Relationships between Productivity, Physical Habitat, and Aquatic Invertebrate and Vertebrate Populations of Forest Streams: An Information‐Theoretic Approach

Freshwater ecosystems worldwide are threatened by habitat degradation resulting in widespread population declines or extinction of invertebrate and vertebrate taxa. Millions of dollars are spent annually on efforts to restore the physical complexity of degraded habitats, such as adding wood to streams. The efficacy of these efforts may be limited, however, because basal productivity (availability of nutrients, algae, bacteria, and organic matter) and predation also constrain the abundance and diversity of stream biota. Using light input as a surrogate for basal productivity, we examined the relative importance of predation, productivity, and physical habitat in explaining the variation in invertebrate and vertebrate populations in small forest streams of coastal Oregon. Our results suggest that aquatic populations are limited by these factors individually and in combination as well as by their interactive effects. For example, invertebrate taxa richness was best predicted by basal productivity, and richness was positively correlated with light. Total vertebrate biomass was positively related to both basal productivity and physical habitat, and the top‐ranked model included light input and velocity. The top‐ranked model explaining variation in the biomass of the coastal giant salamander Dicamptodon tenebrosus was a single‐variable model that included the interaction of basal productivity with channel slope. Streams are dynamic environments, and species living within these ecosystems are probably affected by multiple, interacting factors. Our results support this notion and suggest that, in some cases, restoring physical habitat may have minimal effect on stream communities if productivity at lower trophic levels is also limiting.