Effects of Population Size on Seed Production and Germinability in an Endangered, Fragmented Grassland Plant

Abstract: Fragmentation and isolation of plant populations are thought to affect demographic processes such as seed production and cause reductions in fitness. I followed seed set over a 3-year period in eight populations of the endangered Rutidosis leptorrhynchoides (Asteraceae) that differed in population size from 13 to over 5000 flowering plants. Germinability of the resultant seed was also examined to determine whether small populations had lower fitness than large populations. Seed set was significantly associated with population size in 2 of the 3 years. Small populations (<30 flowering plants) produced significantly fewer seeds per head in 1994 and 1995 than did large populations (500 to over 5000 flowering plants), which did not differ significantly from one another. There was, however, substantial variation within populations. In 1993 seed production did not follow any simple relationship with population size, possibly because environmental stress from low rainfall had an overriding impact. Differences in seed germinability between populations were largely not evident, suggesting that this aspect of fitness has not declined substantially in small populations relative to large populations. This study suggests that nongenetic, demographic factors are of immediate importance to the persistence of small populations of R. leptorrhynchoides because of their potential impacts on seedling recruitment.     

Genetic Diversity, Population Size, and Fitness in Central and Peripheral Populations of a Rare Plant Lychnis viscaria

Abstract: Genetic diversity is expected to decrease in small and isolated populations as a consequence of bottlenecks, founder effects, inbreeding, and genetic drift. The genetics and ecology of the rare perennial plant Lychnis viscaria (Caryophyllaceae) were studied in both peripheral and central populations within its distribution area. We aimed to investigate the overall level of genetic diversity, its spatial distribution, and possible differences between peripheral and central populations by examining several populations with electrophoresis. Our results showed that the level of genetic diversity varied substantially among populations (  H _exp = 0.000–0.116) and that the total level of genetic diversity (mean H _exp = 0.056) was low compared to that of other species with similar life-history attributes. The peripheral populations of L. viscaria had less genetic variation (mean H _exp = 0.034) than the central ones (0.114). Analysis of genetic structure suggested limited gene flow (mean F _ST = 0.430) and high differentiation among populations, emphasizing the role of genetic drift (  N _e m = 0.33). Isolation was even higher than expected based on the physical distance among populations. We also focused on the association between population size and genetic diversity and possible effects on fitness of these factors. Population size was positively correlated with genetic diversity. Population size and genetic diversity, however, were not associated with fitness components such as germination rate, seedling mass, or seed yield. There were no differences in the measured fitness components between peripheral and central populations. Even though small and peripheral populations had lower levels of genetic variation, they were as viable as larger populations, which emphasizes their potential value for conservation.     

Translating effects of inbreeding depression on component vital rates to overall population growth in endangered bighorn sheep

Evidence of inbreeding depression is commonly detected from the fitness traits of animals, yet its effects on population growth rates of endangered species are rarely assessed. We examined whether inbreeding depression was affecting Sierra Nevada bighorn sheep (Ovis canadensis sierrae), a subspecies listed as endangered under the U.S. Endangered Species Act. Our objectives were to characterize genetic variation in this subspecies; test whether inbreeding depression affects bighorn sheep vital rates (adult survival and female fecundity); evaluate whether inbreeding depression may limit subspecies recovery; and examine the potential for genetic management to increase population growth rates. Genetic variation in 4 populations of Sierra Nevada bighorn sheep was among the lowest reported for any wild bighorn sheep population, and our results suggest that inbreeding depression has reduced adult female fecundity. Despite this population sizes and growth rates predicted from matrix-based projection models demonstrated that inbreeding depression would not substantially inhibit the recovery of Sierra Nevada bighorn sheep populations in the next approximately 8 bighorn sheep generations (48 years). Furthermore, simulations of genetic rescue within the subspecies did not suggest that such activities would appreciably increase population sizes or growth rates during the period we modeled (10 bighorn sheep generations, 60 years). Only simulations that augmented the Mono Basin population with genetic variation from other subspecies, which is not currently a management option, predicted significant increases in population size. Although we recommend that recovery activities should minimize future losses of genetic variation, genetic effects within these endangered populations-either negative (inbreeding depression) or positive (within subspecies genetic rescue)-appear unlikely to dramatically compromise or stimulate short-term conservation efforts. The distinction between detecting the effects of inbreeding depression on a component vital rate (e.g., fecundity) and the effects of inbreeding depression on population growth underscores the importance of quantifying inbreeding costs relative to population dynamics to effectively manage endangered populations.     

