological Communities of the United States
by
Dennis H. Grossman
The Nature Conservancy
Kathleen Lemon Goodin
The Nature Conservancy
Rare Terrestrial Ecological Communities of the United States | ||
|
|
by Dennis H. Grossman The Nature Conservancy Kathleen Lemon Goodin The Nature Conservancy |
| Federal agencies and conservation organizations have shifted their focus from managing individual species to managing entire ecosystems to protect biological diversity and conserve natural resources. Although ecological communities provide a more appropriate level of biological organization for characterizing ecosystems than individual species, the lack of a standard ecological community classification has impeded progress for ecosystem protection and management. | ||
| The Nature Conservancy and the Association of Natural Heritage Programs and Conservation Data Centers (Natural Heritage Network) have developed a framework for the classification of ecological communities. The first product from this effort is a preliminary list of rare terrestrial communities across the conterminous United States. This list was completed for the U.S. Fish and Wildlife Service (Grossman et al. 1994). This article provides a summary of the information from the Grossman et al. report, including a review of the status of information concerning rare communities of the United States, an analysis of regional patterns of rarity, and a discussion of the application of this information toward protection efforts. The use of ecological communities as a coarse conservation unit promotes conservation of the underlying ecological processes and biotic interactions that sustain the ecosystems across the landscape and ensures protection of biological diversity and rare species. | ||
The Nature Conservancy/Natural Heritage Network Classification System |
||
| The basic goal of the community classification effort is to provide a complete listing of all communities that represent the variation in ecological systems. The classification hierarchy for terrestrial communities is based on the biological characteristics of existing vegetation types. These types range from early successional through climax associations and include seral stages that are maintained by natural and human-induced management and disturbance regimes. | ||
| The classification hierarchy is partitioned into terrestrial, aquatic, and subterranean "systems." The upper levels of the terrestrial system have been derived through the modification of United Nations Educational, Scientific and Cultural Organization (1973) and Driscoll et al. (1984) and refer to the physiognomic attributes (structural) of the vegetation. The two finest levels of the classification hierarchy are based on floristic analysis and are determined through the identification of diagnostic species (Westhoff and van der Maarel 1973). | ||
Ranking System |
||
| The Nature Conservancy and Natural Heritage Network rank all elements of natural biological diversity according to their relative rarity and vulnerability to aid in ranking critical areas for conservation. The community ranks are consistent with the overall conservation ranking approach applied to all elements of natural diversity within The Nature Conservancy/ Natural Heritage Network methodology (Master 1991). The communities described in this report have been ranked G1 and G2 according to The Nature Conservancy/Natural Heritage Network ranking system (see Table 1). |
Table 1. The Nature Conservancy/Natural Heritage Network conservation ranks for rare communities. | |
Patterns of Community Rarity |
||
| Within the lower 48 United States, 371 globally rare terrestrial vegetated communities have been documented (Grossman et al. 1994). Preliminary evaluation of the proportion of G1 and G2 types indicates that these will account for about 10%-15% of all terrestrial communities. It is premature to attempt detailed national analysis and synthesis of existing data because of the preliminary nature of the overall classification and the unevenness in available community information among regions. We can, however, provide a preliminary examination of the relative proportion of rare communities in each physiognomic class within each region. | ||
| Eastern Region | ||
| Fourteen percent of the nationally rare communities occur in the eastern region (Anderson et al. 1994). Many new community types are still being identified. Most of the rare communities reported from the eastern region were forest, followed by sparse woodland and herbaceous types (Figure). The rarity of these communities is either related to the suitability of these habitats for land conversion or due to association with naturally rare habitats. |
Figure. Rare terrestrial communities by region and physiognomic class. Numbers in parentheses refer to number of community types. |
|
| Southeastern Region | ||
