As has been shown in the previous chapter we have inherited a wide variety of genetically diverse livestock populations from our ancestors. There is concern, however, that due to current high rates of extinction, our descendants will inherit a far less genetically rich and diverse selection of livestock breeds and thus agricultural options, unless we take action to conserve them.
The International Union for the Conservation of Nature and Natural Resource's (IUCN) World Conservation Strategy has defined the need for conservation as:
“The management for human use of the biosphere so that it may yield the greatest sustainable benefit to present generations while maintaining its potential to meet the needs and aspirations of future generations. Thus conservation is positive, embracing preservation, maintenance, sustainable utilization, restoration and enhancement of the natural environment.” (International Union for the Conservation of Nature, 1980)
Within this concept of conservation the FAO Expert Consultations have defined animal genetic resources as: “all species, breeds and strains of animals particularly those of economic scientific and cultural interest to mankind for agriculture either at present or in the future” (Weiner, 1989).
FAO and other international agricultural agencies more commonly use the term ‘animal genetic resources’ to apply to breeds or strains of the common domestic species of sheep, goats, cattle, pigs, buffalo and poultry. Horses, donkeys, camels, elephants, reindeer and other domesticated animals are given less attention and are often considered to be of marginal interest. In fact the FAO definition includes all these domesticated species and those species on the fringe of domestication or with potential for domestication. It incorporates, for example, a number of Asian ungulates including the Banteng, Mithan, Yak, Guar, Kouprey, Tamaraw and Anoas (Veitmeyer, 1983); several species of antelope and deer; relatives of the domestic pig including warty pigs, pigmy hogs and babirusa; species of rodent including rabbits, capibara and guinea pig (Veitmeyer, 1991); poikilothermic (cold blooded) animals including alligators, lizards, turtles, fish, shell fish and crustacea; and domesticated insects including honey bees and silk worms.
This manual is written with the major agricultural species in mind but all domesticated animals, and animals with potential for domestication and feral populations should be considered when preparing local or regional conservation strategies.
The FAO definition of animal genetic resources eligible for conservation includes animal populations with economic potential, scientific use and cultural interest.
Endangered populations should be conserved for their potential economic use in the future. Their economic potential may be the production of meat, milk, fibre, skin or draught power. This potential production may be in diverse climatic and environmental conditions. Endangered populations with economic potential may have regional adaptation developed for the country of origin, or adaptations which may be beneficial in other areas of the world where similar or complementary conditions exist. For example the successful use of the Zebu cattle breeds in diverse regions of the world and the use of the trypanotolerant N'Dama cattle breed from the Republic of Guinea in other countries of West Africa (Devillard, 1983). Animals with distinct characteristics may be beneficially incorporated into the breeding programmes of other countries, for example, the prolific characteristics of the once rare Finnsheep (Maijala et al, 1990).
Economic potential cannot be measured by looking simply at performance. Rare or endangered breeds are often highly adapted and their performance should be measured comparatively, within their own environmental conditions. They should not be compared with other breeds in improved or modified conditions or under intensive management. Furthermore, they should be examined with respect to the products for which they were selected and valued in the conditions under which they evolved.
There are many examples where growth rate, prolificacy, or milk production have been measured and used to illustrate the inferiority of purebred indigenous stock over that of exotic imported breeds or their crosses (Hodges, 1986). However, when survivability of the offspring, fertility and longevity are taken into account the indigenous stock are often found to be very productive overall. Two examples of this type of productivity are the Panteneiro cattle from the Pantenal or swampy region of Brazil, and Tswana goats of Botswana in Southern Africa both of which are described in more detail in chapter 5.
It is important to remember when considering economic potential that bioefficiency is not the same as bioeconomic efficiency. That is to say the genetically controlled ability of a population or breed to survive and produce in a region is only one function of its economic efficiency. The economic success of a breed or agricultural system at any one time is dependent upon many manmade variables. These variables include the value of land, the cost of oil and other fuels, the international currency markets and exchange rates, the production efficiency of other breeds and populations in this and other regions of the world, the product shelf-life, travel and storage characteristics, health controls, current marketing strategies, consumer preferences and international political objectives. Changes in any one of these features may shift the balance and enhance the economic value of one breed type over another. For example, a shift in oil prices will affect the cost of cereal production, this in turn affects the cost of feeding grains to livestock and may affect the choice of breeds used in human food production towards more forage efficient stocks.
