Conservation Biology Case Studies
text pp. 420-426,169, 432-441, 380-381,433, 553, 649, 83, 89, 288.
Conservation efforts have classically been aimed at species rather than habitats, communities or ecosystems. A number of studies may be identified which document successful recovery of species from critically endangered status (e.g. peregrine falcon, whooping crane), while others have proven either less successful (black-footed ferret) or unsuccessful (passenger pigeon, Pacific Island tree snail). Among the most controversial species currently embroiled in conservation programs are the African cheetah (Acinonyx jubatus) and northern spotted Owl (Strix occidentalis caurina). We will review three cases (black-footed ferret, cheetah, northern spotted owl) because they provide interesting insights regarding conservation problems or because they involve major controversies.
1) Black-footed ferret (Mustela nigripes): see www site listed on course homepage. Meffe & Carroll (1997) pp. 83, 420-426.
This species is a small member of the weasel family that formerly occupied plains and prairie habitat from Saskatchewan to Texas. The species was listed as threatened in the USA in 1967 and endangered in 1973. An initial recovery plan was devised by the US Fish and Wildlife Service in 1978. By then, however, there were no known wild black-footed ferrets. The species was re-discovered at a single site in Wyoming in 1981. A Species Survival Plan (SSP) was developed based on captive propagation of ferrets to eventually re-release ferrets into the wild.
Decline of the species coincided with, and may have been caused by, the tremendous decline (90-95%) in prairie dog abundance earlier this century. Prairie dogs are the primary (90%) food of black footed ferrets. Prairie dogs were targeted as pests because their burrows damaged farm implements and tractors. In addition, the prairie dogs appear to have suffered from introduction of Sylvatic plague. A severe decline in prairie dog abundance was evident by 1985, when ferrets were collected to begin a captive propagation program. The ferret's behaviour and ecology had been studied in the Wyoming population from 1981 to 1985.
Ferrets also suffered from habitat fragmentation as the plains were developed for agricultural and other purposes. By 1985, the species was limited to ~10 individuals in two populations (South Dakota and Wyoming). One population (from South Dakota) was placed in a captive breeding program without success (Was this good conservation biology?). The other population was surveyed but not captured; this population increased to 128 animals by 1984. This population soon suffered, however, after plague was discovered among its prairie dog prey. Ferret numbers appeared to decline sharply in 1985 despite treatment of 80,000 prairie dog burrows to control plague. Six ferrets were captured for a captive breeding program but all died of canine distemper. Additional collections of 6 and 12 animals were made for captive breeding programs. The last free-ranging individuals died from distemper, resting the fate of the species in the last 12 individuals in the captive breeding program. Goals were quickly set to maintain as much (90%) genetic diversity as possible for a minimum of 50 years. Two litters of kits were born in 1987, and by 1988 the first two isolated captive populations were initiated (to minimize catastrophic extinction of the species).
Extensive research on ferret reproductive biology, immunology and behaviour was conducted to maximize offspring production and survival, and to prepare the animals for eventual release in the wild. By the early 1990s the success of the captive breeding program was evident in the birth annually of >100 kits. Ferrets were reintroduced to southern Wyoming in 1991 with the permission and cooperation of landowners (228 ferrets over the period 1991-4). Some legal battles pitted ranchers' organizations and some environmental groups against the Fish and Wildlife Service and its reintroduction plan. (A similar problem arose with Florida panther captive breeding programs). Forty-nine young individuals were given a soft-release (food provided at cage site after the animals were exposed to the area [but still confined] for 10 days). Some of these individuals (12%) survived the winter and reproduced successfully. Ninety additional ferrets were released in 1992. Coyotes appeared to be the primary predator and source of mortality, though survival was moderate (20-25% for 30 days).
The ferret is still in jeopardy owing to distemper virus, plague and low population numbers (genetic bottleneck). However, the population is growing and introductions of ferrets to a site in South Dakota (90 ferrets) and one in southern Montana (78 ferrets) in 1994-5 occurred. Care is exercised in selecting sites to minimize the possibility of contact with plague (prairie dog prey) or canine distemper.
Local responses to ferret re-introduction are also significant. The re-introduction sites are a first example of the problem imposed by political opposition. The re-introduced ferrets are (in the language of the U.S. Endangered Species legislation) a nonessential, experimental population. Under this designation, the animals are protected at the re-introduction site, but are left unprotected should they move into a farmer's field or a rancher's pastureland.
