Interior Low Plateau Distributional Patterns 185 



narrow habitat or niche requirements, remained close to the points of 

 entry (as in the case of some of the headwater species), or they may have 

 spread inexorably through systems, progressively coming into compe- 

 tition with members of the preexisting fauna. Such competition may 

 have caused some minor extinctions here and there by way of niche 

 replacement, but it probably stimulated subdivision of niches and, 

 apparently, considerable sequences of divergence, speciation, and other 

 types of faunal readjustments. Some speciation may have been stimu- 

 lated by the presence of unoccupied niches, by the habitat fluctuations 

 associated with long-term geologic influences, and by differences between 

 various segments of Low Plateau drainages. 



All these influences have acted in concert to create a superior aquat- 

 ic fauna in the streams and rivers of the Interior Low Plateau, but one 

 which has remained relatively poorly understood until recent times, par- 

 ticularly in its relationships to extralimital drainages. Now a new factor 

 has been superimposed upon natural ones, a factor that not only threat- 

 ens the integrity of the fauna and flora (Branson 1977), but one that 

 also threatens our ability to understand this biota. The results of pre- 

 impoundment studies indicate that some species, present prior to dam- 

 ming of streams, are now either very rare or unknown from afflicted 

 areas. An example is Hybognathus nuchalis (pers. comm., David Etnier 

 and students, following study of University of Michigan holdings). 

 Every major stream that flows through the Low Plateau bears dams, 

 sometimes multiple ones. These structures have vastly modified habi- 

 tats, upset temperature regimens, and diminished nutrient flow through 

 the systems. Exotic species and transplanted ones, such as striped bass, 

 trout, and threadfin shad, have gotten into the system, creating stress 

 that was not earlier present. Previously autonomous drainages are now 

 interconnected. For example, a navigation canal joins the Cumberland 

 and Tennessee rivers at Land Between the Lakes; other connections are 

 planned, such as the Tennessee-Tombigbee Waterway. Coal mining and 

 channelization continue to inflict adverse changes in many Low Plateau 

 streams. Other recent influences are steadfastly afflicting this unique 

 fauna, bringing many species to levels of concern (Branson et al. 1981). 

 Most are associated with human population pressures, the changing 

 financial picture, and the energy crisis. Hopefully, we have learned 

 enough to preserve at least the principal genomes of the Low Plateau 

 biota. 



ACKNOWLEDGMENTS.— I greatly appreciate the helpful and crit- 

 ical comments of Mr. Melvin L. Warren, Jr., Kentucky Nature Pre- 

 serves Commission, and the criticism of Dr. David Etnier, University of 

 Tennessee. 



