Caribou Seasonal Fidelity

Over 90% of caribou calve within 7 days of the date of calving the previous year. Calving is a likely driver for the high fidelity to the timing of spring migration.

NPS/Kyle Joly

Fidelity, the tendency to return to certain areas, is thought to be related to several factors, including the predictability of the resources in that area, habitat quality, density and behavior of the species, the presence of predators, human disturbance, and other factors. Barrenground caribou are known to have high fidelity to their calving grounds, which is why caribou herds have largely been defined by where they calve. However, less is known about other seasons.

Wildlife biologists with the NPS, University of Maryland, and Alaska Department of Fish and Game analyzed the movements of adult female Western Arctic Herd caribou from 2010-2019 to assess how much fidelity they showed to various seasonal ranges and how close in time they calved and migrated from year to year. As expected, caribou showed high fidelity to their calving grounds, but also to places where they could reduce insect harassment, which can be extreme in the Arctic. The amount of fidelity varied with spatial scale: at the largest scale, caribou showed very high fidelity, but individual caribou calved an average of 35 miles (57 km) from where they calved the year before (which is not particularly close). Calving takes place where abundant, high-quality forage becomes available predictability every year. Likewise, cool, windy, barren habitats that tend to reduce insect harassment are also more predictable from year to year. Fidelity was lower at the end of summer and winter, seasons that are less predictable due to variability in weather conditions. Spring migrations showed higher fidelity than fall migrations, perhaps due to the demand of calving. Over 90% of the females had a calf within 7 days of when they had a calf the previous year, a remarkable consistency given that they range over an area the size of Wyoming and all the variability they encounter in terms of weather and habitat. A better understanding of fidelity can help us better manage and conserve caribou for this and future generations.

Seasonal patterns of spatial fidelity and temporal consistency in the distribution and movements of a migratory ungulate

Abstract

How animals use their range can have physiological, ecological, and demographic repercussions, as well as impact management decisions, species conservation, and human society. Fidelity, the predictable return to certain places, can improve fitness if it is associated with high‐quality habitat or helps enable individuals to locate heterogenous patches of higher‐quality habitat within a lower‐quality habitat matrix. Our goal was to quantify patterns of fidelity at different spatial scales to better understand the relative plasticity of habitat use of a vital subsistence species that undergoes long‐distance migrations. We analyzed a decade (2010–2019) of GPS data from 240 adult, female Western Arctic Herd (WAH) caribou (Rangifer tarandus) from northwest Alaska, U.S.A. We assessed fidelity at 2 spatial scales: to site‐specific locations within seasonal ranges and to regions within the herd's entire range by using 2 different null datasets. We assessed both area and consistency of use during 6 different seasons of the year. We also assessed the temporal consistency of migration and calving events. At the scale of the overall range, we found that caribou fidelity was greatest during the calving and insect relief (early summer) seasons, where the herd tended to maximally aggregate in the smallest area, and lowest in winter when the seasonal range is largest. However, even in seasons with lower fidelity, we found that caribou still showed fidelity to certain regions within the herd's range. Within those seasonal ranges, however, there was little individual site‐specific fidelity from year to year, with the exception of summer periods. Temporally, we found that over 90% of caribou gave birth within 7 days of the day they gave birth the previous year. This revealed fairly high temporal consistency, especially given the spatial and temporal variability of spring migration. Fall migration exhibited greater temporal variability than spring migration. Our results support the hypothesis that higher fidelity to seasonal ranges is related to greater environmental and resource predictability. Interestingly, this fidelity was stronger at larger scales and at the population level. Almost the entire herd would seek out these areas with predictable resources, and then, individuals would vary their use, likely in response to annually varying conditions. During seasons with lower presumed spatial and/or temporal predictability of resources, population‐level fidelity was lower but individual fidelity was higher. The herd would be more spread out during the seasons of low‐resource predictability, leading to lower fidelity at the scale of their entire range, but individuals could be closer to locations they used the previous year, leading to greater individual fidelity, perhaps resulting from memory of a successful outcome the previous year. Our results also suggest that fidelity in 1 season is related to fidelity in the subsequent season. We hypothesize that some differences in patterns of range fidelity may be driven by seasonal differences in group size, degree of sociality, and/or density‐dependent factors. Climate change may affect resource predictability and, thus, the spatial fidelity and temporal consistency of use of animals to certain seasonal ranges.

Joly, K., E. Gurarie, D. A. Hansen, and M. D. Cameron. 2021. Seasonal patterns of spatial fidelity and temporal consistency in the distribution and movements of a migratory ungulate. Ecology and Evolution.