224 R. W. PENNAK 



2. From a purely environmental standpoint, the freshwater 

 fauna of the West does not have the advantages of stable flo wages 

 and many associated large lakes. Western topography is violent and 

 broken, the climates are highly demanding because of wide annual 

 variations, and barriers are abundant and rigorous. Many of our 

 rivers are laden with silt during much of the year, the gradients 

 are steep, and the water levels sometimes vary from trickles (or dry 

 beds) in late autumn and winter to rushing torrents during the 

 spring runoff. Droughts in the West are often extreme and exten- 

 sive; undoubtedly they are effective in inhibiting or exterminating 

 local populations. Except in mountainous regions, the West has 

 relatively few natural lakes, and many of these are small, ephemeral, 

 and pondlike. 



3. We assume that many species of freshwater invertebrates in 

 the eastern half of the United States could become well established in 

 western areas, but for several important reasons these eastern 

 species in certain taxa are apparently prevented from naturally 

 spreading westward. The following barriers, for example, are un- 

 doubtedly effective: prevailing westerly winds, the topography and 

 headwater drainages of the Continental Divide area, steep stream 

 gradients, intermittent rivers, streams, and ponds, and extensive 

 deserts and semi-arid regions. 



4. Nevertheless, when examined closely, the western states 

 appear to present a set of conditions that should encourage isolation 

 and speciation, especially in certain taxa containing macroscopic 

 forms, and the West should theoretically have a unique population 

 of freshwater invertebrates. Some of these conditions may be 

 enumerated briefly, as follows: (a) the abundance of barriers to 

 gene flow (mountains, deserts, closed drainage systems, variety of 

 climates, etc.); (b) the abundance of natural and artificial lakes 

 and springs with peculiar chemistry (alkali lakes, saline lakes, 

 saline springs, etc.); (c) thermal springs; (d) the generally wide 

 variety of lakes and streams. Indeed, there is already evidence to 

 show that the western aquatic invertebrate fauna is much richer 

 and more varied than is indicated in the literature. Our personal 

 experience with alkali lakes, thermal springs, and high-altitude 

 lakes and ponds convince us of this. 



Some of the most striking examples of western endemism are to 

 be found in the arid and semi-arid Great Basin and adjacent regions, 



