Landscape Factors Inhibiting Mountain Goat Movements: A Contribution to Delineating Demographic Units

Harris, Richard; Hayes, Forest; Rice, Clifford; Vales, David

Because mountain goat (Oreamnos americanus) populations are sensitive to anthropogenic mortality, managers of recreational harvests typically restrict hunting opportunity to a small percentage of estimated abundance within some defined boundary (and often, only if abundance meets a defined minimum). In addition to difficulties of estimating abundance in the field, goat managers face uncertainty in geographically delineating where one “population” ends and another begins. Mountain goats typically remain close to steep escape terrain, yet they are sometimes seen in atypical habitats and occasional migrants are known to traverse considerable distances across inhospitable terrain. Molecular approaches provide understanding of barriers to gene flow, but aggregations that can potentially be overexploited likely operate on smaller spatio-temporal scales. Managing a small subset of goats may understate the scale on which demography operates; conversely, managers may face pressure to aggregate units inappropriately. Localized, detailed information to answer these questions are unavailable, and goats exhibit considerable heterogeneity in movement patterns. Thus, we used of GPS-collar data from 184 mountain goats from previous studies in Washington and Montana to quantify movement patterns that may be generalizable across their range south of Alaska. We used U.S. Forest Service digital maps of unconfined valley bottoms (Nagle et al. 2014) as a common currency to quantify goats’ willingness to cross atypical habitat, and thus provide a proxy for topography likely to constrain populations at the management scale. As expected, mountain goat movement paths (reflecting ~ 285 goat/years of data) rarely intersected unconfined valleys, particularly those larger than 100 ha in size (𝑥𝑥̅ = 0.007/goat/year). This suggests that unconfined valleys, as defined, may provide useful surrogates of barriers to movement at the demographic spatio-temporal scale. Such valleys would not appropriately be used directly as demographic unit boundaries, but because they are mapped across the U.S. Northern Rockies can be referenced in assessing if goat hunting boundaries are likely to encompass > 1 demographic unit.