RESOURCE NOTES NO. 35 DATE 08/01/00

maintain sufficient DEFINITION: Metapopulation refers Summary within these populations. to two or more local breeding popula- tions which are linked to one another by Recommendations- DISCUSSION: Reproductive capac- dispersal activities of individual animals. ity is, to a large degree, dictated by These populations may have unique BLM Wild Horse the genetic of a population. demographic features (birth and death Generally speaking, the higher the rates) but ultimately may share some and Burro level of genetic diversity, within the genetic material if interbreeding is

herd, the greater its long-term repro- occurring between individuals. This Wild Horse and Burro Program Population ductive capacity. , random sharing of genetic material may act to matings (), and/or envi- enhance genetic diversity within par- Viability Forum, ronmental catastrophes can all lead to ticipating herds, and as such, these the loss of genetic diversity within the populations should be evaluated as April 21, 1999 population. In most herds, though, one larger metapopulation. by Linda Coates-Markle, genetic resources will tend to be lost Montana/Dakotas Wild Horse slowly over periods of many generations DISCUSSION: A complete popula- (~10 years/generation), and there is tion census of each herd management and Burro Specialist, little imminent risk of inbreeding or area is unrealistic, especially for the larg- Bureau of Land Management, population extinction. Potential nega- er populations (>200 total census Montana State Office tive consequences of reduced diversity, size). However, can however, may include reduced foal pro- and should be estimated using reliable duction and survival, as well as reduced scientific techniques. These survey The final Note in a series adult fitness and noted physical defor- techniques are under continual revision of 13, Session 4 mities. Smaller, isolated populations and BLM continues to participate in (<200 total census size) are particularly these research efforts. On a more critical RECOMMENDATION #1: vulnerable when the number of ani- level, however, is the determination of BLM should carefully consider its man- mals participating in breeding drops size of the many smaller populations date (The Wild Free-Roaming Horse below a minimum needed level. This (<200 total census size) over which and Burro Act) with respect to long- minimum level can be calculated and BLM has responsibility. Available data term genetic viability of populations of is different for each population (see indicates that almost 70% of the man- wild horses and burros. subsequent recommendations). aged herds have AMLs (appropriate management levels) set at 150 animals EXISTING POLICY: BLM regula- RECOMMENDATION #2: or less. In fact, almost 40% of the herds tions and policy state that wild horses BLM should continue to use (and in Nevada, Utah, Wyoming, Colorado, and burros Rangeshall be managed as viable, improve upon) defensible scientific aer- and Arizona (71 out of 177 total self-sustaining populations of healthy ial and/or ground survey techniques in HMAs) are indicated to have popula- animals in balance with other multiple census activities for all managed wild tion sizes of less than 50 animals. There uses and the productive capacity of horse and burro herds. In order to fully is a real possibility that some of these their (CFR 4700.0-6). evaluate genetic viability issues, popula- populations will be unable to maintain tions which participate in a measurable self-sustaining reproductive ability, over the long term, unless there is a natural DEFINITION: Self-sustaining refers level of natural ingress or egress activity or management induced influx of to the process whereby established and which are, in reality, a component genetic information from neighboring populations are able to persist and of larger metapopulations, should be herds. An exchange of only 2 to 3 breed- successfully produce viable offspring identified, and the genetic impact of ing age animals (specifically females), which shall, in turn, produce viable this activity should be estimated. every 10 years, is often sufficient to offspring, and so on over the long term. maintain genetic diversity within a given The absolute size which a population EXISTING POLICY: BLM regula- herd. Estimates of existing genetic diver- must attain to achieve a self sustaining tions and policy state that HMAs sity can be calculated for each wild horse condition varies based on the demo- should be inventoried and monitored and burro population (see subsequent graphic and sociological features of the for population size, animal distribu- recommendations). herd (and adjoining herds), and these tion, herd health and condition and aspects should be evaluated on a case by habitat characteristics at least every 4 case basis. In many cases it is not neces- years (CFR 4710.2). As such, BLM is RECOMMENDATION #3: sary that populations be isolated genetic required to provide reliable estimates of BLM should establish baseline genetic units, but both naturally-occurring and population size and distribution with- diversity information for each popula- management-induced ingress and egress in each herd management area on a tion over which it has management activity can be considered, in order to regular interval. responsibility. Range

