To forage or flee: lessons from an elk migration near a protected area

Abstract

Alteration of wide‐ranging wildlife migrations can drastically impact the structure and function of ecosystems, yet the causes and consequences of shifting migration patterns remain largely unknown. Management decisions made in one portion of a landscape may induce spatial and temporal shifts of wildlife use in another, creating tension among private, state, and federal lands with varying missions. Recent declines in migratory behavior have initiated studies focused primarily on spring migration, but the timing and benefits of autumn migration have received limited attention due to the difficulty in assessing the extreme asynchrony in autumnal events, although nutrition during this time period is crucial to winter survival and reproduction. Here, we used five years of data from 73 female elk (Cervus canadensis) which utilize a landscape managed by 4 federal agencies, a state, and private landowners, to identify the driving factors behind the initiation of fall migration in two subpopulations, one of which migrates to a protected area where hunting is prohibited. Most elk departed summer range prior to frost or snow, with 67% of elk that used the protected area migrating prior to the onset of archery hunting season (1 September), preemptively avoiding risk, while no elk from the other subpopulation left prior to archery season. However, departure from productive summer range prior to frost or snow, nearly two months before vegetation senescence, led to an important tradeoff. Early migrants gave up 0.30% of late summer‐fall integrated NDVI (iNDVI) per day when they moved to the safety of the protected area, leading to an average difference of 15.81% in iNDVI between an individual departing on 30 August (the mean departure date) vs. an individual departing on 1 November. Our results suggest that in areas where migratory ungulates span multiple jurisdictions, the benefits of migratory behavior may be dramatically impacted by unevenly distributed anthropogenic disturbance. As this is a common scenario globally, our work highlights the urgent need to improve our understanding of subtle changes in migratory behavior, both spatially and temporally, which may erode the resilience of migration to future change.

Sex‐Specific Behaviors of Hunted Mule Deer During Rifle Season

“…Avoidance of motorized routes may not have been significant in the pre‐rifle period most likely because approximately 18% of males in the sample were migrating or had begun migration, likely exposing them to areas closer to motorized routes. Mule deer generally avoid roads and associated human disturbances (Lendrum et al 2012, Coe et al 2018), as do other ungulates (Meisingset et al 2013, Montgomery et al 2013), especially during hunting season (Bonnot et al 2013); however, we did not observe mule deer moving to private, or restricted lands, as has been previously observed in elk (Proffitt et al 2009, 2010; DeVoe et al 2019; Mikle et al 2019). Our landscape configuration allowed animals to remain on a mix of public and private lands but in areas where motorized route density (<0.6 km/km 2 on average in our study area) mediated disturbance.…”

“…Recent work suggests that disturbance from hunting can influence ungulate movements and distribution patterns (Paton et al 2017, Brown et al 2020) and may even trigger migration onset (Rivrud et al 2016, Mikle et al 2019). Our study suggests that nearby refugia or security areas where mule deer can reduce their exposure to hunting may mitigate potential effects on autumn migration (Proffitt et al 2013).…”

“…There is growing concern that hunting can influence broad‐scale movements and trigger autumn migration to occur earlier than environmental cues would dictate (Rivrud et al 2016, Mikle et al 2019, Rickbeil et al 2019). For example, Rivrud et al (2016) reported the likelihood of migration was increased on opening day of hunting season, 1–2 days before and after opening day, and the week after hunting onset for red deer ( Cervus elaphus ) in Norway (Rivrud et al 2016).…”

“…For example, Rivrud et al (2016) reported the likelihood of migration was increased on opening day of hunting season, 1–2 days before and after opening day, and the week after hunting onset for red deer ( Cervus elaphus ) in Norway (Rivrud et al 2016). Elk herds from studies in the Greater Yellowstone Ecosystem (Rickbeil et al 2019) and southwest Wyoming (Mikle et al 2019) that were exposed to hunting pressure also left summer ranges earlier. These findings challenge the notion that weather variables such as snowfall and cold temperatures (Garrott et al 1987, Jones et al 2014, Rittenhouse et al 2015, Rickbeil et al 2019), and intrinsic factors such as age (Monteith et al 2011) and sex (Peters et al 2019) are primarily responsible for influencing autumn migration.…”

Comparative Quality and Trend of Remotely Sensed Phenology and Productivity Metrics across the Western United States

“…In addition, spatial heterogeneity of plant phenology, which may be declining due to warming temperatures, relates to the reproduction rates of caribou [26]. Autumn phenology, which has received considerably less attention than that of spring, can influence body mass and overwinter survival of mule deer [4] and migratory patterns of elk and red deer [18,27,28]. In addition to phenology, vegetation productivity is closely correlated to greenness indices such as Normalized Difference Vegetation Index (NDVI) [8] and explains ungulate habitat use [24,29], health characteristics [30], and demographic parameters [16].…”