A Six Month Analysis of the Desert Bighorn Sheep (Ovis canadensis nelsoni) Translocation Project in the Santa Catalina Mountain Range
Essay by Meagan B.
Introduction
As human encroachment increases and changes to the climate, environment and landscape occur, the loss of a species in a particular area is becoming familiar across the globe. In some cases, the species appears forever lost to the region. Such was the case in the Santa Catalina Mountain range where desert bighorn sheep (Ovis canadensis nelsoni) once flourished but disappeared from the landscape in the 1990’s. The decline of the bighorn population has not been attributed to any single factor, but to a number of factors including fire suppression and natural predation (Etchberger et al., 1999, Czech and Krausman 1997, Papouchis et al., 2001).
In other parts of the southwestern United States, desert bighorn sheep (Ovis canadensis nelsoni) still thrive. The sheep in those regions have many factors in common with the past Catalina population. They survive in an arid environment with rugged terrain and sheer cliffs to afford protection and escape from predators such as mountain lions, golden eagles, bobcats, and coyotes. They consume desert plants that are commonly found in the Catalinas such as fluff grass, catsclaw, ocotillo spurges, buckwheat, and variety of seasonal weeds (AZGF 2013). And just as they once did here, the female bighorn sheep, known as ewes, tend to select steeper terrain rather than more open habitats that mature males select (Bleich et al., 1997, Mooring et al., 2004). And so consideration was given to whether a reintroduction back into the area would be successful.
Launay (2014), Chair of the IUCN SSC Re-introduction Specialist Group, cautions that the success of a reintroduction largely depends on whether the cause of the previous local extinction has been corrected, and upon the research and planning that must ensure the viability of the population. When the cause of extirpation of mega fauna such as the bighorn has been reversed or removed, a particular type of reintroduction, called translocation, can be proposed. Translocation is the removal of a sample of a prospering population, then placing those individuals into a habitat where the species formerly survived in order to re-populate the area. Translocation of mega fauna has, in some cases, had great success. In 1973, twenty-two bighorn sheep were reintroduced back into Aravaipa Canyon and the Galiuro Mountain Range in Arizona using translocation. Today, the reintroduced population is once again flourishing after experiencing considerable initial problems due to mountain lion predation. Three releases of thirty bighorn sheep each took place between 2007 and 2010 in the Mineral Mountains of Arizona, setting the stage for the current release of bighorn sheep in this project. Little is published about the status of the Mineral Mountain herd, which is comprised of many individuals taken from the same region as the current study- the Trigo Mountains, near Yuma, AZ.
The Arizona Game and Fish Department (AZGFD) assessed the Santa Catalina Mountain range in 2011 to determine whether the region was suitable for bighorn sheep reintroduction. The fires that occurred in 2003 and 2004 cleared dense vegetation, reducing the threat of predation on bighorn sheep, which was one of the original factors that may have contributed to their disappearance. AZGFD used a Cunningham/Hansen evaluation that divided the region into seventy 4km2 cells to evaluate the mountain range in greater detail. Each block was individually examined for five criteria: topography, vegetation, precipitation, water sources, and human use. The results were categorized into four categories labeled as excellent, good, fair, and poor. There were 24 poor cells, 41 fair cells, 5 good cells, 0 excellent cells (AZGF 2014 BHS update). The results were considered sufficient to proceed with the planning of a reintroduction of the bighorn sheep back into the Santa Catalinas.
Thirty-one bighorn sheep from the Trigo and Plumosa Mountain Ranges near Yuma, AZ were released on November 18, 2013 into the Pusch Ridge Wilderness of the Santa Catalina Mountains near Tucson, AZ by Arizona Game and Fish. Each individual was fitted with Global Positioning System (GPS) collars that update six times a day and provide the animal’s location and mortality status. The question of whether the translocated bighorn sheep will ultimately survive their initial reintroduction is the topic of this project.
Research Question
How will two populations of bighorn sheep disperse, traverse the terrain, interact (with each other and with humans) and survive during the six-month period immediately following their translocation from the Trigo and Plumosa Mountains near the Yuma Proving Grounds to the Pusch Ridge Wilderness area in the Santa Catalina Mountains?
Null Hypotheses
Hypothesis One: There will be no differences between the distances traveled and elevations climbed when comparing the first two weeks of the study window with the final two weeks, when comparing individuals from Trigo versus Plumosa, and when comparing males to females.
