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Demographic stochasticity in population fragments of the declining distylous perennial Primula veris (Primulaceae)
Basic Appl. Ecol. 4, 197–206 (2003) © Urban & Fischer Verlag http://www.urbanfischer.de/journals/baecol
Basic and Applied Ecology
Demographic stochasticity in population fragments of the declining distylous perennial Primula veris (Primulaceae) Marc Kéry1,2,*, Diethart Matthies2, Bernhard Schmid3 1
Patuxent Wildlife Research Center, Laurel, MD, USA Department of Biology, Plant Ecology, University of Marburg, Germany 3 Institut für Umweltwissenschaften, Universität Zürich, Winterthurerstrasse 190, CH - 8057 Zürich, Switzerland 2
Received February 20, 2002 · Accepted June 10, 2002
Abstract We studied ecological consequences of distyly for the declining perennial plant Primula veris in the Swiss Jura. Distyly favours cross-fertilization and avoids inbreeding, but may lead to pollen limitation and reduced reproduction if morph frequencies deviate from 50%. Disassortative mating is promoted by the reciprocal position of stigmas and anthers in the two morphs (pin and thrum) and by intramorph incompatibility and should result in equal frequencies of morphs at equilibrium. However, deviations could arise because of demographic stochasticity, the lower intra-morph incompatibility of the pin morph, and niche differentiation between morphs. Demographic stochasticity should result in symmetric deviations from an even morph frequency among populations and in increased deviations with decreasing population size. If crosses between pins occurred, these would only generate pins, and this could result in a pin-bias of morph frequencies in general and in small populations in particular. If the morphs have different niches, morph frequencies should be related to environmental factors, morphs might be spatially segregated, and morphological differences between morphs would be expected. We tested these hypotheses in the declining distylous P. veris. We studied morph frequencies in relation to environmental conditions and population size, spatial segregation in field populations, morphological differences between morphs, and growth responses to nutrient addition. Morph frequencies in 76 populations with 1–80000 flowering plants fluctuated symmetrically about 50%. Deviations from 50% were much larger in small populations, and six of the smallest populations had lost one morph altogether. In contrast, morph frequencies were neither related to population size nor to 17 measures of environmental conditions. We found no spatial segregation or morphological differences in the field or in the common garden. The results suggest that demographic stochasticity caused deviations of the morph ratio from unity in small populations. Demographic stochasticity was probably caused by the random elimination of plants during the fragmentation of formerly large continuous populations. Biased morph frequencies may be one of the reasons for the strongly reduced reproduction in small populations of P. veris. Wir untersuchten die Folgen der Distylie für die seltene, ausdauernde Pflanzenart Primula veris im Schweizer Jura. Distylie begünstigt Fremdbestäubung und vermeidet Inzucht, könnte aber, falls die Häufigkeit der beiden Morphen unterschiedlich ist, zu Pollenlimitierung und verringerter Reproduktion führen. Bei distylen Pflanzen werden dissortative Paarungen durch die unterschiedliche Position von Narben und Antheren bei den beiden Morphen (lang- und kurzgriffelig) sowie durch
*Corresponding author: Marc Kéry, Patuxent Wildlife Research Center, U.S. Geological Survey, 11510 American Holly Drive, Laurel, MD 20708-4017, USA. Phone: 301-497-5632, Fax: 001-301-497-5666, E-mail: [email protected]; Current address: Department of Biology, Plant Ecology, University of Marburg, 35032 Marburg, Germany
1439-1791/03/04/03-197 $ 15.00/0
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M. Kéry et al. weitgehende Selbstinkompatibilität der Morphen begünstigt, was ein ausgeglichenes Morphenverhältnis zur Folge haben sollte. In kleinen Populationen könnte jedoch demographische Stochastizität zu einem unausgeglichenen Morphenverhältnis führen. Bei P. veris könnten zudem Kreuzungen zwischen gleichen Morphen auf Grund der größeren Selbstkompatibilität der langgriffeligen Pflanzen diese häufiger werden lassen. Auch Nischendifferenzierung zwischen den Morphen könnte das Morphenverhältnis beeinflussen. In diesem Fall sollten die Morphen räumlich segregiert sein, morphologische Unterschiede aufweisen und das Morphenverhältnis sollte durch Umweltfaktoren beeinflusst werden. In den untersuchten 76 Populationen von P. veris mit 1–80000 blühenden Pflanzen variierte die relative Häufigkeit der Morphen symmetrisch um 50%. Abweichungen von 50% waren in kleinen Populationen viel größer und sechs der kleinsten Populationen hatten eine Morphe völlig verloren. Im Gegensatz dazu wurde die Häufigkeit einer Morphe nicht durch die Populationsgröße oder durch 17 untersuchte Umweltfaktoren beeinflusst. Wir fanden keine räumliche Segregation zwischen den Morphen und weder in der Natur noch im Versuchsgarten Unterschiede in vegetativen oder reproduktiven Merkmalen. Die Ergebnisse deuten darauf hin, dass demographische Stochastizität als Folge der Fragmentierung ehemals großer Populationen für die ungleiche Häufigkeit der Morphen in kleinen Populationen verantwortlich ist. Das unausgeglichene Morphenverhältnis könnte eine der Ursachen für die stark reduzierte Reproduktion der Pflanzen in kleinen Populationen sein. Key words: dioecy – heterostyly – morph differences and frequency – sex ratio – small populations
Introduction Heterostylous plant species have two (distyly) or three (tristyly) genetically-determined flower morphs that are often self- and intra-morph incompatible. Heterostyly is thought to have evolved as a means of inbreeding avoidance and to ensure the effective exchange of pollen between mating types (Barrett 1992). However, it may have a cost in terms of limited mating opportunities when morph frequencies move away from 50%. Furthermore, there is an increased risk of population extinction if a small isolated population consists only of plants of one morph. An analogous situation is found in dioecious plants and animals, in which small populations may consist almost entirely of one sex. Well-known examples are the last dusky seaside sparrows (Ammodramus maritimus nigrescens) and heath hens (Tympanuchus cupido cupido; Fuller 2001). In heterostylous plants with strict intramorph-incompatibility, disassortative mating between the different morphs should result in an equilibrium of equal morph frequencies (Fisher 1930, Heuch 1979). However, in tristylous species, morph frequencies have frequently been found to deviate from equality in natural populations (Weller 1976, Andersson 1994, Mal & Lovett-Doust 1997). This has been attributed to genetic drift, founder effects and population bottlenecks (Eckert & Barrett 1992). Drift and founder effects in particular have been stressed as factors influencing morph ratios, e.g. in Eichhornia paniculata (Husband & Barrett 1992ab), Lythrum salicaria (Eckert et al. 1996) and Pontederia cordata (Morgan & Barrett
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1988), while population bottlenecks have been invoked as an explanation for biased morph frequencies in Decodon verticillatus (Eckert & Barrett 1995) and in Lythrum salicaria (Ågren & Ericson 1996). In contrast, little is known about the factors influencing morph frequencies in populations of distylous species (Endels et al. 2002, Jacquemyn et al. 2002). We investigated what factors affect morph frequencies in populations of the distylous perennial Primula veris L. in the Swiss Jura. Distyly in this species is coded by several separate loci that are tightly linked and inherited as a single gene complex (Lewis & Jones 1992). Pin plants are the recessive (ss) and thrum plants the heterozygous (Ss) genotype (Richards & Ibrahim 1982). Intramorph-incompatibility in P. veris is strong but not strict. Hand cross-pollination between the two morphs resulted in a seed set (seeds per ovule) of 75.7%, while pin-pin crosses produced only 14.5% and thrum-thrum crosses just 0.6% seed set (Wedderburn & Richards 1990). However, disassortative mating is also promoted by the different position of anthers and stigmas in the two morphs. P. veris has been listed as locally vulnerable in the Swiss Red Data book (Landolt 1991). In our study area, it occurs mainly in remnant fragments of calcareous grassland. The extent of this habitat has greatly declined due to changes in land-use during the last 20–30 years (Zoller & Wagner 1986). We hypothesized that three mechanisms could lead to biased morph frequencies in P. veris; demographic stochasticity, pin advantage and niche differentiation. First, demographic stochasticity caused by the random elimination of established plants during the fragmen-
Demographic stochasticity in a distylous plant
tation of formerly larger populations could lead to morph frequencies that are more biased, i.e. further away from 50%, especially in very small populations. The loss of an entire morph is most likely in the smallest populations. Because the process leading to such population bottlenecks is completely random and the result of the removal of a part of the population, the average morph frequency across populations would not be different from 50%, deviations from unity should be symmetric and there should be no relationship between morph frequencies and population size. Second, pin plants of P. veris could have a reproductive advantage over thrum plants because they are more fertile when paired with the same morph. This pin advantage could lead to pin-biased morph frequencies in populations of any size. This effect should be strongest in small populations, where legitimate pollination could be a problem and could result in a negative relationship between the frequency of the pin morph and population size. Third, niche differentiation between the morphs could cause biased morph frequencies. Heterostylous species may have functional genders, i.e. one morph functions more as a female, and the other more as a male (Casper & Charnov 1982). Natural selection would then be expected to shape other morphological and physiological traits as well, as is the case with sexes in dioecious species (Putwain & Harper 1972, Harper 1977, Onyekwelu & Harper 1979), another breeding system with dimorphic incompatibility. Differential mortality or clonal growth could bias morph frequencies in a population if one morph is favoured at a site, or locally, if the morphs track environmental heterogeneities within a site and become aggregated (see references on dioecious species in Bierzychudek & Eckhart 1988, Lovett-Doust & Laporte 1991). In the case of niche differentiation it would be expected that morph frequencies are related to environmental conditions, and that morphological differences exist between the morphs. To test these hypotheses, we investigated morph frequencies, spatial relationships and environmental conditions of P. veris in the field and studied morph differences in vegetative and reproductive traits both in the field and in the common garden. We also compared the response of the two morphs to fertilization. We addressed the following specific questions: Is there variation in morph frequencies in natural populations of P. veris? Are deviations of morph frequencies from unity greater in small than in large populations? Is there spatial segregation between the morphs? Are morph frequencies related to population size or to environmental conditions? Are there morph differences in morphological traits or in the response to nutrient addition that would suggest different ecological niches?