Finally, crosses between unrelated breeds are not completely predictable in their production characteristics. There are many instances where two pure breeds produce crosses which far exceed the production characteristics of either parent breed due to heterosis. This may be particularly important between breeds which are historically distant or which are each inbred.
This may be due to the two breeds carrying genes of different allelic pairs which complement each other. Highly inbred lines may also demonstrate specific combining ability. This is probably because one line or breed will have become homozygous for some favourable dominant genes and by chance, homozygous for other unfavourable recessive ones. A second unrelated breed would be unlikely to be homozygous for the same undesirable genes. Thus a cross between the two would result in more vigorous offspring (Warwick, 1979). This ‘matching’ of breeds is not predictable. The total number of possible crosses is potentially infinite, and many un-tried crosses could produce valuable production stocks. The South American Criollo cattle whose numbers were in dramatic decline due to replacement by European and Zebu imports, have now been shown to have great potential in commercial crossing where the Criollo first crosses out-produce purebred imported and indigenous stocks.
Populations should not therefore be discarded on the grounds of economic efficiency as measured at any one time, but should be considered with respect to biological efficiency operating within the context of a wide range of possible political and economic situations and a wide range of possible breed crosses (Henson, 1986).
Endangered populations should be conserved for their possible scientific use. This may include the use of conservation stocks as control populations, in order to monitor and identify advances and changes in the genetic makeup and production characteristics of selected stocks. They may include basic biological research into physiology, diet, reproduction or climatic tolerance at the physiological and genetic level. Genetically distinct breeds are needed for research into disease resistance and susceptibility which could help in the development of better medication or management of disease. It could also help with the identification of specific genes involved in natural disease or parasite control. Some populations may also be used as research models in other species, including man. This is already the case in the use of Ossabaw Island Hogs in the USA. These feral pigs from an isolated island off the east coast of the USA have been shown to have a natural insulin disorder making them a useful research model for human diabetes (Brisbin, 1985).
Many populations have played an important role in specific periods of national or regional history. For example, Texas Longhorn cattle in the colonization of the USA, Spanish Merino sheep in the creation of Spain's seventeenth century wealth, or llamas, important as pack animals and fibre producers for the Inca nation of Peru. There are also breeds which have been associated with social and cultural development; the Navajo-Churro sheep whose wool is essential in the production of the native rugs of the Navajo Indians in the USA, or elephants involved in the religious ceremony of the Perehera in Sri Lanka.
There are also many breeds which may be conserved for their aesthetic value. These might include the strain of performing Lippizan horses in Austria, the multihorned and spotted Jacob sheep of Britain, the cork-screw horned Racka sheep of Hungary or many of the ornamental poultry breeds.
The idea of conserving animal genetic resources focusses on two separate but interlinked concepts. The first is the conservation of ‘genes’ and the second, the conservation of ‘breeds’ or populations.
The conservation of ‘genes’ refers to action to ensure the survival of individual genetically controlled characteristics inherent within a population or group of populations. It could, for example, be trypanotolerance, polledness, wool shedding, or a specific milk protein. Such programmes require that the characteristic to be conserved is clearly recognized and identified. It does not, however, require that the genetic function at the chromosome or DNA level be understood. Such a characteristic may in fact be a complicated biochemical function controlled by several sections of DNA on more than one chromosome, but provided the characteristic can be identified in the appearance or function of the animals that exhibit it, a programme can be developed to conserve it as a gene within the population.
The conservation of populations or breeds refers to action to ensure the survival of a population of animals as defined by the range of genetically controlled characteristics that it exhibits. This form of conservation is applied to endangered species as well as to breeds, and is developed to ensure the conservation of all the characteristics inherent with a given population, including many which may not have been recognized, defined, identified or monitored. The differences between breeds may often be due to differences in the frequency of quantitative genes rather than the presence or absence of unique genes. Such a difference in gene frequency may result in dramatically different populations with respect to appearance and production in a given environment.
All proposed conservation projects should clearly define whether the project proposes to conserve ‘genes’ or ‘breeds’. Conservation methods and strategies are not exactly the same for the two objectives.
Opinions have varied over the past forty years as to which animal genetic resources are candidates for conservation. Estimates have been influenced by the relative cost/benefit of conserving all genetic variation as compared to those that can be demonstrated to have predictable economic, scientific or cultural value as described above.
