Habitat: The Missing Element in Montana's Bighorn Strategy

Let's face it. Bighorn conservation and management throughout the West, maybe especially in Montana, has been a failure. We have mostly small, isolated, inbreeding herds scattered across a miniscule portion of their native range. There are periodic die-off's followed by persistent lamb mortality. Abundant habitat, too close to ubiquitous domestic sheep, is off limits for bighorns. Montana has been unable to transplant animals to establish a new herd in over a decade. A huge demand for hunting permits is unfulfilled. It is time to reevaluate strategies and test new tactics. 

Aside from its social/political aspects, wildlife management has two parts: population management and habitat management. Montana’s bighorn strategy emphasizes population management, largely neglecting habitat management. Activities include transplants, harvest management, and elimination of foraying bighorns.

Bighorn transplants were once common, returning the species to many areas where they had been eliminated. Some transplants flourished temporarily. But the many small, struggling herds of today are the primary legacy of past transplants. Montana has not found a suitable new transplant site in almost ten years.

Today’s herd management consists largely of harvest control. Rams are taken, even from the smallest herds, despite probable negative effects on already compromised herd genetics. Rarely, ewes are harvested to limit already small herd sizes, largely to forestall “excess animals” from traveling toward disease-carrying domestic sheep or goats.

The major emphasis of management is to prevent contact of bighorns with domestic sheep or goats. Such contact almost always produces lung disease in bighorns, often resulting in a dieoff of over half the bighorn herd. Preventing contact includes eliminating any bighorn approaching a domestic flock.

The policy of maintaining small and sedentary bighorn herds has not been satisfactory. Small bighorn herds are normally limited or diminished by predation. Their inbreeding produces low-quality animals that are especially susceptible to disease and some inbred animals persist as carriers that infect lamb crops for years. The process of genetic drift further diminishes and disorganizes the once-adapted herd genomes.

Is there a solution to this bighorn problem in habitat management? First, we must consider the history and nature of bighorn habitat.

Bighorn sheep are habitat specialists. They are designed to forage in large groups within secure habitat. Habitat security is largely a function of visibility and proximity to steep, broken terrain where they may outmaneuver predators. Visibility is important in detecting danger, and in visually communicating any detected threats among animals. Habitat security is reduced by trees or shrubs that diminish visibility of the surroundings.

A robust bighorn herd uses six seasonal ranges connected with secure migration corridors. Males and ewe/juvenile groups have separate winter and summer ranges and there is a rutting area and a lambing area, the latter on especially steep, cliffy terrain.

But such a complete bighorn habitat may be diminished as some habitats are degraded by visibility-obstructing forest or shrub encroachment and/or by human developments including housing, roads, railroads, reservoirs and fences. Bighorn herds that have declined may lose their herd-memory of such degrading ranges and migration corridors. Moreover, newly transplanted herds will require years to reestablish a complete pattern of habitat use, especially if the herd remains small, and if attractive habitat is not nearby a transplant site.

The historical decline of bighorns from unregulated hunting and livestock diseases was followed by decades of fire suppression that allowed visibility-obstructing tree encroachment to degrade bighorn ranges and migration corridors. Seasonal ranges and migration corridors were abandoned and lost from herd memories. Bighorns, males and females, concentrated more in time and space. The number of separate seasonal ranges declined, increasing impacts on forage resources and enhancing transmission of diseases, such as lungworm, that are transmitted through the environment.

Are there opportunities to improve bighorn habitat on Forest Service lands?

In western Montana, much historical bighorn range is on Forest Service land where tree encroachment has degraded habitat, especially in the interiors of Forest Service lands. Much remaining bighorn habitat is at and near the upper tree line and, especially, the lower tree line where forest encroachment has least degraded bighorn habitats.

Lower tree lines tend to be along Forest boundaries. Thus, many of today’s bighorn ranges include considerable private land, or Bureau of Land Management land, where domestic sheep are grazed in commercial herds or in small herds on small family tracts. Research suggests that the more private land within a herd’s range, the more likely the herd has suffered a pneumonia dieoff, most likely from contacting domestic sheep or goats.

