Year of Award
Dissertation - Campus Access Only
Doctor of Philosophy (PhD)
Department or School/College
Division of Biological Sciences
Jonny Armstrong, Lisa Eby, Bonnie Ellis, Chris Servheen
University of Montana
A key challenge for ecologists is understanding how organisms achieve a positive live history energy balance in spite of resources which vary in abundance across space and through time. Recently, two foraging ecology themes have emerged which contribute to our understanding of this topic. First, resource waves describe how animals can use spatial variation in resource phenology to extend access to foods. Several publications have highlighted animals using resource waves caused by elevational or latitudinal gradients, however, none have demonstrated animals tracking more complex resource waves. Second, the macronutrient optimization hypothesis (MOH) provides a more nuanced model animal diet selection; rather than simply maximizing energy intake, the MOH says animals also attempt to minimize digestive costs by consuming diets with specific mixtures of macronutrients (protein, carbohydrates, and fat). In this dissertation, I used the foraging behavior of Kodiak brown bears in southwest Kodiak Island, Alaska to contribute to these two foraging ecology themes: resource waves and macronutrient optimization. The body of the dissertation consists of four chapters, detailed below.
First, to understand how bears respond to sockeye salmon spawning in tributaries, I developed a monitoring method that did not disturb foraging bears, was inexpensive, and could be deployed in remote locations. The system used time-lapse photography and video to observe passing salmon accurately, but at a fraction of the equipment costs and footage review time required by conventional methods. I used these systems to monitor 9-11 streams from 2013-2015. A manuscript detailing this method is currently in review at PeerJ.
In southwest Kodiak Island, sockeye salmon spawning phenology varies among different spawning locations, creating a resource wave. While spawning at each of these rivers, lake beaches, and streams may only last for 30-40 days, salmon are spawning somewhere in the study area for over three months. I used data from GPS collared bears to determine the extent to which bears used phenological variation in spawning to extend their access to salmon. Bears used an average of 3 different streams, rivers, and lakes to access salmon, and they visited these sites in the order predicted by spawn timing. More importantly, the number of spawning sites used was positively correlated with salmon feeding duration, suggesting phenological variation allowed bears to increase their access to salmon, a resource linked to bear fitness. These findings were reported in a paper entitled “Kodiak brown bears surf the salmon red wave: direct evidence from GPS collared individuals” published in Ecology in May, 2016.
In 2014 and 2015, I observed periods where few bears seemed to be foraging on salmon despite strong salmon returns. The explanation from local Kodiak naturalists was bears were abandoning salmon to eat seasonally abundant red elderberries (Sambucus racemosa). Although this behavior seemed maladaptive from an energetics perspective, the macronutrient optimization hypothesis (MOH) predicts more efficient weight gain by bears foraging on elderberries compared to salmon. I used three years of bear distribution data and natural variation in elderberry phenology to test whether bears foraged according to the MOH in the wild.
Elderberry phenology was relatively early in 2014 and 2015, overlapping the second half of the salmon runs, whereas the elderberry crop and salmon were discrete in time in 2013. In both 2014 and 2015, bear detection along streams dropped considerably when elderberries became ripe, while in 2013 bear activity was synchronous with salmon abundance. During the lull in bear activity on streams, collared bears were using elderberry habitat. Together, these data suggest wild bears facing real-world foraging constraints forage according to the MOH. Although bears preferred berries to salmon, salmon were available for much longer, and likely contribute more to bear annual energy budgets.
In addition to creating a salmon monitoring method that expands the breadth of sites where salmon monitoring is feasible, I contributed to the foraging ecology literature by testing two aspects of foraging theory. I used the movements of brown bears to determine whether a mobile consumers can track a complex resource wave caused by variation in salmon run phenology, and I used natural variation in red elderberry phenology to test whether wild bears forage according to the macronutrient optimization hypothesis, foraging on red elderberry when abundant salmon are available.
Deacy, William Welling, "FORAGING BEHAVIOR OF BROWN BEARS ON KODIAK ISLAND, ALASKA" (2016). Graduate Student Theses, Dissertations, & Professional Papers. 10884.
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© Copyright 2016 William Welling Deacy