Recent FAO Expert Consultations and meetings of other interested scientists have concluded that all ‘breeds’ or populations which are ‘unique and endangered’ are eligible for inclusion in some form of conservation programme (FAO, 1989b; National Academy of Science, 1992; Office of Technology Assessment, 1987; Wilson, 1988; Weiner, 1989). This definition is based on the belief that it is impossible to determine which characteristics have potential value in the future, because it is impossible to envisage all future eventualities which might include climate change, mutations in disease or parasite populations, the affects of political change, wars, and the availability of energy. This definition has been frequently extended to include popular breeds in which there is rapid genetic change (see section 3.3.5).
Uniqueness is difficult to define with respect to livestock populations. There are clearly some populations with obviously unique characteristics or traits. For example naked neck chickens (Bodo et al, 1990), seaweed eating North Ronaldsay sheep (Henson, 1978), or the Kuri cattle of Lake Chad whose hollow horns enable them to swim to the lake islands (Adeniji, 1983). There are also breeds or strains which exhibit extremes of quantitative production traits for example, the miniature Dexter cattle of Ireland (Ark, 1976), the prolific Taihu pigs of China (Peilieu, 1984), and the excessively fat Mangalitza pigs of Hungary (Baltay, 1982).
For the vast majority of populations their uniqueness is subjective. It refers to the fact that no other population has the same ancestry, environmental adaptation, human selection, appearance or production characteristics. In effect, the difference between two populations may only be a function of the relative frequencies of the same genes. From the point of view of conservation any population which is historically or geographically isolated or which has had little genetic influence from other breeds over a long period of time, or which exhibits unusual characteristics or traits should be considered to be a unique population.
The concept of what constitutes an endangered population varies considerably. In wildlife conservation, that is to say in the conservation of endangered species, a population is said to be endangered when the chance of the survival in the wild is unlikely unless action is taken to conserve that population.
There is no simple numerical level at which a population is defined as being endangered or eligible for consideration as a candidate for conservation. Rather it is dependent upon a number of factors: the actual numbers of animals; the rate of decline in the population size; the closeness of relationship between individuals within the population; the geographical range and the rate of reduction of that range; special threats from introduced species; rapid changes in the environmental conditions including climate, predators and parasites.
The classification for endangered status is based upon the long term survival change of the population being considered. This survival chance can be estimated using population models which incorporate all the relevant variables. These variables include rates of population decline and effective population size (Ne) which incorporates: the sex ratio and age structure of the population; inbreeding rate and genetic drift; genetic diversity within the population; and the length of time the survival plan needs to operate. As a rough guide, a population of less than 10,000 animals may be considered in need of some form of intervention with respect to species conservation (see appendix 3.1).
Effective Population Size
Effective population size (Ne) is determined by the relative genetic contribution of each animal to the next generation. For a more detailed discussion of the calculation of effective population size see appendix 3.2.
Effective population size is greatly affected by the ratio of males to females active within the population such that 4 males and 4 females constitute the same effective population size (Ne), as 100 females and only 2 males.
|Nos of |
|Effective population size (in whole nos)|
with varying nos of females
Effective population size is also affected by relative fertility, family size and longevity. This is because it results in some individuals contributing more progeny to the next generation than others. This is especially important in a small population with limited population growth.
Depression of effective population size (Ne) due to inequality of lifetime family sizes - that is the total number of surviving and reproducing offspring each adult animal produces (after Foose, 1983).
Heterozygosity is the measure of genetic variation within a population and, as already described in chapter 2, is closely linked to the long term survival chances of a population. Heterozygosity refers to the number of genetic options available within a population at a single address on the chromosome. The amount of heterozygosity or genetic variation begins to decrease at an accelerated rate, once the effective size of the population (Ne) falls below 100.
Loss of genetic diversity (as measured by heterozygosity) due to random drift for various effective population sized (Ne) for a total number of 250 animals, based on a rate of decline in heterozygosity of 1/2Ne x 100% per generation (after Foose, 1983).
As a general rule, programmes for the conservation of captive endangered species are limited by the number of spaces available within the participating zoos. This may be a serious limiting factor in the survival chances of a population, especially when the population limit is close to the minimum effective population size needed for long term survival. When the increase in population size is not limited, population increasing from very low numbers can reach an inbreeding equilibrium and completely recover their chances of survival (Yamada, 1983).