2) African cheetah (see www site listed on course homepage)
In 1983, O'Brien and colleagues reported that cheetahs had remarkably little genetic variation. The species is limited principally to regions in sub-Saharan Africa, though a small population remains in Iran as well. The population is estimated to have declined by 50% in abundance (to ~10,000 to 20,000) by the mid-1970s from the previous decade, largely as a result of habitat destruction and hunting by humans. The population has continued to decline though accurate estimates are not available of current population size. O'Brien et al. [1983, 1985] speculate the total population is between 1500 and 25000 individuals).
In a study that commenced the controversy over reasons for endangerment of the cheetah, O'Brien and colleagues (1983) reported that 55 captive and wild-caught cheetahs derived from two separate populations were monomorphic at all 47 allozymes surveyed. They compared their results with those for 43 fruit fly (Drosophila) species, 2 mouse (Mus) populations, 1 cat (Felis) population, and a large number of human (Homo) populations. The cheetah had the lowest frequency of polymorphic loci (0.0) and lowest average heterozygosity (0.0). Overall, the cheetah had between 10 and 100 times less genetic variability than other mammals.
O'Brien attributed the patterns in cheetah to a severe population bottleneck followed by inbreeding. The bottleneck would reduce genetic diversity as a result of selection pressures and genetic drift. They attribute the bottleneck to decimation of the population by legal and illegal hunting by African cattle farmers about 100 cheetah generations ago. They suggested this genetic pattern would also be consistent with a bottleneck 100 generations ago coupled with a low population growth rate (which would allow drift to reduce diversity). They suggest that current cheetah distribution is but a remnant of its once global distribution (Africa, Asia, Europe, North America), and that a low sperm count and abnormal sperm is evidence of a bottleneck and inbreeding.
O'Brien et al. (1985) also reported significant mortality of cheetahs in captive breeding programs in zoos. Non-inbred cheetah mating had among the highest infant mortality rates of all mammals surveyed. The same pattern was observed for inbred matings of cheetahs. As well, infant mortality rates for inbred and non-inbred cheetah mating did not differ significantly, suggesting that inbreeding has no pronounced effect today (largely because strong effects were evident earlier).
They also added 5 additional enzyme surveys to the previously studies 55 individuals, with the same results. As well, 7 different skin grafts were performed on non-inbred pairs of cheetahs (14 individuals) to determine whether different cheetahs could serve as skin donors. Successful grafts depend on acceptance of 'donor' tissue by the 'recipient' individual, which in turn is governed by a group of genes called the 'major histocompatability complex' (MHC). In all vertebrate, the MHC is the most polymorphic region of the genome, thus it should be most useful in differentiating genetically different individuals. All of the grafts succeeded through the typical stage of rejection, though control grafts of house cat tissue were rejected; thus, the cheetahs were incapable of immunologically identifying other cheetah's tissues, though they did recognize cat tissue. This suggested strong genetic relatedness of the cheetahs. As well, an infectious feline virus wiped out a cheetah colony in captivity in Oregon; O'Brien attributed the widespread success of the virus to genetic uniformity of the cheetahs.
In a third study O'Brien and colleagues (1987) again visited the cheetah issue to look at frequency of enzyme polymorphisms and heterozygosity levels in subspecies of the cheetah, the south African form A. jubatus jubatus and its east African relative A. jubatus raineyi. Again they found very low levels of polymorphism (2 - 4%) and average heterozygosity (0.0004 - 0.014) in both groups; as well, the genetic distance between subspecies was minimal (0.004) indicating that the cheetah became genetically impoverished before the subspecies diverged. They again stated that these genetic patterns are most consistent with 2 bottlenecks (one 10,000 years ago and another during the past century) followed by inbreeding.
Menotti-Raymond and O'Brien (1993) used two DNA analysis techniques - hypervariable minisatellite loci and mitochondrial loci - to time the bottleneck in the cheetah population. Genetic variation was observed with these techniques. Based on expected mutation rates and current levels of diversity, they back calculated the bottleneck to between 3500 - 12,700 years and 28,000 - 36,000 years, respectively, for mitochondria and minisatellite techniques. The latter variable is likely an overestimate of the true time, though both techniques place a bottleneck during the late Pleistocene (when many extinctions occurred). These techniques also identified only 1 to 10% of DNA diversity found among other out-crossed cat species.
More recently, however, fireworks erupted regarding the cheetah's genetic and demographic status. The lead argument was fired by Merola (1994) who compared the cheetah's genetic variability with that of other carnivorous vertebrates. She suggested that of 24 terrestrial carnivores surveyed, 8 had no heterozygosity (H = 0), while the remaining ones averaged H = 0.042 (vs. H = 0.014 for the cheetah).
She also stated that the lack of breeding success and high infant mortality rates were due to poor captive breeding program procedures, and that the feline virus that decimated the Oregon cheetahs was effective because the cheetahs were held at very high density. Cheetahs are solitary creatures in the wild and would therefore rarely encounter densities like the Oregon situation.
