C. Habitat Degradation or Loss
Vehicle Disturbance While bighorn sheep collisions with vehicles are not a leading cause of mortalities, they still effect the growth of populations, and are the third leading cause of deaths in Nebraska’s populations (Nordeen et al., 2019). Two of the counties that bighorn sheep have been introduced to, (Scotts Bluff and Dawes) were in the top twenty Nebraska counties for animal related crashes in 2020 (Department of Transportation [DOT], n.d.). Additionally, road traffic through bighorn habitat can alter the behavior and landscape use of the sheep. Bighorn sheep behaviors and adaptations to vehicle traffic has been studied in different populations, and while the sheep typically avoid high disturbance areas, access to resources and escape terrain will also affect the need to cross roads even with high disturbance (Lowrey & Longshore, 2017).
A study in Rocky Mountain National Park, Colorado, looked at the impacts of vehicle traffic disturbance on a population of sheep’s use of a popular natural mineral lick, the locationof which requires the sheep to cross a heavily used road within the park (Keller & Bender, 2007). The study found that bighorn sheep visits to the mineral lick decreased in the summer when the vehicle traffic increased, and that daily sheep visitation numbers peaked on the day with the lowest vehicle use (Keller & Bender, 2007). The study also found that hourly visits to the location corresponded with the peak visitations by humans to the area. This is likely due to bighorns being diurnal, and the study notes that bighorn activity and peak vehicle disturbance are likely to consistently overlap in the future. Keller & Bender (2007) also observed that out of 135 different attempts by the sheep to cross the road, 11 attempts were unsuccessful and 22 were initially unsuccessful, but subsequent successful attempts were made, with the addition that maximum vehicle numbers correlated with higher number of attempts and increased crossing time.
A similar study in Canyonlands National Park, Utah, on desert bighorn sheep responses to human disturbances found that the sheep tended to avoid roads of high human use, maintaining a greater average distance from the road than in areas of low human road use (Papouchis et al., 2001). Sheep close to the road were also found to flee from vehicle disturbance more often than those a farther distance away. Papouchis et al. (2001) also noted that roads can create a zone of influence, or an area larger than the road in which the sheep avoid due to the disturbance at the road. This study cited an additional 15.3% of available habitat that was avoided, likely due to the zone of influence from the road.
Recreation
Human recreation activities have been found to cause disturbances to bighorn sheep that can influence behaviors. Recreational activities like hiking, motor biking, and hunting are common in bighorn sheep habitats, and though hiking and motor biking does not directly involve sheep, disturbances in the bighorn habitat can have the same risk effects as predation on bighorn sheep (Lowrey & Longshore, 2017). Disturbances mimicking predation can cause stress to the sheep populations, as well as altering their behavior to the point of temporarily or permanently losing habitat if the disturbance persists over an extended period of time (Wiedmann & Bleich, 2014). Altering behavior can lead to a reduction in fitness for the sheep, as studies have found that behaviors associated with predator risk lead to increased heart rate (MacArthur et al., 1982) as well as changes in feeding and vigilance, and increased environmental sensitivity, (Taylor & Knight, 2003).
Studies show that hiking activities through bighorn habitat causes more disruption to the sheep than other disturbances like biking or vehicle traffic. Papouchis et al. (2001) found that bighorn sheep fled 61% of the time in interactions with hikers, as well as moving more than 100 meters father from hikers than when fleeing from other disturbances. This study also noted that hikers at the same level of sheep caused sheep to flee in 87% of interactions but were less likely to flee when hikers were approaching from above or below them. Greater response to hikers is likely due to the unpredictability of their movements, unlike vehicles and bikes that are confined to roads and biking trails (Papouchis et al., 2001). Despite greater predictability in motor biking activities, consequential behavior in bighorns is still prominent. Lowrey and Longshore’s (2017) study of Nevada desert bighorns found that bighorn sheep were likely to completely abandon suitable but less attractive habitat when disturbed by unpredictable mountain biking. Additionally, the probability of bighorns inhabiting the area increased as distance from the bike trails increased.
Populations of female bighorns are particularly vulnerable to the loss of lambing habitat to human recreation activities. Wiedmann and Bleich (2014) found that on four subpopulations of bighorns in North Dakota, despite inhabiting areas with similar ecological factors, one population was found to avoid historic lambing sites. The only difference found between the populations was high levels of human recreation (primarily hiking) disturbance in this subpopulation’s lambing ranges, suggesting the loss of the habitat area is due to human recreation. The females were found to be displaced from the area for six years and abandoned the habitat for another additional six years. Female bighorn sheep in California changed their habitat use in response to increased recreational activities on weekends (Longshore et al., 2013). Sheep were more likely to occur in areas with higher slope and farther from hiking trails on the weekends compared to weekdays (Longshore et al., 2013).
Hunting is an additional recreation activity with potential effects on populations. Current hunting of bighorns primarily allows for harvest of males, removal of which commonly has no effect on resources available to reproductive female herds if population numbers are stable (Jorgenson et al., 1993). However, harvest of bighorns has potential to alter sheep behavior. Bighorns begin to perceive any human disturbance as predatory behavior in populations that are recreationally hunted (Hutchins & Geist, 1987), heightening stress to the individual sheep. Trophy hunting of male bighorns also has the potential to alter fitness of younger males, as removal of trophy sized males can lead to selective pressure of younger males to compensate for the loss of more mature rams (Kuparinen & Festa-Bianchet, 2017). Further, genetic analysis of trophy hunting shows that trophy hunting targets rams with high genetic quality, but often harvests them before they reach the peak of their reproduction (Coltman et al., 2003). This reduces the genetic qualities of early growth in both body and horns from the gene pool, and a continuation of this practice alludes to future genetic deterioration and added stress to the population’s remaining males (Coltman et al., 2003).
Continue with onX Maps
Sign in with Facebook
