Native forage mediates influence of irrigated agriculture on migratory behaviour of elk

Abstract

Ungulates migrate to maximize nutritional intake when forage varies seasonally. Populations of ungulates often include both migratory and non‐migratory individuals, but the mechanisms driving individual differences in migratory behaviour are not well‐understood. We quantified associations between hypothesized drivers of partial migration and the likelihood of migration for individual ungulates that experienced a range of environmental conditions and anthropogenic influences. We evaluated the effects of forage variation, conspecific density, and human land uses on migratory behaviour of 308 adult female elk in 16 herds across western Montana. We found irrigated agriculture on an individual's winter range reduced migratory behaviour, but individuals were more likely to migrate away from irrigated agricultural areas if better forage was available elsewhere or if they experienced high conspecific density on their winter range. When the forage available during the summer growing season varied predictably between years, elk were more likely to migrate regardless of whether they had access to irrigated agriculture. Our study shows that predictable availability of beneficial native forage can encourage migration even for ungulates with irrigated agriculture on their winter range. Perturbations that can affect the forage available to ungulates include wildfires, timber harvest, livestock grazing and changing weather patterns. If these or other disturbances negatively affect forage on summer ranges of migrants, or if they cause forage to vary unpredictably across space and time, our results suggest migratory behaviour may decline as a result.

Behavior‐specific habitat models as a tool to inform ungulate restoration

“…Our work was conducted with both resident and migrant populations with little variation among individuals within a population (Appendix S2). Recent work however has highlighted the diversity of migratory behaviors and that a single ungulate population can contain residents as well as short-, mid-, and long-distance migrants (Cagnacci et al 2011, Sawyer et al 2016, Barker et al 2019). Moreover, residency, which occurred exclusively on low elevations in our study populations, can also occur at high elevations where bighorn sheep remain at elevations ≥3000 m annually (Courtemanch et al 2017, Spitz et al 2020.…”

The plasticity of ungulate migration in a changing world

“…Species that appeared in multiple studies often exhibited multiple changes, with some species exhibiting every type of migration change (e.g., elk [ Cervus canadensis ], Box 2).

Elk ( Cervus canadensis ) exhibit particularly plastic migratory behavior. Most populations of elk are partially migratory, with some individuals migrating seasonally and others residing on a shared range year‐round (Barker et al 2019 a ). Despite past belief that migratory behavior is fixed at the individual level, recent long‐term studies reveal that some elk switch between migratory and resident behaviors (Eggeman et al 2016).

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“…Altered migration propensity has been speculated to improve fitness by decreasing risk (e.g., predation risk avoidance hypothesis) or by increasing nutrition (e.g., forage maturation hypothesis; Middleton et al 2013, Eggeman et al 2016). Changes to the distribution of high‐quality forage can also affect migration propensity (Barker et al 2019 a ). Among the 34 propensity change events documented in the literature we reviewed, 5 were influenced by predation (14.7%; e.g., Hebblewhite and Merrill 2011, White et al 2014) and 11 by interannual climate conditions (32.4%; e.g., Fieberg et al 2008, Cagnacci et al 2011).…”

“…For example, in early studies of partial migration, migratory propensity was often treated as a dichotomy of resident vs. migratory (Berg et al 2019). More recent work in which partial migration was observed along a behavioral continuum marked a step toward applying reaction norms to understand variation in migration propensity (Cagnacci et al 2011, Gurarie et al 2017, Barker et al 2019 a ). Given the interdependence of multiple dimensions of migratory plasticity, the concept of behavioral continuum could also be applied to the temporal and spatial dimensions.…”

Toward a new framework for restoring lost wildlife migrations

“…Rather, migration is best conceived as a continuum that also includes intermediate movement tactics (e.g., making multiple trips; moving for abbreviated times or distances; Boel et al., 2014). This population‐level variation in behavior has proven far more diverse than previously recognized, and populations even within the same species and geographic area can exhibit markedly different variations (Barker et al., 2019; Weimerskirch et al., 2017).…”