Habitat improvement has great potential for reestablishing larger, more resilient bighorn herds on Forest Service lands in Montana. The goal must be to reestablish a complete suite of year-round ranges and migration corridors in the interiors of Forest Service lands where contact with domestic sheep is less likely. Apparently, the Forest Service does not recognize this need or potential. It seems satisfied with several small, struggling bighorn herds, mostly along the boundaries of Forests.

Usually, the Forest Service contends that its mandate to provide secure bighorn habitat is fulfilled using a “coarse filter” strategy to guide habitat assessment. This strategy assumes that if the amounts of several broad categories of vegetation (aspen, meadows, mixed-conifer forests, etc.) are maintained within their historical ranges of variation across the Forest, the habitat needs of all native species must be met. The approach fails for bighorn by neglecting the species need for the spatially combined habitat factors of visibility and steep terrain, and for spatial connectivity of habitat units. Moreover, the approach neglects substantial separation from domestic sheep or goats as a habitat-security requirement.

The potential for mechanical tree or shrub removals to enhance bighorn habitat by increasing visibility is limited - because they are expensive. Offsetting expenses by cutting commercial timber will generally fail in that it directs activity toward areas with larger densities of larger trees. Such areas lack steep terrain and have environmental conditions favoring rapid reforestation. Prescribed fire, located strategically to benefit bighorns, has the most habitat management potential. Aside from enhancing habitat visibility, forage conditions on recent burns often attract bighorns. Wildfires can likewise benefit bighorn habitat, but such fires are, at best, located randomly, not strategically, in potential bighorn ranges.

There has been a tendency to focus big-game habitat improvements on the currently known and used ranges of the animals, especially winter ranges. For bighorns, this is only an attempt to maintain the status quo. To enhance most bighorn herds, habitat management must also be focused on expanding the currently inadequate suite of bighorn ranges.

How to proceed?

1. Montana Fish, Wildlife & Parks and the Forest Service should jointly recognize the need to enhance the abundance, security and resiliency of bighorn herds in western Montana. A joint commitment to a long-term strategy is necessary.
2. Based on existing knowledge of bighorn herds, and environmental and social conditions, at least two herds should be selected for implementing an initial recovery strategy.
3. Using marked bighorns and radio telemetry, year-round use of habitat should be determined. Using these data with vegetation and topographic data, the focus would be to locate existing sex-specific winter and summer ranges, lambing areas, rutting areas and likely migration corridors. The data would also be used to identify areas for likely expansion of seasonal bighorn use toward the interiors of the Forests.
4. Strategically located habitat improvements should be initiated, most likely with prescribed fire. Strategic locations would include the perimeters of current use-areas and likely migration corridors. Cliffy areas, southward-facing slopes and ridgelines will be the best candidates for long-lasting forest removal. Annual evaluations of habitat changes and bighorn responses would be used to sequentially expand the program, generally in the interiors of the Forests.
5. It is possible that a small bighorn herd may not respond to habitat improvements due to population limitation by predation and/or due to poor genetic quality of a herd that has been small for a long time. It may be necessary to rectify these limitations. Predation might be rectified with temporary control of likely predators, especially mountains lions, and by improving visibility and habitat security first in areas where bighorns, especially lambs, are known to be killed. Genetic limitations may be removed by adding new animals for outbreeding. Local transplants may be used to hasten herd recognition of newly improved habitats that have been abandoned for years.

Jim Bailey, Belgrade December 2019

The Small-population Strategy of

Bighorn Sheep Does Not Work

by Dr. Jim Bailey

Montana Fish, Wildlife & Parks claims that 125 bighorn is a “minimum viable population”. Abundant science disagrees. In the long term, 125 bighorn is not a minimum population that is surely viable. In the short term, it may be a population that is minimally viable. (“Minimally” is the more appropriate term, as it inserts a realistic degree of uncertainty and it clearly modifies “viable”, not “population”.)

Compared to other big game, abundant resources are dedicated to bighorn management. Yet, bighorn herds struggle throughout the West. Very many herds remain small and are subject to periodic outbreaks of disease. The most common and most apparent source of bighorn problems is pneumonia that is most often related to contact with domestic sheep.