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Materials and methods Study species Primula veris L. (Primulaceae) is a distylous, clonal, perennial herb that grows in nutrient-poor calcareous grasslands and along the edges of woodlands from Spain to Eastern Asia (Hegi 1927). The two mating types differ in flower morphology. Pin plants are longstyled and their anthers are near the base of the corolla, whereas in thrum plants the anthers are positioned above the short style. Vegetative daughter rosettes are produced adjacent to the mother plant. Only the below-ground parts, i.e. roots, rhizomes and buds remain alive over winter to form a new rosette and to produce one to four inflorescences in April. The flowers are pollinated mainly by Hymenoptera and Lepidoptera (Hegi 1927). Study sites Field studies were carried out in 1994 and 1995 in pastures and meadows of the Sundgau hills approximately 12 km south-west of the city of Basel, Switzerland, and in adjacent parts of the Jura mountains. The altitude of the study sites ranged from 330–770 m. Groups of plants of P. veris were defined as separate populations if they were separated by at least 50 m from the nearest conspecific plants. As in other parts of Europe (Keymer & Leach 1987), nutrient-poor grasslands have greatly declined in the study area in the last decades (Zoller & Wagner 1986). Therefore, most of the extant populations of P. veris in the study area are probably remnant fragments from far larger and less isolated populations only 20–30 years ago. Because P. veris is long-lived (Tamm 1972), populations cannot have been isolated for more than a few generations. The studies were carried out in haphazard subsets of a total of 76 remnant populations. We did not select study populations with respect to environmental conditions or morph frequencies, but rather studied all populations that we located in the area. The outermost study populations described a triangle with side lengths of 12, 17, and 26 kms. The distance of a study population to the nearest neighbouring population ranged from 50–1000 m (median: 88 m, quartile range: 50–150 m). Flowering population size ranged from 1–80000 flowering rosettes (median: 245, quartile range: 56–2190; see also Fig. 2). Morph frequencies and their relationship with population size In April 1994 and 1995, the number of flowering rosettes (called plants subsequently) was assessed in all
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76 field populations. To estimate the morph frequency, in each population one to 272 plants (mean: 82, SD: 57) were randomly selected and their flower morph recorded. In a clonal distylous plant, biased morph frequencies could also arise if clones grew at different rates. Therefore, we scored plants separated by c. 0.5 m. Given the restricted vegetative growth of P. veris, this made clonal growth an unlikely explanation for biased morph frequencies. Morph frequency was defined as the number of pin plants divided by the total number of sampled plants. Spatial distribution of the morphs within populations We recorded the spatial distribution of morphs in a total of 34 populations in either April 1994 or 1995. Along a transect across each population, one plant was selected approximately every 0.5 m and its flower morph recorded. Because even the largest genets did not extend over more than 0.15 m, the distance of 0.5 m made it unlikely that successive plants along a transect belonged to the same clone. The distribution of genet sizes was highly skewed and ranged from 1–17 with a median of 2 and a quartile range of 1–4. Depending on population size, 30–272 plants per population (mean: 117, SD: 52) were scored for their flower morph. The sequence of morphs in each population was tested for serial independence by a runs test. Relationship between morph frequency and environmental conditions We used several habitat factors to investigate the relationship between morph frequency and environmental conditions. From a map (Schweizer Landestopographie, 3084 Wabern, scale 1 : 25000) we read altitude (to the nearest 10 m), aspect (to the nearest 5°) and slope (to the nearest 5%) for each of 27 P. veris sites (a subset of the total of 76). Aspect was expressed as the absolute deviation in degrees from South. In May 1994, we recorded the composition of the vegetation and determined all vascular plant species and their cover in five randomly selected 2 × 2 m plots in 23 of these P. veris sites. Because of heavy grazing of cattle, the vegetation could not be analysed at the other four sites. We used the number of vascular plant species and their total cover, averaged over the five plots per site, as another measure of habitat quality. Furthermore, we calculated mean indicator values for humidity, light, temperature, continentality, soil reaction, nutrients, soil humus content and soil dispersion, by taking weighted means of the specific indicator values of the species present (Landolt 1977), with weights according to their abundance. These indicator values are similar to those of Ellenberg (Persson 1981,
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Ellenberg et al. 1992), but based on conditions in Switzerland. Next, we analysed gradients in the vegetation composition of the sites by correspondence analysis using the program CANOCO 4.0 (ter Braak & Smilauer 1998) and computed the first four canonical axes. We used the Spearman correlation coefficient to study the relationship between morph frequencies and environmental conditions as measured by each of these 17 habitat variables. Field survey of morphological differences between pin and thrum morphs In April 1994, in each of the 27 populations where environmental factors were studied, 20–24 plants were randomly selected and individually marked unless population size was less than 20, in which case all plants were marked (425 plants in total). The number sampled per population depended on the size of the population (mean: 15.7, SD: 5.8). For each plant, we recorded flower morph, length of the longest leaf, number and length of the flower stalks, number of flowers and flower buds, and number of flowering rosettes of P. veris within concentric rings of 0.2 m, 1 m and 5 m around each target plant. We recorded genet size by counting all plants that touched the target plant and belonged to the same morph. The proportion of flower buds that had developed into open flowers was used as an indicator of the phenological state of a plant. In July 1994, we harvested the above-ground parts of 80 of the marked plants in 12 of the 23 populations. For conservation reasons, no plants were harvested in the four smallest populations, and in the remaining 11 populations all marked plants had been lost to grazing cattle. The seeds of the 80 harvested plants were removed and counted and the remaining plant material was dried at 70 °C for two days. Vegetative parts (leaves) and reproductive parts (flower stalks, fruits and seeds) were weighed separately. Reproductive allocation was calculated as the ratio of reproductive to total biomass. Differences between morph types were analysed with a mixed-model ANOVA. Morph type as a fixed effect was tested against the morph-by-population interaction, while the population main effect and the interaction (random effects) were tested against the residual variation among plants. Variables were logtransformed if necessary to achieve normality and homoscedasticity of residuals (see table legends). Binary data were analysed with logistic regression models (McCullagh & Nelder 1989) and significance tests performed by dividing the mean deviance due to a factor by the appropriate error mean deviance, analogous to the calculation of F-values in ordinary analy-
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sis of variance. Ratios of mean deviances follow an approximate F-distribution (McCullagh & Nelder 1989).
