Genetic aspects of ex situ conservation Jennie Håkansson Email: [email protected] Introductory paper, 2004 Department of Biology, IFM, Linköping University ___________________________________________________________________________ Abstract More than 50 percent of all vertebrate species are classified as threatened today and this has become a central concern for people all over the world. There are several ways of dealing with preservation of species and one of the techniques receiving most attention is ex situ conservation. In ex situ conservation animals are bred in captivity in order to eventually be reintroduced into the wild when their habitats are safer. This is often called conservation breeding. Since threatened species usually have small and/or declining populations, the effect of small population size is a major concern in conservation breeding. The aim of the present paper is to review genetic aspects of ex situ conservation and to discuss how populations should be managed in captivity in order to be successful in a reintroduction situation. Populations in captivity may deteriorate due to loss of genetic diversity, inbreeding depression, genetic adaptations to captivity and accumulation of deleterious alleles. These factors could seriously jeopardize the successfulness of ex situ conservation and need to be investigated thoroughly in order to optimize conservation breeding programs. Keywords: Genetics; Ex situ conservation; Conservation breeding ___________________________________________________________________________ Introduction which extinction occurs today is at least partly due to human influences. It is possible It has been estimated that there are that up to 18 000 species have become between 3 and 30 million species of various extinct since 1600AD (Magin et al., 1994). life living today (May, 1992). This can be This means that the extinction rate in the last compared to the 5 to 50 billion species that 400 years may have been 45 animals per may have existed at some point since the year if rates of extinctions among mammals beginning of life (Raup, 1991). Such and birds are evenly extrapolated to other comparisons indicate the magnitude of taxa. This rate is 15 times higher than the extinctions that have occurred in the past. estimated natural extinction rate and What is important to remember is that some conservation of species has now become a degree of extinction is an entirely natural central concern for people all over the occurrence. Magin et al. (1994) estimated world. that roughly three described animal species There are several ways of preserving became extinct each year before the threat of species and the focus of this paper is on ex human influence. However, the rate of situ conservation of animals. In ex situ 1 conservation animals are bred in captivity vertebrates may have to be bred in captivity for an eventual reintroduction to the wild. in order to survive. To preserve all This is called conservation breeding and is threatened species is practically an one of the species preservation techniques impossible mission but, hopefully, the receiving most attention. establishment of conservation programs can Today, over 50 percent of all at least reduce the extinction rates. vertebrate species are classified as Species in need for conservation threatened (Frankham et al. 2002). The first actions are categorized into different groups; convention on biodiversity took place in Rio critically endangered, endangered and de Janeiro in 1992 and the zoos’ response to vulnerable species (IUCN, 2001). this was The World Zoo Conservation Additionally, there are some species that are Strategy (Wheater et al., 1993) with the aim completely extinct in the wild and survives to help conserve the earth’s fast- only in cultivation, in captivity or as a disappearing wildlife and biodiversity. naturalized population outside the past Many species require conservation breeding range. Threatened species are found in all to save them from extinction as they may taxonomic groups and the number of species not be capable of surviving in the hostile in each category is shown in table 1. The natural environment (Ralls and Ballou, survival of many of these species depends 1983). Soulé et al. (1986) estimated that upon well-designed ex situ conservation approximately 2000 species of terrestrial actions. Taxonomic group EW CR EN VU Mammals 4 184 337 609 Birds 3 182 331 681 Reptiles 1 57 78 158 Amphibians 0 30 37 90 Table 1. Number of species extinct in the wild (EW), critically endangered (CR), endangered (EN) and vulnerable (VU) in four different taxonomic groups. (After IUCN, 2003) The ultimate goal of ex situ to prevent immediate extinction as well as conservation is to provide support for the re-establishment and support of natural survival of species in the natural populations to ensure ultimate survival of environment (e.g. Wheater et al., 1993). species in their natural environment. Hence, conservation breeding programs are Conservation breeding programs must be not an alternative for, but rather a complemented with for example habitat complement to, in situ conservation, i.e. preservation and elimination of the original conservation through biotope protection and threats in order to give the reintroduced preservation. This can be accomplished animals a fair chance to survive. through captive propagation of rare species 2 Reintroduction is the intentional of deleterious alleles (Couvet and Ronfort movement of captive-born animals into the 1994). All these factors could seriously wild. Animals can be moved into (or near) jeopardize the successfulness of a the species’ historical range either to re- conservation breeding program and despite a establish a new population or to boost a wild certain overlap; they will be discussed one population. According to Beck et al. (1994) by one. only 11 percent (16 of 145) vertebrate reintroductions where captive-born Loss of genetic diversity individuals were released into the wild can From a genetic point of view, the be considered successful. Causes of failure most critical point of ex situ conservation is or problems in reintroduction of captive- to keep high levels of genetic diversity so bred animals vary from case to case but a that the population’s ability to face new common cause is behavioural deficiencies in environmental challenges is not at risk and released animals (e.g. Fleming and Gross, to prevent adaptation to captive conditions 1993; Kleiman, 1989; Shepherdson, 1994). (Fernández and Caballero, 2001). Genetic Still, few data are available to explain why diversity is reflected in the differences reintroductions fail and despite guidelines among individuals, including coat colour, stating the need for evaluation, the results of behavioural patterns and DNA sequences. reintroductions are rarely published (Beck et Maintenance of genetic diversity must be in al., 1994; Kleiman et al., 1994). For focus in conservation breeding because conservation breeding to be a successful environmental change is a continuous technique to save species from extinction, process and genetic diversity is required for more knowledge is needed about what evolving to adapt to such change. happens with behaviour and genetics in Furthermore, loss of genetic diversity is small captive populations bred in captivity. often associated with inbreeding and reduction in reproductive fitness. According Aim to Tegelström and Sjöberg (1995), high The aim of the present paper is to levels of homozygosity and loss of allelic review genetic aspects of ex situ variation may affect a population negatively conservation and to discuss how populations in at least three ways. First, homozygous should be managed in captivity in order to genotypes may be more susceptible to be successful in a reintroduction situation. environmental variation (Beardmore, 1983). Secondly, if individuals share genes by Genetic considerations of ex situ descent, deleterious genes expressed in the conservation homozygous condition result in abnormalities, low viability and death Given that threatened species usually (Chambers, 1983; Bensch et al., 1994). have small and/or declining populations, the Thirdly, a low degree of allelic variation effect of small population size is a major limits the ability of a population to respond concern in ex situ conservation. According to temporal and spatial environmental to Woodworth et al. (2002), populations in variation (Beardmore, 1983). captivity deteriorate due to loss of genetic Most threatened species and diversity, inbreeding depression and genetic populations have lower genetic diversity adaptations to captivity that are deleterious than related, non-threatened species with in the wild. Ex situ conservation may also larger population sizes (Frankham et al., lead to genetic changes due to accumulation 2002). According to evolutionary theory, 3 populations with low genetic variation are where r is the total number of loci and rp is expected to have lower adaptive potentials the number of polymorphic loci. to cope with environmental changes than The genetic composition of a populations with higher levels of genetic population can also be described by allele variation. The environment changes frequencies. The allele frequency or the annually, seasonally and daily but also over frequency of the F allele (p) in a population time due to, for example, climatic changes. is the proportion of all alleles examined Genetic variation in a population is therefore which are F. The number of each important