Implications of Preliminary Genetic Findings for Grizzly Bear Conservation in the Central Canadian Rockies


Gibeau, Michael L. Implications of preliminary genetic findings for grizzly bear conservation in the Central Canadian Rockies. Eastern Slopes Grizzly Bear Project, University of Calgary, Calgary, AB.
Abstract:
Genetic considerations are one of the foundations of conservation biology. DNA analysis has quickly become the technique most investigators use to explore the genetics of wild populations. I used results from both nuclear and mitochondrial DNA analysis to evaluate the genetic variability of grizzly bears captured during 1994 in the Bow River Watershed, Alberta. Mitochondrial DNA analysis suggests that there is little maternal gene flow between the Eastern Slopes population and the more divergent Northern Continental Divide Ecosystem population. Evidence also suggests a recent genetic bottleneck. Analysis of nuclear DNA using the microsattelite approach has uncovered abundant genetic variation. This variation is probably mostly contributed by the adult male cohort. However, genetic diversity, measured as the level of heterozygosity, is lower than several other northern grizzly bear populations sampled to date. Results are discussed in relation to demographic, environmental, and anthropogenic pressures as they affect population viability.
Eastern Slopes Grizzly Bear Project Report: 1995
It is increasingly important to understand the ecological and evolutionary dynamics of wildlife populations as we approach the close of the twentieth century. Many wildlife populations have been reduced to small fractions of their former abundance during modern times due to anthropogenic pressures such as habitat loss and overexploitation. However, even in the absence of such pressures small populations may be threatened by stochastic (chance or random) events. Factors containing elements of chance that may influence population viability include: demographic factors, environmental variability and genetic uncertainty (Allendorf and Servheen 1986, Lande 1988, Berger 1990, Caughley 1994).
The objectives of this paper are to: review the theory of the genetics of small populations; describe the current techniques for assessing modern molecular genetics; present the current knowledge of grizzly bear genetics in the Central Canadian Rockies; and discuss the relevance of recent findings for conservation of grizzly bears given some of the other factors mentioned above.
Genetic considerations are one of the foundations of conservation biology. In a recent review, Caughley (1994) points out that about half of the conservation biology books published in the 1980’s (Soule and Wilcox 1980, Frankel and Soule 1981, Schonewald- Cox et al. 1983, Soule 1986, 1987, Western and Pearl 1989) were on genetics. Here the theory of minimum viable population, effective population size, population viability analysis and extinction vortices have all been explored. Much of the literature surrounding these topics has focused on implications for captive breeding programs and on the design of reserve systems. The central theme throughout is that we are faced with a population in risk of extinction because its numbers are small and those numbers are capped. This area of research is what Caughley (1994) terms the small-population paradigm that abounds with theory but has yet to be broadly applied in wild populations.
Two key concepts appear to be at the heart of conservation biology’s theoretical genetic literature: 1) inbreeding and inbreeding depression and 2) genetic variability and genetic drift.
Breeding among closely related individuals has long been recognized as detrimental to overall fitness in domesticated stock (Wright 1977, Falconer 1981). Fertility, birth weight, growth rate, survival rate, disease resistance and productivity are all depressed when farm animals are inbred. Similarly, Ralls and Ballou’s (1983) review of inbreeding in captive populations of wild animals documented higher juvenile mortality among inbred animals of 45 species bred in zoos. Overall, inbreeding reduces heterozygosity of the offspring below that of the rest of the population and thus reduces fitness because it reveals harmful genes in homozygotes (Lande 1988, Lacy 1993, Jimenez et al. 1994).
The effects of inbreeding and inbreeding depression have not been well documented in wild populations. The case of the cheetah (Acinonyx jubatus) has been put forward as one of the few examples of inbreeding depression in wild populations (O’Brien 1991). Several recent reviews of the cheetah’s situation reject the notion that inbreeding is the major problem and suggest overexploitation and habitat fragmentation are more immediate threats (Caro and Laurenson 1994, Caughley 1994, Merola 1994). Some of these other factors which I will discuss later in this paper may override the effects of inbreeding in the short term. Lacy (1994) summarizes the effect of inbreeding, stating that while no natural population is known to have gone extinct because of inbreeding depression, there is increasing evidence that inbreeding depression is contributing to the decline, or impeding recovery, of some wild populations.
Genetic variability within wild populations is generally regarded as important in maintaining high levels of fitness and allows for adaptation to a changing environment. In small populations, random fluctuation in gene frequency tends to reduce genetic variation, leading eventually to homozygosity and loss of evolutionary adaptability to environmental change. Small population size sustained for several generations can severely deplete genetic variability (Franklin 1980, Lande 1988). There is good evidence that individuals within a population that has recently lost some of its heterozygosity are less fit on average than individuals of the same species within populations that have not suffered a recent loss of heterozygosity (Caughley 1994, emphasis added). In addition to reducing fitness and reduced ability of populations to adapt to changing environments, Heschel and Paige (1995) suggest that loss of variation increases susceptibility to pests, pathogens and other environmental stresses. Caughley (1994) summarized, stating that both genetic drift and inbreeding reduce heterozygosity. For both, the rate of loss accelerates with declining numbers.