Genetic Structure of Fragmented Populations of the Endangered Daisy Rutidosis leptorrhynchoides

Abstract: The endangered grassland daisy Rutidosis leptorrhynchoides has been subject to severe habitat destruction and fragmentation over the past century. Using allozyme markers, we examined the genetic diversity and structure of 16 fragmented populations. The species had high genetic variation compared to other plant species, and both polymorphism and allelic richness showed strong positive relationships with log reproductive population size, reflecting a loss of rare alleles (frequency of q < 0.1) in smaller populations. Fixation coefficients were positively related to size, due either to a lack of rare homozygotes in small populations or to Wahlund effects (owing to spatial genetic structure) in large ones. Neither gene diversity nor heterozygosity was related to population size, and other population parameters such as density, spatial contagion, and isolation had no apparent effect on genetic variation. Genetic divergence among populations was low , despite a large north-to-south break in the species' current distribution. To preserve maximum genetic variation, conservation strategies should aim to maintain the five populations larger than 5000 reproductive plants, all of which occur in the north of the range, as well as the largest southern population of 626 plants at Truganina. Only one of these is currently under formal protection. High heterozygosity in smaller populations suggests that they are unlikely to be suffering from inbreeding depression and so are also valuable for conservation. Erosion of allelic richness at self-incompatibility loci, however, may limit the reproductive capacity of populations numbering less than 20 flowering plants.     

Inbreeding Depression Accumulation across Life‐History Stages of the Endangered Takahe

Abstract: Studies evaluating the impact of inbreeding depression on population viability of threatened species tend to focus on the effects of inbreeding at a single life‐history stage (e.g., juvenile survival). We examined the effects of inbreeding across the full life‐history continuum, from survival up to adulthood, to subsequent reproductive success, and to the recruitment of second‐generation offspring, in wild Takahe ( Porphyrio hochstetteri ) by analyzing pedigree and fitness data collected over 21 breeding seasons. Although the effect size of inbreeding at individual life‐history stages was small, inbreeding depression accumulated across multiple life‐history stages and ultimately reduced long‐term fitness (i.e., successful recruitment of second‐generation offspring). The estimated total lethal equivalents (2B) summed across all life‐history stages were substantial (16.05, 95% CI 0.08–90.8) and equivalent to an 88% reduction in recruitment of second‐generation offspring for closely related pairs (e.g., sib–sib pairings) relative to unrelated pairs (according to the pedigree). A history of small population size in the Takahe could have contributed to partial purging of the genetic load and the low level of inbreeding depression detected at each single life‐history stage. Nevertheless, our results indicate that such “purged” populations can still exhibit substantial inbreeding depression, especially when small but negative fitness effects accumulate across the species’ life history. Because inbreeding depression can ultimately affect population viability of small, isolated populations, our results illustrate the importance of measuring the effects of inbreeding across the full life‐history continuum.     

The immediate costs and long-term benefits of assisted gene flow in large populations

With the genetic health of many plant and animal populations deteriorating due to climate change outpacing adaptation, interventions, such as assisted gene flow (AGF), may provide genetic variation necessary for populations to adapt to climate change. We ran genetic simulations to mimic different AGF scenarios in large populations and measured their outcomes on population-level fitness to determine circumstances in which it is worthwhile to perform AGF. In the absence of inbreeding depression, AGF was beneficial within a few generations only when introduced genotypes had much higher fitness than local individuals and traits affecting fitness were controlled by a few genes of large effect. AGF was harmful over short periods (e.g., first ∼10–20 generations) if there was strong outbreeding depression or introduced deleterious genetic variation. When the adaptive trait was controlled by many loci of small effect, the benefits of AGF took over 10 generations to realize—potentially too long for most climate-related management scenarios. The genomic integrity of the recipient population typically remained intact following AGF; the amount of genetic material from the donor population usually constituted no more of the recipient population's genome than the fraction of the population introduced. Significant genomic turnover (e.g., >50% replacement) only occurred when the selective advantage of the adaptive trait and translocation fraction were extremely high. Our results will be useful when adaptive management is used to maintain the genetic health and productivity of large populations under climate change.     