The International Union for the Conservation of Nature (IUCN) provides clear definitions for the rarity of species in its international Red Data Books. These definitions relate to the survival chances of the populations and take all the variables of population structure and environmental factors into account.
Wildlife conservation, based upon these categories is most commonly centered around the in situ conservation of populations in their natural environments. This involves the protection of wildlife habitats and requires that sufficiently large reserves are maintained to enable the target species to exist in large numbers. The population size must be sufficient to enable the necessary genetic diversity to survive within the population, so that it has a good chance of continuing to adapt and evolve over time. This reserve size can be calculated for target species by examining the population density in naturally occurring situations. The reserves must then be protected from intrusion, or destruction by man, and against other catastrophes.
IUCN Categories for Wildlife
|Ex||Extinct||Not found in the wild for 50 years|
|E||Endangered||In danger of extinction, survival unlikely if causal factors continue to operate|
|V||Vulnerable||Likely to become extinct in the near future if causal factors continue to operate|
|R||Rare||Small population not endangered or vulnerable but at risk|
|I||Indeterminate||Known to be in category E, V or R above but insufficient information to determine which|
|O||Out of danger||A population which was on the list but is now recovered|
For severely endangered populations, or those for which long term habitat protection is not a viable option, or where sufficiently large reserves cannot be secured, breeding programmes must be designed. These might simply involve the controlled movement of males from one ‘island’ reserve to another in order to enhance the gene flow within several geographically isolated subgroups. Other programmes for which suitable habitat is no longer available must be based entirely on captive breeding programmes and the uncertain hope of maintaining a viable remnant population in a closely controlled environment.
Similar categories and strategies should apply when dealing with domestic animals in which the entire species, rather than just strains or varieties within a common species are involved, for example the South American camelidae, the yak, camel and elephant.
In common domestic species for which varieties, strains or breeds are in danger of extinction, the population levels at which action needs to be taken can be much lower. In these cases the common strains or breeds can be used for cross breeding, grading up or as surrogate mothers in an embryo transfer programme. Despite this the range of numbers used to determine the point at which a population is in need of conservation varies considerably from many European estimates in the hundreds to FAO estimates in the thousands (Hodges, 1990c).
The large difference between the European estimates and the FAO estimates stems from the fact that they are based on completely different objectives. The European estimates are based on the minimum number of animals needed to maintain a viable population. Such viable populations are the living parallel to the ex situ cryogenic store designed to conserve extractable genetic material. The FAO estimates, on the other hand, are based on the minimum number needed to maintain a population or breed in which future selection and improvement can be carried out.
In general terms the endangered status of domestic populations can be considered in exactly the same way as those described above for wild species. (see Table 3.)
Categories for Domestic Populations
|Extinct||No possibility of restoring the population, no pure bred males or females can be found.|
|Critical||Close to extinction, genetic variability reduced to below that of the ancestral population, action to increase the population size is essential if it is to survive.|
|Endangered||In danger of extinction because the effective population size (Ne) is too small to prevent genetic loss through inbreeding which will result in a reduction in the viability of the breed. Preservation must be enacted.|
|Insecure||Population numbers decreasing rapidly.|
|Vulnerable||Some disadvantageous affects endanger the existence of the population and some precautionary measures should be taken to prevent further decline.|
|Normal||Population not in danger of extinction, can reproduce without genetic loss, no visible changes in population size.|
|(after Bodo, 1989)|
In order to convert these general terms into figures of population size, birth and survival rates, sex ratios and levels of variation must be taken into account. Table 4 has been proposed as a basic term of reference for uniparous populations of cattle, horses and buffalo (Bodo, 1989).
|status||No breeding females||Estimated effective average population size|
|(after Bodo, 1989)|
The mechanism for the conservation of individual genes within populations is closely linked to the conservation of species and breeds. The most important feature of a small population conservation programme is the rate of genetic loss. The increase in homozygosity within a small population results in the loss of ability to adapt, inbreeding depression and, ultimately, extinction. Viable population size must therefore be linked to the ability to conserve genetic diversity within any conservation population.
The maintenance of genetic diversity is linked to the effective population size and to the number of founder animals (see 3.3.2 above).