Hypothesis Two: By the end of the study window and there will be no signs of herding behavior among individuals that were translocated from the Trigo and Plumosa Mountains and individuals will not migrate into one habitat classification (i.e. “Good”) more than another as indicated by the Cunningham Hansen Habitat Evaluation Map (April. 21, 2014) .
Hypothesis Three: By the end of the study, bighorn sheep survival rates will remain the same as at the beginning of the study, with no mortalities caused by mountain lions, or any other means.
Figure 2: Cunningham/Hansen Habitat Evaluation Map for the Catalina Mountains
System to evaluate habitat quality using five criteria: topography, vegetation, precipitation, water sources, and human use. Classified into four groups: excellent (0), good (5), fair (41), and poor (24).
Operational Definitions
Desert Bighorn sheep (Ovis canadensis nelsoni): Diurnal ungulates, Found in southwestern US and Northern MX
Extirpation: The local extinction of a species in a specific region
Reintroduction: The release of species into the wild from another region where species thrives
Translocation: The transport and release of a species from one location to another
Reintroduction site: Pusch Ridge Wilderness, Santa Catalina Mountain Range located in Southern Arizona
Habitat evaluation: Attributes of an environment as scored by Cunningham/Hansen model to describe habitat
Escape terrain: Steep Slopes above 80%, with rock outcroppings, and little vegetation cover. Used by sheep to escape predators
Extinction: Using a High Mortality model, extinction is defined as a population less than or equal to 2 individuals.
Results and Data Analysis
Hypothesis One: Movement
Distance Traveled and Elevation for First Three Data Sets
Overall, Individuals travelled significantly more in the first two weeks after the release than they did in either the third or six month after release (see figure 16). A Friedman's Test displayed that there was a significant difference in the distance travelled by the remaining sheep over the six months (Χ²(2) = 10.86, p < .005). A Dunn Post-Hoc Test displayed that the sheep travelled significantly farther in the first two weeks after the release (M=4.55, SD= 2.07) than they three months later (M= 2.24 km, SD= 2.67; p<.05), and far more that six months later (M=2.01 km, SD= 3.34; p< .01). There was not a significant difference in distance travelled between the three and six month marks in the surviving population (p>.999). A Wilcoxon Signed-ranks test confirmed this trend as the 20 surviving sheep in February sheep travelled farther in the first two weeks after release (M=5.07 km, SD=2.51 ) than in the eleventh and twelfth weeks after the release (M=3.11 km, SD=3.08; Z = -1.98, p < .05).
A Friedman's Test displayed that there was a significant difference among the altitude climbed by the remaining sheep over six months (Χ²(2) = 10.43, p > .005). A Dunn Post-Hoc Test of the sheep surviving at the six month mark displayed that they climbed significantly higher in the first two weeks after the release (M= 640.9 m, SD=290.4) than six months later (M= 122.3 m, SD=32.69; p< .005). Interestingly, there was not a significant difference in distance travelled between immediately following the release and the three month mark (M= 324.9 m, SD=325.2; p=.178) and the three and six month marks in the surviving population (p=.558). This lack of significance demonstrates the decrease in altitude climbed throughout the six month period: Month 1 M= 640.9m, Month 3 M= 324.9m, Month 6 M=116.8m (See figure 16). However, when the surviving population at the three month mark was examined, a Wilcoxon Signed-ranks test indicated that the sheep climbed significantly higher in the first two weeks after release (M=706.662m, SD=285.87) than in the third month after the release (M=307.86 m, SD=285.34; Z = -3.061, p < .01).
Distance Traveled and Elevation by Sex for First and Third Data Sets
No difference in distance travelled by sex was present when analyzed for the surviving population in February using a Mann-Whitney U Test. The distance travelled between males and females was not significantly different in either the first two weeks (M= 4.32 vs 5.33 km; p=.48), or the three month sample of the data set (M= 1.81 vs 3.55 km; p=.48). An analysis could not be conducted at the six month mark as only one male survived.
A Mann-Whitney U Test of the three month population indicated that Males (M=999.33 m, SD=159.09) climbed significantly higher than females (M=609.11 m, SD=250.77) in the first two weeks after the release (Z = -2.44, p < .05). Interestingly, there was no significant in elevation travelled between males (M=198.87 m, SD=140.17) and females (M=344.19 m, SD=315.01) three months after the release (U=30, p=.52). An analysis could not be conducted at the six month mark as only one male survived.
