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Desert Bighorn Sheep: Collar and Conserve

Photo Credit: Josh Schulgen

Due to numerous factors including habitat loss, disease and overhunting, desert bighorn sheep were driven to near extinction in the 1800s. Thanks to persistent conservation efforts, populations were able to rebound, and currently reside at their highest numbers since the early 20th century. Through trial and error, it has become apparent that monitoring these animals has become crucial in preserving California’s desert bighorn population.

Photo Credit: Josh Schulgen

Capturing and collaring these bighorns has been a way for scientists to track populations for decades. Tracking these animals provides scientists with valuable information in order to determine a variety of possible solutions to fulfill the needs of a given herd. Originally, radio collars used for tracking wildlife only contained Very High Frequency(VHF) technology, which emits radio signals that can be picked up via antenna when in sight of the collared animal. Technology has since improved leading to the use of Global Positioning System(GPS) collars, which provide a more frequent and accurate location which is retrieved from satellites and can be monitored remotely. The combination of these two technologies has allowed for scientists to track bighorn sheep more closely than ever before.

One of the first uses of VHF radio collars on desert bighorn sheep in California took place between 1977 and 1978 after concerns over resources arose in a population of these animals within Joshua Tree National Monument. Springs and other bodies of water within the monument had been drying up for decades, leaving the Stubbe Spring herd of bighorns to depend on a singular water source. This meant the entirety of this herd had to live within a relatively small area in order to have access to water. With so many sheep in a tight area, overgrazing can quickly become a problem, further hindering an increase in population numbers.

Radio collars, fit around the neck of the animal, would seek to provide information pertaining to the range of sheep utilizing Stubbe Spring. This information would aid in the placement of a guzzler, which would be implemented in hopes of expanding the herd’s summer range into additional unused habitat. This would ideally help the herd expand their population without the risk of overutilizing their native range.

In July of 1977, 9 sheep, including 4 rams and 5 ewes, were trapped and fitted with radio-telemetry collars around the Stubbe Spring area. Unfortunately, the battery life on these collars did not live up to the expected 18-24 months, and more sheep needed to be collared in order to collect precise data. The following year, 3 more ewes and 1 more ram were fitted with collars between the months of July and August. After further analysis, data collected from these collars indicated the guzzler would most likely be found and used by bighorns ~1 mile west to southwest of Stubbe Spring(Desert Bighorn Council, 1979).

Photo Credit: Josh Schulgen

Collars using GPS technology were first seen used on California desert bighorns during a project that took place in the Peninsular Range between 2001 and 2003. 34 bighorns from 5 different subpopulations were fitted with GPS collars, along with additional VHF collars. These GPS collars provided over 16,000 daytime location points between October 2001 and November 2003. The sex and location of collared animals are as stated, Vallecito Mountains: 2 M (ale), 1 F (emale); San Ysidro Mountains: 6 M , 10 F ; Coyote Canyon: 3 M , 4 F ; Santa Rosa Mountains SE of Hwy 74: 5 M , 3 F (CDFG, “Assessment of Predictive Habitat Models for Bighorn Sheep in California's Peninsular Ranges'', 2003). Desert bighorn sheep are rather inactive at night, meaning only daytime locations were used in order to accurately display how habitat was prominently utilized. The precise locations collected from these populations during the two year window were used to create a predictive habitat model for the Peninsular Range desert bighorn sheep population.

Being the most recent innovation in tracking technology, GPS collars have continued to be used alongside VHF collars for wildlife tracking since the early 2000s. With modern urbanization taking place over the majority of Southern California, humans have continued to interfere with optimal habitat that is/was once inhabited by desert bighorns. Freeways in the Mojave Desert have created massive barriers in both migratory paths and home ranges of these sheep. Although it may seem like these massive roadways are avoidable for the sheep, many of these corridors are critical to population numbers. Migration and traveling allows for a consistent gene flow among the species, keeping a population more diverse and sustainable. Without a safe way to do so, many animals resort to attempting to cross oncoming traffic, putting drivers and wildlife at risk.

In order to solve problems regarding desert bighorns and the freeways that divide their ranges, Oregon State University captured and fitted 41 of these animals, from 9 different populations, with GPS collars. No GPS data exists from before the construction of these freeways, so the information collected from these collars was used to create a simulation in order to see what these corridors may have looked like historically. Lead research associate, Christina Aiello(Oregon State University), used the data to simulate 200 years of movement from each individual bighorn. Results revealed that the most preferred crossing corridors for Desert Bighorns in the area were through valleys currently blocked by freeways. These valleys directly correlate with historic bighorn roadkill events, as well as predicted routes seen in landscape genetic models(Christina Aiello, OSU, 2023).

Photo Credit: Josh Schulgen

Along with the already restrictive freeways, a proposed high-speed electric railway looks to connect Southern California to Las Vegas, thus splitting bighorn habitat even further. This railway would follow the center divide of I-15 and would see the construction of 6-foot-high concrete walls to separate traffic from the railway(Los Angeles Times, 2021). This wall would create a total blockade in historic desert bighorn routes, and would cut off essential corridors for many herds who rely on migration and traveling.

Fortunately, the California Department of Fish and Wildlife(CDFW) partnered with the companies tasked with constructing this railway and were able to agree upon the introduction of three new wildlife overpasses(CDFW News Room, 2023). These overpasses will be placed in travel corridors that directly correlate with predicted historic routes, determined from GPS data collected by CDFW. Placement of these crossings hopes to see these populations return to historic migration patterns, giving them complete access to necessary resources.

Wildlife crossings, like those mentioned, drastically increase safety for drivers while also decreasing fatalities for animals. Thanks to modern GPS technology, along with the help of data simulations, locations for these critical mitigation sites are much easier to pinpoint. In places where wildlife friendly crossings, paired with fences, have been implemented, we have seen a decrease in wildlife-vehicle collisions of greater than 80%(ScienceDaily). This means by fitting these animals with GPS collars, scientists gather crucial information which can immediately reduce population threats.

Conclusively, gathering information on desert bighorn populations is the first step to conserving and managing these populations. The Winter Range Foundation is currently seeking funds in order to capture and collar sheep from the Last Chance, Dry Mountain, and North Hunter Mountain bighorn populations. Little is known about these individual populations, and although they have never been extirpated, they are thought to have population numbers well below historic levels. The Winter Range Foundation needs to purchase 15 GPS collars in order to facilitate a larger collaring project to be conducted in November 2023 by CDFW. This project currently calls to collar 39 desert bighorns across 10 units. Information gathered by these collars provides critical data that will greatly help CDFW in future conservation and management of these populations.

Photo Credit: Josh Schulgen

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