Individual variation creates diverse migratory portfolios in native populations of a mountain ungulate

Abstract

Ecological theory and empirical studies have demonstrated population‐level demographic benefits resulting from a diversity of migratory behaviors with important implications for ecology, conservation, and evolution of migratory organisms. Nevertheless, evaluation of migratory portfolios (i.e., the variation in migratory behaviors across space and time among individuals within populations) has received relatively little attention in migratory ungulates, where research has focused largely on the dichotomous behaviors (e.g., resident and migrant) of partially migratory populations. Using GPS data from 361 female bighorn sheep (Ovis canadensis) across 17 (4 restored, 6 augmented, 7 native) populations in Montana and Wyoming, USA, we (1) characterized migratory portfolios based on behavioral and spatial migratory characteristics and (2) evaluated the relative influence of landscape attributes and management histories on migratory diversity. Native populations, which had been extant on the landscape for many generations, had more diverse migratory portfolios, higher behavioral switching rates, reduced seasonal range fidelity, and broad dispersion of individuals across summer and winter ranges. In contrast, restored populations with an abbreviated history on the landscape were largely non‐migratory with a narrow portfolio of migratory behaviors, less behavioral switching, higher fidelity to seasonal ranges, and less dispersion on summer and winter ranges. Augmented populations were more variable and contained characteristics of both native and restored populations. Differences in migratory diversity among populations were associated with management histories (e.g., restored, augmented, or native). Landscape characteristics such as the duration and regularity of green‐up, human landscape alterations, topography, and snow gradients were not strongly associated with migratory diversity. We suggest a two‐pronged approach to restoring migratory portfolios in ungulates that first develops behavior‐specific habitat models and then places individuals with known migratory behaviors into unoccupied areas in an effort to bolster migratory portfolios in restored populations, potentially with synergistic benefits associated with variation among individuals and resulting portfolio effects. Management efforts to restore diverse migratory portfolios may increase the abundance, resilience, and long‐term viability of ungulate populations.

Regional variability in pregnancy and survival rates of Rocky Mountain bighorn sheep

“…Mountain ungulates in particular may be sensitive to changing climatic conditions (Jacobson et al 2004, Aublet et al 2009, White et al 2011, 2018) as these species occupy extreme and highly seasonal environments that are predicted to experience changes in vegetation phenology and community composition in response to predicted climatic changes (Inouye 2008, Wipf et al 2009, Gottfried et al 2012). Within our study region, bighorn sheep populations occupy diverse landscapes from prairie populations occupying relatively arid landscapes of prairie badlands, to nonmigratory populations occupying low elevation mountain foothills, to seasonally migratory populations occupying alpine environments in summer and low elevation foothills in winter, to populations with a significant component of animals that remain year‐round in high elevation alpine environments (Lowrey et al 2020). Given the wide diversity of regional environmental conditions experienced by the populations in our study, we would expect the impacts of climate change in the demographic performance of these populations to also be diverse.…”

“…The populations include a variety of management histories, including native, augmented, and restored populations. Restored populations were largely resident on low elevation winter ranges, while augmented and native populations had varying degrees of migratory behavior (Lowrey et al 2019(Lowrey et al , 2020. A variety of Pasteurellaceae bacterial pathogens associated with respiratory disease in bighorn sheep have been detected in all populations, and Mycoplasma ovipneumonia was known to be resident in all populations except Galton, Grand Teton National Park, Middle Missouri, Paradise, and Petty Creek (Butler et al 2018).…”

Broadening the migratory portfolio of altitudinal migrants

“…Diverse migratory portfolios are associated with large, robust populations of large herbivores (Fryxell et al 1988, Sawyer et al 2016) and greater resilience with respect to population persistence. Further, movement behaviors have everincreasing prominence in evolutionary ecology and conservation (Lowrey et al 2020). Here, we report a novel movement behavior for altitudinal migration observed in Sierra Nevada bighorn sheep (Ovis canadensis sierrae; hereafter Sierra bighorn), which we suspect could have important demographic and fitness consequences.…”

“…Movement allows animals to exploit spatially and temporally variable resources; these resources in turn influence individual fitness and demographic rates of populations (Fryxell et al 1988, Bolger et al 2008, Mueller and Fagan 2008, Hebblewhite and Merrill 2009. A diverse movement portfolio, wherein a variety of movement behaviors are represented across space and time and among individuals in a population, likely is important for population persistence in the face of environmental change (Lowrey et al 2020) and may be especially important for managers to re-establish in populations of endangered species. Diverse movement portfolios include some combination of movement behaviors such as range residency (single, year-round range), migration (movement between two nonoverlapping ranges using the same route at regular time intervals), and nomadism (movements across the landscape that do not follow the same route; Mueller and Fagan 2008), although other intermediate behaviors may occur (e.g., abbreviated migration [Courtemanch et al 2017]; commuting between ranges [Cagnacci et al 2011]).…”

“…Because vacillating migrants experienced a different environment than individuals with other movement behaviors in the same area, we expect their demography also may differ. Hence, we emphasize the importance of identifying vacillating and other atypical migratory strategies, particularly for demographic studies (Lowrey et al 2020). An alternative explanation for vacillating migration is that it represents seasonal habitat use or forays rather than a type of migration; however, high-and low-elevation ranges were separated by hundreds to thousands of meters in elevation (Appendix S1: Fig.…”