Abstracts from Selected Publications

Bradshaw, H. D., Jr., K. G. Otto, B. E. Frewen, J. K. McKay and D. W. Schemske. 1998. Quantitative trait loci affecting differences in floral morphology between two species of monkeyflower (Mimulus). Genetics 149:367-382.

Conspicuous differences in floral morphology are partly responsible for reproductive isolation between two sympatric species of monkeyflower because of their effect on visitation of the flowers by different pollinators. Mimulus lewisii flowers are visited primarily by bumblebees, whereas M. cardinalis flowers are visited mostly by hummingbirds. The genetic control of 12 morphological differences between the flowers of M. lewisii and M. cardinalis was explored in a large linkage mapping population of F2 plants (n = 465) to provide an accurate estimate of the number and magnitude of effect of quantitative trait loci (QTLs) governing each character. Between one and six QTLs were identified for each trait. Most (9/12) traits appear to be controlled in part by at least one major QTL explaining ³25% of the total phenotypic variance. This implies that either single genes of individually large effect or linked clusters of genes with a large cumulative effect can play a role in the evolution of reproductive isolation and speciation.

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Schemske, D. W. and H. D. Bradshaw, Jr. 1999. Pollinator preference and the evolution of floral traits in monkeyflowers (Mimulus). Proceedings of the National Academy of Sciences 96:11910-11915.

A paradigm of evolutionary biology is that adaptation and reproductive isolation are caused by a nearly infinite number of mutations of individually small effect. Here, we test this hypothesis by investigating the genetic basis of pollinator discrimination in two closely related species of monkeyflowers that differ in their major pollinators. This system provides a unique opportunity to investigate the genetic architecture of adaptation and speciation because floral traits that confer pollinator specificity also contribute to premating reproductive isolation. We asked: (i) What floral traits cause pollinator discrimination among plant species? and (ii) What is the genetic basis of these traits? We examined these questions by using data obtained from a large-scale field experiment where genetic markers were employed to determine the genetic basis of pollinator visitation. Observations of F2 hybrids produced by crossing bee-pollinated Mimulus lewisii with hummingbird-pollinated Mimulus cardinalis revealed that bees preferred large flowers low in anthocyanin and carotenoid pigments, whereas hummingbirds favored nectar-rich flowers high in anthocyanins. An allele that increases petal carotenoid concentration reduced bee visitation by 80%, whereas an allele that increases nectar production doubled hummingbird visitation. These results suggest that genes of large effect on pollinator preference have contributed to floral evolution and premating reproductive isolation in these monkeyflowers. This work contributes to growing evidence that adaptation and reproductive isolation may often involve major genes.

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Schemske, D. W. and P. Bierzychudek. Evolution of flower color in the desert annual Linanthus parryae: Wright revisited. (Evolution, in review)

Linanthus parryae, a diminutive desert annual with white or blue flowers, has been the focus of a long-standing debate among evolutionary biologists. At issue is whether the flower color polymorphism in this species is the product of random genetic drift, as Sewall Wright argued, or of natural selection, as proposed by Carl Epling and his colleagues. Our long-term studies of three polymorphic populations in the Mojave Desert demonstrate that flower color is subject to selection that varies in both time and space in its direction and magnitude. For all sites taken together, blue-flowered plants produced more seeds than white-flowered plants in years of relatively low seed production, while white-flowered plants had higher fitness in years of high seed production. Evidence of selection on flower color was found in two of the three study sites. Differences in fitness between the color morphs were sometimes large, with selection coefficients as high as 0.60 in some years. Our longest period of observations was at Pearblossom site 1, where plants reached appreciable densities in seven of the eleven years of study. Here we found significant differences in the seed production of the color morphs in six years, with three years of blue advantage and three years of white advantage. For all sites taken together, total spring precipitation (March and April) was positively correlated with both absolute and relative seed production of the color morphs. At Pearblossom site 1, blue-flowered plants typically had a fitness advantage in years of low spring precipitation, while white-flowered plants had a fitness advantage in years of high spring precipitation. This temporal variation in selection may contribute to the maintenance of the flower-color polymorphism at Pearblossom site 1, while gene flow from neighboring populations is proposed as the principal factor maintaining the polymorphism at the other study sites. We found no significant differences between the color morphs in pollinator visitation rate or in their carbon isotope ratio, a measure of water-use efficiency. Although the mechanism of selection remains elusive, our results refute Wright's conclusion that the flower color polymorphism in Linanthus parryae is an example of isolation by distance, a key component of his shifting balance theory of evolution.

