A Review of Hypothesized Determinants Associated with Bighorn Sheep (Ovis canadensis) Die-Offs

Table 2

Environmental determinants proposed to limit free-ranging bighorn sheep populations.

Determinant (subcategories)	Geographic distribution of determinant	Locations where determinant reported	Die-off attributed to determinant^1	Selected references
Adverse environmental conditions	Northern and mountain locations	California	Yes	[28]
		Colorado	No	[42]
		Canadian Rockies, Canada	Yes	[72]
		Wyoming	Yes	[43]
Escape terrain	Range-wide	Arizona	No	[83]
		Baja California Sur, Mexico	No	[84]
		Montana	No	[85]
		New Mexico	No	[44]
Range and migration restriction due to human settlement or activities	Range-wide	Alberta, Canada	No	[86]
		Arizona	No	[87]
		California	Unclear^2	[28]
		California	No	[88]
		Colorado	No	[27, 89–91]
		Colorado	Yes	[92] (also see [93])
		Montana	No	[94]
Free-water	Primarily arid regions	Desert, general	No	[95]
		Arizona	No	[96, 97]
		California	No	[54, 79]
		California	No	[53]
		Utah	No	[55]
Food quantity
Limited forage and precipitation	Potentially range-wide	Alberta	No	[59]
		Arizona	No	[60, 98, 99]
		California	Yes	[65]
		Colorado	No	[61]
		Montana	Yes	[49]
		Nevada	No	[50]
		New Mexico	No	[44, 62]
		Texas	No	[63, 66]
		Utah	Yes	[51]
Plant community succession	Potentially range-wide	British Columbia, Canada	No	[73]
		California	No	[74, 75]
		Canadian Rockies, Canada	Yes	[72, 76]
		Montana	No	[94]
Food quality
Protein deficiency	Potentially range-wide	Colorado	No	[27]
		Texas	No	[63]
Dietary mineral availability	Range-wide	Alberta, Canada	No	[100]
		Arizona	No	[60, 101]
		British Columbia, Canada	No	[102, 103]
		California	No	[28]
		Colorado	No	[27]
		Utah	No	[104]
		Wyoming	No	[105]
Selenium deficiency	Localized	Wyoming	Yes	[70]
Competition
Competition for forage-domestic species	Range-wide	Alberta, Canada	Yes	[106]
		Arizona	Unclear^2	[107]
		California	No	[28, 78]
		Colorado	No	[27, 89]
		Nevada	No	[108]
		Oregon	Yes	[109]
		Texas	No	[110]
		Wyoming	Yes	[111]
Competition for forage-bighorn sheep	Potentially range-wide	Colorado	Yes	[112]
		Montana	Yes	[113, 114]
		Tiburon Island, Mexico	No	[3]
		Wyoming	No	[115, 116]
Competition for forage-native ruminant species	Potentially range-wide	Canadian Rockies, Canada	Yes	[72]
		Colorado	No	[27, 89]
		Colorado	Yes	[112]
		Montana	Yes	[117]
		Montana	No	[94]
		Sierra Nevada, California	Unclear^2	[28]
		Wyoming	No	[118]
		Wyoming	Yes	[111, 113, 119]
Seasonal
Competition for forage-limited winter range (environmental or nonspecific)	Potentially range-wide	Alberta and British Columbia, Canada	Yes	[120, 121]
		Colorado	No	[42]
Competition for forage-limited winter range (domestic livestock grazing)		Colorado	No	[4, 27]
Competition for forage-limited winter range (native ruminant species)		Wyoming	Yes	[122]
Competition for space (social impacts)	Potentially range-wide	Alberta, Canada	No	[59, 80]
		California	No	[79]
		Colorado	Yes	[19]
		Idaho	No	[81]

¹Mortality considered to be in excess of baseline levels;
²Mortality in excess of baseline versus endemic disease status was not clear.