Additional Bighorn Sheep Science:


For those who want more information we would also suggest searching out these additional citations and discussions. The first comes from the abstract of Traill et al. (2010) that discusses thousands rather than hundreds of animals are necessary to have a high probability of persistence/viability over the long term (100+ years). Here is the citation and abstract.


Traill, L.W, B. W. Brook, R. R. Frankham and C. J .A. Bradshaw. 2010. Pragmatic population viability targets in a rapidly changing world. Biological Conservation 143(1):28-34.




To ensure both long-term persistence and evolutionary potential, the required number of individuals in a population often greatly exceeds the targets proposed by conservation management. We critically review minimum population size requirements for species based on empirical and theoretical estimates made over the past few decades. This literature collectively shows that thousands (not hundreds) of individuals are required for a population to have an acceptable probability of riding-out environmental fluctuation and catastrophic events, and ensuring the continuation of evolutionary processes. The evidence is clear, yet conservation policy does not appear to reflect these findings, with pragmatic concerns on feasibility over-riding biological risk assessment. As such, we argue that conservation biology faces a dilemma akin to those working on the physical basis of climate change, where scientific recommendations on carbon emission reductions are compromised by policy makers. There is no obvious resolution other than a more explicit acceptance of the trade-offs implied when population viability requirements are ignored. We recommend that conservation planners include demographic and genetic thresholds in their assessments, and recognize implicit triage where these are not met.


The second comes for the Montana Fish, Wildlife & Parks. 2010. Montana Bighorn Sheep Conservation Strategy.  Wildlife Division, Helena MT. pp 313.  Pages 57 & 58 of the Conservation Strategy discuss important genetic issues noting that isolated populations of 200 or less animals are likely not genetically adequate even in the short term (2-3 generations for bighorn sheep or 10-15 years).


From pages 57&58:



There are four main reasons why genetics should be considered in the management of bighorn sheep. First, molecular genetic markers can identify populations experiencing a loss of genetic variation and inbreeding, which may be due to reduced connectivity and small population size (Hogg et al. 2006). Second, genetic data can also help detect potential undesirable effects of selective harvest on important attributes such as horn and body size (Coltman et al. 2003; Allendorf et al. 2008). Third, genetic tools can aid forensics by detecting poaching and illegal sale of body parts such as trophy skulls or horns (Manel et al. 2002). Finally, genetic markers can be used to identify the presence of and track the transmission of pathogens or parasites within and among individuals and populations (Archie et al. 2008). Much of the above information can be obtained using polymerase chain reaction (PCR)-based technologies allowing for noninvasive sampling of feces, hair, urine, or saliva (Taberlet et al. 1999; Luikart et al. 2008b; Beja-Pereia et al. 2009).


Loss of Genetic Variation and Inbreeding

Isolated populations with small size will experience rapid loss of genetic variation and inbreeding (mating between relatives). The rate of loss of genetic variation (heterozygosity) is determined by the effective population size (Ne), not the population census size (i.e., abundance). The rate of loss of variation and Ne can be estimated by analyzing approximately 10 to 20 molecular genetic markers (e.g., microsatellites) and DNA samples from approximately 30 to 50 individuals from the population of interest. In wild populations Ne is almost invariably less than the population census size (Nc). The Ne is reduced below the Nc by phenomena such as skewed sex ratio, variation in reproductive success among individuals, and changes in population size through time. Most estimates of Ne suggest that it is only about 10-50% of Nc (Frankham 1995). Given a breeding structure where few males dominate reproduction, the Ne/Nc ratio of bighorn sheep is probably at the lower end of this range. For populations with 50 to 200 adults, therefore, Ne may be only 10 to 20, resulting in a rapid loss of genetic variation and an accumulation of inbreeding.


Many of Montana’s 45 bighorn sheep populations are relatively small, isolated, and were founded with few individuals. Because of small founding size and low abundance, many are likely to have low Ne, making them susceptible to the random loss of genetic variation, inbreeding, and the random increase in the frequency of harmful genetic variation (deleterious alleles). Loss of genetic variation, especially particular variants (alleles) is also expected to result in reduced adaptability and may also increase the susceptibility of the animals to particular parasites and diseases. Furthermore, because of their small size and isolation over time, the amount of inbreeding in many populations will increase and eventually result in inbreeding depression, which is defined as the loss of fitness in inbred individuals. All of these factors act concurrently to increase the risk of extinction (Berger 1990), and many have been observed in bighorn sheep populations (Hogg et al. 2006; Luikart et al. 2008a).


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