| The rare communities occurring in the southeastern region account for about 18% of the nationally listed types (Patterson et al. 1994). This region is dominated by forest, and numerous diverse and intact rare forest associations remain. Most of the rare types fall within the forest class, followed by the herbaceous and woodland classes (Figure). The communities within the herbaceous class remain poorly defined throughout this region, but this class still represents a large portion of the rare types. We believe that the total number of types and the number of rare types within the herbaceous class will increase as additional information becomes available. Fire suppression has threatened many of the woodland types, and the actual number of rare woodland types is also presumed higher than now reported. | ||
| Midwestern Region | ||
| About 19% of the communities in the list of nationally rare communities occur in the midwestern region (Ambrose et al. 1994). Although the proportion of rare community types in this region is relatively small because of the historically coarse level of classification for this region, the magnitude of land conversion to agricultural production is staggering. The herbaceous class accounts for 40% of the rare types in the midwestern region, and the woodland and sparse woodland types make up another 38% (Figure). The rare herbaceous types reflect the remnant patches of the once-extensive prairie province. The woodland and sparse woodland communities have been heavily affected by the disruption of historical fire regimes and agricultural development. | ||
| Western Region | ||
| Most rare and threatened types identified in the national list of rare communities (about 56%) occur in the western region (Reid et al. 1994). This reflects the region's rich base of ecological and biological data and the consistent application of a detailed level of community classification, as well as a high level of natural diversity in this large region. Most rare types in the western region occur within the forest class, followed by the woodland, herbaceous, and shrubland classes (Figure). Fire suppression as a widespread forest-management practice over many decades has pushed many forest types to this status of rarity. Flood-control and water-diversion projects have similarly affected many of the forest and woodland riparian types. The rarity of the herbaceous communities across the western region is reported to be primarily the result of overgrazing and, to a lesser degree, direct agricultural conversion. | ||
| No regions reported rare communities in the nonvascular class and few were documented within the dwarf shrubland, sparse dwarf shrubland, and sparsely vegetated class. This result may not reflect the actual status of rare communities in these classes throughout the United States but rather the shortage of available information. | ||
Knowledge Gaps |
||
| The rare communities for several states are not documented at this time. This does not mean there are no rare communities in those states but instead indicates the lack of available information. These knowledge gaps were documented during the listing of rare communities. Information gaps at the state level included incomplete or overly coarse classifications, lack of conservation ranks, and the lack of time and support for field verification. Those states where significant work remains are listed in Table 3. | Table 3. Knowledge gaps related to state community classification, ranking, and inventory. | |||||||||||||||||||||||||||||||||
| ||||||||||||||||||||||||||||||||||
| Many communities recognized as rare still require additional work to complete their classification, ranking, and description process. The number of communities in this group presently totals 482. | ||
Limitations |
||
| The number of rare communities varies among regions, reflecting unevenness in the quantity and quality of community information among the regions, along with varying levels of classification development and subtle differences in procedures for conservation ranking. To some extent, the regional variation also reflects the actual differences in ecological and biological diversity, the results of landscape fragmentation, and land-cover conversion. | ||
| While rarity of ecological communities is critical information for biodiversity conservation and management, appropriate protection and management activities should be determined for each individual rare community. Communities assigned a rank of G1 or G2 are very rare and occur generally within a restricted range of environmental conditions. These ranks do not reflect why a particular community is rare; such analysis, however, is fundamental to setting guidelines for protection and long-term management. | ||
