Novel range overlap of three ursids in the Canadian subarctic

This paper describes, for the first time, observations of North America’s three ursid species — American black bears (Ursus americanus Pallas, 1780), grizzly bears (Ursus arctos Linnaeus, 1758), and polar bears (Ursus maritimus Phipps, 1774) — co-occurring in the same locations. These observations were made during an ongoing study focusing on polar bear–human interactions in Wapusk National Park, MB, Canada. We present them here because of their novelty and implications for species conservation in the park and surrounding region and also because they are consistent with expected ecological responses to the amplified impacts of climate change on high-latitude ecosystems (Humphries et al. 2004).

The subarctic is experiencing rapid climate warming with accelerating biophysical, ecological, and social impacts (Callaghan et al. 2010). Responses to climate change by ursids may include changes in distribution, with the potential to result in overlap in habitat use among species and an increase in interspecific interactions (Laidre et al. 2008; Miller et al. 2015). It has been long-postulated that these three species co-occur in the Mackenzie Delta, NT, but not in our study area (Banfield 1977). Black bears are at or near the northern edge of their range in Wapusk, in what Scheick and McCown (2014) consider secondary habitat (defined as lower quality habitat with no or only isolated breeding). Wapusk is at the southeastern edge of an apparent recent range expansion by grizzly bears in the Canadian Arctic and subarctic since the 1990s (Clark 2000; Doupé et al. 2007; Rockwell et al. 2008; Clark and Slocombe 2011; COSEWIC 2012), which may not have been the Arctic’s first (Harington et al. 1962) but appears novel for Manitoba. The source population for grizzlies in Wapusk is not known but is likely to have been from established populations in Nunavut as there are no known grizzly populations to the south or west (Clark 2000). A large, but varying, proportion of polar bears from the Western Hudson Bay subpopulation spend summer and autumn (typically July–November) ashore in Wapusk due to complete annual melting of the sea ice (Stirling et al. 1977, 2004). On shore those bears segregate by age and sex classes, with most bears remaining on the coast and pregnant females traveling inland to a maternity denning area where they will overwinter and give birth (Clark et al. 1997; Clark and Stirling 1998). The conservation status of these three species varies. Internationally, both polar and grizzly bears are listed under Appendix II of the Convention on the International Trade in Endangered Species. Black bears are not considered at-risk anywhere in Canada. Grizzlies are considered a Species of Special Concern in Canada (COSEWIC 2012) and remain listed as extirpated under the province’s Endangered Species and Ecosystems Act as there is no evidence of an established breeding population (W. Watkins, personal communication, 2016). Polar bears are also provincially listed as threatened and are highly vulnerable to a warming Arctic climate, although the Western Hudson Bay subpopulation is considered temporarily stable (Lunn et al. 2016).

We recorded a total of 401 bear-visits at all camps up to the latest complete month of data retrieval for all camps, April 2017: 25 by black bears (6.2%), 10 by grizzlies (2.5%), and 366 by polar bears (91.3%) (Figs. 2, 3, Supplementary Table).¹

These observations represent the first time in peer-reviewed literature that all three North American bear species have been documented in precisely the same locations, although not at the same time. Wapusk National Park has long been known as important seasonal polar bear habitat so it is unsurprising they were commonly observed all three camps, or that their observed camp visitation pattern is consistent with previous studies of their terrestrial distribution on the west coast of Hudson Bay (Stirling et al. 1977; Latour 1981; Derocher and Stirling 1990; Clark and Stirling 1998; Towns et al. 2010). However the distribution and ecology of the other two species in Wapusk — particularly grizzlies — are far less well understood. Although these species appeared to differ in their behavioural responses to the camps, they are unlikely to have been sampled at different intensities as our cameras were fixed and had a constant detection radius (Laforge et al. 2017). However, we do not know how bears may have behaved towards the camps outside that detection radius.