Results
Morphological differences between pin and thrum morphs in a common environment
We recorded the morph of a total of 6232 plants in 76 populations of P. veris ranging in size from 1–80000 plants. The frequency of pin plants (50.7%) in all populations taken together was not significantly different from 50% (sign test, p = 0.27). The frequency distribution of pin plants in 76 populations was symmetrical around 50% and ranged from 0 to 100% (mean: 50.5%; SD: 17.4; Fig. 1). Its mean was again not significantly different from the expected value of 50% (t75 = 0.26, p = 0.80). Seven of the 76 populations consisted of fewer than 10 plants and were not tested statistically for uneven morph frequencies because such a test would have very limited power. Of the remaining 69 populations, 12 (17.4%) had a frequency of pin plants significantly different from 50% (binomial tests, α = 0.05). Ten of these populations were smaller than 450 and two consisted of 1000 and of 1540 plants, with pins predominant in both cases. The frequency of pin plants in a population was independent of population size (Fig. 2a). In contrast, there was a significant negative relationship between the absolute deviation from an even morph frequency and population size (r74 = – 0.61, p < 0.001; Fig. 2b) which held also when the six monomorphic populations were omitted. Six small populations (8% of the 76 sampled), had lost one morph completely. Half of
To test for differences between morphs in a common environment and in response to fertilizer application, a common garden experiment was carried out (see Spillmann 1998 for more details). In summer 1994, we collected seeds in the 12 P. veris populations used in the previous study and in eight additional populations and stored them in paper bags at 7 °C. We germinated them in May 1995 on moist filter paper in petri dishes and planted the seedlings in July 1995 into pots with 8 cm diameter. A total of 2800 P. veris seedlings originating from 113 seed families from 17 populations were planted into 845 pots in the common garden. Seeds from three populations did not germinate. The plants were watered if necessary. There was strong mortality, and in March 1996, 170 surviving plants representing 67 seed families from 14 populations (mean: 4.8, SD: 2.7 seed families per population) were transplanted individually into pots (8 cm diameter) containing commercial potting soil. For each plant the width of the largest leaf and the rosette diameter were measured as an estimate of size. At the end of March 1996, 85 plants each were randomly assigned to either a fertilizer treatment or to the control. Plants in the fertilizer treatment received 1 g of slow release fertilizer (Osmocote) at the start of the experiment and 50 ml of 0.12% solution of a commercial fertilizer (Hauert Flory 2 Typ K, Wädenswil: 18 mg N l–1, 7.2 mg P l–1, 31.2 mg K l–1) in April, May and June 1996. Controls received equivalent amounts of water. In July 1996, we recorded whether the plants flowered and their flower morph and rosette diameter. Relative growth rate in 1996 was expressed as the difference between the logarithms of rosette diameters in July and March. In July, the above-ground parts of the plants were harvested, dried at 70 °C for two days and weighed. Most plants survived the removal of their above-ground parts. In April 1997, we recorded the following traits for all genets that had survived and were flowering: flower morph, number of rosettes, maximum leaf length, number and length of flower stalks, number of open flowers and flower buds per rosette. Both the growth response and the morphological data were analysed with a two-way fixed effects ANOVA. Binary data were analysed using a logistic regression model. For all statistical analyses except the correspondence analysis we used the GenStat statistical package (Payne et al. 1993).
Population morph frequencies and their relationship with population size
Fig. 1. Frequency distribution of the morph frequency (frequency of pin plants) in 76 population fragments of Primula veris. The classes represent proportions of 0, 0.001–0.1, 0.11–0.2, …, and 1. Note that the two outermost bars include exclusively monomorphic populations.
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these populations had lost their pins, and half their thrums. The two largest monomorphic populations consisted of 9 pin and 10 thrum plants, respectively. Spatial distribution of the morphs in the population fragments and population morph frequencies and their relationship with environmental conditions There was no spatial segregation between the morphs at a scale of 0.5 m. Only in two out of 34 populations of P. veris were morph sequences not random (runs test, p < 0.05). The number of significant results thus did not exceed chance expectation. There was no significant relationship between the morph frequency in individual populations and environmental conditions at the sites for any of 17 variables used to quantify environmental conditions (all p-values > 0.05; mean: 0.70, SD: 0.20). Differences between pin and thrum plants in the field and in the common garden There were no morphological differences between the morphs in the field. Of the 425 marked plants in 27 P. veris populations 213 were pin and 212 thrum
Fig. 2. The relationship between morph frequency and population size in 76 population fragments of Primula veris. There are fewer than 76 points because some populations were of the same size. (a) Proportion of pin plants vs. population size. The horizontal line indicates the line of best fit of the relationship between the morph frequency and population size (y = 0.5045 + 0.0002*×). The intercept and the slope are not significantly different from 0.5 and 0, respectively. (b) Deviation from an even morph frequency vs. population size. Deviation was calculated as abs(proportion of pin plants – 0.5). Note the log-scale for population size.
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plants. Pin plants did not differ from thrum plants in any of four vegetative traits, and in only two out of 14 reproductive traits studied. Thrum plants flowered slightly earlier and they produced more flowers per inflorescence. The local density of conspecifics within radii of 0.2 m, 1 m and 5 m around target plants did not differ between the morphs (Table 1). The number of significant differences between morphs did not exceed what would have been expected by chance alone. After a Bonferroni correction of the test-wise error level, morph differences could no longer be called significant. Of the 120 plants that flowered in the common garden experiment, 55 were pin and 65 were thrum. There were no differences between the morphs in the common garden in any of the vegetative or reproductive traits measured (Table 2). Fertilizer addition influenced three traits, but the reaction to fertilizer application was the same in both morphs (no significant morph-by-fertilizer interactions).
Discussion Morph frequencies of heterostylous plants at equilibrium should be even (Fisher 1930, Heuch 1979). However, in the distylous P. veris, some populations exhib-
Fig. 3. The relationship between the deviation from an even population morph frequency and (a) the mean number of seeds per fruit and (b) the mean number of seeds per plant in 16 remnant populations of Primula veris.
Demographic stochasticity in a distylous plant
ited significant deviations from equal morph frequencies. These deviations could have been caused by demographic stochasticity, pin advantage or niche differentiation between morphs. The stochastic mortality of plants in the course of habitat fragmentation would re-
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sult in symmetric deviations from an even morph frequency, more strongly biased morph frequencies and morph loss in smaller populations. This is what we found for P. veris. Such mortality would then be equivalent to demographic stochasticity.
Table 1. Mixed-model ANOVA of the effects of morph type and population (a) on vegetative traits and local density of conspecifics and (b) on reproductive traits in 27 natural populations of Primula veris. Means for the pin and thrum morphs are also given. Genet size, total length of stalks, flowers per plant and seeds per plant were log-transformed prior to analysis. The effects on the proportion of open flowers and fruits per flower were analysed with a logistic regression model, therefore the entries are approximate F-values obtained as appropriate ratios of mean deviances (see text). Error levels are test-wise. ***: P < 0.001; **: P < 0.01; *: P < 0.05. Means
F-values
Pin
Thrum
Res. d.f.
Population
Morph
Pop* Morph
a) Vegetative traits Leaf length [cm] Vegetative biomass [g] Mean leaf mass [g] Genet size Density within 0.2 m radius Density within 1 m radius Density within 5 m radius
9.52 0.63 0.15 2.27 0.77 8.30 56.10
9.55 0.65 0.16 2.26 1.18 7.50 52.10
7.58*** 1.30 1.74 4.31*** 21.30*** 7.37*** 31.20***
0.05 0.05 0.14 1.90 1.18 0.93 0.05
0.44 0.58 0.51 1.76* 1.23 1.24 0.76
373 42 42 373 371 372 372
1.29 13.21 17.20 5.78 7.31 0.89 0.64 4.37 20.60 114.00 0.98 127.00 0.24 0.40
1.33 13.36 18.20 6.31 8.85 0.93 0.64 5.25 18.50 101.00 1.17 122.00 0.30 0.37
2.58*** 6.27*** 3.84*** 2.52*** 2.59*** 9.66*** 3.62** 1.96* 4.47*** 2.88** 3.00** 1.92 2.03* 0.89
0.33 0.66 0.89 6.46* 2.76 5.04* 0.23 0.04 3.54 1.51 3.13 0.61 0.07 0.59
2.22*** 0.72 1.51 0.92 1.56 2.00 0.75 0.76 0.51 0.41 2.17 0.17 1.34 0.76
373 373 373 373 373 373 48 60 52 60 46 46 58 37
b) Reproductive traits Number of flower stalks Mean flower stalk length [cm] Total length of flower stalks [cm] Flowers per inflorescence Flowers per plant Prop. of open flowers Fruits per flower Fruits per plant Seeds per fruit Seeds per plant Single seed mass [mg] Seed mass per plant [mg] Reproductive biomass [g] Reproductive allocation
Table 2. ANOVA of the effects of fertilizer application and morph type on vegetative and reproductive traits of Primula veris in the common garden experiment. Means for the pin and thrum morphs are also given. Genet size 1997, total length of flower stalks 1997, number of flowers in the largest rosette 1997 and number of flowers per genet 1997 were log-transformed prior to analysis. This is a fixed-effects model and all effects were tested over the residual variation among plants. Flowering probability of a genet in 1996 and the proportion of open flowers in 1997 were analysed with a logistic regression model, therefore approximate F-values were obtained as appropriate ratios of mean deviances (see text). The residual d.f was 116 in all cases. Error levels are test-wise. ***: P < 0.001; **: P < 0.01; *: P < 0.05. Means Pin a) Vegetative traits Relative growth rate 1996 Above-ground biomass 1996 [g] Leaf length 1997 [cm] Genet size 1997 b) Reproductive traits Flowering prob. of a genet in 1996 Length of flower stalks 1997 Flowers per largest rosette 1997 Flowers per genet 1997 Prop. of open flowers in 1997
F-values Thrum
Fertilizer
Morph
Fert*Morph
0.30 1.06 9.84 1.93
0.31 1.15 9.73 1.94
0.03 3.92* 12.96*** 1.16
0.16 0.90 0.10 0.00
1.36 1.63 0.00 0.94
0.31 11.78 10.28 20.17 0.61
0.32 11.79 10.68 20.04 0.64
0.06 2.34 0.16 0.02 9.62**
0.02 1.25 0.26 0.03 0.29
0.90 0.09 1.37 0.00 0.89