The two concepts merge in small populations where concerns focus on the loss of genetic diversity occurring deterministically through genetic drift. In the short-term, this loss of diversity threatens the fitness of the population through the effects of inbreeding depression. In the long-term, loss of diversity limits the evolutionary potential of a population as adaptation to a changing environment requires genetic variation (D. Paetkau pers. comm.). An important factor in a population’s ability to maintain diversity is how genetically connected it is to neighbouring populations.
DNA analysis has quickly become the technique most investigators use to explore the genetics of wild populations. The field has expanded rapidly in the last ten years and is now routinely applied to conservation efforts. Two distinct approaches have emerged: mitochondrial (mt) DNA and nuclear DNA. Each has its own methods that rely on different analyses and generate different products (see Moritz 1994 for overview of mtDNA as well as Bruford and Wayne 1993 for overview of nuclear DNA).
Mitochondrial DNA is a circular DNA molecule residing in the mitochondrion of eukaryotic cells, separate from the DNA of the chromosomes, which is found in the nucleus (L.Waits pers. comm). Moritz’s (1994) review of mtDNA analysis in conservation distinguishes two classes of applications: (1) gene conservation – the use of genetic information to measure and manage genetic diversity for its own sake; and (2) molecular ecology – genetic analysis as a complement to ecological studies of demography.
Gene conservation analysis can be used to: (i) measure genetic variation within populations, especially those thought to have declined recently; (ii) to identify evolutionary divergent sets of populations; and (iii) to assess conservation value of populations or geographic areas from an evolutionary or phylogenetic perspective. Analysis of molecular ecology is a tool for ecologists to: (i) define the appropriate geographic scale for monitoring and management; (ii) to provide a means for identifying the origin of individuals in migratory species; and (iii) to test for drastic changes in population size and connectedness.
One type of highly informative nuclear DNA marker used to assess genetic variability is microsatellite repeats. Knowledge of microsatellites are only now starting to unfold and may become the markers of choice for molecular population genetics studies in the future (Bruford and Wayne 1993). Microsatellite analysis can be used to: (1) assess relatedness between individuals; (2) detect variation in genetically depauperate populations; and (3) make inter-population comparisons of genetic relatedness (Amos et al. 1993, Hughes and Queller 1993, Paetkau and Strobeck 1994).
I used results from both nuclear and mitochondrial DNA analysis to evaluate the genetic health of grizzly bears captured during 1994 in the Bow River Watershed, Alberta as part of the Eastern Slopes Grizzly Bear Project (Gibeau and Herrero 1995). Analysis of mtDNA was done by Lisette Waits at the University of Utah. Her analysis detected two maternal lineages including 29 bears in one linage and only one bear in a second linage. Further analysis by Waits and Ward (1995) determined the two lineages are not unique to the Eastern Slopes of the Canadian Rockies. Both lineages are included in the five lineages detected from grizzly bears in the Northern Continental Divide ecosystem, and one of the two lineages in the Yellowstone ecosystem. When comparing the Eastern Slopes sample to mtDNA data from five other bear populations sampled to date, only the Kodiak Island population has fewer lineages (Waits and Ward 1995).
These preliminary results suggest that there is little maternal gene flow between the Eastern Slopes population and the more divergent Northern Continental Divide Ecosystem population to the south. In addition, the fact that 29 of 30 samples collected form a single mtDNA lineage suggests a recent genetic bottleneck. The reason for this bottleneck is unknown, although I suspect that human induced change is a likely cause. This may occur when movement patterns are blocked by human structures such as major transportation corridors or through direct mortality when bears come into contact with humans.
Microsatellite DNA analysis was done by David Paetkau at the University of Alberta. He reported the microsattelite approach has uncovered abundant genetic variation that will allow several types of analyses. Eastern Slopes grizzlies were found to have a 1 in 6,300,000 chance of being identical to another random, unrelated grizzly in the population. This variability makes it possible to identify independently obtained samples from the same individual with great confidence (ie. DNA fingerprinting). In addition, first order (parent-offspring) relationships between individuals can be assessed. This approach is most powerful when a mother-offspring relationship is known and one wants to determine if a particular male is the father. In this situation, we expect that 99.4% of random males (males that are not the father) will be excluded in an assessment of paternity involving Eastern Slopes grizzlies (Paetkau unpubl. data).
