Can polar bears use terrestrial foods to offset lost ice‐based hunting opportunities?

Abstract

Increased land use by polar bears (Ursus maritimus) due to climate‐change‐induced reduction of their sea‐ice habitat illustrates the impact of climate change on species distributions and the difficulty of conserving a large, highly specialized carnivore in the face of this global threat. Some authors have suggested that terrestrial food consumption by polar bears will help them withstand sea‐ice loss as they are forced to spend increasing amounts of time on land. Here, we evaluate the nutritional needs of polar bears as well as the physiological and environmental constraints that shape their use of terrestrial ecosystems. Only small numbers of polar bears have been documented consuming terrestrial foods even in modest quantities. Over much of the polar bear's range, limited terrestrial food availability supports only low densities of much smaller, resident brown bears (Ursus arctos), which use low‐quality resources more efficiently and may compete with polar bears in these areas. Where consumption of terrestrial foods has been documented, polar bear body condition and survival rates have declined even as land use has increased. Thus far, observed consumption of terrestrial food by polar bears has been insufficient to offset lost ice‐based hunting opportunities but can have ecological consequences for other species. Warming‐induced loss of sea ice remains the primary threat faced by polar bears.

Polar Bear Foraging Behavior During the Ice-Free Period in Western Hudson Bay: Observations, Origins, and Potential Significance

“…It is more likely, however, that the commonalities reflect a shared genetic and evolutionary legacy of the two species (Hailer et al, 2012;Cahill et al, 2013;Weber et al, 2013), even after taking into account that polar bears have developed traits (especially morphological ones) that enable them to exploit an ice-based environment (Stirling, 2011;Rode et al, 2014). While the genetic architecture underlying flexible foraging is not known, it is likely that it depends to some degree on phenotypic behavioral plasticity that shifts depending on the circumstances and targets (e.g., Weaver et al, 1996;Agosta and Klemens, 2008;Geinapp et al, 2007;Gormezano and Rockwell, 2013a).…”

“…Simultaneously, there is a need to establish the extent of specialization among various demographic classes as has been done with grizzly bears (Edwards et al, 2011). It will also be necessary to move from opportunistically based observations (e.g., this study, Russell, 1975;Smith et al, 2010) to detailed individual studies that will resolve questions on specialization regarding terrestrial foods, and also allow the construction of precise time and energy budgets (Dyck and Kebreab, 2009;Rode et al, 2014). And, finally, we must conduct detailed physiological studies on both captive and free-ranging bears that allow us to estimate both the costs of obtaining land-based prey and their nutritional and energetic value.…”

Invariant polar bear habitat selection during a period of sea ice loss

“…During this same period, there has been a significant increase in the number of bears summering onshore [10] even though opportunities to forage terrestrially are probably limited [37]. This implies that opportunities to hunt while on sea ice during summer are probably becoming fewer or that fewer polar bears can access productive areas in summer given the reduction in highly selected habitat area.…”

“…Thus, if polar bears are able to compensate for lost hunting opportunities with terrestrial foods [38], then increased land use might not be overly problematic for the CS population. If, however, terrestrial foods will never be able to provide sufficient energy to compensate for lost seal hunting opportunities [37], then polar bears in the CS might begin to experience negative population-level effects of sea ice loss in the near future. Table 2.…”

Polar bears observed climbing steep slopes to graze on scurvy grass in Svalbard

“…However, none of these animals make up a significant part of their diet (Russell 1975;Derocher et al 2000;Brook & Richardson 2002). It is thought that polar bears cannot derive sufficient caloric intake from terrestrial food (Ramsay & Hobson 1991;Rode et al 2015). However, Gormezano and Rockwell (2015) have calculated that the probability of land-locked polar bears surviving climate change depends largely on the duration of the ice-free season and local availability of novel high-energy terrestrial foods (geese, eggs, caribou).…”