Abstract
Aquatic biological diversity in Pacific Northwest (PNW) forests was examined after two disturbance types: natural (flooding with and without associated debris flows); and anthropogenic (canopy removal). Within the region two multi-decade studies on aquatic insects in western Oregon establish the likely upper ends of forested stream richness, with richness values of ≈300 taxa collected at Berry Creek and 449 taxa within a small watershed (Lookout Creek, 6400 ha). Therefore, compared to intensively studied sites, at least 100 to 200 species have not been documented at these sites. We examine disturbance impacts on assemblage richness in the PNW with data from three studies characterized by similar levels of sampling and taxonomic effort. Rare species were important contributors to richness, as 20 to 30% of taxa within each study area were found at only one site. Mature, clearcut, high flow, and debris flow disturbance states were compared. Ephemeroptera (p=<0.001) richness increased after debris flows and high flows, and Chironomidae (p=0.04) increased after debris flows and clearcutting. Site variability was high, with assemblage structure weakly clustered by disturbance severity as debris flow disturbance (characterized by both streambed and canopy removal) mostly separated from high flow and clearcut disturbances.
Highlights
Maintaining biological diversity in managed landscapes is a stewardship challenge for forest managers
Two comprehensive studies on aquatic insects in western Oregon are available, with richness values of ≈300 taxa [16] and 449 taxa [17] (Table1). We used these values as the probable upper ends of richness and focused on the role of disturbance in influencing richness in our three study areas, which each include at least two of the treatments
Total richness for all sites combined in those projects was about half that of the Berry Creek and HJ Andrews small watersheds
Summary
Maintaining biological diversity in managed landscapes is a stewardship challenge for forest managers. Examination of whole communities of more speciose-stream benthic invertebrate species will provide a qualitatively different and better test to understand stream networks in managed landscapes. These landscapes experience infrequent very large storm events and systematic canopy removal disturbances. Stream length consists mostly of small streams These headwater and low-order streams provide habitat that supports a large number of species, many of which are not found anywhere else in the river system, and enhance biological diversity of the entire watershed [1,2]. Meyer et al.[1] estimated the macroinvertebrate diversity for three fishless first-order streams in the southern Appalachian Mountains to be just under 300 taxa. Because the non-insect taxa are less intensively studied and have less robust taxonomic clarity, the authors suggested that the true richness of the streams may be twice their reported estimates
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