Consequently, avoiding contact between wild and domestic sheep has become the most overriding management prescription. Small bighorn herds, with no wandering/exploring animals, on small bighorn ranges have become acceptable goals. Many herds have limited or variable recruitment and remain small on very limited ranges for several reasons discussed below. However, ewes are sometimes harvested to maintain small populations. Wandering bighorns that may encounter widely distributed domestic sheep have been “administratively removed”, limiting range expansion.

Bighorn were once very abundant and widely distributed in mountains, canyons and river breaks across the West. They have declined and disappeared for a variety of reasons, most of which persist. Today, most herds are limited by combinations of more than one of these same problems. As a result, management that is focused on only the most apparent problem will fail, or at best provide little improvement in bighorn numbers, productivity or distribution.

Five interrelated problems of bighorn herds are: habitat degradation and loss, small populations, predators, disease, and genetic deterioration. Note, the interrelationships are just as important as the problems themselves. Hence, some redundancy below.

Habitat Loss: Very much bighorn habitat has been lost and degraded by human developments, agriculture, highways, railroads and reservoirs. Partly due to fire control, habitats have been degraded by forest and shrubland growth that diminishes visibility, a key component of good bighorn habitat.

Once, healthy herds accessed four to several seasonal ranges connected by consistently used migration corridors. Loss of these range components has left many herds with fewer and smaller seasonal ranges and few options for responding to variation in weather and plant phenology. Limited diversity of range resources limits herd health and productivity and herd size.

Small Populations: Small bighorn herds have little resiliency for responding to random or periodic losses of even a few animals due to accidents, weather events, predation or disease. They have inadequate genetic diversities and are predisposed to genetic deterioration. They produce few animals that will explore and foster range expansion.

Predation: Mountain lions and perhaps coyotes are the primary killers of bighorn in the United States. Effects of predation are enhanced by small herd size and by poor habitat security. Small herds may experience high predator/prey ratios and have ineffective communal vigilance. Habitat security is often limited by relatively dense vegetation resulting in poor visibility along migration corridors, at watering sites and at foraging areas near escape terrain. Poor visibility inhibits predator detection and visual communication of danger among bighorn sheep.

Disease: Bighorn are subject to several diseases. In recent decades, pneumonia acquired from almost any contact with domestic sheep has been, by far, the most significant concern. Periodic all-age dieoffs followed by years with persisting lamb infections and mortality have been common across the West. Populations have declined by 50-90 percent, exacerbating other small-population effects, including predation and genetic deterioration.

Limited data indicates that larger bighorn herds suffer smaller rates of mortality and their numerical recovery occurs more rapidly. Within small herds, inbred animals have poorer disease resistance, higher mortality rates, and – if they survive – longer recovery times. Inbred animals are more likely to persist as disease carriers and may be responsible for years of post dieoff lamb mortalities.

With the high frequency of bighorn dieoffs related to contact with domestic sheep, almost all bighorn management programs emphasize complete separation of the species. This involves limiting the sizes of bighorn ranges and herds to minimize the numbers of wandering/exploring bighorn. Bighorn with or approaching domestic sheep have been euthanized. It is a small-population strategy used abundantly throughout the West, generally exceeding most other management activity.

Genetic Deterioration: Among the problems of bighorn herds, genetic deterioration has been largely ignored until recently. It may be the least understood problem, especially among practicing wildlife managers. Three components of genetic deterioration are inbreeding, loss of genetic diversity and evolutionary potential, and a dismantling of the adapted wild genome. The latter components are due to a process called genetic drift. Inbreeding effects can become serious within a few generations. Serious effects of genetic drift generally require several generations, but some are irreversible.

Inbreeding can negatively affect many aspects of behavior, physiology and anatomy, often in unnoticeable ways. The most frequently described effects have been poor reproduction and juvenile survival, and poor disease resistance. Obvious effects of inbreeding may be episodic, depressing survival and reproduction more during times of stress, such as severe weather or during periodic disease challenges. As carriers, inbred animals can prolong the persistence of a population disease event. This may account for persisting lamb mortality following a bighorn dieoff. Likely, inbred animals have an increased susceptibility to predation. Thus, disease, predation and inbreeding can be intertwined.