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In contrast, a reproductive advantage of pins, which are less intramorph incompatible than thrums (Wedderburn & Richards 1990), would have resulted in a general pin-bias of morph frequencies that was particularly strong in small populations. The niche differentiation explanation for morph frequency deviations predicted that morph frequencies would be related to environmental factors, and that morphs would be spatially segregated and differ in their morphology. None of these predictions was true for P. veris. Collectively, our results suggest that the random survival of fragments of formerly larger populations of P. veris resulted in deviations of morph frequencies from 50%. This is a case of demographic stochasticity which is difficult to study experimentally, because the process may span many generations. Most studies of demographic stochasticity are therefore modelling exercises (e.g. Goodman 1987, Shaffer 1987, Gabriel & Burger 1992). Stochasticity in a key demographic parameter such as morph frequency has only rarely been demonstrated in plants. Endels et al. (2002) studied 89 declining remnant populations of the distylous Primula vulgaris and found a strikingly similar relationship between morph frequency and population size as we did here. Morph frequencies were equal for large populations (>100 plants). Symmetric deviations were noticeable for smaller populations, and there were several monomorphic populations containing 8 or fewer plants. Very similar patterns have also been found for 18 populations of Primula elatior (Jacquemyn et al. 2002). It might be argued that the larger variance of morph frequencies around 0.5 in smaller populations (Fig. 2a) is a statistical artefact; fewer plants were sampled in small populations to determine the morph frequency. However, this argument is flawed. A much larger fraction of the entire population (and sometimes the entire population) was scored for morph type in small populations. Therefore, the sampling variance around the estimate of the population morph frequency is smaller in small populations or even zero in some very small populations. In contrast, for large populations, our study actually overestimates the variance of morph frequencies around 0.5, because we sampled a much smaller fraction of their plants. Demographic stochasticity is random variation in the survival and fecundity of individuals in a population. Genetic stochasticity in its strict sense is random variation of gene frequencies arising from the sampling error when two gametes form a zygote. In small populations of P. veris, mortality of plants (a demographic process) will alter the morph ratio, i.e. a demographic parameter. However, because the genetic system is so simple, this will automatically map onto a
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changed gene frequency. We prefer to call this process demographic instead of genetic stochasticity, although the distinction is somewhat arbitrary in a distylous plant. In the following, we compare our findings with patterns found in tristylous and dioecious species and discuss some implications of our results. Stochastic processes including founder effects and population bottlenecks have been found to cause biased morph frequencies in several tristylous species (Morgan & Barrett 1988, Eckert & Barrett 1992, Husband & Barrett 1992ab, Eckert & Barrett 1995, Ågren & Ericson 1996, Eckert et al. 1996). An important difference between distylous and tristylous species is that populations of tristylous species that have lost one morph are viable (Eckert et al. 1996) while isolated, monomorphic populations of a distylous species with strict intra-morph incompatibility are doomed to extinction. Biased mating type frequencies in small populations have also been found in one dioecious Silene species (S. otites; Soldaat et al. 1997), but not in another (S. dioica, our analysis of data from Table 1 in Carlsson-Granér et al. 1998). Founder effects have repeatedly been put forward as an explanation of biased morph frequencies in tristylous species (Morgan & Barrett 1988, Eckert & Barrett 1995). It may be argued that they could have played a role in P. veris, too. If new populations were founded by unequal numbers of pins and thrums, morph frequencies may not yet have returned to equilibrium. However, the recent history of the habitat of P. veris in our study area makes such an explanation unlikely. Hardly any new suitable grasslands have been created recently, the fragmentation of P. veris habitats and therefore the distance to possible source populations has greatly increased and dispersal has become more difficult. As an anecdote, our finding of a monomorphic population of 10 thrum plants supports the hypothesis that population fragmentation was responsible for biased morph frequencies and not founder effects, because any reproduction in this population would have produced new pins. We found no morphological differences between the morphs in the field, in the common garden or in their reaction to experimental fertilizer treatment, nor were the morphs spatially segregated. This is consistent with the results of other studies of heterostylous plants (e.g. Husband & Barrett 1992c, Mal & Lovett-Doust 1997, but see Levin 1974, McCall 1996). The loss of an entire morph in a population is the most extreme form of a biased morph frequency. Theoretical models for tristylous species predict that large populations are more resistant to morph loss than small populations (Heuch 1980). The critical size of a population, above which morph loss becomes unlikely, may be quite small. In P. veris (this study), all
Demographic stochasticity in a distylous plant
monomorphic populations consisted of 10 or fewer plants, in P. vulgaris (Endels et al. 2002), of 8 or fewer, and in P. elatior (Jacquemyn et al. 2002), the only monomorphic population consisted of 4 plants (H. Jacquemyn, pers. comm.). In the tristylous Lythrum salicaria, morph loss occurred in populations with fewer than 8 (Eckert et al. 1996), 9 (Ågren 1996), 100 (Mal & Lovett-Doust 1997) and 230 plants (Ågren & Ericson 1996). Populations of intra-morph incompatible, distylous plants with a biased morph frequency may experience reduced reproduction and monomorphic populations may even completely fail to reproduce. There is support for this hypothesis in P. veris. From a study on the reproduction of P. veris in populations of different size (Kéry et al. 2000), we had data on the reproductive success in 16 of the 27 populations used to study morph differences in the field. There was a significant negative relationship between the absolute deviation from an even morph frequency and both the mean number of seeds per fruit (Fig 3a; r = – 0.72, p < 0.01) and the mean number of seeds per plant (Fig. 3b; r = –0.59, p < 0.05). Part of the pattern was probably caused by inbreeding depression or reduced pollinator activity in small populations (Kéry et al. 2000). However, these relationships suggest that insufficient mate availability in small populations resulted in pollen limitation and reproductive failure in the smallest populations (see Washitani et al. 1994). Distylous species with a heteromorphic incompatibility system with two mating types may only be a special case of the more general situation of a monomorphic incompatibility system with multiple mating types. For instance, most strictly outcrossing species in small isolated populations are likely to experience pollination limitation (Sih & Baltus 1987). DeMauro (1993) demonstrated the loss of rare mating types in small populations of the endangered plant Hymenoxys acaulis, and Byers (1995) identified only four incompatibility alleles in two populations of a rare monomorphic and outcrossing species. Similarly, in the endangered grassland herb Rutidosis leptorrhynchoides, Young et al. (2000) found a strong relationship between the number of mating types and population size. Demographic stochasticity of mating type frequencies of the kind suggested in this paper may thus exacerbate the problems of small populations not only of heterostylous species but of all those with multiple mating types. Acknowledgements. The authors wish to thank Jon Ågren, Giorgina Bernasconi, Markus Fischer, Benedikt Schmidt and the members of the Cosecha 2001 of the Institut für Umweltwissenschaften Journal Club for valuable comments on draft versions of this work. Hans-Heiner
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“John” Spillman provided us with additional data and with his experimental plants. The late Karl Altenbach and Stefan Peter saved some of our study populations from mowing. Josiane Rey was a great help in the field and also elsewhere. Many thanks to them all. This study was supported by the Swiss National Science Foundation, Priority Programme Environment (grants No. 5001-35231 to D. Matthies and B. Schmid and No. 5001-44626 to D. Matthies, B. Schmid and P. J. Edwards).
References Ågren J (1996) Population size, pollinator limitation, and seed set in the self-compatible herb Lythrum salicaria. Ecology 77: 1779–1790. Ågren J, Ericson L (1996) Population structure and morphspecific fitness differences in tristylous Lythrum salicaria. Evolution 50: 126–139. Andersson S (1994) Unequal morph frequencies in populations of tristylous Lythrum salicaria (Lythraceae) from southern Sweden. Heredity 72: 81–85. Barrett SCH (1992) Heterostylous genetic polymorphisms: Model systems for evolutionary analysis. In: Barrett SCH (ed) Evolution and function of heterostyly. Springer-Verlag, Berlin, pp 1–29. Byers DL (1995) Pollen quantity and quality as explanations for low seed set in small populations exemplified by Eupatorium (Asteraceae). American Journal of Botany 82: 1000–1006. Bierzychudek P, Eckhart V (1988) Spatial segregation of the sexes of dioecious plants. American Naturalist 132: 34–43. Carlsson-Granér U, Elmqvist T, Ågren J, Gardfjell H, Ingvarsson P (1998) Floral sex ratios, disease and seed set in dioecious Silene dioica. Journal of Ecology 86: 79–91. Casper BB, Charnov EL (1982) Sex allocation in heterostylous plants. Journal of Theoretical Biology 96: 143–149. DeMauro MM (1993) Relationship of breeding system to rarity in the lakeside daisy (Hymenoxys acaulis var. glabra). Conservation Biology 7: 542–550. Eckert CG, Barrett SCH (1992) Stochastic loss of style morphs from populations of tristylous Lythrum salicaria and Decodon verticillatus (Lythraceae). Evolution 46: 1014–1029. Eckert CG, Barrett SCH (1995) Style morph ratios in tristylous Decodon verticillatus (Lythraceae): Selection vs. historical contingency. Ecology 76: 1051–1066. Eckert CG, Manicacci D, Barrett SCH (1996) Genetic drift and founder effect in native versus introduced populations of an invading plant, Lythrum salicaria (Lythraceae). Evolution 50: 1512–1519. Ellenberg H, Weber HE, Düll R, Wirth V, Werner W, Paulissen D (1992) Zeigerwerte von Pflanzen in Mitteleuropa. 2. Aufl. Scripta Geobotanica 18: 1–258. Endels P, Jacquemyn H, Brys R, Hermy M, de Blust G (2002) Temporal changes (1986–1999) in populations of primrose (Primula vulgaris Huds.) in an agricultural landscape and implications for conservation. Biological Conservation 105: 11–25.
