Longitudinal Zonation of Pacific Northwest (U.S.A.) Fish Assemblages and the Species-Discharge Relationship
Fish ecologists often use species-discharge relationships (SDRs) to understand how species richness varies with aquatic habitat availability, but few SDR studies have considered whether the reported SDRs are scale-dependent, or attributed the SDR to a specific causal mechanism. Here, we assessed whether the SDR is scale-dependent by using individual river reaches, rather than complete river basins, as sampling units in a SDR analysis. We also determined whether the SDR is a function of among-reach habitat diversity. To do so, we first tested for longitudinal zonation along three major Pacific Northwest (U.S.A.) rivers. Our zonation tests consistently detected ‘lower,’ ‘middle,’ and ‘upper’ river fish assemblages, each of which was characterized by common patterns in adult habitat use, feeding guild structure, and reproductive behavior, and was associated with predictable habitat conditions. When these longitudinal zones were used as sampling units in a SDR analysis (i.e., total discharge and species richness within each zone), we detected strong linear relationships between discharge and species richness (log10 data). Because individual zones predicted species richness more effectively than complete basins, we conclude that the SDR is scale-dependent. And we infer that among-zone habitat shifts are an important determinant of the SDR, as the slope of the SDR is a function of the differential richness found in each zone.Abstract
Locations of the three study basins in Oregon. River courses that were converted to longitudinal profiles are shown as heavy black lines. The Willamette course flows from June Lake (43.39°N, 122.13°W) to the Middle Fork Willamette River, to the mainstem Willamette River. The Umpqua course flows from Last Creek (43.17°N, 122.75°W) to Boulder Creek, to the South Umpqua River, to the mainstem Umpqua River. The John Day course flows from Hidaway Creek (45.08°N, 118.58°W) to Camas Creek, to the North Fork John Day River, to the mainstem John Day River. Arrows indicate the direction of flow. Scale bar length is 250 km.
Longitudinal profiles of the Willamette, Umpqua, and John Day Rivers. Dashed vertical lines reflect the approximate locations of longitudinal zone boundaries (i.e., demarcations between lower, middle, and upper zones).
Nonmetric multidimensional scaling (NMDS) ordinations of the Willamette, Umpqua, and John Day River segments. Points within each plot are identified by their respective longitudinal positions (e.g., point ‘100’ represents the 100-km through 149-km segment). Segments that were at least 75% similar in cluster analyses (i.e., within the same zone) are circled. Mean within-zone similarities (Wavg), mean between-zone similarities (Bavg), and mean similarity test P-values are shown for each river. Lower, middle, and upper zone segments are indicated by plain, bold-italic, and underscored characters, respectively.
Functional characteristics of the lower, middle, and upper zone fish assemblages in each of the study basins, including (A) primary adult habitats, (B) feeding guilds, and (C) reproductive habits. All data are expressed as percentages. In each panel, longitudinal zone membership is shown on the primary (lower) x-axis, while basin identity is shown along the secondary (upper) x-axis.
Species-discharge relationships for (A) all species data (i.e., native and non-native species) and (B) native species only. Uppercase letters indicate river basins: W = Willamette; U = Umpqua; J = John Day. Triangles, circles, and crosses represent upper, middle, and lower zones, respectively.
Contributor Notes
Associate Editor: J. W. Snodgrass.