Restoration of urban salmon habitat has limited effects on a key ecosystem function

Abstract

Pacific Salmon have the potential to act as umbrella species in western North America. Urban streams have warmer temperatures, higher nutrient loads, and flashier hydrographs from impervious surroundings (urban stream syndrome). These changes are likely to affect ecosystem functions that are key to biodiversity and ecosystem services. Here, we ask whether restoration to improve urban stream habitat for Pacific Salmon can naturalize organic matter decomposition rates. Using structural equation models, we investigated the relationship between physical habitat, surrounding impervious surface, restoration intensity, and cotton strip decomposition rates, for streams in Vancouver, British Columbia, Canada. We hypothesized that 1) decomposition rates would differ along a gradient of urbanized to more natural reaches, and 2) sites receiving more intense restoration would have decomposition rates more similar to less-urbanized sites, partly because restoration would mitigate the environmental changes associated with urban stream syndrome. Streams surrounded by more impervious surface tended to have faster decomposition rates (standardized path coefficient = 0.38, the strongest direct effect on decomposition out of all the variables we considered). Reaches with higher restoration showed altered physical habitat characteristics compared to less-restored urban reaches. However, restoration intensity did not have a significant effect on decomposition rates in our structural equation model. Human impacts on freshwater ecosystems occur through pathways at broad spatial scales, and our results suggest that local efforts focusing on physical habitat restoration for umbrella species may not be the most effective way to address changes to ecosystem functioning.