Predator and Prey, Past, Present, and Projected: Modelling Polar Bears and Ringed Seals in a Dynamic Arctic

  • Author / Creator
    Reimer, Jody Renae
  • Climate change is causing the Arctic to warm faster than anywhere else on earth. The projected effects of a warmer Arctic include changes in population dynamics and distributions, biodiversity, food web structure, and ecosystem services. Our ability to successfully monitor ecological changes and manage vulnerable populations relies on our ability to predict these responses. Mechanistic mathematical models are a powerful tool for exploring the unprecedented nature of these environmental changes, allowing us to make quantitative, testable predictions—a hallmark of scientific understanding—against which we can compare future observations. Unfortunately, we lack even baseline population estimates for many ice-associated species. Polar bears and ringed seals, however, are two species for which we have data spanning multiple decades, making them suitable indicator species for detecting broader ecological change. There is a strong predator-prey relationship between polar bears and ringed seals, and both species rely on the sea ice. Ringed seals rely on the sea ice, and the snow on top, for moulting and for the creation of the protective snow lairs in which they give birth. Polar bears rely on the sea ice for travel, mate finding, hunting, and, in some regions, for maternity denning. Changes in the sea ice thus affect both species directly as well as indirectly through their predator-prey relationship. Historically, environmental conditions with negative effects on ringed seals and polar bears came in the form of anomalously cold winters, resulting in heavier ice cover. In these years, ringed seal reproductive rates declined, changing the prey availability for polar bears. Due to climate change, however, these years of extreme cold are being replaced by years of extreme heat. In the Beaufort Sea and Amundsen Gulf, Canada, this is resulting in earlier spring sea ice breakup and a later autumn ice freezeup. This later freezeup results in reduced snow accumulation on the ice, as the early winter snow falls on open water. For ringed seals, their reliance on stable sea ice and sufficiently deep snow drifts in which to dig their spring birth lairs makes them vulnerable to these changes. For polar bears, earlier ice breakup shortens the length of the important spring hunting season, with energetic consequences. In this thesis, I explored the responses of ringed seals and polar bears to past, present, and predicted environmental challenges. To do so, I used matrix population models and stochastic dynamic programming (SDP). I found that polar bears typically strongly select for ringed seal pups, but switch to disproportionately select older ringed seals in those years with low pup availability corresponding to anomalously cold winters—a novel ecological phenomenon I termed intraspecific prey switching. Looking ahead, I coupled a ringed seal population model to ice and snow forecasts, modelling the population to the end of this century. These projections showed median declines in population size of more than 50%, with concurrent changes in population structure. Data collected through the current monitoring program is not sufficient to detect these changes, highlighting the need to re-evaluate existing field programs in light of emerging stressors. Anticipating the shorter spring feeding season, I modelled shifts in a female polar bear's optimal behavioural and physiological strategies and the consequences for her expected lifetime reproductive output. This highlighted the effect that seemingly small annual changes may have over the entire lifetime of a long-lived species. Additionally, the intuition developed through the application of matrix population models in this thesis proved useful in understanding patterns which emerge in ecological applications of SDP. A rich body of mathematical results on SDP exist, but have not been popularized in the ecological SDP literature. I applied relevant mathematical results to two canonical SDP equations in ecology, demonstrating their utility both for solving SDP models and for interpreting their biological implications.This thesis contributes to our understanding of Arctic marine ecology, provides examples of appropriate mathematical tools and interpretive paradigms with which to explore ecological effects of climate change, and suggests new methods for applications of SDP in ecology.

  • Subjects / Keywords
  • Graduation date
    Fall 2019
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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