9709 - Laserfiche WebLink

Home Browse Search

2 R. FRANKHAM vertebrate species were being bred in captivity in zoos and associated facilities (Magin et aI. 1994). For plants, the Royal Botanic Gardens at Kew, England, alone houses 2700 out of about 25 000 threatened species (Primack 2006) and the Center for Plant Conservation in the USA now maintains over 600 endangered US plants (Center for Plant Conser- vation 2006). Captive breeding programs typically seek to preserve the option of release into the wild (Frankham et al. 2002). Reintroductions are currently being carried out for at least 121 bird and mammal species, 68 of which are threatened (Wilson & Stanley Price 1994). Examples of endangered species that have been captive-bred or propagated and reintroduced into the wild include Arabian oryx, black- footed ferrets, Przewalski's horse, California condors, Guam rail (Gallirallus owstoni), Lord Howe Island woodhen (Gallirallus syIvestris) and the Mauna Kea silversword plant (Argyroxiphium sandwicense). Adverse genetic changes in captivity jeopardize the ability of captive populations to reproduce and survive when returned to the wild. Four types of harmful genetic changes can occur during captivity, namely: (i) loss of genetic diversity, (ii) inbreeding depression, (ill) accumu- lation of new mildly deleterious mutations, and (iv) genetic adaptation to captivity (Frankham et al. 2002). The first three of these are associated with small population size and they are addressed, directly or indirectly, by current recommended management regimes. This contribution reviews the final issue, genetic adaptation to captivity. Genetic adaptation to the captive environment occurs through natural selection. As captive environments differ from wild ones, the genetic variants favoured in captivity differ somewhat from those favoured in natural environ- ments. Darwin (1868) pointed out that natural selection for tameness and other adaptations to the captive environment were inevitable, especially in the context of domestication. Genetic adaptations to captivity has been documented in mammals, fish, insects, plants and bacteria (see Zouros et aI. 1982; Allard 1988; Frankham & Loebe11992; Latter & Mulley 1995; Levin et al. 2001; Lewis & Thomas 2001; Nunney 2001; Woodworth et aI. 2002; Heath et aI. 2003a, b). The extent of genetic adaptation to captivity may be very large. For example, after 25 generations in captivity, a popu- lation of wild rats (Rattus norvegicus) showed 55% earlier age at first reproduction, more than doubled duration of reproductive life, almost three-times as many litters, a reduction in female sterility from 35% to zero, increased docility and much improved mothering ability in females (King 1939). Further, a threefold increase in reproductive fitness over 84 generations and a doubling of fitness over eight generations in captivity have been reported in Drosophila (Frankham & Loebe11992; Gilligan et aI. 2003) and a dou- bling in barley (Allard 1988). Fecundity in captivity of a large white butterfly (Pieris brassicae) population that had been in captivity for 100-150 generation was about 13-fold higher than that in a new wild strain (Lewis & Thomas 2001). Characteristics selected for under-captive conditions are overwhelmingly disadvantageous in the natural environ- ment (Leopold 1944; Bush 1978; Fraser 1981; Chilcote et aI. 1986; Frankham et aI. 1986; Allard 1988; Kohane & Parsons 1988; Lachance & Magnan 1990; Leider et al. 1990; Hindar et al. 1991; Fleming & Gross 1993; Reisenbichler & Rubin 1999; Waples 1999; Fleming et al. 2000; Chilcote 2003; Heath et al. 2003a, b; McGinnity et aI. 2003; Kraaijeveld-Smit et aI. 2006; Araki et al. 2007). This effect has been reported for turkeys, amphibians, plants and many species of fish and biocontrol insects. The evidence from fish is extensive and reveals highly deleterious effects (Allendorf & Luikart 2006). For example, a hatchery stock of chinook salmon (Oncorhynchus tshawytscha) in Canada evolved smaller eggs and supplementation of wild populations using this stock reduced the egg size of wild populations, resulting in reduced wild fitness (Heath et al. 2003a, b). Lifetime reproductive success of hatchery fish stocks when returned to the wild was found to be only 5-15% of that for the wild fish (Leider et aI. 1990). Farmed fish, that have been domesticated by adapting to captivity and being artificially selected had a fitness only 16% that of wild fish in the study by Fleming et al. (2000) and 2-4% in that of McGinnity et al. (2003). Genetic adaptation to captivity is also strongly deleterious in biological control programs, where Myers & Sabath (1980) reported that success of such programs was negatively related to time in captivity. Considerable difficulty has been encountered in the reintroduction of endangered species into their natural habitats (Griffith et aI. 1989; Beck et aI. 1994; Serena 1995; Wolf et al. 1996; Fischer & Lindenmayer 2000), and genetic adaptations to captivity that reduce reproductive fitness in the wild are one of several possible reasons for this low success rate (Frankham et aI. 2002). Only 38% of introduc- tions or reintroductions were considered to be successful by Griffith et aI. (1989), while the figure had risen to 53% in a later survey (Wolf et al. 1996). Beck et al. (1994) using dif- ferent criteria concluded that 11 % of reintroductions were successful, while Fischer & Lindenmayer (2000) reported 26% success, 27% failures, and 47% unknown results. Reintroduction projects using translocated wild animals were more successful than those using captive-bred animals (75% vs. 38%; Griffith et aI. 1989; 71 % vs. 50%; Wolf et al. 1996; 31% vs. 13%; Fischer & Lindenmayer 2000). The impact of genetic adaptation to captivity is likely to be an increasing problem in the future, as many species may face periods of 100-200 years in captivity before suitable habitat for reintroductions becomes available following a reduc- tion in the human population (Soule et aI. 1986). Genetic adaptation to captivity is not currently addressed directly as an issue in captive breeding and reintroduction @ 2007 The Author Journal compilation @ 2007 Blackwell Publishing Ltd