Diverse migratory portfolios drive inter‐annual switching behavior of elk across the Greater Yellowstone Ecosystem

Abstract

A growing body of evidence shows that some ungulates alternate between migratory and nonmigratory behaviors over time. Yet it remains unclear whether such short‐term behavioral changes can help explain reported declines in ungulate migration worldwide, as opposed to long‐term demographic changes. Furthermore, advances in tracking technology reveal that a simple distinction between migration and nonmigration may not sufficiently describe all individual behaviors. To better understand the dynamics and drivers of ungulate switching behavior, we investigated 14 years of movement data from 361 elk in 20 herds across the Greater Yellowstone Ecosystem (GYE). First, we categorized yearly individual behaviors using a clustering algorithm that identified similar migratory tactics across a continuum of behaviors. Then, we tested seven hypotheses to explain why some ungulates switch behaviors, and we evaluated how behavioral changes affected the proportions of different behaviors across the system. We identified four distinct behavioral tactics: residents (4.8% of elk‐years), short‐distance migrants (53.7%), elevational migrants (21.9%) and long‐distance migrants (19.6%). Of the 20 herds, 18 were partially migratory, and 5 had all four movement tactics present. We observed switches between migratory tactics in all sets of consecutive years during our study period, with an average of 22.5% of individual elk changing movement tactics from one year to the next. Elk in herds with higher movement tactic diversity were significantly more likely to switch tactics and often responded more effectively to adverse environmental conditions, compared to those in herds with low movement tactic diversity. During our study period, switching increased the prevalence of both short‐ and long‐distance migrants, decreased the prevalence of elevational migrants, and had no effect on the prevalence of residents. Our findings suggest that rather than contributing to the declining migratory behavior found in the GYE, switching behavior may enable greater resiliency to continuously changing environmental and anthropogenic conditions.

Migration, movements, and survival in a partially migratory elk (Cervus canadensis) population

“…In the 1982–1996 study, 8 out of 40 cows migrated 42–84 km, compared with only 1 out of 9 cows in the current study captured in the same area which migrated 41 km, with the remaining elk migrating <10 km or being resident. These changes are analogous to changes in migratory tactics from long‐distant migrants to elevational or short‐distant migrants, which can be seen as elk responding dynamically to changes in natural and anthropogenic environments (Zuckerman et al, 2023), in this case, the ~45% increase in mine footprint over the past 30 years, which may reduce predation risk, and the increase in reclamation that provide high quantity and quality forage (Poole, 2013; Smyth, 2014). Local observations from hunters and guide outfitters confirmed that elk still inhabit the upper Elk Valley, albeit at much lower densities than in the past.…”

“…Within the current study, migration strategies used by individual elk switched at a mean rate of 14% per year, with relatively subtle changes in seasonal movements and a subtle shift towards increased migration. Snow depths and winter severity likely influenced the extent (the probability of an elk migrating increases following a severe winter) and timing (spring migration is delayed with increased severity and snow depth) of migration within the Elk Valley among years (cf Eggeman et al, 2016; Kauffman et al, 2018; Lendrum et al, 2013; Zuckerman et al, 2023). In the current study, elk occasionally but not consistently appeared to switch migration strategies according to winter severity.…”

“…In addition, migration may in part be density‐dependent as a result of intraspecific competitive release, where migrants may switch to a resident strategy at low abundance and residents may switch to a migrant strategy at high abundance, resulting in increased proportion of migrants with higher density on shared range (Berg et al, 2019; Eggeman et al, 2016; Mysterud et al, 2011). Switching migratory behavior may also increase resiliency to changing environmental and anthropogenic conditions (Zuckerman et al, 2023).…”

Predation risk drives long‐term shifts in migratory behaviour and demography in a large herbivore population

“…In our long-term study, Eggeman et al (2016) showed ~15% of individual female elk changed tactics between years, and that both top-down (predation risk) and bottom-up (forage) drivers were potential drivers of year-toyear changes. Migratory plasticity has been broadly documented in dozens of elk populations in the Greater Yellowstone Ecosystem (Zuckerman et al, 2023). Martin et al (2021) showed that such facultative switching was driven by female elk switching to the migratory tactic which demonstrated higher survival and reproduction in the previous year (similar to northern pike, Esox lucius, Haugen et al, 2006).…”

Energetic trade-offs in migration decision-making, reproductive effort and subsequent parental care in a long-distance migratory bird

“…Given spatially and temporally varying resources along migratory paths, there is substantial heterogeneity in decision-making during migration both among and within animal populations [ 4 , 5 ]. For example, populations of elevational migrants like elk ( Cervus canadensis ) [ 6 ] and bighorn sheep ( Ovis canadensis ) [ 7 ] exhibit considerable plasticity in elevational and geographical migration distance, and many latitudinal migrant bird species demonstrate heterogeneity in energy acquisition during migration stopovers [ 8 , 9 ]. While previous studies have demonstrated individual variation in reproductive success of migratory animal populations [ 10 , 11 ], the degree to which decisions throughout the annual cycle lead to differences in reproductive outcomes is often poorly understood.…”