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Turelli, M., D. W. Schemske and P. Bierzychudek. Stable two-allele polymorphisms maintained by fluctuating fitnesses and seed banks: Protecting the blues in Linanthus parryae. (Evolution, in press)

Motivated by data demonstrating fluctuating relative and absolute fitnesses for white- vs. blue-flowered morphs of the desert annual Linanthus parryae, we present conditions under which temporally fluctuating selection and fluctuating contributions to a persistent seed bank will maintain a stable single-locus polymorphism. In L. parryae, blue flower color is determined by a single dominant allele. To disentangle the underlying diversity-maintaining mechanism from the mathematical complications associated with departures from Hardy-Weinberg genotype frequencies and dominance, we successively analyze a haploid model, a diploid model with three distinguishable genotypes, and a diploid model with complete dominance. For each model, we present conditions for the maintenance of a stable polymorphism, then use a diffusion approximation to describe the long-term fluctuations associated with these polymorphisms. Our protected polymorphism analyses show that a genotype whose arithmetic and geometric mean relative fitnesses are both less than one can persist if its relative fitness exceeds one in years that produce the most offspring. This condition is met by data from a population of L. parryae whose white morph has higher fitness (seed set) only in years of relatively heavy rainfall. The data suggest that the observed polymorphism may be explained by fluctuating selection. However, the yearly variation in flower color frequencies cannot be fully explained by our simple models, which ignore age structure and possible selection in the seed bank. We address two additional questions—one mathematical, the other biological—concerning the applicability of diffusion approximations to intense selection and the applicability of “long term” predictions to data sets spanning decades for populations with long-lived seed banks.

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Schemske, D. W. and C. C. Horvitz. 1984. Variation among floral visitors in pollination ability: A precondition for mutualism specialization. Science 225:519-521.

The unusual floral biology of a neotropical herb provided an opportunity to determine that floral visitors varied significantly in their ability to effect fruit-set. Pollination efficiency and visitation frequency varied among Hymenoptera (five taxa), which were responsible for 99 percent of all fruits set. Lepidoptera (four taxa) were common visitors but poor pollinators. These results indicate that flower visitors vary in their beneficial effects on plants, fulfilling one of the primary conditions required for the specialization of plants on pollinators.

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Schemske, D. W. and C. Horvitz. 1988. Plant animal interactions and fruit production in a neotropical herb: a path analysis. Ecology 69:1128- 1137.

We used path analysis to estimate the direct and indirect effects of antguards, an ant-tended herbivore, and pollinators, on flower and fruit production in Calathea ovandensis, a neotropical herb. The natural variation in these plant-animal interactions was quantified for individual plants in Veracruz, Mexico in 1984 and 1985. We conclude that variation in plant-animal interactions has significant, and complex, effects on flower and fruit production in this species. Antguards had a positive direct effect on flower number in both years, and because of the positive effect of flower number on fruit number, had a positive indirect effect on mature fruit number. The herbivore had a negative direct effect on flower number and mature fruit number in both years. Thus, direct and indirect effects contribute to the large negative effect of herbivores on mature fruit number.

The composition of the pollinator community was very different in the two years, with a marked increase in the number of visits by efficient pollinators in 1985. This change was reflected in between-year differences in the effects of pollinators on number of initiated fruits. In 1984, pollinators had no significant effect on initiated fruit number, while the two pollinators that increased in abundance in 1985 had significant effects on initiated fruit number in that year. These results are consistent with experimental data for 1985, which indicate that supplemental pollination causes a significant increase in the number of fruit initiated. While pollinators may limit the number of fruit initiated in some years, our observational and experimental data suggest that number of mature fruits is primarily limited by resources, not by pollination.

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Horvitz, C. C. and D. W. Schemske. 1995. Spatiotemporal variation in demographic transitions for a neotropical understory herb: projection matrix analysis. Ecological Monographs 65:155-192.