Our observations demonstrate occupancy over multiple years by >1 bear, but do not extend the geographic range of grizzly bear observations in Wapusk (Rockwell et al. 2008). Subadult grizzlies disperse widely in tundra habitat (Gau et al. 2004) so the observation of at least one mature bear signifies that some bears observed may be resident rather than transient. Edwards et al. (2009) observed that space use by Arctic grizzlies varied annually, so in that context the spring observations across years at Owl River suggests that this area is ecologically significant. Denning is a possibility, as that delta contains the largest exposed alluvial deposits in the park (Dredge and Nixon 1992), a substrate which is the preferred denning habitat for Arctic grizzlies (McLoughlin et al. 2002). Evans et al. (2016) found that timing of den emergence in European brown bears varies among individuals but is driven by physiological mechanisms rather than environmental cues. If so, the synchrony of these observations could be explained by recent emergence of the same individual and, by inference, correspondingly lower likelihood that it denned farther away. The dates of these observations are more than three weeks later than emergence dates documented farther north in tundra habitat (McLoughlin et al. 2002), so denning habitat may be at best only a partial explanation of why grizzlies are present there. However, the delta is distant from concentrated terrestrial food sources such as snow goose (Chen caerulescens) colonies (Iles et al. 2013) and forage plants are sparse at that time of year. Alternatively, that bear may simply have been traveling through, possibly towards the still-frozen bay for either feeding or mating opportunities (Taylor 1995; Pongracz et al. 2017). Better understanding of grizzly bear ecology in this region should be a high scientific and conservation priority.

Taken together, these observations demonstrate clear potential for interactions to occur between these three species, raising questions about how interspecific interactions might affect conservation efforts for them in the region. We examined literature on interactions between these three species to assess what implications, if any, such interspecific interactions may have for their conservation (Table 1). To be clear, we did not observe any such interactions but discuss them here because of their potential significance for conservation and management. Interactions between each species have been documented across multiple ecosystems elsewhere, but these may not represent the full range of ecological possibilities in Wapusk.

Under current ecological conditions, the most benefits from these specific interactions would likeliest accrue to grizzlies, with neutral to negative outcomes for polar and black bears (Table 1). Some potential interactions clearly warrant further investigation. As interspecific avoidance between any of these three species cannot be quantitatively demonstrated from our present data, we recommend research into this phenomenon in Wapusk, testing hypotheses formulated from the overview in Table 1. Specific foci for such research include competitive exclusion of female or subadult grizzly bears by male black bears (Mattson et al. 2005), which may functionally limit how far south breeding populations of grizzly bears will establish in the Hudson Bay Lowlands. Conversely, grizzlies could functionally block any northward range expansion by black bears there too, and the geographic overlap of these species is likely to continue changing spatially and temporally. Black bears appear most commonly at Owl River in the months of the on-shore season when polar bears visit least, suggesting the possibility of avoidance (Fig. 1). Such behaviour could go either way, though, or even be mutual given the predominance of female polar bears with cubs who would likely avoid situations where cub predation might be a risk. Hybridization between polar and grizzly bears is possible in and offshore from Wapusk National Park due to the range overlap there. Very little is known about the ecology or behaviour of such hybrid individuals (Pongracz et al. 2017) so close study of any hybrid individuals’ ecology would be important. Consequently, we recommend that range overlap of these three ursid species in Wapusk National Park not be viewed as a threat to any of them, but rather as an ecological response to environmental change that needs to be further examined through a holistic, dynamic, and non-equilibrium ecological perspective.

This research was supported by Parks Canada, the University of Saskatchewan, and the Churchill Northern Studies Centre. Field assistance was provided by Sheldon Kowalchuk, Melissa Gibbons, Jill Larkin, Jessica Lankshear, Natalie Asselin, Heather MacLeod, and David Britten (Parks Canada), Nils Lokken and Aimee Schmidt (University of Saskatchewan), and Jim Roth (University of Manitoba). Frank Baldwin of Manitoba Conservation kindly granted permission to place cameras at Nester One. We are grateful to Hudson Bay Helicopters for their exemplary logistical support. We thank Sujata Manandhar (Yukon College and University of Saskatchewan) for reviewing drafts of this manuscript.