Diversity of Juvenile Anadromous Salmonid Assemblages in Coastal Oregon Basins with Different Levels of Timber Harvest

-We examined the relationships of timber harvest, stream habitat complexity, and diversity of juvenile anadromous salmonid assemblages in 14 smallto intermediate-sized basins in coastal Oregon between 1985 and 1989. Diversity (the inverse of a species dominance index) of assemblages in streams in basins with low harvest levels (<25% of the basin area harvested) was greater than in streams in basins with high harvest levels (>25% of the basin area harvested) (P = 0.02). Assemblages in basins with high levels of harvest were more dominated by a single species than were assemblages in basins with low harvest. Percent of basin harvested was more strongly associated with assemblage diversity (P = 0.07) than were basin area (P = 0.90) or gradient (P = 0.22) when the influence of the other two factors was controlled. Habitat features were compared between three pairs of streams. Streams in basins with low timber harvest had more complex habitat, as manifested by more large pieces of wood per 100 m (P < 0.01). We conclude that a community and basin-level perspective is necessary to fully assess the effects of timber harvest and other human activities on stream fish. Alteration of ecological processes and environmental conditions may affect several levels of ecological organization. Individual and population responses may vary with the magnitude and duration of the impact, species-specific requirements (Kelly and Harwell 1990; Yount and Niemi 1990), and the presence of refugia (Sedell et al. 1990). Because of variability in responses by individuals and populations, members of a community are unlikely to exhibit a uniform response to a disturbance or an environmental alteration. The effect of disturbance on communities depends, in part, on the combined effect on both individuals and populations as well as on the extent to which processes that influence the structure and composition of communities are altered (e.g., Reeves et al. 1987; Baltz et al. 1982). A primary factor influencing the diversity of stream fish communities is habitat complexity (Gorman and Karr 1978; Schlosser 1982; Angermeier and Karr 1984). More diverse habitats support more diverse communities. Habitat diversity can also mediate biotic interactions such as competition (Kalleberg 1958; Hartman 1965) and predation (Crowder and Cooper 1982; Schlosser 1988). Simplification of fish habitat in streams may be a primary consequence of timber harvest (Bisson and Sedell 1984; Hicks et al. 199 1). Consequences of simplification include a decrease in the range and variety of hydraulic conditions (Kaufmann 1987) and reductions in structural elements (Bisson et al. 1987), frequency of habitat units, and diversity of substrates (Sullivan et al. 1987). Ef-