Basic Appl. Ecol. 4, 3 (2003)
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M. Kéry et al.
Fisher RA (1930) The genetical theory of natural selection. Dover, New York. Fuller E (2001) Extinct birds. Oxford University Press, Oxford. Gabriel W, Burger R (1992) Survival of small populations under demographic stochasticity. Theoretical Population Biology 41: 44–71. Goodman D (1987) The demography of chance extinction. In: Soulé ME (ed) Viable populations for conservation. Cambridge University Press, Cambridge, pp 11–34. Harper JL (1977) Population Biology of Plants. Academic Press, London. Hegi G (1927) Illustrierte Flora von Mitteleuropa. Lehmanns Verlag, München. Heuch I (1979) Equilibrium populations of heterostylous plants. Theoretical Population Biology 15: 43–57. Heuch I (1980) Loss of incompatibility types in finite populations of the heterostylous plant Lythrum salicaria. Hereditas 92: 53–57. Husband BC, Barrett SCH (1992a) Effective population size and genetic drift in tristylous Eichhornia paniculata (Pontederiaceae). Evolution 46: 1875–1890. Husband BC, Barrett SCH (1992b) Genetic drift and the maintenance of style length polymorphism in tristylous populations of Eichhornia paniculata (Pontederiaceae). Heredity 69: 440–449. Husband BC, Barrett SCH (1992c) Pollinator visitation in populations of tristylous Eichhornia paniculata in northeastern Brazil. Oecologia 89: 365–371. Jacquemyn H, Brys R, Hermy M (2002) Patch occupancy, population size and reproductive success of a forest herb (Primula elatior) in fragmented landscape. Oecologia 130: 617–625. Kéry M, Matthies D, Spillmann H-H (2000) Reduced fecundity and offspring performance in small populations of the declining grassland plants Primula veris and Gentiana lutea. Journal of Ecology 88: 17–30. Keymer RJ, Leach SJ (1987) Calcareous grassland – a limited resource in Britain. In: Hillier SH, Walton DWH, Wells DA (eds) Calcareous grasslands – ecology and management. Bluntisham Books, Bluntisham, pp 11–17. Landolt E (1977) Ökologische Zeigerwerte zur Schweizer Flora. Veröffentlichungen des Geobotanischen Institutes der ETH, Stiftung Rübel, Zürich. Landolt E (1991) Gefährdung der Farn- und Blütenpflanzen in der Schweiz. Bundesamt für Umwelt, Wald und Landschaft (BUWAL), Bern. Levin DA (1974) Spatial segregation of pins and thrums in populations of Hedyotis nigricans. Evolution 28: 648–655. Lewis D, Jones DA (1992) The genetics of heterostyly. In: Barrett SCH (ed) Evolution and function of heterostyly. Springer-Verlag, Berlin, pp 129–150. Lovett-Doust J, Laporte G, (1991) Population sex ratios, population mixtures and fecundity in a clonal dioecious macrophyte Vallisneria americana. Journal of Ecology 79: 477–490. Mal KM, Lovett-Doust J (1997) Morph frequencies and floral variation in a heterostylous colonizing weed, Lythrum salicaria. Canadian Journal of Botany 75: 1034–1045. McCall C (1996) Gender specialization and distyly in Hoary Puccoon, Lithospermum croceum (Boraginaceae). American Journal of Botany 83: 162–168.
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McCullagh P, Nelder JA (1989) Generalized linear models. Chapman and Hall, London. Morgan MT, Barrett SCH (1988) Historical factors and anisoplethic population structure in tristylous Pontederia cordata: A reassessment. Evolution 42: 496–504. Onyekwelu SS, Harper JL (1979) Sex ratio and niche differentiation in spinach (Spinacia oleracea L.). Nature 282: 609–611. Payne RW, Lane PW, Digby PGN, Harding SA, Leech PK, Morgan GW, Todd AD, Thompson R, Tunnicliffe Wilson G, Welham SJ, White RP (1993) Genstat 5, Release 3, Reference Manual. Clarendon Press, Oxford. Persson S (1981) Ecological indicator values as an aid in the interpretation of ordination diagrams. Journal of Ecology 69: 71–84. Putwain PD, Harper JL (1972) Studies in the dynamics of plant populations. V. Mechanisms governing the sex ratio in Rumex acetosa and R. acetosella. Journal of Ecology 60: 113–129. Richards JH, Ibrahim HB (1982) The breeding system in Primula veris L. II. Pollen tube growth and seed-set. New Phytologist 90: 305–314. Shaffer ML (1987) Minimum viable populations: coping with uncertainty. In: Soulé ME (ed) Viable populations for conservation. Cambridge University Press, Cambridge, pp 69–86. Sih A, Baltus M-S (1987) Patch size, pollination behavior, and pollinator limitation in Catnip. Ecology 68: 1679–1690. Soldaat LL, Vetter B, Klotz S (1997) Sex ratio in populations of Silene otites in relation to vegetation cover, population size and fungal infection. Journal of Vegetation Science 8: 697–702. Spillmann H-H (1998) Studies on the population biology of Primula veris L. s.str. and Gentiana lutea L. Diploma thesis, University of Zürich. Tamm CO (1972) Survival and flowering of perennial herbs III: The behaviour of Primula veris on permanent plots. Oikos 23: 159–166. ter Braak CJF, Smilauer P (1998) CANOCO Reference Manual and User’s Guide to Canoco for Windows: Software for Canonical Community Ordination (version 4). Microcomputer Power, Ithaca, NY. Washitani I, Osawa R, Namai H, Niwa M (1994) Patterns of female fertility in heterostylous Primula sieboldii under severe pollinator limitation. Journal of Ecology 82: 571–579. Wedderburn F, Richards AJ (1990) Variation in within-morph incompatibility inhibition sites in heteromorphic Primula L. New Phytologist 116: 149–162. Weller SG (1976) Breeding system polymorphism in a heterostylous species. Evolution 30: 442–454. Young AG, Brown ADH, Murray BG, Miller CH (2000) Genetic erosion, restricted mating and reduced viability in fragmented populations of the endangered grassland herb Rutidosis leptorrhynchoides. In: Young AG, Clarke GM (eds) Genetics, demography and viability of fragmented populations. Cambridge University Press, Cambridge, pp 335–359. Zoller H, Wagner C (1986) Rückgang und Gefährdung von Mesobromion-Arten im Schweizer Jura. Veröffentlichungen des Geobotanischen Institutes der ETH 87: 239–259.