Our goal was to elucidate the population dynamics of the perennial understory herb Calathea ovandensis in a rain forest in southern Mexico using matrix projection model analysis. We emphasize the magnitude and consequences of spatiotemporal variation in (1) basic demographic parameters (growth, survival, and reproduction) (2) asymptotic demographic properties of a given environment (the asymptotic population growth rate and the associated stable-stage distribution and reproductive values) and (3) demographic sensitivities associated with a given environment (sensitivity and elasticity).

We obtained 6 yr (1982-1987) of empirical data from four study plots (differing in substrate, light, and density) from which we used the first 5 yr (1982-1986) to construct 16 plot-year and 1 pooled population projection matrices. This stage- structured population was characterized by a long-lived seed bank, temporally variable seedling recruitment (10 fold variability among years), high mortality of seedlings (>90%), very low mortality of reproductives (usually <10%), fertility that increased markedly with plant size, and the ability of large plants to shrink rather than die under adversity. Within these broad outlines, the magnitudes of transitions representing demographic fates exhibited considerable variation through space and time, some parameters varying much more than others (cv from 22 to 400%).  Growth and reproduction were positively correlated across environments. The least variable parameters were seed dormancy and stasis of small reproductives.

Observed stage distributions were reasonably close to the stable stage distributions (mean = 86.1% similar). In most plot-years, the stable-stage distribution was dominated by seeds, followed by seedlings, and then small reproductives and the reproductive values increased with size class.

Population growth rates, given by the dominant eigenvalue of the matrices, ranged from 0.73 to 1.25. Analysis of the mean dynamics gave l = 0.97 (using a variety of analytical approaches) and our analysis of the overall pooled dynamics gave a l = 0.99, indicating that the habitat at the study site favored the persistence of Calathea ovandensis. An el Nino event coincided with the year of the highest population growth rate.

Survival, growth, and reproduction varied significantly through space and time, and different plot-years were beneficial to different stages. Most interestingly, stage-specific sensitivity parameters (sensitivity and elasticity) also varied through space and time. Spatiotemporal variability of sensitivity structure has important implications. Determination of stages most "critical" to population dynamics will depend upon knowledge of this variation. Population growth rate was significantly positively correlated with elasticity of seed production, seed germination, and seedling growth. These results indicate that the opportunity for selection on plant characters affecting particular life history stages varies through space and time even if the effect on the single-stage transition probability does not vary. Selection on characters affecting juvenile stages may be stronger in populations of higher growth rates.

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Schemske, D. W. 1980. The evolutionary significance of extrafloral nectar production by Costus woodsonii (Zingiberaceae): An experimental analysis of ant protection. Journal of Ecology 68:959-967.

(1) Observations and experiments were carried out in central Panama to examine the hypothesis that ants increase the reproductive success of Costus woodsonii.

(2) The plant secretes extrafloral nectar from bracts on the inflorescence and the nectar is harvested by ants.

(3) The fly Euxesta sp., a specialist on Costus woodsonii, oviposits beneath the bracts on immature fruit, and its larvae destroy the seeds and arils.

(4) In both the dry and wet seasons, plants with ants experimentally removed had a higher rate of fly-oviposition and produced only one-third as many seeds as control plants with ants.

(5) The ant Camponotus planatus was dominant in the dry season, and had less influence on seed production than had Wasmannia auropunctata, the wet-season dominant.

(6) Among dry-season control plants, the probability of fruit maturation was significantly correlated with an index of ant activity based on the consistency of ant occupation.

(7) As a consequence of larval damage to the arils, seeds from plants with ants excluded had a lower probability of dispersal in both dry and wet seasons than those from plants with ants.

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Schemske, D. W. 1981. Floral convergence and pollinator sharing in two bee-pollinated tropical herbs. Ecology 62:946-954.