EXISTING POLICY: BLM regula- a result, a comparison of genetic viabil- DISCUSSION: Once baseline genetic tions and policy state that HMAs ity levels in the tested population can data has been established, the main should be inventoried and monitored be made to existing information from focus of genetic management, especial- for population size, animal distribu- over 100 domestic and wild horse ly for the smaller populations (<200 tion, herd health and condition and populations representing different total census size), becomes the attempt habitat characteristics at least every 4 herd sizes and demographic back- to preserve as much of the existing years (CFR 4710.2). Furthermore, the grounds. genetic diversity as possible. Establishing purpose of monitoring is to collect a genetic conservation goal will require data necessary to evaluate progress (or re-testing of herd diversity on at least lack thereof) in achieving the objectives RECOMMENDATION #4: a five-year cycle, with subsequent eval- of management. Within the context BLM should establish a realistic man- uations of the potential impact of of wild horse and burro populations, agement goal for maintenance of management decisions (including the ability to maintain the quality of genetic diversity within all managed the establishment and/or revision of “reproductively self sustaining” is populations. Previous wildlife conser- appropriate management levels) on required. This can primarily be vation research, and current efforts that diversity. Management may need accomplished through evaluation and with wild horses, suggest management to evaluate ways to introduce genetic the maintenance of an acceptable level should allow for a 90% probability of material into a herd which appears of genetic diversity within the popu- maintaining at least 90% of the exist- genetically deficient in order to be lation over the long term. ing population diversity over the next self-sustaining over the long-term (see 200 years. Existing diversity should be subsequent recommendations). sufficient to ensure a self-sustaining Baseline genetic data can also be incor- DEFINITION: Establishing baseline (see earlier definition) reproductive porated into PVA (population viability genetic diversity, for a wild horse popu- capacity within the herd. analysis) models, which attempt to lation, often refers to typing up to 29 predict the impact of management genetic marker systems from a sample of EXISTING POLICY: BLM regula- decisions (as well as environmental cat- individual animals (~25 individuals or tions and policy state that wild horses astrophes) on existing diversity levels. up to 25% of the population) within a and burros shall be managed as viable, Most models require reasonably accu- specific herd. Traditionally, these marker self-sustaining populations of healthy rate data in terms of age class foaling systems have included blood group and animals in balance with other multiple and mortality rates, as well as individ- biochemical systems, and have required uses and the productive capacity of ual genetic information. As such, the fresh blood samples. These systems their habitat (CFR 4700.0-6). By defi- means to collect accurate data neces- were originally developed for verifying nition this requires BLM to manage to sary for a genetically-based PVA, for parentage or founder animals within a allow established populations to suc- most herds, is probably unavailable herd. Analysis of genetic diversity, how- cessfully produce viable offspring at the present time. ever, can also be done through the use which shall, in turn, produce viable of DNA genetic marker systems, and offspring, and so on over the long direct testing can utilize almost any bod- RECOMMENDATION #5: term. This suggests that management ily product including hair or even feces. BLM should, in its efforts to evaluate monitor levels of genetic diversity Only DNA marker analysis can be used the genetic diversity and self sustaining within the population in order to for burros, however, due to the very nature of managed herds, estimate the mitigate the effects of genetic drift limited variation in blood protein genes. genetic effective population size (Ne) of and possible inbreeding and popula- Range all populations, or metapopulations, Range tion-associated problems due to loss with a total census size of 200 animals DISCUSSION: Most wild horse of diversity. or less. herds, sampled to date, have shown fairly high levels of genetic diversity. In DEFINITION: Genetic diversity, some cases, however, this diversity is within wild horse and burro popula- EXISTING POLICY: BLM regula- attributed to a large number of low tions, refers to the entire complement of tions and policy state that wild horses frequency and relatively rare genetic genetic material representative of all and burros shall be managed as viable, material which is often easily lost individuals (or a sample of individuals) self-sustaining populations of healthy from the herd. Thus, it becomes from within the population. Some animals in balance with other multiple important to understand the genetic populations may possess genetic uni- uses and the productive capacity of their makeup of individual herds. Baseline formity to a certain “type” or breed of habitat (CFR 4700.0-6). By defini- data needed to establish current levels horse, but management interests are tion this requires BLM to manage to of genetic diversity in populations is specific to maintaining a maximum allow established populations to suc- relatively easy to gather. Individual diversity of genetic material which cessfully produce viable offspring which samples cost about $25 to process, and appears representative of each herd. shall, in turn, produce viable offspring, if ~25-50 individuals are sufficient to Promotion of diversity will minimize and so on over the long term. establish baseline information for herds the effects of genetic drift, or the ran- ranging in size from 100 to 200 ani- dom loss of genetic material due to DEFINITION: The genetic effective mals, then the cost would be approxi- mating processes, and maximize genet- population size (Ne) is a measure of mately $1250 for herds of this size. As ic health of the herds. the total number of mares and stallions Range