Distance Traveled and Elevation by Site of Origin for First, Second and Third Data Sets
There was no difference in distance travelled between individuals from the Plumosa and Trigo sites according to Mann-Whitney U Tests for immediately after the release (M= 4.14 vs 6.00km; U=30, p=.14), three months after (M= 1.90 vs 4.32km; U=30, p=.14), and six months after (M= 2.35 vs 1.66km; U=16, p=.31) .
There was no difference in elevation travelled between individuals from the Plumosa and Trigo sites according to Mann-Whitney U Tests for immediately after the release (M= 821.2 vs 559.1m; U=28, p=.10), three months after (M= 193.3 vs 422.4m; U=30, p=.14), and six months after (M= 174.0 vs 59.71m; U=17, p=.38) .
Hypothesis Two: Grouping and Site Selection
A indicated on Figure 17, it is apparent that individuals have begun to group together, and that individuals are moving to better habitat. Six months after release two groups have formed, one composed of Trigo Females, and the other of Plumosa Females, and both have settled into “Good” habitat by the Cunningham/Hansen habitat model. Nine individuals have moved into habitat that is considered “Good” and four remain in what is considered “Fair” as compared to three months post release where there were three groups and eight individuals in “Fair” habitat and three individuals in “Good” habitat.
Three months (left) after release versus six months after release (right)
Hypothesis Three: Survival
During the course of the study, there were sixteen mortalities. The cause of one death was cardiomyopathy (ID# 52), and 15 were caused by predators, most likely mountain lion (ID#’s 36, 37, 38, 42, 45,46,47,48,50,51,57,58,59,61,64). Five births occurred within the study period bringing the total number of sheep to an altered sample size of twenty. (see figure 20)
Four population simulations were run with 100 iterations each to determine the status of the population in ten years. The four simulations (Default, Moderate, High, Extreme) were based on rates of mortalities and reproduction based on age and sex with chance of extinction noted. Using High Mortality based on current mortality rates, chances of extinctions are 18.1% . Extinction defined as N ≤ 2.
Discussion
Based on the data, Hypothesis One must be rejected. An analysis of data revealed that there were significant differences in both distance traveled and elevations climbed from the beginning of the study to the end, and some differences when comparing that of females to males, although none when comparing individuals from their source of the Trigo area versus the Plumosa area.
• The pattern of bighorn sheep distribution initially started as a scattering of individuals, none in direct conjunction with others from their previous herd. When examining the total bighorn population as a whole, the span of distance from the farthest point east that any bighorn traveled to the farthest point west was 27.78 kilometers from the release site. The largest range of elevation any one sheep travelled was 1855.43 meters from the lowest point (783.75m) to the highest point (2639.18m) The bighorns may have traversed farther than planned since some individuals entered unprotected areas. The fact that the sheep travelled significantly farther in the first two weeks after release than in the others may indicate a reaction to the stress of an unknown location. How the they traversed the terrain during the initial two weeks after release was not typical of how they moved once settled.
• The way that the bighorn males responded initially was noteworthy. Males climbed significantly higher than females in the first two weeks after the release (Z = -2.44, p < .05) however, there was no significant difference in elevation climbed between males and females in the third month (p=.54), while the sixth month could not be analyzed since only one male survived. The prior locations where the bighorn sheep originated had no elevations as high as the Catalinas. Male elevations appeared to be climbing up “to the top” as they might have in their previous location – except here the “top” was a great deal higher. The difference in distance travelled between males and females was not significant in either the first two weeks (p=.48), or the last two weeks (p=.48). Both quickly traversed terrain initially as compared to later in the study, perhaps as a reaction to the stress of the release and exploring the terrain.
• Individuals from the Plumosa versus the Trigo Mountians did not differ in either their distance travelled or elevation climbed over the six months so the population source appeared not to be a factor. Differences by age could not be calculated as the sample size was not robust enough.
Based on the data, Hypothesis Two must be rejected. By the end of the study window there were signs of herding behavior among some of the individuals that were translocated. Furthermore, by the end of the study window, individuals started to migrate into the areas classified as “Good” as compared to the initial weeks after the release
• The initial distribution maps of individual sheep indicate that some herding behavior has begun. The February distribution maps depict three separate groups of sheep that had moved in close proximity to each other, while currently there are two groups in even closer proximity, indicating that grouping behavior in this study is not permanent yet.
• The groups in February contained, four adult ewes (635,656,662,663), two female ewes (660, 639), and two four-year old males (658,641) While the groups in April consisted of Two adult ewes (635, 663) in the first group, and two ewes, one adult (644) and one a yearling (655) in the second group. In the time between February and April only one of the sheep died, the male (658) possibly indicating the groups size is stabilizing.