The hypothesis that the understory herbs Costus allenii and C. laevis (Zingiberaceae) have converged in floral characteristics to use the same pollinator was investigated in central Panama. Observations and experiments indicated that these species (1) occupy the same habitats, (2) flower synchronously, (3) are identical in flower color, morphology, and nectar secretion patterns, (4) share the same pollinator, the bee Euglossa imperialis, (5) are self-compatible, but not autogamous, and (6) have strong barriers to hybridization. Both grow in low density along streamsides and produce a single flower per day for an extended period (up to 4 mo). Flower density is depressed through extensive predation by the weevil Cholus cinctus, which damaged 31% of all C. allenii and 60% of all C. laevis inflorescences. Direct observation of foraging bees indicated that individuals regularly visit both plant species. An experimental analysis of interspecific pollen transfer using powdered paint as a marker verified these results; 97% of the flowers checked had received heterospecific visits. The high probability of interspecific pollination did not affect fruiting success.

I suggest that low flower density, exaggerated by extreme floral predation, has selected for floral similarity and pollinator sharing in these species. Floral convergence increases effective flower density and nectar supplies, and probably increases the regularity and rate of pollinator visitation.

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Schemske, D. W. 1982. Ecological correlates of a neotropical mutualism: Ant assemblages at Costus extrafloral nectaries. Ecology 63:932-941.

The species richness and composition of ant assemblages at the extrafloral nectaries of four Costus spp. (Zingiberaceae) in Panama were quantified to determine (1) the relative specificity of these ant-plant mutualisms, and (2) the ecological factors that influence ant assemblages. Although inflorescences of Costus spp. commonly produce extrafloral nectar for several months (species means 1-2 mo, range to 4 mo), there was little turnover in ant-species composition on individual inflorescences. A total of 34 ant species in five subfamilies were observed, but plant species varied significantly in ant species richness. The number of ant species expected from equal sampling (40 inflorescences per Costus sp.) was 9 for C. pulverulentus, 13 for C. scaber, 15 for C. allenii, and 20 for C. laevis. Among-species differences in ant species richness were not a function of plant distribution, the volume or composition of extrafloral nectar, or the duration of nectar flow. The number of ant species was positively correlated with inflorescence height, which varied from 0.6 to 2.0 m. The differences in ant species richness with inflorescence height were due primarily to the addition of arboreal ants, which increased from 3 species representing 3% of all ant species observations on the shortest plant, C. pulverulentus, to 12 species representing 62% of all ant observations on the tallest plant, C. laevis. The number of terrestrial ant species varied only from six to eight.

Cumulative ant-species curves as a function of number of inflorescences sampled (N = 41-80) did not plateau for any Costus sp., indicating that a large number of ant species can utilize extrafloral nectar in this system. The high ant species richness and the similarity in nectar characteristics among Costus spp. suggest these are relatively generalized mutualisms with limited coevolution between the plants and their attendant ants. The brief period of extrafloral nectar production (mean = 36-55 d), coupled with spatial variation in ant species distributions, probably limit the degree of specialization among mutualists. This is in contrast to more specialized ant-plant mutualisms where plants are relatively predictable sources of food and/or nest sites for ants.

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Schemske, D. W., J. Ågren and J. Le Corff. 1996. Deceit pollination in the monoecious, neotropical herb Begonia oaxacana. pp. 292-318. In D. G. Lloyd and S.C.H. Barrett (eds.), Floral Biology. Chapman and Hall.

A suite of traits in female flowers contribute to the resemblance between male and female flowers in Begonia oaxacana, and despite the striking preference of pollinators for male flowers, females receive enough pollen to achieve high reproductive success. Our experiment to examine the relationship between female flower size and pollinator visitation represents a first step in identifying the selective forces operating in deceit pollination systems. Other experiments, such, as altering the size, color and spectral qualities of stigmas as compared to the characteristics of anthers, would be useful for evaluating the potential for intersexual mimicry in Begonia. In addition, estimating the genetic variances and covariances for male and female flower traits would elucidate the importance of genetic constraints on floral evolution. Finally, further study of deceit pollination and intersexual mimicry would benefit from consideration of phylogenetic data which could be used to answer questions such as: 1) Has the degree of floral resemblance evolved from the "ancestral state" in insect-pollinated Begonia due to changes in the mating system and/or the selective value of resemblance? and 2) Does resemblance involve different floral characters in clades/species pollinated by different kinds of pollinators? We conclude that a synthetic study of deceit pollination and intersexual mimicry will require information on the form and magnitude of selection on male and female floral characters, the genetic architecture of floral traits, and the phylogenetic relationships among species.

