Phenological tracking associated with increased salmon consumption by brown bears

William W Deacy,William B Leacock,Jonathan B Armstrong,Jack A Stanford,Joy A Erlenbach,Charles T Robbins

doi:10.1038/s41598-018-29425-3

Abstract

There is growing interest in the ecological significance of phenological diversity, particularly in how spatially variable resource phenologies (i.e. resource waves) prolong foraging opportunities for mobile consumers. While there is accumulating evidence of consumers moving across landscapes to surf resource waves, there is little data quantifying how phenological tracking influences resource consumption due to the challenge of documenting all the components of this ecological phenomenon (i.e., phenological variation, consumer movement, resource consumption, and consumer fitness). We examined the space use of GPS collared female brown bears to quantify the exploitation of a salmon resource wave by individual bears. We then estimated salmon consumption levels in the same individuals using stable isotope and mercury analyses of hair. We found strong positive relationships between time spent on salmon streams and percent salmon in assimilated diets (R2 = 0.70) and salmon mass consumed (R2 = 0.49). Salmon abundance varied 2.5-fold between study years, yet accounting for salmon abundance did not improve salmon consumption models. Resource abundance generally is viewed as the key variable controlling consumption levels and food web dynamics. However, our results suggest that in intact watersheds of coastal Alaska with abundant salmon runs, interannual variation in salmon abundance likely has less effect on salmon consumption than individual variation in bear foraging behavior. The results complement previous work to demonstrate the importance of phenological variation on bear foraging behavior and fitness.

Highlights

Spatial variation in the timing of food resources generates resource waves in which mobile consumers can exploit resource pulses as they propagate across landscapes
We quantified assimilated diets (% of carbon and nitrogen assimilated by bears that came from sockeye salmon, plant matter, and black-tailed deer (Odocoileus hemionus columbianus)) and salmon consumption of individual GPS-tracked female brown bears that spent varying amounts of time foraging for salmon
As in a previous study[7], time spent at fishing sites increased with the number of fishing sites used by bears (n = 18, t = 2.51, p < 0.05, Fig. 1), suggesting that bears were surfing salmon resource waves as a strategy to increase their access to salmon

Summary

Introduction

Spatial variation in the timing of food resources generates resource waves in which mobile consumers can exploit resource pulses as they propagate across landscapes. A handful of studies have quantified fitness proxies for mobile consumers under contrasting levels of phenological variation expressed among locations[10] or across time[11,12]. There is a strong need for research that quantifies how this variation in individual behavior affects levels of resource consumption, a variable which is closely linked to fitness. Recent research established that brown bears (Ursus arctos) prolong their access to spawning sockeye salmon (Oncorhynchus nerka) by tracking consistent patterns of landscape-level variation in spawning phenology[7,14]. We quantified assimilated diets (% of carbon and nitrogen assimilated by bears that came from sockeye salmon, plant matter, and black-tailed deer (Odocoileus hemionus columbianus)) and salmon consumption (kg consumed) of individual GPS-tracked female brown bears that spent varying amounts of time foraging for salmon. The study was conducted in a ~1,000,000 km[2] area in the vicinity of Karluk Lake in southwestern Kodiak Island, where salmon enter a large suite of spawning streams as a classic resource wave and are utilized by abundant, large brown bears[7,14]

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Conclusion