Unprecedented Low Levels of Genetic Variation and Inbreeding Depression in an Island Population of the Black-Footed Rock-Wallaby

Abstract: It has been argued that demographic and environmental factors will cause small, isolated populations to become extinct before genetic factors have a significant negative impact. Islands provide an ideal opportunity to test this hypothesis because they often support small, isolated populations that are highly vulnerable to extinction. To assess the potential negative impact of isolation and small population size, we compared levels of genetic variation and fitness in island and mainland populations of the black-footed rock-wallaby ( Petrogale lateralis [Marsupialia: Macropodidae]). Our results indicate that the Barrow Island population of P. lateralis has unprecedented low levels of genetic variation (  H _e = 0.053, from 10 microsatellite loci) and suffers from inbreeding depression (reduced female fecundity, skewed sex ratio, increased levels of fluctuating asymmetry). Despite a long period of isolation ( ∼ 1600 generations) and small effective population size (  N _e ∼ 15), demographic and environmental factors have not yet driven this population to extinction. Nevertheless, it has been affected significantly by genetic factors. It has lost most of its genetic variation and become highly inbred (  F _e = 0.91), and it exhibits reduced fitness. Because several other island populations of P. lateralis also exhibit exceptionally low levels of genetic variation, this phenomenon may be widespread. Inbreeding in these populations is at a level associated with high rates of extinction in populations of domestic and laboratory species. Genetic factors cannot then be excluded as contributing to the extinction proneness of small, isolated populations.     

Genetic Variation and Outcrossing Rate in Relation to Population Size in Gentiana pneumonanthe L

The amount of genetic variation in the rare perennial herb Gentiana pneumonanthe L. was determined to explore its relation to population size. Differences in isozyme variation between maternal plants and their offspring were used to investigate the relationship between population size and outcrossing rate. In 25 populations in The Netherlands, differing in size from 1 to more than 50,000 flowering individuals, 16 allozyme loci were analyzed on leaves of maternal plants and offspring grown in a greenhouse. Population size was significantly positively correlated with the proportion of polymorphic loci, but only marginally with heterozygosity and the mean effective number of alleles. Most of the studied populations were characterized by a complete absence of rare alleles, and F -statistics suggest relatively high levels of genetic differentiation among populations and thus a low level of gene flow. Leaf samples (maternal) were mostly in Hardy-Weinberg equilibrium, while several offspring samples showed an excess of homozygotes, which suggests selection favoring heterozygotes. Because most small populations consist only of adult survivors from formerly larger populations, this may partly explain the absence of a clear relationship between genetic variation of the maternal plants and population size. A significant positive correlation was found between the level of cross-fertilization and population size. From these results, we conclude that, to some degree, small populations have a reduced level of genetic variation, while their present isolation in nature reserves has resulted in a very limited interpopulational gene flow level. At present a higher level of inbreeding in small populations contributes to a further loss of genetic variation and may also result in reduced offspring fitness.     

Mutation and Conservation

Mutation can critically affect the viability of small populations by causing inbreeding depression, by maintaining potentially adaptive genetic variation in quantitative characters, and through the erosion of fitness by accumulation of mildly detrimental mutations. I review and integrate recent empirical and theoretical work on spontaneous mutation and its role in population viability and conservation planning. I analyze both the maintenance of potentially adaptive genetic variation in quantitative characters and the role of detrimental mutations in increasing the extinction risk of small populations. Recent experiments indicate that the rate of production of quasineutral, potentially adaptive genetic variance in quantitative characters is an order of magnitude smaller than the total mutational variance because mutations with large phenotypic effects tend to be strongly detrimental. This implies that, to maintain normal adaptive potential in quantitative characters under a balance between mutation and random genetic drift (or among mutation, drift, and stabilizing natural selection), the effective population size should be about 5000 rather than 500 (the Franklin-Soulé number). Recent theoretical results suggest that the risk of extinction due to the fixation of mildly detrimental mutations may be comparable in importance to environmental stochasticity and could substantially decrease the long-term viability of populations with effective sizes as large as a few thousand. These findings suggest that current recovery goals for many threatened and endangered species are inadequate to ensure long-term population viability.     

