Timber Harvest Transforms Ecological Roles of Salmon in Southeast Alaska Rain Forest Streams

Abstract

Although species commonly modify habitats and thereby influence ecosystem structure and function, the factors governing the ecological importance of these modifications are not well understood. Pacific salmon have repeatedly been shown to positively influence the abundance of benthic biota by annually transferring large quantities of nutrients from marine systems to the nutrient-poor freshwaters in which they spawn. Conversely, other studies have demonstrated that salmon can negatively influence the abundance of freshwater biota, an effect attributed to bioturbation during upstream migration and nest construction. The factors determining which of these contrasting ecological effects predominates are unknown, including how human activities, such as land use, influence ecological responses to salmon. We sampled a key basal food resource, sediment biofilm, in seven southeast Alaskan streams impacted to varying degrees by timber harvest. Biofilm abundance (measured as chlorophyll a and ash-free dry mass) was positively related to timber-harvest intensity prior to salmon arrival. However, during the salmon run, an inverse relationship emerged of more abundant biofilm in less-harvested watersheds. Among-stream variability in biofilm response to salmon was largely explained by sediment particle size, which was larger in less-harvested watersheds. Collectively, these results suggest that, by altering stream sediment size, timber harvest transformed the dominant effect of salmon from nutrient enrichment to physical disturbance, thus modifying nutrient linkages between marine and freshwater ecosystems.

Effects of subsidies from spawning chum and pink salmon on juvenile coho salmon body size and migration timing

“…Our study is the first to separate effects by age class (including age 0 with no direct contact and age 1 with direct contact with spawning salmon), to examine effects on coho age composition, and to include the comparative influences of habitat characteristics. Contrary to our expectation and indications from previous research (Tiegs et al 2008, Armstrong et al 2010, we found habitat characteristics did not mediate the relationship between spawning chum and pink, and juvenile coho salmon. This may be related to a comparatively high density of spawning fish obscuring any effects of habitat.…”

“…No previous research has examined effects on juvenile coho salmon age composition, nor has there been consideration of the mediating effects of habitat characteristics that are known to affect juvenile salmonids (Tiegs et al 2008). For example, juvenile coho salmon can be affected by habitat and food availability, cover or refugia from predation, and temperature (e.g., Bradford et al 1997, Sharma andHilborn 2001).…”

Importance of Riparian Zone: Effects of Resource Availability at Land-water Interface

“…Land use -Almost all ecosystems suffer from some degradation due to impacts of land use, such as by agriculture, urbanization and deforestation or forest harvesting [108,118,119], which were the dominant changes in the past 100 years and seriously impacted biodiversity and ecosystem functioning at local, regional and global scales [120]. During the past 50 years, agricultural land use was and will continue to be the main reason of ecological changes in both aquatic and terrestrial ecosystems [121].…”

Willingness to pay for ecosystem conservation in Alaska’s Tongass National Forest: a choice modeling study

“…River and stream degradation stemming from timber production adversely affect many other ecosystem services, beyond those just associated with consuming, viewing, or knowing that wild salmon exist in historically abundant levels. For example, documented examples on the Tongass include the degradation of nutrient cycling (Tiegs et al 2008), water regulation (Gomi et al 2004), and erosion control services (K. Kahklen and W. Hartsog 1998, unpublished manuscript).…”