Polar bears and terrestrial food
A study published in the April 2015 issue of Frontiers in Ecology and the Environment provides a review of polar bear use of terrestrial foods as they are increasingly forced onshore due to sea ice loss. The study found that fewer than 30 individual polar bears from any one population have been observed eating calorically-dense terrestrial foods, such as bird eggs. Feeding on dispersed, low calorie terrestrial foods, such as berries and vegetation, is more widespread, but requires extensive time spent foraging by large-bodied polar bears. The study found that some individual bears are likely able to offset some of their lost opportunities hunting seals by consuming higher quality foods, such as bird eggs. But thus far, the proportion of any population documented feeding on high quality terrestrial food has been very small.
In places where terrestrial foraging has been best documented, such as western Hudson Bay, survival rates of sex and age classes important to population dynamics, such as adult females, were directly related to sea ice conditions and the duration of time bears spend onshore (Regehr et al. 2007; Lunn et al. 2014). Thus, female polar bears had lower survival rates when they spent more time onshore despite recent observations of terrestrial feeding.
The study notes that over much of the polar bear’s range, terrestrial habitats are already occupied by grizzly bears. Those grizzly bears occur at low densities and are some of the smallest of their species due to low food quality and availability. Further, they are a potential competitor as polar bears displaced from their sea ice habitats increasingly use the same land habitats as grizzly bears.
Polar bears are the largest of all bears because their primary diet consists of energetically-dense marine prey. Studies suggest that polar bears consume the highest lipid diet of any species, which provides all essential nutrients and is ideal for maximizing fat deposition and minimizing energetic requirements. Potential foods found in the terrestrial environment are dominated by high protein, low fat animals and vegetation. Polar bears are not well-suited for digesting plants and because of their much larger body size compared to Arctic grizzly bears, it is difficult for them to ingest the volumes required to support their size.
The study concluded that focused research will help determine whether terrestrial foods can contribute to polar bear nutrition despite the physiological and nutritional limitations and the low availability of most terrestrial food resources. However, the evidence thus far suggests that increased consumption of terrestrial foods by polar bears is unlikely to offset declines in body condition and survival resulting from sea ice loss.
Published Thursday April 16 2015 by Dag Vongraven