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LeCorff, J., J. Ågren, and D. W. Schemske. 1998. Floral display, pollinator choice and female reproductive success in two monoecious Begonia species pollinated by deceit. Ecology 79:1610-1619.

In many plant species with unisexual flowers, the female flowers lack pollinator rewards and are pollinated by visitors who mistake the female flowers for rewarding male flowers. Here we present the results of observations and experiments designed to investigate pollinator discrimination and its consequences for female reproductive success in two monoecious Begonia species pollinated by deceit. Male and female flowers of B. urophylla are similar in shape, but female flowers are smaller than male flowers, while in B. tonduzii, male and female flowers have strikingly different shapes, but their petaloid tepals are similar in total area. Flower visitors displayed marked discrimination against the rewardless female flowers of both species. Seven bee species were observed on B. urophylla, and for all species taken together the relative visitation rate to male flowers was 8.6 times that to female flowers. The bee Trigona fulviventris accounted for 95% of all visits to B. tonduzii and visited male flowers 15.4 times as often as female flowers. The flower visitors of B. urophylla varied significantly in their degree of discrimination against female flowers, with visitation rates to male flowers 3.7-21.8 times those to female flowers. The two most abundant flower visitors displayed the greatest discrimination against female flowers and, although they comprised nearly 54% of all visits, were responsible for only 15% of all visits to female flowers. Thus, the relative abundance of different flower visitors on B. urophylla is a poor predictor of pollinator importance.

In both species the inflorescence sex ratio was male-biased early in the season and female-biased later. Hand-pollination experiments revealed that fruit set in B. tonduzii was pollinator limited both during peak and late flowering, with supplemental pollination having its greatest effect late in the season when male flowers were rare. In contrast, fruit set was only weakly affected by supplemental hand-pollination in B. urophylla.  The significant pollinator limitation observed in B. tonduzii and the lack of it in B. urophylla are consistent with our observation that the visitation rate to female flowers was much higher in B. urophylla than in B. tonduzii, which was due in part to the lower level of pollinator discrimination against female flowers of B. urophylla. Our finding that female reproductive success is limited by pollinator visitation in B. tonduzii indicates an opportunity for the evolution of increased resemblance between male and female flowers in this species.

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Schemske, D. W. Ecological and evolutionary perspectives on the origins of tropical diversity, In R. Chazdon and T. Whitmore (eds.), Foundations of Tropical Biology: Key papers and commentaries. Univ. of Chicago Press. (in press)

"Since the animals and plants which exist in the world are products of the evolutionary development of living matter, any differences between tropical and temperate organisms must be the outcome of differences in evolutionary patterns." (Dobzhansky 1950)

How do we explain the extraordinary biological diversity of tropical regions? Few questions have generated such interest, yet we still struggle to find the answers. One of the greatest obstacles is that the question is multi-faceted, so a single mechanism is unlikely to provide both a necessary and sufficient explanation. Most of the hypotheses put forth to explain tropical diversity are purely ecological, and therefore do not identify the causal mechanisms responsible for diversity gradients. For example, energy is one of the best predictors of diversity, yet the energy hypothesis fails to identify how or why new species are produced in productive sites.

Here I review the major ideas concerning the origins of tropical diversity, with the goal of linking ecological and evolutionary perspectives. I first discuss the papers reprinted in this section, illustrating their individual contributions and their relationship to a general theory of tropical diversity. This is followed by a review of the evidence and explanations for latitudinal diversity gradients, a discussion of the mechanisms that may contribute to the origin and maintenance of diverse communities, and recommendations for future research directions.

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Ramsey, J. and D. W. Schemske. 1998. Pathways, mechanisms and rates of polyploid formation in flowering plants. Annual Review of Ecology and Systematics 29:477-501.