His analysis also included assessing the level of gene flow occurring between adjacent areas, or measuring the level of diversity an isolated population has been able to maintain. This analysis, measured as the expected level of heterozygosity between different loci, is aided by the availability of comparison data from other populations – data which, in the case of grizzlies, Paetkau has obtained from twelve North American populations based on a sample of nearly seven hundred individuals. Based on preliminary results, we are able to get a reasonably accurate picture of the amount of genetic diversity within the study area. The level of diversity is certainly lower than in several northern populations, but is no lower than is seen in relatively undisturbed habitat at the eastern end of the barren ground distribution (Paetkau unpubl. data).
Results of these analyses appear contradictory at first glance, however further explanation and interpretation are needed. First, the primary mode of inheritance of mtDNA is clonal and maternal via the egg cytoplasm. This mode of inheritance results in a rapid rate of mtDNA differentiation among populations relative to nuclear genes, and allows assessment of matriarchal population history and structure (Cronin 1993). Dispite being only a single genetic marker, mtDNA is more sensitive than nuclear DNA in assessing population bottlenecks because: (1) the effective population size is 1/4 the effective population size for nuclear markers meaning variation will be lost through drift much more rapidly; and (2) male-mediated gene flow would replenish nuclear diversity, but not mtDNA diversity. Hence, an assessment of matriarchal population history through mtDNA results suggests the Eastern Slopes grizzly bear population has gone through a recent genetic bottleneck and may be isolated from other populations. This is significant in that it reaffirms speculations that there may be a loss of connectedness to other populations or in other words the effective population size is very small. Several studies have suggested, however, that a combination of mtDNA and nuclear genes is needed for a more complete description of genetic relationships (Cronin 1993, Moritz 1994).
Results from microsatellite analysis demonstrate that there is still genetic diversity within the Eastern Slopes population although the level of heterozygosity is lower than several other northern grizzly bear populations sampled to date. Allendorf (1983) and Allendorf and Servheen (1986) suggested that adequate gene flow would be maintained with immigration of at least one successfully breeding individual per generation. Given the wide ranging movements of male Eastern Slopes grizzly bears, it is reasonable to assume that the diversity documented by microsatellite analysis is being provided by the male segment of the population. This underscores the need for landscape connectivity and sharpens concerns about linkages across the Bow River Valley and the Crowsnest Pass region.
This recent DNA analysis raises concerns about the genetic history of the grizzly bear population along the Eastern Slopes of the Canadian Rockies although inbreeding depression and genetic diversity may not, in the short term, be the crucial factors in determining this population’s long term viability. There are other factors which override genetic factors in wild populations as mentioned earlier in this paper. These include random factors affecting demography and environmental variability, as well as anthropogenic pressures such as overexploitation and habitat loss.
Balances in the basic demographics (population growth and age structure) within a population are governed by the law of averages. By chance alone, small populations are at more risk of extinction than are large populations because of the effects of skewed sex ratios and/or negative growth rates (Shaffer 1987, Lande 1988, 1993). Environmental variability (food supply, weather, fire) adds another element of uncertainty acting upon an entire population. These random events may generate fluctuations in numbers that become progressively more severe (May 1973). There has been some debate about what factor is more important, however Lande (1993) suggests when the long-run growth of a population is negative, regardless of whether the cause is deterministic or stochastic, the average extinction time scales logarithmically with initial population size. These findings corroborate Berger’s (1990) empirical assessment of bighorn sheep where population size alone was a marker of persistence trajectories.
Overexploitation and habitat loss at the hands of humans also exert pressures upon small populations. Diamond’s (1984, 1989) investigations identified four human induced agents of decline including overkill, habitat destruction and fragmentation, impact of introduced species, and chains of extinction. His analysis indicated that humans are the dominant cause in most post- Pleistocene extinctions. Caughley (1994) summarizes the declining population paradigm by suggesting that a species in trouble ends up not in the habitat most favourable to it, but in the habitat least favourable to the agent of decline.
Grizzly bears in the Central Canadian Rockies are probably being influenced by all the factors outlined in this paper. Preliminary genetic results suggests the population has gone through a genetic bottleneck and currently exhibit little gene flow with populations to the south. However, the population continues to exhibit genetic diversity, presumably through males. These results raise concerns about the connectivity of the population as a whole and certainly questions the long held belief that grizzly bears freely move up and down the length of the Rocky Mountains.
Continued DNA sampling is required to determine what the extent of the affected area is. What effect this situation will have on the population is unknown at this time. However, detrimental genetic factors may not have enough time to manifest themselves before this population could succumb to some other short term threat. Demographic and environmental stochasticity usually pose more immediate and potent dangers than do genetic drift and inbreeding depression (Lande 1988); and external agents of decline are more prevalent and dangerous than either (Caughley 1994). Hence, researchers and managers need to remain focused on reducing immediate threats to this population while remaining cognizant of the need for long term genetic health of grizzly bears.
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