Basic and Applied Ecology
Demographic stochasticity in population fragments of the declining distylous perennial Primula veris (Primulaceae) Marc Kéry1,2,*, Diethart Matthies2, Bernhard Schmid3 1
Patuxent Wildlife Research Center, Laurel, MD, USA Department of Biology, Plant Ecology, University of Marburg, Germany 3 Institut für Umweltwissenschaften, Universität Zürich, Winterthurerstrasse 190, CH - 8057 Zürich, Switzerland 2
Received February 20, 2002 · Accepted June 10, 2002
Abstract We studied ecological consequences of distyly for the declining perennial plant Primula veris in the Swiss Jura. Distyly favours cross-fertilization and avoids inbreeding, but may lead to pollen limitation and reduced reproduction if morph frequencies deviate from 50%. Disassortative mating is promoted by the reciprocal position of stigmas and anthers in the two morphs (pin and thrum) and by intramorph incompatibility and should result in equal frequencies of morphs at equilibrium. However, deviations could arise because of demographic stochasticity, the lower intra-morph incompatibility of the pin morph, and niche differentiation between morphs. Demographic stochasticity should result in symmetric deviations from an even morph frequency among populations and in increased deviations with decreasing population size. If crosses between pins occurred, these would only generate pins, and this could result in a pin-bias of morph frequencies in general and in small populations in particular. If the morphs have different niches, morph frequencies should be related to environmental factors, morphs might be spatially segregated, and morphological differences between morphs would be expected. We tested these hypotheses in the declining distylous P. veris. We studied morph frequencies in relation to environmental conditions and population size, spatial segregation in field populations, morphological differences between morphs, and growth responses to nutrient addition. Morph frequencies in 76 populations with 1–80000 flowering plants fluctuated symmetrically about 50%. Deviations from 50% were much larger in small populations, and six of the smallest populations had lost one morph altogether. In contrast, morph frequencies were neither related to population size nor to 17 measures of environmental conditions. We found no spatial segregation or morphological differences in the field or in the common garden. The results suggest that demographic stochasticity caused deviations of the morph ratio from unity in small populations. Demographic stochasticity was probably caused by the random elimination of plants during the fragmentation of formerly large continuous populations. Biased morph frequencies may be one of the reasons for the strongly reduced reproduction in small populations of P. veris. Wir untersuchten die Folgen der Distylie für die seltene, ausdauernde Pflanzenart Primula veris im Schweizer Jura. Distylie begünstigt Fremdbestäubung und vermeidet Inzucht, könnte aber, falls die Häufigkeit der beiden Morphen unterschiedlich ist, zu Pollenlimitierung und verringerter Reproduktion führen. Bei distylen Pflanzen werden dissortative Paarungen durch die unterschiedliche Position von Narben und Antheren bei den beiden Morphen (lang- und kurzgriffelig) sowie durch
*Corresponding author: Marc Kéry, Patuxent Wildlife Research Center, U.S. Geological Survey, 11510 American Holly Drive, Laurel, MD 20708-4017, USA. Phone: 301-497-5632, Fax: 001-301-497-5666, E-mail: [email protected]; Current address: Department of Biology, Plant Ecology, University of Marburg, 35032 Marburg, Germany
1439-1791/03/04/03-197 $ 15.00/0
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M. Kéry et al. weitgehende Selbstinkompatibilität der Morphen begünstigt, was ein ausgeglichenes Morphenverhältnis zur Folge haben sollte. In kleinen Populationen könnte jedoch demographische Stochastizität zu einem unausgeglichenen Morphenverhältnis führen. Bei P. veris könnten zudem Kreuzungen zwischen gleichen Morphen auf Grund der größeren Selbstkompatibilität der langgriffeligen Pflanzen diese häufiger werden lassen. Auch Nischendifferenzierung zwischen den Morphen könnte das Morphenverhältnis beeinflussen. In diesem Fall sollten die Morphen räumlich segregiert sein, morphologische Unterschiede aufweisen und das Morphenverhältnis sollte durch Umweltfaktoren beeinflusst werden. In den untersuchten 76 Populationen von P. veris mit 1–80000 blühenden Pflanzen variierte die relative Häufigkeit der Morphen symmetrisch um 50%. Abweichungen von 50% waren in kleinen Populationen viel größer und sechs der kleinsten Populationen hatten eine Morphe völlig verloren. Im Gegensatz dazu wurde die Häufigkeit einer Morphe nicht durch die Populationsgröße oder durch 17 untersuchte Umweltfaktoren beeinflusst. Wir fanden keine räumliche Segregation zwischen den Morphen und weder in der Natur noch im Versuchsgarten Unterschiede in vegetativen oder reproduktiven Merkmalen. Die Ergebnisse deuten darauf hin, dass demographische Stochastizität als Folge der Fragmentierung ehemals großer Populationen für die ungleiche Häufigkeit der Morphen in kleinen Populationen verantwortlich ist. Das unausgeglichene Morphenverhältnis könnte eine der Ursachen für die stark reduzierte Reproduktion der Pflanzen in kleinen Populationen sein. Key words: dioecy – heterostyly – morph differences and frequency – sex ratio – small populations
Introduction Heterostylous plant species have two (distyly) or three (tristyly) genetically-determined flower morphs that are often self- and intra-morph incompatible. Heterostyly is thought to have evolved as a means of inbreeding avoidance and to ensure the effective exchange of pollen between mating types (Barrett 1992). However, it may have a cost in terms of limited mating opportunities when morph frequencies move away from 50%. Furthermore, there is an increased risk of population extinction if a small isolated population consists only of plants of one morph. An analogous situation is found in dioecious plants and animals, in which small populations may consist almost entirely of one sex. Well-known examples are the last dusky seaside sparrows (Ammodramus maritimus nigrescens) and heath hens (Tympanuchus cupido cupido; Fuller 2001). In heterostylous plants with strict intramorph-incompatibility, disassortative mating between the different morphs should result in an equilibrium of equal morph frequencies (Fisher 1930, Heuch 1979). However, in tristylous species, morph frequencies have frequently been found to deviate from equality in natural populations (Weller 1976, Andersson 1994, Mal & Lovett-Doust 1997). This has been attributed to genetic drift, founder effects and population bottlenecks (Eckert & Barrett 1992). Drift and founder effects in particular have been stressed as factors influencing morph ratios, e.g. in Eichhornia paniculata (Husband & Barrett 1992ab), Lythrum salicaria (Eckert et al. 1996) and Pontederia cordata (Morgan & Barrett
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1988), while population bottlenecks have been invoked as an explanation for biased morph frequencies in Decodon verticillatus (Eckert & Barrett 1995) and in Lythrum salicaria (Ågren & Ericson 1996). In contrast, little is known about the factors influencing morph frequencies in populations of distylous species (Endels et al. 2002, Jacquemyn et al. 2002). We investigated what factors affect morph frequencies in populations of the distylous perennial Primula veris L. in the Swiss Jura. Distyly in this species is coded by several separate loci that are tightly linked and inherited as a single gene complex (Lewis & Jones 1992). Pin plants are the recessive (ss) and thrum plants the heterozygous (Ss) genotype (Richards & Ibrahim 1982). Intramorph-incompatibility in P. veris is strong but not strict. Hand cross-pollination between the two morphs resulted in a seed set (seeds per ovule) of 75.7%, while pin-pin crosses produced only 14.5% and thrum-thrum crosses just 0.6% seed set (Wedderburn & Richards 1990). However, disassortative mating is also promoted by the different position of anthers and stigmas in the two morphs. P. veris has been listed as locally vulnerable in the Swiss Red Data book (Landolt 1991). In our study area, it occurs mainly in remnant fragments of calcareous grassland. The extent of this habitat has greatly declined due to changes in land-use during the last 20–30 years (Zoller & Wagner 1986). We hypothesized that three mechanisms could lead to biased morph frequencies in P. veris; demographic stochasticity, pin advantage and niche differentiation. First, demographic stochasticity caused by the random elimination of established plants during the fragmen-
Demographic stochasticity in a distylous plant
tation of formerly larger populations could lead to morph frequencies that are more biased, i.e. further away from 50%, especially in very small populations. The loss of an entire morph is most likely in the smallest populations. Because the process leading to such population bottlenecks is completely random and the result of the removal of a part of the population, the average morph frequency across populations would not be different from 50%, deviations from unity should be symmetric and there should be no relationship between morph frequencies and population size. Second, pin plants of P. veris could have a reproductive advantage over thrum plants because they are more fertile when paired with the same morph. This pin advantage could lead to pin-biased morph frequencies in populations of any size. This effect should be strongest in small populations, where legitimate pollination could be a problem and could result in a negative relationship between the frequency of the pin morph and population size. Third, niche differentiation between the morphs could cause biased morph frequencies. Heterostylous species may have functional genders, i.e. one morph functions more as a female, and the other more as a male (Casper & Charnov 1982). Natural selection would then be expected to shape other morphological and physiological traits as well, as is the case with sexes in dioecious species (Putwain & Harper 1972, Harper 1977, Onyekwelu & Harper 1979), another breeding system with dimorphic incompatibility. Differential mortality or clonal growth could bias morph frequencies in a population if one morph is favoured at a site, or locally, if the morphs track environmental heterogeneities within a site and become aggregated (see references on dioecious species in Bierzychudek & Eckhart 1988, Lovett-Doust & Laporte 1991). In the case of niche differentiation it would be expected that morph frequencies are related to environmental conditions, and that morphological differences exist between the morphs. To test these hypotheses, we investigated morph frequencies, spatial relationships and environmental conditions of P. veris in the field and studied morph differences in vegetative and reproductive traits both in the field and in the common garden. We also compared the response of the two morphs to fertilization. We addressed the following specific questions: Is there variation in morph frequencies in natural populations of P. veris? Are deviations of morph frequencies from unity greater in small than in large populations? Is there spatial segregation between the morphs? Are morph frequencies related to population size or to environmental conditions? Are there morph differences in morphological traits or in the response to nutrient addition that would suggest different ecological niches?
