Population size and environment are obviously essential to survival of a species. Less obvious are genetic factors that contribute to survival and reproduction of individuals, and thus species, in nature. Modern population genetics has demonstrated that genes have a strong influence on almost all important characteristics that help a species to adapt to its environment. Such inborn inherited influences reach into every aspect of the life cycle.
Of crucial importance is the great genetic variability between individuals in a population. Individuals show genetic variations that affect not only their overall size, but also the sizes, shapes, and functioning of particular organs. For example, genetic differences may occur in the ability of animals to obtain mates and thus reproduce successfully, in the acuteness of sensory perceptions that will affect their efficiency in finding food and in detecting and escaping from predators, and in such fundamental attributes as the immune system that enables them to withstand attacks of disease.
Breeding in nature normally provides a species with a population consisting of an array of genetically different individuals. Through natural selection, the gene combinations that work well for the species are identified; the hardiest individuals among them leave more offspring. That results in a system for perpetuating and multiplying the "good" variations. Natural selection is not a process that merely eliminates the "bad."
The system is not mysterious. Indeed, practical animal and plant breeders used the same system to select animals and plants long before modern knowledge of genetics emerged. The system essentially retains considerable minor gene variability from which it generates and puts out for trial new variants of the gene combinations that have worked well in the past. It keeps the population large and variable: when there are a lot of combinations (i.e., individuals) to choose from, selective adaptation can keep up genetically with the ebb and flow of environmental shifts.
Even though it works well for most animals and plants, the natural system of selective breeding nevertheless extracts a price from the population, in that it only rarely solidifies and fixes a particular invariable or "ideal" combination of genes. Rather, it retains a balance of many present and potential combinations of genes. The outcome is that some individuals in the population are inferior to others in how they are able to react to the problems posed by their environment.
A further, biologically expensive genetic price is that DNA is prone to pick up and carry along some new mutant genes. Those genes might affect an essential organ system or process negatively; they can thus have a negative effect on the organisms that carry them. Such deleterious genes commonly exist in heterozygous form. When genetic recombination occurs as two carrier parents are producing offspring, the probability increases that one or more of the progeny will get a double dose of (i.e., become homozygous for) one of these "lethal" or "semilethal" genes. The effect is likely to manifest itself by causing the early death of the organism in the embryo stage. If it does not actually kill, it might seriously handicap the organism (a semilethal gene). The existence of such genes in many organisms has been well established. They constitute the notorious ''inborn errors," genetic conditions often referred to as our "load of mutations." They have also been studied in experimental genetics of mice and insects, all of which carry genetic diseases in their populations. Many occur naturally, having been hidden in the genome for many generations, but some are caused in current generations by ionizing radiation or mutagenic chemicals.
When populations are large, genetic variation is maintained by selection and mutation, and the general health of the population is not seriously affected when some members die without reproducing or fail to reproduce as well as others. That is the normal situation in the large, natural populations of most species. What happens when the size of the population is reduced, as in an endangered species? Generally, genetic variation is progressively eroded, initially by the loss of rare alleles during the population decline (Denniston, 1977) and later by the reduced effectiveness of selection relative to the chance genetic changes due to the random sampling of alleles in each generation (genetic drift) and the increase in mating between genetically similar relatives (inbreeding) (Frankel and Soulé, 1981; Allendorf and Leary, 1986; Lacy, 1987; Lande, 1988; Simberloff, 1988). The resulting increase in homozygosity leads to increased expression of lethal and semilethal genes that were hidden in the larger population. The situation is well known to animal and plant breeders and to those who deal with small, captive populations of exotic species (see Ralls et al., 1980a). To avoid trouble, breeders must continually strive to keep the stock outbred—to arrange matings between unrelated parents. Indeed, most wild populations have, as part of their inherited breeding systems, many kinds of adaptions that prevent the mating of close relatives; that is, there is a strong tendency for outbreeding, e.g.; sex-biased dispersal in birds and mammals (Greenwood, 1980). Progeny of outbred animals are less likely to manifest the effects of lethal or semilethal genes.
If a population is decreasing in size, generation by generation, its members will be forced by circumstances either to mate with relatives or not to mate at all. That greatly increases the chance of homozygous combinations of lethal or semilethal genes. A common result is a condition called "inbreeding depression"; the term describes the increased difficulty of maintaining genetically healthy animals (Ralls et al., 1980a, b; Ralls and Ballou, 1983; Hedrick and Miller, 1992). It has been dramatically demonstrated in many artificial breeding experiments. For example, using the rapidly breeding, newly domesticated Japanese Quail (Coturnix japonica), Sittmann et al. (1966) compared reproductive performance in purposely
and Miller, 1992). It has been dramatically demonstrated in many artificial breeding experiments. For example, using the rapidly breeding, newly domesticated Japanese Quail (Coturnix japonica), Sittmann et al. (1966) compared reproductive performance in purposely inbred lines and normally outbred lines and determined that successive full-sibling matings led to considerable inbreeding depression in all traits that were considered. Fitness traits—including fertility, egg hatchability, chick survivorship, age at sexual maturity, and total egg production—were affected most. For each 10% increase in inbreeding, fertility declined by 11% (of which 4% resulted from complete male infertility), egg hatchability declined by 7%, chick mortality at 0–5 weeks of age increased by 2–4% and at 5–16 weeks of age by 0.8%, sexual maturity in females was delayed by about 1 day, and total egg production declined by about 1.5 eggs.
