Animal migration amid shifting patterns of phenology and predation: lessons from a Yellowstone elk herd

Abstract

Migration is a striking behavioral strategy by which many animals enhance resource acquisition while reducing predation risk. Historically, the demographic benefits of such movements made migration common, but in many taxa the phenomenon is considered globally threatened. Here we describe a long-term decline in the productivity of elk (Cervus elaphus) that migrate through intact wilderness areas to protected summer ranges inside Yellowstone National Park, USA. We attribute this decline to a long-term reduction in the demographic benefits that ungulates typically gain from migration. Among migratory elk, we observed a 21-year, 70% reduction in recruitment and a 4-year, 19% depression in their pregnancy rate largely caused by infrequent reproduction of females that were young or lactating. In contrast, among resident elk, we have recently observed increasing recruitment and a high rate of pregnancy. Landscape-level changes in habitat quality and predation appear to be responsible for the declining productivity of Yellowstone migrants. From 1989 to 2009, migratory elk experienced an increasing rate and shorter duration of green-up coincident with warmer spring-summer temperatures and reduced spring precipitation, also consistent with observations of an unusually severe drought in the region. Migrants are also now exposed to four times as many grizzly bears (Ursus arctos) and wolves (Canis lupus) as resident elk. Both of these restored predators consume migratory elk calves at high rates in the Yellowstone wilderness but are maintained at low densities via lethal management and human disturbance in the year-round habitats of resident elk. Our findings suggest that large-carnivore recovery and drought, operating simultaneously along an elevation gradient, have disproportionately influenced the demography of migratory elk. Many migratory animals travel large geographic distances between their seasonal ranges. Changes in land use and climate that disparately influence such seasonal ranges may alter the ecological basis of migratory behavior, representing an important challenge for, and a powerful lens into, the ecology and conservation of migratory taxa.

Large herbivores surf waves of green-up during spring

“…A mismatch between the true, current locations of high-quality food and locations of high-quality food in memory is likely exacerbated in areas where weather patterns are rapidly changing, because of climate and landuse change. For example, previous work on the Absaroka elk population in our analysis indicates that rate of spring greenup has increased, leading to shorter spring periods [37]. Shorter springs likely make it difficult for individuals with strong site fidelity to adapt to new environmental conditions.…”

“…Finally, monitoring changes in IRG over the growing season can provide insight into how climate change is affecting the timing and rate of spring green-up, which is emerging as a key component of annual habitat quality, and ultimately, fitness and population performance (e.g. [18,19,37,38]). Incorporating IRG into such analyses will prove useful for quantifying the loss in forage availability-because of climate change or land development-that animals might experience in the future.…”

Grizzly bear predation links the loss of native trout to the demography of migratory elk in Yellowstone

“…These patterns suggest that summer predation has contributed to low calf-cow ratios in migratory populations [21,24].…”

“…Most strikingly, segments of the northern Yellowstone herd that summer near Yellowstone Lake have been observed with calf-cow ratios below 0.1 in July and August [29]. Such low calf numbers, relatively soon after calving, suggest a combination of low pregnancy [21], low birth weights [27] and/or high rates of predation [24]. However, pregnancy rates in the northern Yellowstone herd have been more than 80 per cent in recent years [29,30], and In studies conducted during the post-decline period, the proportion of trout in the grizzly bear diet (black) at peak calving/spawning time has decreased, whereas the proportion of ungulate tissue (grey) has increased (estimates from Fortin et al [10], Mattson & Reinhart [14] and Mattson [26]).…”

Sustained disruption of narwhal habitat use and behavior in the presence of Arctic killer whales

“…When large enough, such effects have even been suggested to lead to trophic cascades (16,17,3). However, there is disagreement about whether nonconsumptive effects can be strong enough to cause trophic cascades, even in well-studied exemplar systems (18)(19)(20).…”