Risk Assessment of Inbreeding and Outbreeding Depression in a Captive‐Breeding Program

Captive‐breeding programs can be implemented to preserve the genetic diversity of endangered populations such that the controlled release of captive‐bred individuals into the wild may promote recovery. A common difficulty, however, is that programs are founded with limited wild broodstock, and inbreeding can become increasingly difficult to avoid with successive generations in captivity. Program managers must choose between maintaining the genetic purity of populations, at the risk of inbreeding depression, or interbreeding populations, at the risk of outbreeding depression. We evaluate these relative risks in a captive‐breeding program for 3 endangered populations of Atlantic salmon (Salmo salar). In each of 2 years, we released juvenile F₁ and F₂ interpopulation hybrids, backcrosses, as well as inbred and noninbred within‐population crosstypes into 9 wild streams. Juvenile size and survival was quantified in each year. Few crosstype effects were observed, but interestingly, the relative fitness consequences of inbreeding and outbreeding varied from year to year. Temporal variation in environmental quality might have driven some of these annual differences, by exacerbating the importance of maternal effects on juvenile fitness in a year of low environmental quality and by affecting the severity of inbreeding depression differently in different years. Nonetheless, inbreeding was more consistently associated with a negative effect on fitness, whereas the consequences of outbreeding were less predictable. Considering the challenges associated with a sound risk assessment in the wild and given that the effect of inbreeding on fitness is relatively predictable, we suggest that risk can be weighted more strongly in terms of the probable outcome of outbreeding. Factors such as genetic similarities between populations and the number of generations in isolation can sometimes be used to assess outbreeding risk, in lieu of experimentation. Evaluación del Riesgo de Depresión por Endogamia y Exogamia en un Programa de Reproducción en Cautiverio     

Relative Contribution of Inbreeding Depression and Eroded Adaptive Diversity to Extinction Risk in Small Populations of Shore Campion

Abstract:  To study the relative importance of inbreeding depression and the loss of adaptive diversity in determining the extinction risk of small populations, we carried out an experiment in which we crossed and self-fertilized founder plants from a single, large population of shore campion ( Silene littorea Brot.). We used the seeds these plants produced to colonize 18 new locations within the distribution area of the species. The reintroduced populations were of three kinds: inbred and genetically homogeneous, each made up of selfed seed from a single plant; inbred and mixed, made up of a mixture of selfed seeds from all founder plants; and outbred and mixed, made up of a mixture of seeds obtained in outcrosses between the founders. We compared the inbred homogeneous populations with the inbred mixed to measure the effect of genetic diversity among individuals and the inbred mixed with the outbred mixed to measure the effect of inbreeding. Reintroduction success was seriously limited by inbreeding, whereas it was not affected by genetic diversity. This observation and the nonsignificant interaction between family and reintroduction location for individual plant characters suggest that the fixation of overall deleterious genes causing inbreeding depression posed a more serious threat to the short-term survival of the populations than the loss of genes involved in genotype and environment interactions. Thus, reintroduction success was related to adaptive diversity. Preventing such fixation might be the most important consideration in the genetic management and conservation of shore campion populations.     

Effects of Population Size and Food Stress on Fitness-Related Characters in the Scarce Heath, a Rare Butterfly in Western Europe

Abstract: Knowledge about the effects of inbreeding in natural populations is scarce, especially in invertebrates. We analyzed to what extent fitness-related traits in the scarce heath (  Coenonympha hero ), a butterfly, are affected by population size and isolation and whether differences in food quality influence these effects. We categorized nine populations as either large or small and isolated. Full-sib groups of offspring from 27 females were followed under seminatural conditions. Because of increased zygote mortality, egg hatchability was significantly lower in the small and isolated populations than in the large ones. Population category had no effect on larval weight under optimal conditions, but weight was significantly lower in the small-isolated category with low food quality. The effects of inbreeding can thus be hidden when conditions are benign but can appear under stress. Survival also differed significantly between population categories, and larval developmental time tended to be longer in the small-isolated category, irrespective of food conditions. We suggest that the differences in fitness between offspring from large and small isolated populations are at least partly due to inbreeding. This adds a further threat to a species that is already suffering from decreasing population sizes and increasing isolation among populations.     