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Materials and methods Study species Primula veris L. (Primulaceae) is a distylous, clonal, perennial herb that grows in nutrient-poor calcareous grasslands and along the edges of woodlands from Spain to Eastern Asia (Hegi 1927). The two mating types differ in flower morphology. Pin plants are longstyled and their anthers are near the base of the corolla, whereas in thrum plants the anthers are positioned above the short style. Vegetative daughter rosettes are produced adjacent to the mother plant. Only the below-ground parts, i.e. roots, rhizomes and buds remain alive over winter to form a new rosette and to produce one to four inflorescences in April. The flowers are pollinated mainly by Hymenoptera and Lepidoptera (Hegi 1927). Study sites Field studies were carried out in 1994 and 1995 in pastures and meadows of the Sundgau hills approximately 12 km south-west of the city of Basel, Switzerland, and in adjacent parts of the Jura mountains. The altitude of the study sites ranged from 330–770 m. Groups of plants of P. veris were defined as separate populations if they were separated by at least 50 m from the nearest conspecific plants. As in other parts of Europe (Keymer & Leach 1987), nutrient-poor grasslands have greatly declined in the study area in the last decades (Zoller & Wagner 1986). Therefore, most of the extant populations of P. veris in the study area are probably remnant fragments from far larger and less isolated populations only 20–30 years ago. Because P. veris is long-lived (Tamm 1972), populations cannot have been isolated for more than a few generations. The studies were carried out in haphazard subsets of a total of 76 remnant populations. We did not select study populations with respect to environmental conditions or morph frequencies, but rather studied all populations that we located in the area. The outermost study populations described a triangle with side lengths of 12, 17, and 26 kms. The distance of a study population to the nearest neighbouring population ranged from 50–1000 m (median: 88 m, quartile range: 50–150 m). Flowering population size ranged from 1–80000 flowering rosettes (median: 245, quartile range: 56–2190; see also Fig. 2). Morph frequencies and their relationship with population size In April 1994 and 1995, the number of flowering rosettes (called plants subsequently) was assessed in all
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76 field populations. To estimate the morph frequency, in each population one to 272 plants (mean: 82, SD: 57) were randomly selected and their flower morph recorded. In a clonal distylous plant, biased morph frequencies could also arise if clones grew at different rates. Therefore, we scored plants separated by c. 0.5 m. Given the restricted vegetative growth of P. veris, this made clonal growth an unlikely explanation for biased morph frequencies. Morph frequency was defined as the number of pin plants divided by the total number of sampled plants. Spatial distribution of the morphs within populations We recorded the spatial distribution of morphs in a total of 34 populations in either April 1994 or 1995. Along a transect across each population, one plant was selected approximately every 0.5 m and its flower morph recorded. Because even the largest genets did not extend over more than 0.15 m, the distance of 0.5 m made it unlikely that successive plants along a transect belonged to the same clone. The distribution of genet sizes was highly skewed and ranged from 1–17 with a median of 2 and a quartile range of 1–4. Depending on population size, 30–272 plants per population (mean: 117, SD: 52) were scored for their flower morph. The sequence of morphs in each population was tested for serial independence by a runs test. Relationship between morph frequency and environmental conditions We used several habitat factors to investigate the relationship between morph frequency and environmental conditions. From a map (Schweizer Landestopographie, 3084 Wabern, scale 1 : 25000) we read altitude (to the nearest 10 m), aspect (to the nearest 5°) and slope (to the nearest 5%) for each of 27 P. veris sites (a subset of the total of 76). Aspect was expressed as the absolute deviation in degrees from South. In May 1994, we recorded the composition of the vegetation and determined all vascular plant species and their cover in five randomly selected 2 × 2 m plots in 23 of these P. veris sites. Because of heavy grazing of cattle, the vegetation could not be analysed at the other four sites. We used the number of vascular plant species and their total cover, averaged over the five plots per site, as another measure of habitat quality. Furthermore, we calculated mean indicator values for humidity, light, temperature, continentality, soil reaction, nutrients, soil humus content and soil dispersion, by taking weighted means of the specific indicator values of the species present (Landolt 1977), with weights according to their abundance. These indicator values are similar to those of Ellenberg (Persson 1981,
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Ellenberg et al. 1992), but based on conditions in Switzerland. Next, we analysed gradients in the vegetation composition of the sites by correspondence analysis using the program CANOCO 4.0 (ter Braak & Smilauer 1998) and computed the first four canonical axes. We used the Spearman correlation coefficient to study the relationship between morph frequencies and environmental conditions as measured by each of these 17 habitat variables. Field survey of morphological differences between pin and thrum morphs In April 1994, in each of the 27 populations where environmental factors were studied, 20–24 plants were randomly selected and individually marked unless population size was less than 20, in which case all plants were marked (425 plants in total). The number sampled per population depended on the size of the population (mean: 15.7, SD: 5.8). For each plant, we recorded flower morph, length of the longest leaf, number and length of the flower stalks, number of flowers and flower buds, and number of flowering rosettes of P. veris within concentric rings of 0.2 m, 1 m and 5 m around each target plant. We recorded genet size by counting all plants that touched the target plant and belonged to the same morph. The proportion of flower buds that had developed into open flowers was used as an indicator of the phenological state of a plant. In July 1994, we harvested the above-ground parts of 80 of the marked plants in 12 of the 23 populations. For conservation reasons, no plants were harvested in the four smallest populations, and in the remaining 11 populations all marked plants had been lost to grazing cattle. The seeds of the 80 harvested plants were removed and counted and the remaining plant material was dried at 70 °C for two days. Vegetative parts (leaves) and reproductive parts (flower stalks, fruits and seeds) were weighed separately. Reproductive allocation was calculated as the ratio of reproductive to total biomass. Differences between morph types were analysed with a mixed-model ANOVA. Morph type as a fixed effect was tested against the morph-by-population interaction, while the population main effect and the interaction (random effects) were tested against the residual variation among plants. Variables were logtransformed if necessary to achieve normality and homoscedasticity of residuals (see table legends). Binary data were analysed with logistic regression models (McCullagh & Nelder 1989) and significance tests performed by dividing the mean deviance due to a factor by the appropriate error mean deviance, analogous to the calculation of F-values in ordinary analy-
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sis of variance. Ratios of mean deviances follow an approximate F-distribution (McCullagh & Nelder 1989).
Results
Morphological differences between pin and thrum morphs in a common environment
We recorded the morph of a total of 6232 plants in 76 populations of P. veris ranging in size from 1–80000 plants. The frequency of pin plants (50.7%) in all populations taken together was not significantly different from 50% (sign test, p = 0.27). The frequency distribution of pin plants in 76 populations was symmetrical around 50% and ranged from 0 to 100% (mean: 50.5%; SD: 17.4; Fig. 1). Its mean was again not significantly different from the expected value of 50% (t75 = 0.26, p = 0.80). Seven of the 76 populations consisted of fewer than 10 plants and were not tested statistically for uneven morph frequencies because such a test would have very limited power. Of the remaining 69 populations, 12 (17.4%) had a frequency of pin plants significantly different from 50% (binomial tests, α = 0.05). Ten of these populations were smaller than 450 and two consisted of 1000 and of 1540 plants, with pins predominant in both cases. The frequency of pin plants in a population was independent of population size (Fig. 2a). In contrast, there was a significant negative relationship between the absolute deviation from an even morph frequency and population size (r74 = – 0.61, p < 0.001; Fig. 2b) which held also when the six monomorphic populations were omitted. Six small populations (8% of the 76 sampled), had lost one morph completely. Half of
To test for differences between morphs in a common environment and in response to fertilizer application, a common garden experiment was carried out (see Spillmann 1998 for more details). In summer 1994, we collected seeds in the 12 P. veris populations used in the previous study and in eight additional populations and stored them in paper bags at 7 °C. We germinated them in May 1995 on moist filter paper in petri dishes and planted the seedlings in July 1995 into pots with 8 cm diameter. A total of 2800 P. veris seedlings originating from 113 seed families from 17 populations were planted into 845 pots in the common garden. Seeds from three populations did not germinate. The plants were watered if necessary. There was strong mortality, and in March 1996, 170 surviving plants representing 67 seed families from 14 populations (mean: 4.8, SD: 2.7 seed families per population) were transplanted individually into pots (8 cm diameter) containing commercial potting soil. For each plant the width of the largest leaf and the rosette diameter were measured as an estimate of size. At the end of March 1996, 85 plants each were randomly assigned to either a fertilizer treatment or to the control. Plants in the fertilizer treatment received 1 g of slow release fertilizer (Osmocote) at the start of the experiment and 50 ml of 0.12% solution of a commercial fertilizer (Hauert Flory 2 Typ K, Wädenswil: 18 mg N l–1, 7.2 mg P l–1, 31.2 mg K l–1) in April, May and June 1996. Controls received equivalent amounts of water. In July 1996, we recorded whether the plants flowered and their flower morph and rosette diameter. Relative growth rate in 1996 was expressed as the difference between the logarithms of rosette diameters in July and March. In July, the above-ground parts of the plants were harvested, dried at 70 °C for two days and weighed. Most plants survived the removal of their above-ground parts. In April 1997, we recorded the following traits for all genets that had survived and were flowering: flower morph, number of rosettes, maximum leaf length, number and length of flower stalks, number of open flowers and flower buds per rosette. Both the growth response and the morphological data were analysed with a two-way fixed effects ANOVA. Binary data were analysed using a logistic regression model. For all statistical analyses except the correspondence analysis we used the GenStat statistical package (Payne et al. 1993).
