What are the objectives of a conservation effort?

Utilisation and conservation of farm animal genetic resources 137

Chapter 6. Selection of breeds for conservation

of between breed and within breed diversity over time due to extinction of breeds and genetic drit is not accounted for. Hence, it seems to be better to rank the breeds according to their contribution to the expected future diversity, as it will be shown next. Assume that there are a number of N breeds included in the analysis and for each breed the probability is known that it will go extinct within a deined future time horizon t e.g. 50 years, the extinction probability z. At the end of the deined time horizon there are 2 N diferent possibilities of present and extinct breed combinations possible, i.e. there Box 6.2: Relative importance of between and within breed genetic diversity. In chapter 3 it was shown how diversity can be partitioned into between-breed diversity D B and within-breed diversity D w . he question is, which component is more relevant for conservation and hence for the selection of breeds for a conservation plan. Fabuel et al. 2004 investigated the impact of a diferent weighing factor λ in the combination of the two components, i.e. λ D w + D B , and obtained diferent breed conservation priorities, depending on λ. Following this, the determination of the relative importance of these two components is crucial and it determines the choice of the diversity measure chapter 5 that has to be used in the maximum-diversity strategy for the selection of breeds. For instance, if only the between-breed diversity shall be considered λ = 0, the Weitzman diversity measure might be appropriate. However, if both between- and within-breed diversity shall be taken into account, the core set diversities maybe used. he Maximum-Variance-Total core set diversity measure maximises the variance of a hypothetical quantitative trait and gives more weight to the between-breed diversity λ = 0.5, as pointed out by Toro et al. 2006 than the Maximum-Variance-Ofspring λ = 1 does. A stronger weighing of the between-breed diversity might be appropriate when conserved breeds are considered to be used in a crossbreeding plan, because both heterosis and complementarity are a function of this type of diversity Fabuel et al. 2004. Additionally, as argued by Piyasatian and Kinghorn 2003 and Bennewitz et al. 2006, the between-breed diversity is more accessible, because of the speciic gene and allele combination that can be found in diferent breeds. his allows a faster adaptation of commercial breeds to changes in the production environment or in market needs by introducing genetics from a conserved breed. However, a too strong weighing of between-breed diversity results in ignoring most of the total diversity. he within-breed diversity might be important for the creation of a new synthetic breed that copes with a challenging environment. On the other hand, an over-emphasis on within-breed diversity will favour large breeds, which are not endangered. Additionally, the better accessibility of the between-breed diversity is not accounted for. In summary, it seems that a diversity measure should be used that considers both components of diversity with a weight that is not arbitrarily chosen by the researcher but is given by the deinition of the appropriate diversity measure and chosen according to a well deined conservation objective. 138 Utilisation and conservation of farm animal genetic resources Jörn Bennewitz, Herwin Eding, John Ruane and Henner Simianer are 2 N diferent breeds sets K, each with a certain probability P K , which depends solely on the extinction probabilities of the breeds. Each breed set K shows a diversity D K . Following this, the expected conserved diversity at the end of the time horizon t is z N N P K K D K t ¦ K K t D K P D E , which is a projection of the actual diversity into the future. It depends on the extinction probability z and on the expected drit, if the loss of within breed diversity is accounted for; see Simianer, 2005b; Bennewitz and Meuwissen, 2006. he efect of a breed could be assessed by how much the expected diversity would be changed with respect to a small change in the breed extinction probability. his is essentially the concept of marginal diversities. A marginal diversity of a breed md i is deined as the change of conserved diversity at the end of the considered time horizon, when the extinction probability of the breed, z i , would be changed by one unit by a conservation efort Weitzman, 1993. It can be estimated as the irst partial derivative of ED t with respect to z i , i.e. md i E D t z i i t i z D E md w w . fulfils the ‘monotonicity’ and ‘non-negativity’ CP i = md i z i that the maximum-diversity-strategy using the the conservation plan Reist-Marti et al., 2003, other arguments are included, the maximum- be extended to the maximum-utility-strategy as it will be shown in he minus sign makes it positive, i.e. it relects the change of the conserved diversity, when the extinction probability would be lowered by one unit. he estimation of marginal diversities can be done with any diversity measure that fulils the ‘monotonicity’ and ‘non-negativity’ properties of the Weitzman criteria for a proper diversity measure chapter 5. For computational details see Simianer et al. 2003 and Bennewitz et al. 2006. It is important to note that the marginal diversity of a particular breed is independent of its extinction probability. he marginal diversities can be multiplied by the extinction probabilities in order to obtain the conservation potentials of the breeds, i.e. CP i = md i z i Weitzman, 1993. he conservation potential gives an idea how much diversity can be conserved additionally if a particular breed would be made completely safe. It was shown that the maximum-diversity-strategy using the conservation potentials for prioritising breeds is very eicient in selection of breeds for conservation, when diversity is the objective of the conservation plan Reist-Marti et al., 2003, Simianer et al., 2003. If, however, also other arguments are included, the maximum-diversity-strategy has to be extended to the maximum-utility-strategy as it will be shown in the next section. he diiculties in this approach are that extinction probability estimates are required for the breeds, and these are not easy to obtain. In Box 6.3 the diiculties in their estimation are described. However, it was observed that the marginal diversities are not very sensitive to changes in extinction probabilities and that these have to be known