Geneticists refer to a genetically-effective population size (Ne) for evaluating inbreeding and other genetic trends. Ne varies among species according breeding habits; does not include young pre-breeding animals; varies with the sex ratio of breeders, population fluctuations, and other factors. For a bighorn herd, Ne may be only 10-15 percent of the total population (N).

Over 35 years ago, geneticists suggested an Ne of 50 animals is necessary to avoid “significant” levels of inbreeding in wildlife. More recent suggestions, based on additional information, are that an Ne of 100 is necessary. Note: even if Ne is 15% of N, an Ne of 50 requires N = 333, and an Ne of 100 requires N = 667.

For wildlife, genetic drift occurs when alleles (types of genes) that are passed between generations are determined by random factors rather than by natural selection. Ultimately, natural selection is necessary to maintain wild-adapted genomes. Genetic drift results in a slow loss of alleles from the population, diminishing its evolutionary potential for responding to future changes in the environment. Drift also dismantles the adapted genome, producing fewer animals with the best combinations of alleles for survival and reproduction in the local environment.

Negative effects of drift are most pronounced in smaller populations. For wildlife, dismantling of the wild genome is also enhanced by human actions that weaken or replace natural selection, including wildlife feeding, maintaining a stable population, vehicle and train mortalities, and harvests. For some wildlife populations, very few adult animals are exposed to natural selection and drift replaces natural selection. Likewise, over 35 years ago, geneticists suggested that an Ne of 500 animals is needed to avoid “significant” losses of alleles due to drift. Again, recent suggestions, including a concern for loss of natural selection, are that an Ne of 1000 is necessary. Moreover, if Ne is 15% of N, an Ne of 500 requires N = 3333; an Ne of 1000 requires N = 6667.

A Minimum Viable Population

Montana Fish, Wildlife & Parks claims that 125 bighorn is a minimum viable population (MVP), one that has a good chance to persist for a long time. By contrast, in a review of MVP estimates, in the journal Biological Conservation, the vast majority of MVP estimates, for long-term survival of mammal populations, were a few to several thousand adult breeding-age animals! 

FWP’s claim is bizarre in light of experience throughout the West, discussions of MVP in scientific literature, and abundant genetics information. FWP makes no allowance for genetic deterioration. Its MVP claim suggests that many Montana bighorn populations are safe from extinction. Yet, FWP’s objectives are even less than 125 for 16 of 43 herds with objectives. It is a small-population strategy, offered as sufficient, when it is not.

Ram Harvest and Genetics

In small wildlife populations, a skewed sex ratio among breeding animals has a relatively large negative effect on Ne, the genetically-effective population size. Montana allows harvest of mature rams from herds as small as 75 animals, and from even smaller herds if they exhibit appropriate sex-age structures. Note, a herd of 75 bighorn has a predicted Ne of fewer than 12 animals. It is likely that removal of some, but not all, prime-age rams will significantly reduce Ne, increasing the number of half-siblings born to the herd and accelerating inbreeding within a few years.

Complex Limiting Factors

With multiple, interrelated problems limiting most bighorn herds, we must expect that solving one problem while ignoring others will eventually fail. In particular, if we are able to isolate bighorn from domestic sheep (or solve this disease issue with some yet undiscovered technology), we will still have small populations on inadequate ranges, subject to serious predation, with deteriorating genetics, and liable to still other types of disease. A more comprehensive strategy is necessary.

Failure of the Small-Population Strategy

In practice, the small population strategy is often doubly small. Small bighorn herds are managed by dealing with a small number of their problems. Montana is not alone in this situation. Many states are similarly, and expensively, struggling with the small population strategy.

Wildlife managers may respond that, given the widespread distribution of domestic sheep, losses of habitat due to increasing human demands, and strong, effective political opposition from competing interests, they have no alternative but the small population strategy. However, the small population strategy should not be presented to the public as a viable solution. Agency quiescence, avoiding the conflict, and resulting public ignorance, only perpetuate management that will be ineffective in the long term. Moreover, promoting the small population strategy as a compromise with other, incompatible land uses is a delusion.

Jim Bailey, Belgrade, 2017

Supporting references available on request, jabailey34@aol.com

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.

Available online October 8, 2009: 

http://www.sciencedirect.com/science/article/pii/S0006320709004017

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:

Genetics

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).