Populations based on very small numbers of founder animals are less likely to survive than those based on a larger number of founders. A population based on less than 5 or 6 females is highly unlikely to survive. However, only marginally larger populations based on 9 or 10 founder females may survive provided the population is allowed to increase rapidly. If this happens a maximum level of inbreeding is reached after about 30 generations and will then level out (Yamada, 1983). There are examples of populations which have recovered from extremely small founder groups including the Pere David's Deer (Elaphurus Davidianus) and European Bison (Bison Bonasus) (Frankel & Soule, 1981).
Average percentage of genetic diversity (as measured by heterozygosity) contained in founder populations of various sizes. It is assumed that founders are unrelated and non inbred. Diversity preserved is equal to (1–1/2 Ne) x 100% (after Foose, 1983).
Concern has also been expressed for common breeds undergoing periods of rapid genetic change. These are the intensively selected breeds, often involving the use of high levels of advanced technology including artificial insemination (AI) and embryo transfer. These breeds are producing at a very high level under intensive management, veterinary care and feeding regimes. They include breeds likely to be affected by the introduction of transgenic technology. They are the intensively selected dairy cattle breeds of the temperate regions and the industrialized pig and poultry stocks. Conservation may be needed of samples of these populations as they change to ensure that alternative selection options exist. Collection of cryogenic samples would be a useful precaution enabling future changes in direction within these breeds. The establishment and maintenance of live control populations as in situ conservation projects, would not, however, appear to be necessary provided cryogenic storage was possible.
In cases where the disappearance of a population is imminent, action to conserve that breed should be taken immediately.
In general there is a range of information needed in order to identify populations which should be considered as candidates for conservation. The following types of information should therefore be gathered and used in the definition and planning of a conservation strategy:
General descriptive information on the species, breed or type and geographical location. The basis for this information is available in Ian Mason's Dictionary of Livestock Breeds (Mason, 1988). Additional information is being collected throughout the world and will be held on the EAAP/FAO Global database (Simon, 1989).
Estimate of the number of animals, males, females and totals and the population trend. This information is more readily available in some countries than others but many nations carry out regular livestock censuses which could be extended to include breed specific information.
The percentage of the female population being used in cross breeding. By estimating the number of purebred males and females alongside the number of young stock with evidence of cross breeding it is possible to estimate the rate of breed dilution. Even a 20% per year decrease in pure bred young stock will result in a very dramatic crash in population size over a relatively short space of time.
The affects of annual rates of decrease on population size (after Hodges, 1990a).
The number of herds or breeding units. A few very large herds may be more vulnerable to diseases, or the affects of economic or political changes than a large number of smaller herds.
Estimates of the health risk. Populations in regions where lethal epidemics are endemic may be at greater risk than those in regions where such diseases are not present.
Estimates of other risks, political, climatic or economic. In particular the risk of draught, storms, flooding, war or rapid socio-economic change which could result in the disappearance of indigenous populations.
Characterization of the breed which includes the measurement and description of external appearance, production characteristics, climatic adaptation, disease resistance, parasite tolerance, management and any other special feature. It may also involve the collection of biochemical information from blood types, milk proteins and the comparative analysis of DNA fragments. All of this information is useful in determining the long term conservation strategy with respect to a breed but is not essential in establishing an initial programme to prevent the early loss of a breed or population.
Existence of conservation projects. Projects already in existence to maintain, utilize or conserve breeds at the local, national, or regional level may influence the need for further action in breed conservation.
The conservation of animal genetic resources is deemed to be essential in the light of the rapid loss of varieties and breeds through dilution and breed replacement. All varieties of domestic species and species with potential for domestication are considered to be important candidates for conservation. Populations with economic potential, scientific use and cultural or aesthetic interest are of particular importance but all populations which are unique and endangered should be incorporated into conservation efforts.
The general definition for species and breeds in need of conservation ranges from minimum numbers in the hundreds needed to conserve a living survival unit, to much larger populations in the thousands required to maintain populations able to adapt, evolve and be utilized in the future. The effects of small population size on the rate of loss of genetic variation within populations is closely linked to the number of parent animals contributing offspring to the next generation and this effective population size (Ne) must be kept as high as possible in order to prevent dramatic loss of variation in small populations.
Finally, the information used to identify populations in need of conservation is shown to range from simple information available from the farmers familiar with the breeds, to that supplied by detailed biochemical and scientific research.
5. The last remaining herd of Mulefoot Hogs in the USA has been conserved by an individual farmer.
6. Hariana cows conserved at a religious ‘Gupsala’ in India.