She argued that as long as recessive alleles (deleterious) were slowly purged from the population, the resulting population could be relatively homozygous but without inbreeding effects. The inbreeding effects observed in cheetahs would thus be an artifact of the artificial captive breeding environment.
Merola acknowledged that the cheetah is suffering, but it is from a loss of habitat and other adverse human effects. For example, habitat destruction has resulted in population densities of one cheetah per 6 km2 rather than the old rate of 1 per 100 km2. High densities facilitate transmission and spread of disease and 'focusing' of cheetah predators in the small reserves. Cheetah cub death rate is 93% in the Serengeti reserves, of which 73% was attributed to predation by lions and spotted hyenas (Laurenson et al. 1995). It seemed as though reserves promoted predation because of the 'focusing' effect of predators on remaining 'islands' of intact cheetah and predator habitat. Cheetahs are probably also suffering due to food reductions associated with destruction of natural habitat used by the cheetah's ungulate prey, and to hunting.
O'Brien (1994) fired back in the same issue of Conservation Biology that Merola's comparisons used only allozyme data for a relatively small number of loci (< 22), while the cheetah allozymes data consisted of 52 loci. Thus, the cheetah studies used more loci but still found less diversity. In addition, other more useful modes of genetic testing (described above) yielded the same results - that the cheetah is genetically depauperate relative to other mammals, terrestrial vertebrates and other cats. He also suggested that the high natural mortality rate of cheetah cubs observed in Africa may have resulted from discovery of dens (thus multiple killings of cubs rather than independent killings) and to predators learning of den locations from researcher movements. O'Brien does not preclude environmental problems as one of the issues confronting survival of the cheetah, though he maintained that the larger problem was genetically-based.
Laurenson at al. (1995) flatly rejected O'Brien's notion that they led lions and other predators to cheetah dens because they took precautions to prevent this possibility. They also rejected the idea that predation rates were inflated by predators killing all offspring in a den, because predation was a major mortality factor for all dens, not just specific ones. They argued that mortality of cheetah cubs in captivity was attributable to genetic factors (<4%) and animal husbandry (78%). Thus, in the wild they suffer from predation, while in captivity they suffer from poor breeding practices.
Robert May (1995) reviewed the two sides and felt that there was evidence for genetic problems, but that environmental effects were also very important. The issue is unlikely to be resolved soon because of the entrenched views each side has adopted.
See Abstracts of:
The most recent addition to the debate was the contribution of demographic modeling contributed by Crooks et al. (1998). Using published demographic data from the Serengeti. They found that the importance of elevated cub mortality was relatively minor relative to the large effects from variation in adult survivorship. This finding makes sense from a 'demographic' perspective since the adults have high reproductive value and cubs low value. Further, they argue that focusing too much attention on reducing cub mortality could be counteracted by small increases in adult mortality. They summarize by stating that the genetics vs. ecology debate is not helpful since both factors affect cubs and/or adults.
3) Northern Spotted Owl (S. occidentalis caurina): Meffe & Carroll pp. 380-381,433, 553, 649, 83, 89, 288.
Northern Spotted Owls occur in the southwest region of British Columbia and in Oregon and Washington. In all instances, the owl is rare (low abundance) even in the best of habitats. In southwestern B.C., the owl was found at 14 sites, with a total population of as few as 100 individuals (Dunbar et al. 1991). They attributed its rarity to habitat destruction (logging, fires, development) and to Barred Owls which live in the same old-growth habitat and which respond aggressively to spotted owl calls (thus potentially limiting its habitat availability).
In the USA, the northern spotted owl has pitted environmentalists against loggers to the point where decisions regarding the fate of public lands have gone all the way to the Supreme Court. (As an interesting aside, the case was resolved during summer 1995 by the conservative-leaning Supreme Court in favour of preservation of essential lands for owl habitat). The reason why this is such a contentious issue is that the bird is heavily dependent on old-growth forests for nesting habitat, the same forest currently under assault in the western USA and Canada. Because any decision made by the U.S. Fish and Wildlife Service regarding preservation of old-growth forests would be targeted legally by logging interests, the ecologists involved knew that their science would have to stand up.
Tracts of forest lands suitable to spotted owls in the western USA have declined at an alarming rate.
Bart and Forsman (1992) and Bart (1995) looked at spotted owl density and breeding success in habitats of differing quality in Washington and Oregon. In sum, the higher the percentage of old growth forest (good habitat), the higher the owls/km2, breeding pairs/km2, young fledged/km2, young fledged/km2, and adult survival.