which contribute genetically, through with reasonable estimates of foaling DISCUSSION: Simply increasing the successful breeding, to the next genera- and mortality rates, will enable man- total herd size by adding additional ani- tion. Although no standard goal for Ne agers to evaluate the genetic health of mals (adjusting the management AML currently exists for wild horse and most herds. upward) is not the only viable technique burro herds, a goal of Ne=50, which for enhancing the genetic effective pop- comes from domestic breeding ulation size (Ne) of a wild horse and guidelines, can be conservatively RECOMMENDATION #6: burro population. With sound applied. Populations, where Ne is BLM should evaluate viable manage- knowledge of existing herd demo- calculated to be less than 50, may ment alternatives for conserving or graphic information, management experience higher rates of loss of enhancing genetic diversity within alternatives for specific populations genetic diversity than would be consid- populations (or metapopulations) can be evaluated through research ered acceptable under recommended having a known limited level of diver- modeling efforts. As such, manage- management goals (see earlier recom- sity, a total census size of less than 200 ment also has the option of adjusting mendation). animals and/or an estimated genetic certain aspects of herd structure in effective population size (Ne) of less order to promote genetic conserva- DISCUSSION: Limited research into than 50. tion. Major options for consideration wild horse herds (Pryor Mountain Wild were presented in the above defini- Horse Range and Assateague Island EXISTING POLICY: BLM regula- tion. It should also be noted that any National Seashore populations) has tions and policy state that wild horses adjoining herds, which are naturally par- demonstrated that the “Ne”, for a herd and burros shall be managed as viable, ticipating in an exchange of animals and under a natural age structure, is about self-sustaining populations of healthy genetic material through interbreeding, 30-35% of the total census population animals in balance with other multiple are probably self-maintaining their size. In other words, a total population uses and the productive capacity of genetic diversity and management size of about 150 animals might sup- their habitat (CFR 4700.0-6). By defi- should consider both supporting and port only a minimum (Ne=50) genetic nition this requires BLM to manage to estimating this type of activity. effective population size. Ne, however, allow established populations to suc- is difficult to calculate for wild horses, cessfully produce viable offspring RECOMMENDATION #7: since the calculation is complicated by which shall, in turn, produce viable BLM should continue to evaluate a number of issues. The harem struc- offspring, and so on over the long incidences of club foot and parrot ture of the population, for example, term. This suggests that management mouth, and other such physical defi- greatly limits male participation in monitor levels of genetic diversity with- ciencies, within individuals of wild breeding, creating an uneven ratio of in the population in order to mitigate horse and burro populations, on a breeding sexes which reduces Ne and the effects of genetic drift and possible case by case basis. Currently, there is contributes to a high variation in indi- inbreeding. no solid evidence that these physical vidual reproductive success. Extreme conditions are purely genetically- fluctuations in population size, due to DEFINITION: Viable management based and that they may contribute the effects of removals, can also act to alternatives for conserving genetic diver- to a long-term loss of genetic health reduce the value of Ne. Ne is also sity within managed wild horse and in the herd. highly influencedRange by the sex ratio and burro herds may take several forms. age class structure of a population. A Some options to be considered might sex ratio which favors males and results include: altering population age struc- EXISTING POLICY: BLM regula- in larger numbers of smaller sized ture (through removals) to promote tions and policy state that HMAs harems, within the herd, will act to higher numbers of reproductively-suc- should be inventoried and monitored increase Ne (and male participation in cessful animals; altering breeding sex for population size, animal distribu- breeding) to a point. A population ratios (through removals) to encourage a tion, herd health and condition and with an age structure involving high more even participation of breeding habitat characteristics at least every 4 numbers of young animals (<5 years of males and females; increasing generation years (CFR 4710.2). Furthermore, the age) will have a lower value of Ne than a intervals (and reducing the rate of loss of purpose of monitoring is to collect similar sized population with a larger genetic material) by removing (or con- data necessary to evaluate progress (or component of older breeding-age ani- tracepting) younger versus older mares; lack thereof) in achieving the objec- mals (>5 years of age). Also, there is no and/or introducing breeding animals tives of management. single, uniformly accepted method to (specifically females) periodically from calculate Ne. However, researchers have other genetically similar herds to help in DEFINITION: Physical deficiencies used and applied several formulas to conservation efforts. In this last scenario, may be encountered at different rates for certain wild horse herds and have only one or two breeding animals per different wild horse and burro herds. found this comparative approach to generation (~10 years) would need to be Conformational deformity and/or mis- provide the best estimates. Generally, introduced in order to maintain the alignment is often expressed in the legs, the best possible data on population genetic resources in small populations of feet and mouth but may be apparent sex ratios and age structures, coupled less than 200 animals. in other structural areas as well. Range

