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42 GREAT BASIN NATURALIST MEMOIRS 197 apparently not been involved in threats to any eastern species On the list but have been factors for about 4 percent of the western fishes. It is probable that this difference re- sults from the fact that information regarding incidence of disease and parasitism in native fishes is relatively sparse. In addition, though the initial major decline in abundance and distribution of eastern fishes probably oc- curred prior to 1850 (Trautman 1957), in the West the similar event occurred subsequent to 1850 (Miller 1961). Because increased in- cidence of disease and/ or parasitism as an important factor in a population decline be- COmes most apparent during the major de- cline, it must be detected at that time to be recognized. The generally earlier decline of eastern fishes during a time when increased incidence of disease or parasitism would have beim less likely to have either been detected or associated with the decline probably ex- plains its absence from aSsociation with the eastern fauna. This factor doubtless has been a more important contributor to decline of both eastern and western fish populations than is apparent. It has specifically been identified by Wilson et al. (1966) and Seetha- ler (1978) as a factor in the decline of West- ern fishes. Biological interactions of various kinds (Category 4) contribute to the problems faced by 54 percent of the threatened West- ern fauna but only 9 percent of the threat- ened eastern fauna. The marked differences in Category 4 point to distinctions of the western fish fauna that have been repeatedly diSCussed. Physical barriers to dispersal have resulted in relatively low colonization rates throughout the West, with the consequence that western fish faunas are not especially No.3 speciose (Smith 1978). Because their evolu- tionary experiences have been with relatively depauperate faunas, western fishes have rela- tively low tolerances to biological inter- actions (Smith 1978, Deacon and Minckley 1974, Hubbs et al. 1974). A restricted range (occurring in only a single spring, a single group of springs, or a short stretch of stream [Category 5]) is a fac- tor involved in giving a threatened status to 21 percent of the western fishes listed, but only about 7 percent of the eastern fishes. Category 5 illustrates the fact that one group of western fishes appears to have a high de- gree of "extinction resistance" (Smith 1978). The consequence is that many western taxa exist as relict populations in single habitats. They found their way onto the AFS list of threatened fishes because of that fact. They, like many western fishes, generally have high tolerances to physical extremes but low toler- ances to biological interactions (Deacon and Minckley 1974). of ~ the foll( dam irrig 193( pro" IOn cline mail close dams and ~ cline: Occm wate] suit i Recel proce Zona. simila Utah, PHYSICAL MODIFICATION OF HABITATS While western fishes have in general de- veloped considerable resistance to the phys- ical extremes imposed upon them by climatic factors, they have also been most strongly af- fected by general and specific alterations of physical habitats imposed upon them by man. Miller (1961), Hastings and Turner (1965), and Cottam (1961) have dramatically shown the impact of slight climatic shifts su- perimposed on removal of vegetative caver by overgrazing between about 1880 and 1900. The arroyo cutting, siltation, and de- watering that occurred during this period were probably the most detrimental 20 years T..u 1. C=P<ri'<m of ",,"e,,' bod, of th~" to the th,~t""" fred>w",,, fl,h moO, of W""'m ',d ""'m North America, north of Mexico. Western Fishes Eastern Fishes: Number P ercen t Number P ercen t of taxa of fauna of taxa of fauna General threat category (N = 1I2) affected (N=90) affected 1. Habitat modification 109 97.3 90 100 Lal 2. OverexpJoitation 0 0 6 6.7 .3. Parasitism and disease 5 4.4 0 0 4. Biotic interactions 60 54 8 8.9 Fig. 1. ] 5. Restricted range 24 21 6 6.7 dangered ,