Role of brown bears ( Ursus arctos ) in the flow of marine nitrogen into a terrestrial ecosystem

Abstract

We quantified the amount, spatial distribution, and importance of salmon (Oncorhynchus spp.)-derived nitrogen (N) by brown bears (Ursus arctos) on the Kenai Peninsula, Alaska. We tested and confirmed the hypothesis that the stable isotope signature (δN) of N in foliage of white spruce (Picea glauca) was inversely proportional to the distance from salmon-spawning streams (r=-0.99 and P<0.05 in two separate watersheds). Locations of radio-collared brown bears, relative to their distance from a stream, were highly correlated with δN depletion of foliage across the same gradient (r=-0.98 and -0.96 and P<0.05 in the same two separate watersheds). Mean rates of redistribution of salmon-derived N by adult female brown bears were 37.2±2.9 kg/year per bear (range 23.1-56.3), of which 96% (35.7±2.7 kg/year per bear) was excreted in urine, 3% (1.1±0.1 kg/year per bear) was excreted in feces, and <1% (0.3± 0.1 kg/year per bear) was retained in the body. On an area basis, salmon-N redistribution rates were as high as 5.1±0.7 mg/m per year per bear within 500 m of the stream but dropped off greatly with increasing distance. We estimated that 15.5-17.8% of the total N in spruce foliage within 500 m of the stream was derived from salmon. Of that, bears had distributed 83-84%. Thus, brown bears can be an important vector of salmon-derived N into riparian ecosystems, but their effects are highly variable spatially and a function of bear density.

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

“…The consumption of salmon by species varied widely among bears, age class, sex, and location [92,93]. Studies have shown that bears transferred MDN to terrestrial systems through excrement and urine [94] and to riparian forests by physically moving salmon carcasses [91] via foraging activities to enhance soil nitrogen pools, and thus beneficial to many other terrestrial organisms including plants, beetles, flies, and birds [95][96][97][98][99][100][101][102][103].…”

Consumption choice by bears feeding on salmon

“…Bears act as "engineers" of nutrient transfer in riparian areas when they feed on salmon because they defecate (and urinate) digested fish material and drag carcasses into the forest, thereby transferring marine-derived nutrients front the aquatic to the terrestrial systems (Bilby et al 1996;Ben-David et al 1998;Willson et al 1998;Hilderbrand et al 1999b). The marine-derived nutrients and energy in salmon tissue enhances freshwater productivity (Wipfli et al 1999) and is also utilized by riparian plants and animals (Bilby et al 1996;Ben-David et al 1998).…”

Seasonal persistence of marine‐derived nutrients in south‐central Alaskan salmon streams

“…Growth and energy storage among stream-dwelling fishes is increased by MDN (Bilby et al 1996, Wipfli et al 2003, Heintz et al 2004, Rinella et al 2012) and the direct consumption of eggs and carcass material appears to be more important than bottom-up pathways for conveying MDN benefits (Scheuerell et al 2007, Denton et al 2009, Armstong et al 2010. MDN is transferred to the riparian environment by hyporheic processes, flooding, and terrestrial scavengers (Cederholm et al 1989, Ben-David et al 1998, Hilderbrand et al 1999, O'Keefe and Edwards 2002), where it enriches riparian soils (Bartz andNaiman 2005, Drake et al 2005). Riparian vegetation can derive a substantial proportion of nitrogen from this pool (Bilby et al 2003, Reimchen et al 2003, which may lead to enhanced growth rates Naiman 2001, 2002; but see Kirchhoff 2003 and.…”