How Important Is Land-Based Foraging To Polar Bears (ursus Maritimus) During The Ice-Free Season In Western Hudson Bay? An Examination Of Dietary Shifts, Compositional Patterns, Behavioral Observations And Energetic Contributions
Date of Degree
Robert F. Rockwell
Mark E. Siddall
Behavior and Ethology | Biology | Ecology and Evolutionary Biology
climate change, diet, feces, foraging, polar bears, Ursus maritimus
Trophic mismatches between predators and their prey are increasing as climate change causes decoupling of phenological relationships. Predators linked to the life histories of a particular prey will have a more difficult time persisting through environmental change unless they can alter their behavior to maintain the historical match or possess the ability to pursue alternate prey. Arctic predators typically possess flexible foraging strategies to survive in the labile environment, however, quantifying the limits of those strategies can be difficult when life history information is incomplete. In such cases, piecing together different aspects of a predator's foraging behavior, particularly when environmental effects are thought to induce the most nutritional stress, can serve as a basis to understand the species' resiliency in response to climate changes.
Climate change is impacting the Hudson Bay region faster than any other portion of Arctic North America. As a consequence, polar bears (Ursus maritimus) in western Hudson Bay, near the southern extent of their range, are already experiencing a phenological mismatch with their primary prey, ringed seals (Phoca hispida). These polar bears have relied on the energy stores amassed from hunting seal pups in spring to sustain them through the ice-free season on land for 4 to 5 months. As climate change causes the ice in Hudson Bay to melt earlier in spring, polar bears are projected to have less time to hunt seal pups on the sea ice, leaving them with smaller energy reserves to sustain them for longer periods on land. As a result, body condition is expected to deteriorate, leading to eventual declines in reproduction and survival, unless alternative energy sources are utilized.
Polar bears currently hunt and consume a variety of foods during the ice-free season. Few believe, however, that such foraging will compensate for projected energy deficits from lost seal hunting opportunities. This skepticism stems from the perceptions that polar bears are specially adapted to hunting seals on the ice, the behavior has always occurred, but only a few polar bears partake in it, breath-based carbon-isotope analyses suggest that energy expended on land is solely of marine (i.e., seal) origin, pursuing animals on land would be too energetically expensive for polar bears to experience any net gain and there is not enough energy in land-based food to compensate all polar bears in western Hudson Bay for the energy available from seals on the ice.
Many of these arguments are premised on the idea that past (and even present) foraging behaviors are representative of how polar bears will respond to future climate-related changes. Alternatively, the past behaviors may have represented optimal foraging strategies when seals were relatively abundant and easy to catch. Rather than tie the polar bears' fate directly to deteriorating ice conditions and thus availability of a single prey, I consider a more mechanistic approach to evaluating polar bears' reaction to climate changes. In light of the shared genetic legacy with grizzly bears, I analyze different aspects of polar bears' current foraging behavior, as well as known physiological and energetic constraints, to consider an alternative future scenario by which polar bears might persist consuming land-based food during the ice-free season. I explore different aspects of land-based foraging and address aforementioned concerns regarding the potential value of terrestrial foods in a series of interrelated chapters.
In the first chapter, I develop a comprehensive inventory of foods polar bears currently consume on land and compare them to those consumed approximately 40 years earlier, prior to the onset of climate changes, using morphological scat analysis. Changes in the polar bear diet between time periods are compared to changes in availability of specific prey items in the region (Chapter 1) as well as where and when they currently occur most abundantly in the landscape (Chapter 2). Based on compositional patterns, I explore the extent of diet mixing and its implications for weight gain (or rate of weight loss, Chapter 2). In addition to long-term changes in abundance that have made Lesser Snow Geese (Chen caerulescens caerulescens) more available since the 1960s, temporal shifts in their incubation period and earlier ice-breakup is creating a new trophic match between arriving polar bears and eggs. The potential energy available from this increasingly accessible resource and its implications for energy compensation are discussed in Chapter 3. In Chapter 4, I provide total energy values for populations of novel animal foods (snow geese, eggs, caribou (Rangifer tarandus) and vegetation (berries, Lyme grass seed heads (Leymus arenarius) that polar bears consume on land and determine what amounts of each, alone or in combination, would prevent adult males from starving to death as the ice-free season expands to a 180 days as predicted by MolnÃ¡r et al. (2010). In Chapter 5, I reexamine available data on the energetic costs of locomotion at different speeds, develop a new predictive model and challenge past assertions by Lunn and Stirling (1985) that energetic inefficiencies would prevent a polar bear from profiting after a sustained chase. In Chapter 6, I present unpublished observations of polar bears foraging from land and in open water from the Hudson Bay Project archives and my personal observations. I describe different evolutionary pathways for the observed behavior in light of their recent divergence from grizzly bears and the implications of each for future polar bear persistence.
Gormezano, Linda J., "How Important Is Land-Based Foraging To Polar Bears (ursus Maritimus) During The Ice-Free Season In Western Hudson Bay? An Examination Of Dietary Shifts, Compositional Patterns, Behavioral Observations And Energetic Contributions" (2014). CUNY Academic Works.