Modeling Caribou Movements: Seasonal Ranges and Migration Routes of the Central Arctic Herd

Abstract

Migration is an important component of the life history of many animals, but persistence of large-scale terrestrial migrations is being challenged by environmental changes that fragment habitats and create obstacles to animal movements. In northern Alaska, the Central Arctic herd (CAH) of barren-ground caribou (Rangifer tarandus granti) is known to migrate over large distances, but the herd's seasonal distributions and migratory movements are not well documented. From 2003-2007, we used GPS radio-collars to determine seasonal ranges and migration routes of 54 female caribou from the CAH. We calculated Brownian bridges to model fall and spring migrations for each year and used the mean of these over all 4 years to identify areas that were used repeatedly. Annual estimates of sizes of seasonal ranges determined by 90% fixed kernel utilization distributions were similar between summer and winter (X̅ = 27,929 SE = 1,064 and X̅ = 26,585 SE = 4912 km2, respectively). Overlap between consecutive summer and winter ranges varied from 3.3-18.3%. Percent overlap between summer ranges used during consecutive years (X̅ = 62.4% SE = 3.7%) was higher than for winter ranges (X̅ = 42.8% SE = 5.9%). Caribou used multiple migration routes each year, but some areas were used by caribou during all years, suggesting that these areas should be managed to allow for continued utilization by caribou. Restoring migration routes after they have been disturbed or fragmented is challenging. However, prior knowledge of movements and threats may facilitate maintenance of migratory paths and seasonal ranges necessary for long-term persistence of migratory species.

Fig 1. Study area in northern Alaska including the Dalton highway, the Trans-Alaska Pipeline (TAP), major land ownership divisions, and rivers mentioned in text.

Fig 2. Example of seasonal movements of one caribou in northern Alaska during 2004.

The caribou moved from the wintering area, followed the spring migration route to the calving area and summer range and then followed the fall migration route back to its winter range.

Fig 3. Years of use of seasonal ranges of the Central Arctic caribou herd in northern Alaska, 2003–2007.

(A) summer ranges. (B) winter ranges. Ranges were modeled as the 90% isopleths of fixed-kernel utilization distributions for each season and year. Shading indicates the number of years each area was used (i.e., number of intersections of the 4 annual ranges modeled for each season).

Fig 4. Annual estimates of seasonal ranges for the Central Arctic caribou herd in northern Alaska based on the 90% isopleths of fixed-kernel utilization distributions.

(A) 2003–2004 (B) 2004–2005 (C) 2005–2006 (D) 2006–2007.

Fig 5. Relative use of migration routes by the Central Arctic caribou herd in northern Alaska, 2003–2007.

(A) spring (B) fall. Use of 500-m grid cells was estimated as the mean of Brownian Bridge Movement Models for each year. Contours enclose portions of the UD with corresponding levels of use (i.e., 25% contour encloses 25% of the UD with the highest probability of use).

Fig 6. Number of years of use of seasonal migration routes by the Central Arctic caribou herd in northern Alaska, 2003–2007.

(A) spring (B) fall. Each route was defined as the mean of the 95% isopleths of the Brownian Bridge Movement Models of all caribou for each season.