Migration
Many populations of reindeer and caribou migrate across sea ice and frozen lakes and rivers between seasonal ranges and to avoid predators. Rising temperatures are changing the timing of sea, lake, and river freezing, changing migration routes and patterns (Mallory & Boyce, 2017). Disruption to migration routes and the range expansion of predators is also forcing Rangifer range to contract northward.
Threatened Migratory Routes
In Northern Quebec, Canada, caribou travel up to 1000 km from their calving grounds to winter pastures in the boreal forest. These caribou depend on the freezing of numerous lakes and rivers in the region to be able to traverse such vast distances. Caribou can swim across open water, but this method is much less efficient, requires a much greater expenditure of energy, and increases the risk of drowning (Leblond, et al., 2016). Furthermore, expected increases in precipitation due to climate change could increase large river flow, further increasing the risk of drowning (Sharma, et al., 2009). It has been observed that caribou generally avoid open water and will instead travel around bodies of open water for several km. Caribou in Canada were observed to take long pauses before crossing open water bodies, and many caribou that began to swim across would then get out and circumvent the water instead. Such detours were on average 2.6 times longer than direct ice crossings and added a ~28% increase in total distance travelled. Changes in the timing of the freezing and thawing of ice could cause up to 36% of the ice crossings that caribou in the region currently use during fall and spring migrations to be lost by 2070. Caribou may have to begin their migrations sooner to time them correctly with earlier freezing and thawing dates, but it remains unclear whether they will be able to adjust accordingly (Leblond, et al., 2016).
Leblond, et al., 2016
Range Contraction
As ice thaws earlier and freezes later in the season, migratory caribou populations may also have to shift their range northward in order to access frozen migration routes. Furthermore, many caribou populations migrate to calving grounds in the north in order to reduce their risk of predation and increase predator visibility. However, climate change is causing older forests to be replaced by younger ones and is increasing productivity in the north, increasing wolf abundance (Sharma, et al., 2009). Furthermore, increasing abundances of other ungulates such as moose and deer may further increase abundances of predator populations in the north (Mallory & Boyce, 2017). As predator range expands, there may not be enough space for caribou herds to migrate northward to avoid them, causing a contraction in the distribution of migratory caribou herds as well as increased threat from predators (Sharma, et al., 2009).
Researchers modeled the distributions of migratory caribou herds in the Quebec-Labrador peninsula of Canada using climate projections for 2040-2069 and showed that during the winter, spring, and fall, the range of these herds will contract and shift significantly towards the northeast. As ranges contract, the spatial overlap between migratory herds is expected to increase, possibly increasing competition between herds. The overlap in the distribution of the two herds modeled in this study is expected to increase in the spring and fall, which could increase competition for calving grounds (Sharma, et al., 2009).
Sharma, et al., 2009