Precious Heritage: A Watershed Approach to Hot Spots

Excerpted and adapted from Stephen J. Chaplin, Ross A. Gerrard, Hal M. Watson, Lawrence L. Master, and Stephanie R. Flack, "The Geography of Imperilment: Targeting Conservation Toward Critical Biodiversity Areas," in Stein et al., eds., Precious Heritage: The Status of Biodiversity in the United States (2000)

Although at-risk freshwater species are distributed throughout the United States, two particular hydrologic regions dominate, containing 35 % of all vulnerable and imperiled fish and mussel species: 

  • the Tennessee-Cumberland River basins, which includes Tennessee and parts of six other states; and 
  • the Mobile river basin, including Alabama, parts of Georgia and Mississippi, and a bit of Tennessee

Of these at-risk species, 70 % (113) occur nowhere else in the world; they are endemic to one of these two regions. These basins are also rich in other freshwater species, including snails and turtles (Lydeard and Mayden 1995). The Interior Highlands region -- located in Arkansas, southern Missouri, southwestern Oklahoma, and northeastern Texas --is another regional center of diversity, and with 54 species has the next highest count of fish and mussel species at risk.

Of the more than 2,000 small watershed areas found across the continental United States, about 1,300 (61 %) support one or more fish or mussel species at risk. In turn, nearly 100 of these stand out as hot spots, harboring 10 or more vulnerable or imperiled species. These hot spots of aquatic diversity are largely concentrated in the Southeast. Four river basins alone -- the Tennessee, Ohio, Cumberland, and Mobile --contain 18 of the top 20 watersheds. The upper Clinch River on the Virginia-Tennessee border surpasses all other watersheds in the number of imperiled and vulnerable fish and mussel species -- including 21 that are federally listed as endangered or threatened. 

The extraordinary diversity of southeastern rivers results from the coincidence of a diverse physical geography, favorable climate, and a long but dynamic history. The numerous streams of the southeastern United States flow across geologically and topographically diverse landforms. This varied landscape was also spared the repeated advances of continental ice sheets during the Pleistocene, allowing the aquatic fauna to persist and evolve over time. Patterns of evolution were, however, affected by the changes in climate, stream drainage patterns, and coastline position that accompanied the repeated glacial advances and contractions in the North. These cyclic changes isolated many populations, enabling them to diverge genetically and evolve into new species.

For hot spots to be useful in defining conservation priorities, they must be defined at the right scale. For many aquatic species the small watershed units used in this analysis is an appropriate scale. In certain circumstances, however, conservation activities may be effectively employed on a larger scale (for example, in several adjacent watersheds) or a smaller scale (such as in a small headwater stream or a single spring within the watershed), depending on the nature of the threats and the species of concern. Looking beyond small watershed boundaries is also essential when vulnerable streamlife regularly moves between drainages, such as sturgeons, squawfishes, and anadromous fishes like salmon and shad. 

On the other hand, viable populations of at-risk fish and mussel species may be found only in a small portion of the watershed, and not its entire area. Additionally, threats to the continued existence of at-risk species may only be manageable in a small area of the subbasin. Some threats may require attention upstream, such as the effects of large dams and water diversions. Surprisingly, other threats may require abatement downstream, for instance dams blocking fish passage, or channelization and gravel extraction that can cause destructive upstream-progressing river channel erosion.

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