Despite the existence of a specific defi- as the ecological carrying capacity (EEC) known as their food limited ecological ciency, however, an individual animal of the population. This is the level at carrying capacity. At these levels, how- may otherwise be healthy, bear accept- which science has determined that den- ever, the herds would show obvious able condition and be fit enough to sity-dependent population regulatory signs of ill-fitness including poor indi- contribute socially and genetically to mechanisms would take effect within vidual animal condition, low birth the herd. If an individual animal is the herd. Most herds are currently rates, and high mortality rates in all successful in these merits, there seems managed close to their “economic age classes due to disease and/or little reason to remove it simply on the carrying capacity” which is approxi- increased vulnerability to predation. In grounds of physical imperfection by mately 50-65% of EEC. At this level addition, supporting range conditions human standards. of management, health of both the would be noticeably deteriorated, with horse herd and range ecosystem are much of the available habitat showing DISCUSSION: These types of phys- prioritized. symptoms of irreparable over-grazing. ical deficiencies are thought, by Most wild herds are currently man- researchers, to potentially be both EXISTING POLICY: BLM regula- aged close to economic carrying genetically and environmentally (poor tions and policy state that wild horses capacity which allows the herds to be forage base during fetal development) and burros shall be managed as viable, healthy with strong foal production induced. As such, efforts to remove self-sustaining populations of healthy and high individual survival rates. This individual animals bearing this condi- animals in balance with other multiple approach should be continued, as it tion from herds may or may not result uses and the productive capacity of benefits the populations and also in a significant loss of expression of their habitat (CFR 4700.0-6). Thus allows for the maintenance of healthy that trait from the herd. Success in this appropriate management levels (AMLs) and in-balance rangeland systems. area will be related to the source of the are established which provide for a level genotype and whether it results from of use by wild horses and burros which Contact results in a thriving natural ecological inbreeding, founder effect, and/or Linda Coates-Markle balance and avoids deterioration of genetic drift. However, since multiple Program Specialist the range. Furthermore, proper man- genes are probably responsible for the Montana State Office, MT-010 agement requires that wild horses and expression of these traits, it is likely Billings, Montana burros be in good health and reproduc- that the genetic predisposition will phone (406) 896-5223 ing at a rate that sustains the population remain in the herd despite the fact fax (406) 896-5281 and that population control methods be that minimal expression is observed. email [email protected] Eventually over time, then, the trait considered before the herd size causes may continue to reappear. Future damage to the rangeland. research may illuminate different theo- DEFINITION: Ecological carrying ries regarding this situation. In the capacity (EEC) of a population, is a meantime, the impact of human- scientific term which refers to the level induced selection, over factors of natural at which density-dependent population selection, should be evaluated carefully regulatory mechanisms would take and with due consideration as to the effect within specific herds. At this possible long-termRange impacts on individ- Resource Notes are intended to be early level, however, the herds would show ual herds. In other words, if the animal announcements of technical and informational topics for obvious signs of ill fitness including Bureau of Land Management personnel and some of their customers. is otherwise healthy, maintaining a sta- poor individual animal condition, Information in this Resource Note is based on the opinion and tus within the social structure of the experience of the author and has not been peer-reviewed. Conclusions low birth rates, and high mortality and opinions expressed herein do not herd, and contributing to the gene pool rates in all age classes due to disease necessarily represent those of BLM. Use of trade names does not through successful breeding, it might be imply U.S. Government endorsement of commercial products. and/or increased vulnerability to left on the range. However, if a popula- If you have received a copy of or found out about Resource predation. In addition, supporting tion excess has been determined, and Notes in an indirect way and would like to be included in future range conditions would be noticeably mailings, please send the following: an individual animal is young and has deteriorated, with much of the avail- NAME, TITLE, MAILING ADDRESS and a list of the two or three yet to contribute to the gene pool, it subject areas that you are most interested in or that most directly able habitat showing symptoms of may be a candidate for removal and relate to your job. Send this information to Phil Dittberner, BLM, irreparable over-grazing. RS-140, P.O. Box 25047, Denver, CO. 80225-0047 or adoption or sanctuary-placement. [email protected] or FAX 303-236-3508. Likewise, if the animal is older, less DISCUSSION: Populations of wild If you would like to prepare a Resource Note for healthy, and has withdrawn from the distribution, or you have an idea and author in mind for a good horses on western rangelands have the Resource Note, please contact Phil Dittberner at herd, it may also be a candidate for capacity for rates of increase as high as 303-236-1833, FAX 303-236-3508 or [email protected] with removal with sanctuary placement. the topic and the name of writer, including an address, phone num- 20-25% per year. Recent research has ber, and e-mail address. shown that unmanaged populations Thank you for your interest in Resource NoteS. RECOMMENDATION #8: of wild horses and/or burros might BLM should continue to manage wild eventually stabilize (due to density- horse and burro herds, beneath the dependent regulatory mechanisms) level which is scientifically referred to at very high numbers, near what is