| Some communities are naturally rare because of their association with an uncommon habitat. For example, the rarity of the inland salt marsh association (Scirpus maritimus-Atriplex patula-Eleocharis parvula herbaceous vegetation) has been documented, but the community is not noticeably rarer than it was 100 years ago. This kind of community occurs on saturated saline mud flats associated with rare inland salt springs in Illinois, Michigan, and New York (Ambrose et al. 1994; Anderson et al. 1994). The environmental characteristics that support this biological association have similarly restricted the use of this habitat for agricultural production and most other types of land conversion, although some communities have been degraded by salt-extraction operations. Though this community is unlikely to disappear because of human-induced disturbance, individual communities should be protected from degradation due to incompatible land use. | ||
| In contrast, the mesic tall-grass prairie association (Andropogon gerardii-Sorghastrum nutans-Sporobolus heterolepis [Liatrus spp.- Silphium laciniatum] herbaceous vegetation) in the Midwest was common a century ago but is very rare today. The existing occurrences of this association type represent remnants of a community whose acreage has rapidly declined because of the value of its habitat for agricultural production (Ambrose et al. 1994). It has also suffered from the large-scale alteration of historical fire regimes. Rare communities such as this are quite threatened and require immediate protection and management. | ||
Future |
||
| The list of nationally rare communities will help ensure their recognition and set priorities for their protection, an important step for conservation. Even if the list of rare communities were complete, however, it would still be insufficient to conserve and manage biological diversity. A comprehensive national conservation strategy for all communities, including common ones, is necessary to protect and manage the full spectrum of biological diversity and ecological systems. | ||
| The development of a standard community classification system has dramatically increased our capability to make better informed conservation and ecosystem management decisions at multiple geographic scales. The synthesis of existing community data on nationally rare types has identified the strengths and weaknesses of the existing information base, information that will help us decide how to accumulate and analyze data to fill critical gaps in our knowledge. | ||
| The acquisition and management of the ecological and biological data needed to complete the national classification represent a major challenge. The success of many ecosystem management initiatives will depend upon this information. A concerted cooperative effort is necessary to conserve and manage our biological and ecological resources. | ||
|
The Nature Conservancy 1815 N. Lynn St. Arlington, VA 22209 |
||
| References | |
|---|---|
|
Ambrose, D., J. Drake, and D. Faber-Langendoen. 1994. Rare plant communities of the United States: midwestern region. In D.H. Grossman, K.L. Goodin, and C.L. Reuss, eds. Rare plant communities of the conterminous United States: an initial survey. The Nature Conservancy, Arlington, VA. In press. Anderson, M.G., L.A. Sneddon, and K.J. Metzler. 1994. Rare plant communities of the United States: eastern region. In D.H. Grossman, K.L. Goodin, and C.L. Reuss, eds. Rare plant communities of the conterminous United States: an initial survey. The Nature Conservancy, Arlington, VA. In press. Driscoll, R.S., D.L. Merkel, R.L. Radloff, D.E. Snyder, and J.S. Hagihara. 1984. An ecological land classification framework for the United States. U.S. Forest Service Miscellaneous Publ. 1439. Grossman, D.H., K.L. Goodin, and C.L. Reuss, eds. 1994. Rare plant communities of the conterminous United States: an initial survey. The Nature Conservancy, Arlington, VA. In press. |
Master, L. 1991. Assessing threats and setting priorities for conservation. Conservation Biology 5(4):559-563. Patterson, K.D, D.J. Allard, and S. Landaal. 1994. Rare plant communities of the United States: southeastern region. In D.H. Grossman, K.L. Goodin, and C.L. Reuss, eds. Rare plant communities of the conterminous United States: an initial survey. The Nature Conservancy, Arlington, VA. In press. Reid, M., L. Engelking, and P. Bourgeron. 1994. Rare plant communities of the United States: western region. In D.H. Grossman, K.L. Goodin, and C.L. Reuss, eds. Rare plant communities of the conterminous United States: an initial survey. The Nature Conservancy, Arlington, VA. In press. United Nations Educational, Scientific and Cultural Organization (UNESCO). 1973. International classification and mapping of vegetation. Series 6, Ecology and conservation. United Nations Educational, Scientific and Cultural Organization, Paris. 93 pp. Westhoff, V., and E. van der Maarel. 1973. The Braun-Blanquet approach. Pages 626-828 in R.H. Whittaker, ed. Handbook of vegetation science. Part V. Ordination and classification of communities. Junk, The Hague. |