Tactical departures and strategic arrivals: Divergent effects of climate and weather on caribou spring migrations

Abstract

The Arctic has been warming rapidly, affecting ecological processes across the region. Caribou and reindeer (Rangifer tarandus) is a keystone Arctic species undergoing declines in many parts of its range, but definitive links between climate and populations remain elusive. The conspicuous and dramatic mass migration of many caribou populations, during which nearly all pregnant females move from wintering ranges to calving grounds shortly before giving birth, may be an important link between climate and caribou populations. The drivers of migration, however, are similarly mysterious. It is unknown, for example, whether caribou respond to immediate phenological cues, anticipate conditions on calving grounds, or are driven by lagged effects related to physical condition. To investigate the drivers of migration, we analyzed movement data from over 1000 individual caribou from seven major herds, spanning 3000 km across Alaska, Yukon, Northwest Territories (NWT), and Nunavut in Canada, from 1995 to 2017. We developed a hierarchical model to estimate migration departure and arrival times, and analyzed these variables against global climate indices and local weather conditions, exploring immediate and lagged effects, as well as snowmelt timing and vegetation indices. We discovered a continent‐wide synchrony in spring migration departure times, driven mainly by large‐scale, ocean‐driven climate indices (Pacific Decadal Oscillation, Arctic Oscillation, and North Atlantic Oscillation). However, we also found that the speed of migration was highly plastic with later migration departure times followed by shorter migration durations. This plasticity made arrival timing independent of departure timing and its respective drivers. Rather, arrival timing depended strongly on weather conditions from the previous summer: cooler and windier summers generally led to earlier arrival at calving grounds the following year. We suggest that maternal body condition, mainly influenced by conditions that limit insect harassment, is a major factor for earlier spring migration arrival timing, and therefore earlier calving and higher survival rates. We place these results in the context of mechanistic links between climate and caribou population dynamics. Long‐term and large‐scale observations of migratory animals can provide insights into the mechanisms by which long‐distance, collective migrants may adapt to dynamic and unpredictable environments.

Pronounced Fidelity and Selection for Average Conditions of Calving Area Suggestive of Spatial Memory in a Highly Migratory Ungulate

“…Weather conditions, such as precipitation (Le Corre et al, 2017), can influence spring arrival timing, and deeper snow increases the cost of movement for caribou (Fancy and White, 1987) and is hypothesized to delay migration in other arctic caribou herds (Duquette, 1988;Gurarie et al, 2019). We suspect some of the southerly calving sites reported here were caused by such snowy spring conditions impeding migratory movement and delaying arrival to the main calving ground, which resulted in birth en route.…”

“…White polygon indicates the core area in which calving was detected for all 8 years in the study. migration routes for pregnant females are typically snow covered (Boelman et al, 2019;Gurarie et al, 2019), so these segments of the migration occur well before green-up and are unlikely to be a product of perception-based movement along the way used by other ungulates (e.g., Merkle et al, 2016). Considering the spatial consistency of use and selection for average conditions, we suggest that the fidelity of caribou to their calving grounds is supportive of memory-based movement at the landscape scale.…”

Tracking snowmelt to jump the green wave: phenological drivers of migration in a northern ungulate

“…Snow cover had no influence on arrival or departure time for migrating caribou for populations in Québec and Labrador (Le Corre et al 2017). In populations in Alaska and northern Canada, timing of departure from winter range was only weakly associated with timing of snowmelt; however, timing of arrival on calving grounds was significantly correlated with snowmelt (Gurarie et al 2019). The discrepancy in how much populations acclimate the timing of migration as a function of snowmelt may be related to migration distance.…”

Regional variation in green-up timing along a caribou migratory corridor: Spatial associations with snowmelt and temperature

“…and Stellaria longipes and the shrub Betula nana (Post 2019). Rather than relating to departure timing itself or generalized phenological conditions on the departure range, caribou springtime migratory arrival on calving grounds may be influenced by additional proximal factors, such as insect harassment, snow cover, and variation in plant growth along the way, that govern the pace of migration through effects on body condition (Gurarie et al 2019).…”