The extra mile: Ungulate migration distance alters the use of seasonal range and exposure to anthropogenic risk

Abstract

Abstract. Partial migration occurs across a variety of taxa and has important ecological and evolutionary consequences. Among ungulates, studies of partially migratory populations have allowed researchers to compare and contrast performance metrics of migrants versus residents and examine how environmental factors influence the relative abundance of each. Such studies tend to characterize animals discretely as either migratory or resident, but we suggest that variable migration distances within migratory herds are an important and overlooked form of population structure, with potential consequences for animal fitness. We examined whether the variation in individual migration distances (20-264 km) within a single wintering population of mule deer (Odocoileus hemionus) was associated with several critical behavioral attributes of migration, including timing of migration, time allocation to seasonal ranges, and exposure to anthropogenic mortality risks. Both the timing of migration and the amount of time animals allocated to seasonal ranges varied with migration distance. Animals migrating long distances (150-250 km) initiated spring migration more than three weeks before than those migrating moderate (50-150 km) or short distances (<50 km). Across an entire year, long-distance migrants spent approximately 100 more days migrating compared to moderate-and short-distance migrants. Relatedly, winter residency of long-distance migrants was 71 d fewer than for animals migrating shorter distances. Exposure to anthropogenic mortality factors, including highways and fences, was high for long-distance migrants, whereas vulnerability to harvest was high for short-and moderate-distance migrants. By reducing the amount of time that animals spend on winter range, long-distance migration may alleviate intraspecific competition for limited forage and effectively increase carrying capacity. Clear differences in winter residency, migration duration, and risk of anthropogenic mortality among short-, moderate-, and long-distance migrants suggest fitness trade-offs may exist among migratory segments of the population. Future studies of partial migration may benefit from expanding comparisons of residents and migrants, to consider how variable migration distances of migrants may influence the costs and benefits of migration.

Migratory plasticity is not ubiquitous among large herbivores

“…Migration distance strongly influenced the proportion of migratory animals in the Ya Ha Tinda elk herd (Eggeman et al, 2016), but appears to have no such influence with mule deer, as migration distances in our study generally ranged from 40 to 110 km, with extremes in the RD population from 20 to 264 km (Sawyer et al, 2016). Migration distance strongly influenced the proportion of migratory animals in the Ya Ha Tinda elk herd (Eggeman et al, 2016), but appears to have no such influence with mule deer, as migration distances in our study generally ranged from 40 to 110 km, with extremes in the RD population from 20 to 264 km (Sawyer et al, 2016).…”

Effects of distribution, behavior, and climate on mule deer survival

“…There are 2 hypotheses that independently or in synergy might explain this finding. First, although female mule deer could not be legally harvested under the regulations in effect during this study, illegal harvest, or disturbance from legal harvest activities could negatively affect survival rates, and we might expect residents and short‐distance migrants to be affected similarly (Sawyer et al ). In addition, competition for forage and changes in forest structure (Peek et al ) on summer ranges may mean food resources are becoming limiting during October and November (Baker and Hobbs ).…”

Conserving transboundary wildlife migrations: recent insights from the Greater Yellowstone Ecosystem

“…) and western North America (Sawyer et al . ). Coupled with advances in remote sensing and computational analysis, this revolution in tracking technology has led to breakthroughs in several areas of ungulate migration ecology.…”

“…) all appear to track annual patterns of vegetation green‐up, at least to some degree, during migration; some mule deer spend as much as one‐third of the year feeding in the migratory corridor (Sawyer et al . ). As a consequence, investigations have demonstrated not only that migration corridors, like other seasonal ranges, contain habitats that contribute to the annual nutritional cycle but also that the summer range is critical to the nutrition, reproduction, and overwinter survival of ungulates (eg Middleton et al .…”