In what is perhaps the most comprehensive study of the northern spotted owl, Murphy and Noon (1992) formulated a number of important, testable hypotheses regarding the owl. All hypotheses listed below are null models.
1) Is the owl population growing (is lambda [finite rate of growth] >1? Answer: Rejected. In both populations studied (later confirmed with other populations), lambda was significantly lower than 1 (replacement rate). Thus, the population is declining.
2) Owls do not differentiate among forests of different ages or structures. Answer: Rejected. The owls prefer habitats based with old-growth forest in disproportion to the abundance of this habitat type in nature.
3) Habitat type selected by the owls has not changed in aerial abundance. Answer: Rejected. See figure above. 70% of owl habitat is currently on federal lands, and this habitat is declining.
Based on these results, they wanted to devise a protection scheme to stabilize the population by devising a Habitat Conservation Area (HCA) that would, at a minimum, permit the owl to persist for at least 100 years. They thus constructed another set of testable hypotheses:
4) The probability of persistence is not related to the extent of its geographic distribution. Answer: Rejected. Spreading the risk of environmental catastrophes or disease outbreaks by having an extensive distribution reduced the chance of species extinctions. The idea here is that extinction is less likely to occur to species that occupy a large portion of their historic range than a narrow portion.
5) No relationship exists between HCA size and its owl carrying capacity. Answer: Rejected. Owl abundance was positively correlated with forest size, though forests of equal size in different regions supported differing numbers of owls.
6) No relationship exists between HCA size (or carrying capacity) and population stability (likelihood of population persistence). Answer: For British Island birds, persistence time is positively correlated with island size.
7) No relationship exists between habitat fragmentation and persistence likelihood of species using that habitat landscape. Answer: Rejected. Fragmentation results in large edge effects and reduced carrying capacity. Intact forest best promoted growth of the spotted owl population.
8) Distance between habitat patches has no bearing on dispersal success of juvenile owls (i.e. no rescue effect). Answer: Rejected. There is a very strong relationship.
9) Distance between HCAs has no bearing on persistence likelihood. Answer: Rejected. Closer spaced HCAs will foster metapopulation exchanges and thus enhance survival likelihood.
10) No relationship exists between HCA size or shape and carrying capacity. Answer: Rejected. A pronounced negative edge effect was evident.
Based on this exercise of hypothesis testing and available public lands which could serve as HCAs for spotted owls, the Spotted Owl working group devised a patchwork of forests in the western USA to attempt to save the owl (for 100 years).
Bart, J. 1995. Amount of suitable habitat and viability of northern spotted owls. Conservation Biology 9:943-946.
Bart, J. and E.D. Forsman. 1992. Dependence of northern spotted owls (Strix occidentalis caurina), on old-growth forests in the western USA. Biological Conservation 62:95-100.
Crooks, K.R., M.A. Sanjayan, D.F. Doaks. 1998. New insights on cheetah conservation through demographic modeling. Conservation Biology 12: 889-895.
Dunbar, D.L. et al. 1991. Status of Spotted Owl, Strix occidentalis, and Barred Owl, Strix varia, in southwestern British Columbia. Canadian Field Naturalist 105:464-468.
Laurenson, M.K., N. Wielebnowski and T.M. Caro. 1995. Extrinsic factors and juvenile mortality in cheetahs. Conservation Biology 9:1329-1331.
May, R.M. 1995. The cheetah controversy. Nature 374:309-310.
Meffe, G.K. and C.R. Carroll. 1997. Principles of Conservation Biology. Sinauer, Sunderland, MA.
Menotti-Raymond, M. and S.J. O'Brien. 1993. Dating of the genetic bottleneck of the African cheetah. Proceedings of the National Academy of Science 90:3172-3176.
Merola, M. 1994. A reassessment of homozygosity and the case for inbreeding depression in the cheetah, Acinonyx jubatus: implications for conservation. Conservation Biology 8:961-971.
Murphy, D.D. and B.R. Noon. 1992. Integrating scientific methods with habitat conservation planning: reserve design for northern spotted owls. Ecological Applications 2:3-17.
O'Brien, S.J., D. Wildt, D. Goldman, C. Merril and M. Bush. 1983. The cheetah is depauperate in genetic variation. Science 221:469-462.
O'Brien, S.J. and 9 others. 1985. Genetic basis for species vulnerability in the cheetah. Science 227:1428-1434.
O'Brien, S.J. and 7 others. 1987. East African cheetahs: evidence for two population bottlenecks? Proceedings of the National Academy of Science 84:508-511.
O'Brien, S.J. 1994. The cheetah's conservation controversy. Conservation Biology 8:1153-1155.