Similar, but variable, inbreeding effects have been demonstrated in other avian species, including various strains of poultry (Shoffner, 1948; Lerner, 1954), captive Budgerigars (Melopsittacus undulatus) (Daniell and Murray, 1986), captive Mandarin Ducks (Aix galericulata) (Greenwell et al., 1982), captive Hawaiian Geese (Branta sandvicensis) (Kear and Berger, 1980), captive Pink Pigeons (Columba mayeri) (Jones et al., 1989), and wild Great Tits (Parus major) (Bulmer, 1973; Van Noordwijk and Scharloo, 1981). Inbred captive lines are usually difficult to keep and are prone to extinction, unless outcrossed or progressively purged of deleterious recessive traits (Templeton and Read, 1983). Much of the above can be demonstrated in many birds, mammals, lizards, and insects (Ralls et al., 1980 a, b; Schoenwald-Cox et al., 1983; Ralls et al., 1986; see also Soulé, 1987).
For the 'Alala, the major conclusion arrived at in Chapter 2 is that the number of 'Alala is so low that, unless it is somehow increased very soon, extinction because of accidental loss is virtually certain. Indeed, extinction by chance events is possible even in the unlikely event that the genetic health of the birds is unaffected by the serious population decline of the last 20 or more years.
The wild population of 'Alala is now apparently reduced to fewer than 12 birds (J. Engbring, pers. comm., 1992), which are not known to be outside a single geographical area of less than 20 km2 on the McCandless Ranch in the Kona District. Until 11 wild birds were counted by USFWS personnel in March and April 1992 (J. Engbring, pers. comm., 1992), accurate sighting counts were not been made on the McCandless Ranch since about 1980. The proposed number must be viewed in light of the census records cited by Banko and Banko (1980), which covered the 1960s, with 88 birds, and the 1970s, with 346 birds. The latter number was acknowledged to include repeated observation of the same birds in the transect surveys. Nevertheless, in the 1970s, the probable overall population might have been 10–15 times larger than the present estimates. In the 1970s, the birds were distributed along a 75-km portion of the Kona coast from the Pu'uwa'awa'a Ranch south to Alika Homesteads (see Figure 2.1).
In view of their census and restricted geographic range, the wild 'Alala are very likely to be affected to some degree by inbreeding depression. Such a condition can only be surmised, however, because no reliable data on fertility and viability of eggs and young chicks from the wild are available. Most ornithologists studying the birds since 1970 have avoided visiting nests and disturbing the small number of breeding pairs at this crucial stage of the life cycle. Furthermore, substantial inbreeding is now known to characterize some natural and healthy populations of birds that exhibit sedentary habits and group living (Rowley, 1973).
As indicated earlier, the deleterious effects of inbreeding would probably not be expressed solely in reduced fecundity (cf. Hedrick and Miller, 1992). Inbred birds would be expected also to be inferior in vigor, in resistance to disease, and possibly in pair bonding, nest-building, and courtship. Little is known about mate choice in the 'Alala, but in some other birds reduced choice of mates is known to reduce reproductive success (Kepler, 1978; Burley and Moran, 1979; Klint and Enquist, 1980; Bluhm, 1985; Derrickson and Carpenter, 1987; Yamamoto et al., 1989). It should be recalled that in many instances the census count of animals yields a higher number than what population biologists refer to as the effective population size (Ne), which refers to the number of breeding animals (for a discussion of effective population size see Simberloff, 1988). Data on effective population size of the 'Alala have never been obtained or even estimated. Unfortunately for the conservationist, census counts almost never give the exact size of a population, much less its effective size.
Although not all types of population bottlenecks cause genetic disaster (see Carson, 1990; Hedrick and Miller, 1992), the progressive decline in 'Alala numbers has probably had a serious effect on genetic variation. All the 'Alala, both wild and captive, are derived from small populations that have undergone a succession of population bottlenecks or local extinctions extending over many years. Since about 1970, breeding birds have nested in only a narrow elevational and ecological band along the Kona coast that is at most only 75 km long. This very limited breeding range, at an elevation of about 1,000–1,600 m, has been used by crows only at separated local sites (see Figure 2.1) until recently. Only a single small breeding population, on the McCandless Ranch, appears to survive. This persistent pattern of decline suggests that the historical levels of genetic variation might have been progressively eroded as a result of genetic drift and inbreeding. However, because molecular data on both the historical and current levels of genetic variation in the wild not are available, the corroboration of a loss of genetic diversity in 'Alala is lacking. Furthermore, the decline of wild populations has occurred over a relatively long period, so inbreeding might have progressed at a low enough rate to minimize inbreeding depression (Franklin, 1980; Soulé, 1980; Hedrick and Miller, 1992).
If the captive population, which has been shown to be inbred (see discussion in Chapter 4), is to be improved by outcrossing, specimens from McCandless would have to be used. Also as shown in Chapter 4, the McCandless population is so close to the wild sources of some of the captive birds that it would not be expected to provide more than a minor measure of new genetic variability to the captive population. While augmentation of the captive population with
wild-origin stock may benefit the captive population genetically, such augmentation is clearly warranted on demographic grounds alone. Therefore, capture and use of wild birds or eggs solely for genetic purposes (i.e., separate from a full-scale management plan for all populations of the species) cannot be recommended. It is clear that the preservation of genetic diversity in both the captive and wild populations will require that both populations be increased in size as rapidly as possible, and this is best accomplished by improving the reproductive success of both captive and wild crows (Chapter 6).