• The bighorn sheep were released in an area considered “Fair” by Cunningham Hansen Habitat Evaluation Map, however, nine out of thirteen have now moved to a single “good” area in by Pusch Ridge by April 2014. This may be an indication that the selection of habitat by individuals is leading to the eventual grouping of others and possibly, all.
Based on the data, Hypothesis Three must be rejected. By the end of the study window there were 16 sheep mortalities,15 caused by predators, specifically, mountain lions.
• Given the exponential increase in mortalities, in the first few months, the survival of the population was in question. However there have been no mortalities since March 16th, 2014. The resiliency of individuals and how they adapt to their environment must be taken into account. However only one four year old male survives and that could cause a genetic stagnation in the future if the number of males are not replenished.
• The long term population projections mapped four different scenarios, each scenario, contained the mortality rate by sex and age ranked as Default, Moderate, High, and Extreme. Default being with an annual mortality rate of 50%, and Extreme with an annual, mortality rate of 85%. The the current mortalities of Catalina bighorn sheep are the most similar to the high mortality projection, with an 18% chance of extinction.
Implications
There are two more releases of thirty sheep each already planned for the next two years. The obvious question is, given the mortality rate of over half of the translocated population within just six months of their release, should any future releases continue as planned? It can be statistically predicted that given that there were sixteen mortalities by the end of the fifth month of the study, at this continued rate, there is a 18% chance of extinction in the population. However, predation by mountain lions cannot be predicted and so the true number of sheep remaining for the next release is as yet undetermined. And will the mountain lions continue to be removed at the current rate or whether the growing public outcry will deter further removals has an impact on this project.
The date of the next release is scheduled for November 2014, and as of now, the sheep will be translocated to the area with the hope of intermingling with an established population as was initially planned. Although there have been mortalities and the statistics cannot be heavily relied on, the survival of the bighorn may depend on other variables, such as continued new births. There have been five births since February 2014, which may yet play a factor in the survival of this population. This project will continue as long as the releases continue and/or the existing bighorn survive. Data will be tracked and analyzed by age, sex, original herd and interactions with human populations and interference. Mortalities and births will be tracked.
Since humans played a part in reducing populations and even eliminating entire species, do they owe it to the planet to try to restore some of that balance? And in doing so, should they further effect the lives of the other species, such as the mountain lion, in helping the one they are currently trying to assist? Nature exists in a delicate balance. Further research is needed investigate the role of humans in maintaining or correcting this balance.
Limitations
Most conservation studies span over a ten-year period; this population has been studied for six months. This lack of longitudinal data may be a limiting factor. Unfortunately, historically, any other releases of bighorns following translocations have not been well documented and their results were not widely published, so comparison is difficult. Additionally, the fact that data are released in two-week intervals, after an initial delay, prevents real-time analysis of data, and trends
Additionally, the subjects of this study are wild animals in a natural habitat and are subject to environmental factors that could alter the results of this project. Unpredicted variables may play a part, such as weather patterns, disease, or unknown sources. In research there are intervening factors that may have an effect on the project and if corrected, would increase the validity and reliability. Such in the case of this investigation. For further study, more timely releases and access to data could make the study stronger.
Although a population projection and cohort life analysis to predict the likelihood for success with this population was conducted for this study, these projections should not be relied upon for an actuate prediction of what the population will look like in ten years due to the unpredictability of sheep mortality.
References
Bleich, V.C.; R.T. Bower, and J. D. Wehausen. 1997. Sexual segregation in mountain sheep: resources or predation? Wildlife Monographs 134:1–50.
Czech, B and P.R. Krausman (1997). Implications of an ecosystem management literature review, Wildlife Society Bulletin, 25: 667-675.
Etchberger, R. C., and P. R. Krausman. 1999. Frequency of birth and lambing sites of a small population of mountain sheep. Southwestern Naturalist 44:354–360.
Mooring, M. S., T. A. Fitzpatrick, T. T. Nishihira, and D. C. Reisig. 2004. Vigilance, predation risk, and the Allee effect in desert bighorn sheep. Journal of Wildlife Management 68:519–532.
Papouchis, C. M., F. J. Singer, and W. B. Sloan. 2001. Responses of desert bighorn sheep to increased human recreation. Journal of Wildlife Management 65:573–582.
Date: December 27, 2014 Views: 5362 File size:10.4kb, 2758.8kb: 4608 x 1667