Polyploidy is widely acknowledged as a major mechanism of adaptation and speciation in plants. The stages in polyploid evolution include frequent fertility bottlenecks and infrequent events such as gametic nonreduction and interspecific hybridization, yet little is known about how these and other factors influence overall rates of polyploid formation. Here we review the literature regarding polyploid origins, and quantify parameter values for each of the steps involved in the principal pathways. In contrast to the common claim that triploids are sterile, our results indicate that the triploid bridge pathway can contribute significantly to autopolyploid formation regardless of the mating system, and to allopolyploid formation in outcrossing taxa. We estimate that the total rate of autotetraploid formation is of the same order as the genic mutation rate (10-5) , and that a high frequency of interspecific hybridization (0.2% for selfing taxa, 2.7% for outcrossing taxa) is required for the rate of tetraploid formation via allopolyploidy to equal that by autopolyploidy. We conclude that the rate of autopolyploid formation may often be higher than the rate of allopolyploid formation. Further progress toward understanding polyploid origins requires studies in natural populations that quantify: (a) the frequency of unreduced gametes, (b) the effectiveness of triploid bridge pathways, and (c) the rates of interspecific hybridization.

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Husband, B. C. and D. W. Schemske. 2000. Ecological mechanisms of reproductive isolation and coexistence of diploid and tetraploid Chamerion angustifolium. Journal of Ecology 88:689-701.

1. Theoretical models indicate that coexistence of diploid and polyploid individuals in sympatric populations is unlikely when mating is random among cytotypes and hybrids are inviable. However, coexistence may be facilitated by prezygotic isolating mechanisms that reduce intercytotype mating and increase assortative mating.

2. We examined the ecological factors regulating intercytotype mating in Chamerion (formerly Epilobium) angustifolium by measuring floral morphology, flowering synchrony and insect foraging preferences in a mixed population within a diploid-tetraploid contact zone. We also calculated the minimum rate of between-cytotype mating from estimates of the frequency of triploid offspring.

3. Diploids had significantly shorter and narrower petals as well as shorter styles than tetraploids. Inflorescences were significantly taller in tetraploids than diploids, but the mean number of open flowers per inflorescence did not differ. Diploid and tetraploid flowering periods overlapped by 51 %, with diploid stems flowering earlier.

4. In a plot of 20 diploid and 28 tetraploid stems that were flowering simultaneously, only 26% of all bee flights were between flowers on different stems. Of the total flights between inflorescences, only 15% were between different cytotypes. The combined effects of flowering asynchrony and insect foraging reduced the opportunities for intercytotype mating from 49% expected if mating is random to about 2% of the total number of pollinator flights.

5. A computer simulation indicated that the deficiency of pollinator flights between cytotypes was due largely to the spatial structure of cytotypes and limited pollinator flight distances within the observation plot. The frequency of triploid offspring produced during the period when both cytotypes were in flower was 6.6%, similar to the proportion of flights observed between flowers on different cytotypes (4%).

6. The results indicate that flowering phenology and insect behaviour (as influenced by clonal structure) have a significant effect on prezygotic mating isolation and triploid production, and may contribute to the maintenance of mixed cytotype populations in Chamerion angustifolium.

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Husband, B. C. and D. W. Schemske. 1996. Evolution of the magnitude and timing of inbreeding depression in plants. Evolution 50:54-70.

Estimates of inbreeding depression obtained from the literature were used to evaluate the association between inbreeding depression and the degree of self-fertilization in natural plant populations. Theoretical models predict that the magnitude of inbreeding depression will decrease with inbreeding as deleterious recessive alleles are expressed and purged through selection. If selection acts differentially among life history stages and deleterious effects are uncorrelated among stages, then the timing of inbreeding depression may also evolve with inbreeding. Estimates of cumulative inbreeding depression and stage-specific inbreeding depression (four stages: seed production of parent, germination, juvenile survival, and growth/reproduction) were compiled for 79 populations (using means of replicates, N = 62) comprising 54 species from 23 families of vascular plants. Where available, data on the mating system also were collected and used as a measure of inbreeding history. A significant negative correlation was found between cumulative inbreeding depression and the primary selfing rate for the combined sample of angiosperms (N = 35) and gymnosperms (N = 9); the correlation was significant for angiosperms but not gymnosperms examined separately. The average inbreeding depression in predominantly selfing species (d = 0.23) was significantly less (43%) than that in predominantly outcrossing species (d = 0.53). These results support the theoretical prediction that selfing reduces the magnitude of inbreeding depression. Most self-fertilizing species expressed the majority of their inbreeding depression late in the life cycle, at the stage of growth/reproduction (14 of 18 species), whereas outcrossing species expressed much of their inbreeding depression either early, at seed production (17 of 40 species), or late (19 species). For species with four life stages examined, selfing and outcrossing species differed in the magnitude of inbreeding depression at the stage of seed production (selfing d = 0.05, N = 11; outcrossing d = 0.32, N = 31), germination (selfing d = 0.02, outcrossing d = 0.12), and survival to reproduction (selfing d = 0.04, outcrossing d = 0.15), but not at growth and reproduction (selfing d = 0.21, outcrossing d = 0.27); inbreeding depression in selfers relative to outcrossers increased from early to late life stages. These results support the hypothesis that most early acting inbreeding depression is due to recessive lethals and can be purged through inbreeding, whereas much of the late-acting inbreeding depression is due to weakly deleterious mutations and is very difficult to purge, even under extreme inbreeding.