Population Persistence and Offspring Fitness in the Rare Bellflower Campanula cervicaria in Relation to Population Size and Habitat Quality

Abstract: Data from several animal species and a few plant species indicate that small populations face an elevated risk of extinction. Plants are still underrepresented in these studies concerning the relation between population size and persistence. We studied the effect of population size on persistence among natural populations of the rare bellflower Campanula cervicaria in Finland. We monitored 52 bellflower populations for 8 years and found that the mean population size decreased from 24 to 14 during this period. Small populations with ≤5 individuals were more prone to losing all fertile plants than were larger ones. Reduction in population size was nevertheless unrelated to the degree of population isolation, measured as the distance to the nearest known population. To test the hypothesis that offspring fitness is lower in small populations, we germinated bellflower seeds from different-sized populations in a laboratory and found that seed germination ability was independent of population size. The seedlings raised from seeds of small populations grew faster than those taken from larger populations. Population size was negatively related to the amount of shade in the habitats. In conclusion, decreasing population sizes of C. cervicaria seemed not to be caused by lowered germination ability or growth rate in small populations; rather, population size reductions appeared to be due to closing of vegetation in the habitats.     

Estimates of outcrossing rates and of inbreeding depression in a population of the marine angiosperm Zostera marina

Despite the great diversity of pollination and fertilization mechanisms observed in marine plants, little is known about the causes or maintenance of this variation. In this study, I estimated outcrossing rates and levels of inbreeding depression in Zostera marina L. (eelgrass), providing the first empirical test of hypotheses about the evolution of breeding systems in plants with submerged flowers. This study also addressed temporal separation of female and male flowering (dichogamy) in eelgrass as a mechanism promoting an outcrossing mating system, and whether the mating system in eelgrass is related to the degree of dichogamy in the field. Outcrossing rates (0<t<1) estimated from two polymorphic allozyme loci indicate that the Z. marina population in intertidal and subtidal habitats in False Bay, Washington, USA, was highly outcrossing in both 1991 (intertidal t=0.905, subtidal t=1.0) and 1992 (intertidal t=0.775, subtidal t=0.611). The outcrossing rates were positively associated with the degree of dichogamy in 1992; intertidal plants exhibited a greater temporal separation of female and male flowering and a higher outcrossing rate than did subtidal plants. Inbreeding depression at seed set was estimated from hand pollinations (self- and outcross) on 20 reproductive individuals from the False Bay population. Averaged across all maternal parents, a greater proportion of outcrossed flowers set seed than selfed flowers; i.e., inbreeding depression was detected. Plants exhibited genetic variation for inbreeding depression, detected as a significant pollination treatment × maternal family interaction in a log-likelihood analysis. By the end of the seed-maturation period (7 mo after intial seed set) some families showed outbreeding depression, i.e., greater fitness in progeny derived from selfing than in progeny from outcrossing. The inbreeding depression in the False Bay population may be an important selective factor contributing to the maintenance of dichogamy and an outcrossing mating system, as proposed for aquatic plants.     

Relationship between Population Size and Fitness

Abstract:  Long-term effective population size, which determines rates of inbreeding, is correlated with population fitness. Fitness, in turn, influences population persistence. I synthesized data from the literature concerning the effects of population size on population fitness in natural populations of plants to determine how large populations must be to maintain levels of fitness that will provide adequate protection against environmental perturbations that can cause extinction. Integral to this comment on what has been done and what needs to be done, sThe evidence suggests that there is a linear relationship between log population size and population fitness over the range of population sizes examined. More importantly, populations will have to be maintained at sizes of >2000 individuals to maintain population fitness at levels compatible with the conservation goal of long-term persistence. This approach to estimating minimum viable population size provides estimates that are in general agreement with those from numerous other studies and strengthens the argument that conservation efforts should ultimately aim at maintaining populations of several thousand individuals to ensure long-term persistence.     

