Range expansion and population growth of non-native mountain goats in the Greater Yellowstone Area: Challenges for management

Abstract

Population growth and range expansion of nonnative species can potentially disrupt ecosystem function or add conservation value to an area, and evaluation of possible impacts can be a challenge for managers. Nonnative populations of mountain goats (Oreamnos americanus) are present in the Greater Yellowstone Area (GYA) in the U.S. states of Idaho, Montana, and Wyoming because of historical introduction events, but their population trend and range have not been assessed across the area. We used 6,701 location records from 1947 to 2015 to map mountain goat distribution and evaluate, in a descriptive manner, range overlap with native bighorn sheep (Ovis canadensis). We analyzed 136 survey counts using the Exponential Growth State-Space model to estimate population trends and abundance. Mountain goats dispersed 50-85 km from introduction sites to occupy all mountain ranges in the northern GYA and 30-40 km to occupy new areas in the southern GYA. Mountain goat numbers increased in nearly all count units, with the strongest growth rates estimated in areas more recently colonized. Using moderate detection probability (0.70), we estimated approximately 2,355 mountain goats in the GYA. Although not tested in our analysis, the gradual range expansion and population growth rates were consistent with density-dependent processes observed in other introduced large herbivores and demonstrate that mountain goats can successfully disperse over unsuitable locales to colonize new areas. Therefore, we expect mountain goat populations will continue to expand into unoccupied mountain ranges that contain significant numbers of bighorn sheep unless specific management actions are implemented to address their population growth. Ó

Niche similarities among introduced and native mountain ungulates

“…At present, the observed spatial and niche overlap does not appear to be negatively impacting demographics as both species have maintained positive growth rates over the period of mountain goat expansion (Flesch et al. , Butler et al. ).…”

“…Mountain goats have continued to increase in abundance and distribution since their initial introductions (Laundré , Flesch et al. ) and have been sympatric with bighorn sheep for approximately 50 yr (Lemke ). Bighorn sheep and mountain goat minimum counts were 1,500 and 632 individuals in 2016, respectively (Flesch et al.…”

“…Bighorn sheep and mountain goat minimum counts were 1,500 and 632 individuals in 2016, respectively (Flesch et al. , McWhirter , MFWP ). Within the study area, bighorn sheep were never extirpated or augmented, and have maintained relatively stable population demographics with modest harvest rates and nearly ubiquitous presence of respiratory pathogens in sampled animals (Butler et al.…”

“…These efforts have largely been successful and greatly expanded the distribution of mountain goats both within and beyond their native ranges (Côté and Festa‐Bianchet , Flesch et al. ). Although the expanding mountain goat population in nonnative areas has been embraced by some natural resource agencies, there is concern that encroachment into areas occupied by native bighorn sheep ( Ovis canadensis ) may be detrimental to regional restoration efforts (Adams et al.…”

“…The northeast portion of the GYA, where introductions were first initiated and most concentrated, is nearly completely colonized by mountain goats with minimum counts of 632 individuals (Flesch et al. ). Herds of native bighorn sheep occur within and adjacent to the expanding mountain goat population throughout the GYA.…”