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Schemske, D. W., B. C. Husband, M. H. Ruckelshaus, C. Goodwillie, I. M. Parker and J. G. Bishop. 1994. Evaluating approaches to the conservation of rare and endangered plants. Ecology 75:584-606.

Nearly 25% of the estimated 250,000 species of vascular plants in the world may become extinct within the next 50 yr (Raven 1987), and 22% of vascular plant species in the United States are currently of conservation concern (Falk 1992). Plant conservation efforts received a critical boost with the passage of the Endangered Species Act of 1973 (16 U.S.C. §§ 15311544: USFWS 1988a). The Act established a legal mandate of unprecedented proportions to promote the collection, analysis, and exchange of biological information. It requires that for each endangered or threatened species occurring in the United States, a recovery plan be developed which "delineates, justifies, and schedules the research and management actions necessary to support the recovery of a species" (16 U.S.C. §§ 15311544: USFWS 1988a).

The conservation biologist is faced with the daunting task of identifying the biological information needed to evaluate the causes of endangerment and ensuring the continued survival of the target species in nature, as well as developing criteria to determine when recovery is achieved. The writing and implementation of recovery plans is made more difficult by levels of funding that preclude extensive biological research for every species. This problem is especially acute for plants. Although nearly 50% of the 728 federally listed threatened or endangered species are plants (USFWS 1992a), they received only 8% of recovery funds spent by the U.S. Fish and Wildlife Service in 1990 (Campbell 1991).

Deciding on an appropriate course of action is further complicated by contrasting opinions presented in the conservation literature. One important controversy regards the relative importance of genetic and demographic approaches in conservation efforts (Lande 1988). Falk (1992:408) suggests that "the distribution of genetic variation in rare plant species is a key consideration in conservation strategies," while Holsinger and Gottlieb (199 1) note that "populations large enough to mitigate ecological threats to population viability will mitigate genetic ones as well" (1991:205) and that "active management of the genetic structure of an endangered plant species will require an enormous investment of time, money, and expertise. Only a few of the most important species will warrant such heroic efforts" (1991:206). With limited funds and such contrary points of view, the conservation biologist charged with species recovery faces an unenviable challenge.

The successful recovery of an endangered species requires the best scientific information available, yet the present level of understanding and communication among research scientists and resource managers is not sufficient to gather and transmit this information. Our objective in this paper is to integrate the theory of plant population dynamics and academic research on rare plants with the practical task of setting guidelines for species recovery.

First, we provide a general review of the ecological and genetic factors influencing demographic changes at population and metapopulation levels. Second, we review research on rare plants surveyed in the scientific literature and evaluate its usefulness in conservation. Third, we identify the biological information necessary for developing recovery guidelines and outline a research approach. This includes assessing the biological "status" of rare species, identifying life history stages most critical to population growth, and identifying the biological causes of demographic variation at these stages. Fourth, we review recovery plans written for 98 species of plants listed as threatened or endangered by the U.S. Fish and Wildlife Service. Here we present information on the biological status of federally listed species, discuss research recommendations, and evaluate criteria used to establish recovery targets. Finally, we discuss the importance of biological, political, and economic constraints to recovery efforts, suggest biological guidelines for the recovery of endangered plants, and identify research priorities in plant conservation. We conclude with a discussion of the need for more effective communication between academic researchers and conservation managers.

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