Experimental Evidence for Beneficial Fitness Effects of Gene Flow in Recently Isolated Populations

Abstract: A rich theory has been developed to explain the evolution of populations at equilibrium conditions of gene flow, inbreeding, and selection. There are, however, few empirical examples of the effects of gene flow into recently isolated, small populations under nonequilibrium conditions, such as are expected following population fragmentation. We studied the effects of inbreeding and gene flow in small, experimental populations of the mustard Brassica campestris ( rapa ). Replicate populations of five individuals randomly mated in a growth room received treatments of 0, 1, or 2.5 migrants each generation. Plants from the sixth experimental generation were planted in an outdoor common garden to evaluate the effects of the treatments on fitness and the distribution of phenotypic variation. Regression of six fitness components on inbreeding coefficients indicated a negative effect of inbreeding on fitness for five of these components. The 0-migrant treatment had significantly lower fitness than the migrant treatments for four of six fitness components, but fitness did not differ between the 1-migrant and 2.5-migrant treatments. Phenotypic divergence among populations decreased with an increased number of migrants. These data provide empirical evidence of the beneficial fitness effects of a small number of migrants for recently fragmented populations.     

Inbreeding and competitive ability in the common shrew (<Emphasis Type="Italic">Sorex araneus</Emphasis>)

Common shrews (Sorex araneus) maintain a foraging territory for most of their immature life. Possessing a high-quality territory is vital for overwinter survival in the harsh boreal climate, and hence, competitive ability in territorial disputes is expected to be an important component of individual fitness. To test possible association between individual inbreeding and fitness, we used neutral arena trials to assess the competitive performance of young common shrews. The experiment involved pairs of individuals originating from small island populations, where breeding must often occur between related individuals, and from large outbred mainland populations. The percentage of neutral arena tests that an individual won was highly significantly explained by internal relatedness, a surrogate measure of individual inbreeding, measured using ten microsatellite markers. Body size, sex, learning, and population type (mainland vs island) made no significant contributions. Even a low level of individual inbreeding may lead to significant adverse consequences in multiple territorial contests, which may represent a significant cause of inbreeding depression in many wild vertebrate populations.     

An experimental test of alternative population augmentation scenarios

Human land use is fragmenting habitats worldwide and inhibiting dispersal among previously connected populations of organisms, often leading to inbreeding depression and reduced evolutionary potential in the face of rapid environmental change. To combat this augmentation of isolated populations with immigrants is sometimes used to facilitate demographic and genetic rescue. Augmentation with immigrants that are genetically and adaptively similar to the target population effectively increases population fitness, but if immigrants are very genetically or adaptively divergent, augmentation can lead to outbreeding depression. Despite well‐cited guidelines for the best practice selection of immigrant sources, often only highly divergent populations remain, and experimental tests of these riskier augmentation scenarios are essentially nonexistent. We conducted a mesocosm experiment with Trinidadian guppies (Poecilia reticulata) to test the multigenerational demographic and genetic effects of augmenting 2 target populations with 3 types of divergent immigrants. We found no evidence of demographic rescue, but we did observe genetic rescue in one population. Divergent immigrant treatments tended to maintain greater genetic diversity, abundance, and hybrid fitness than controls that received immigrants from the source used to seed the mesocosms. In the second population, divergent immigrants had a slightly negative effect in one treatment, and the benefits of augmentation were less apparent overall, likely because this population started with higher genetic diversity and a lower reproductive rate that limited genetic admixture. Our results add to a growing consensus that gene flow can increase population fitness even when immigrants are more highly divergent and may help reduce uncertainty about the use of augmentation in conservation.     

The Paradox of Forest Fragmentation Genetics

Abstract:  Theory predicts widespread loss of genetic diversity from drift and inbreeding in trees subjected to habitat fragmentation, yet empirical support of this theory is scarce. We argue that population genetics theory may be misapplied in light of ecological realities that, when recognized, require scrutiny of underlying evolutionary assumptions. One ecological reality is that fragment boundaries often do not represent boundaries for mating populations of trees that benefit from long-distance pollination, sometimes abetted by long-distance seed dispersal. Where fragments do not delineate populations, genetic theory of small populations does not apply. Even in spatially isolated populations, where genetic theory may eventually apply, evolutionary arguments assume that samples from fragmented populations represent trees that have had sufficient time to experience drift, inbreeding, and ultimately inbreeding depression, an unwarranted assumption where stands in fragments are living relicts of largely unrelated predisturbance populations. Genetic degradation may not be as important as ecological degradation for many decades following habitat fragmentation.