Population morph frequencies and their relationship with population size
Fig. 1. Frequency distribution of the morph frequency (frequency of pin plants) in 76 population fragments of Primula veris. The classes represent proportions of 0, 0.001–0.1, 0.11–0.2, …, and 1. Note that the two outermost bars include exclusively monomorphic populations.
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these populations had lost their pins, and half their thrums. The two largest monomorphic populations consisted of 9 pin and 10 thrum plants, respectively. Spatial distribution of the morphs in the population fragments and population morph frequencies and their relationship with environmental conditions There was no spatial segregation between the morphs at a scale of 0.5 m. Only in two out of 34 populations of P. veris were morph sequences not random (runs test, p < 0.05). The number of significant results thus did not exceed chance expectation. There was no significant relationship between the morph frequency in individual populations and environmental conditions at the sites for any of 17 variables used to quantify environmental conditions (all p-values > 0.05; mean: 0.70, SD: 0.20). Differences between pin and thrum plants in the field and in the common garden There were no morphological differences between the morphs in the field. Of the 425 marked plants in 27 P. veris populations 213 were pin and 212 thrum
Fig. 2. The relationship between morph frequency and population size in 76 population fragments of Primula veris. There are fewer than 76 points because some populations were of the same size. (a) Proportion of pin plants vs. population size. The horizontal line indicates the line of best fit of the relationship between the morph frequency and population size (y = 0.5045 + 0.0002*×). The intercept and the slope are not significantly different from 0.5 and 0, respectively. (b) Deviation from an even morph frequency vs. population size. Deviation was calculated as abs(proportion of pin plants – 0.5). Note the log-scale for population size.
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plants. Pin plants did not differ from thrum plants in any of four vegetative traits, and in only two out of 14 reproductive traits studied. Thrum plants flowered slightly earlier and they produced more flowers per inflorescence. The local density of conspecifics within radii of 0.2 m, 1 m and 5 m around target plants did not differ between the morphs (Table 1). The number of significant differences between morphs did not exceed what would have been expected by chance alone. After a Bonferroni correction of the test-wise error level, morph differences could no longer be called significant. Of the 120 plants that flowered in the common garden experiment, 55 were pin and 65 were thrum. There were no differences between the morphs in the common garden in any of the vegetative or reproductive traits measured (Table 2). Fertilizer addition influenced three traits, but the reaction to fertilizer application was the same in both morphs (no significant morph-by-fertilizer interactions).
Discussion Morph frequencies of heterostylous plants at equilibrium should be even (Fisher 1930, Heuch 1979). However, in the distylous P. veris, some populations exhib-
Fig. 3. The relationship between the deviation from an even population morph frequency and (a) the mean number of seeds per fruit and (b) the mean number of seeds per plant in 16 remnant populations of Primula veris.
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ited significant deviations from equal morph frequencies. These deviations could have been caused by demographic stochasticity, pin advantage or niche differentiation between morphs. The stochastic mortality of plants in the course of habitat fragmentation would re-
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sult in symmetric deviations from an even morph frequency, more strongly biased morph frequencies and morph loss in smaller populations. This is what we found for P. veris. Such mortality would then be equivalent to demographic stochasticity.
Table 1. Mixed-model ANOVA of the effects of morph type and population (a) on vegetative traits and local density of conspecifics and (b) on reproductive traits in 27 natural populations of Primula veris. Means for the pin and thrum morphs are also given. Genet size, total length of stalks, flowers per plant and seeds per plant were log-transformed prior to analysis. The effects on the proportion of open flowers and fruits per flower were analysed with a logistic regression model, therefore the entries are approximate F-values obtained as appropriate ratios of mean deviances (see text). Error levels are test-wise. ***: P < 0.001; **: P < 0.01; *: P < 0.05. Means
F-values
Pin
Thrum
Res. d.f.
Population
Morph
Pop* Morph
a) Vegetative traits Leaf length [cm] Vegetative biomass [g] Mean leaf mass [g] Genet size Density within 0.2 m radius Density within 1 m radius Density within 5 m radius
9.52 0.63 0.15 2.27 0.77 8.30 56.10
9.55 0.65 0.16 2.26 1.18 7.50 52.10
7.58*** 1.30 1.74 4.31*** 21.30*** 7.37*** 31.20***
0.05 0.05 0.14 1.90 1.18 0.93 0.05
0.44 0.58 0.51 1.76* 1.23 1.24 0.76
373 42 42 373 371 372 372
1.29 13.21 17.20 5.78 7.31 0.89 0.64 4.37 20.60 114.00 0.98 127.00 0.24 0.40
1.33 13.36 18.20 6.31 8.85 0.93 0.64 5.25 18.50 101.00 1.17 122.00 0.30 0.37
2.58*** 6.27*** 3.84*** 2.52*** 2.59*** 9.66*** 3.62** 1.96* 4.47*** 2.88** 3.00** 1.92 2.03* 0.89
0.33 0.66 0.89 6.46* 2.76 5.04* 0.23 0.04 3.54 1.51 3.13 0.61 0.07 0.59
2.22*** 0.72 1.51 0.92 1.56 2.00 0.75 0.76 0.51 0.41 2.17 0.17 1.34 0.76
373 373 373 373 373 373 48 60 52 60 46 46 58 37
b) Reproductive traits Number of flower stalks Mean flower stalk length [cm] Total length of flower stalks [cm] Flowers per inflorescence Flowers per plant Prop. of open flowers Fruits per flower Fruits per plant Seeds per fruit Seeds per plant Single seed mass [mg] Seed mass per plant [mg] Reproductive biomass [g] Reproductive allocation
Table 2. ANOVA of the effects of fertilizer application and morph type on vegetative and reproductive traits of Primula veris in the common garden experiment. Means for the pin and thrum morphs are also given. Genet size 1997, total length of flower stalks 1997, number of flowers in the largest rosette 1997 and number of flowers per genet 1997 were log-transformed prior to analysis. This is a fixed-effects model and all effects were tested over the residual variation among plants. Flowering probability of a genet in 1996 and the proportion of open flowers in 1997 were analysed with a logistic regression model, therefore approximate F-values were obtained as appropriate ratios of mean deviances (see text). The residual d.f was 116 in all cases. Error levels are test-wise. ***: P < 0.001; **: P < 0.01; *: P < 0.05. Means Pin a) Vegetative traits Relative growth rate 1996 Above-ground biomass 1996 [g] Leaf length 1997 [cm] Genet size 1997 b) Reproductive traits Flowering prob. of a genet in 1996 Length of flower stalks 1997 Flowers per largest rosette 1997 Flowers per genet 1997 Prop. of open flowers in 1997
F-values Thrum
Fertilizer
Morph
Fert*Morph
0.30 1.06 9.84 1.93
0.31 1.15 9.73 1.94
0.03 3.92* 12.96*** 1.16
0.16 0.90 0.10 0.00
1.36 1.63 0.00 0.94
0.31 11.78 10.28 20.17 0.61
0.32 11.79 10.68 20.04 0.64
0.06 2.34 0.16 0.02 9.62**
0.02 1.25 0.26 0.03 0.29
0.90 0.09 1.37 0.00 0.89
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In contrast, a reproductive advantage of pins, which are less intramorph incompatible than thrums (Wedderburn & Richards 1990), would have resulted in a general pin-bias of morph frequencies that was particularly strong in small populations. The niche differentiation explanation for morph frequency deviations predicted that morph frequencies would be related to environmental factors, and that morphs would be spatially segregated and differ in their morphology. None of these predictions was true for P. veris. Collectively, our results suggest that the random survival of fragments of formerly larger populations of P. veris resulted in deviations of morph frequencies from 50%. This is a case of demographic stochasticity which is difficult to study experimentally, because the process may span many generations. Most studies of demographic stochasticity are therefore modelling exercises (e.g. Goodman 1987, Shaffer 1987, Gabriel & Burger 1992). Stochasticity in a key demographic parameter such as morph frequency has only rarely been demonstrated in plants. Endels et al. (2002) studied 89 declining remnant populations of the distylous Primula vulgaris and found a strikingly similar relationship between morph frequency and population size as we did here. Morph frequencies were equal for large populations (>100 plants). Symmetric deviations were noticeable for smaller populations, and there were several monomorphic populations containing 8 or fewer plants. Very similar patterns have also been found for 18 populations of Primula elatior (Jacquemyn et al. 2002). It might be argued that the larger variance of morph frequencies around 0.5 in smaller populations (Fig. 2a) is a statistical artefact; fewer plants were sampled in small populations to determine the morph frequency. However, this argument is flawed. A much larger fraction of the entire population (and sometimes the entire population) was scored for morph type in small populations. Therefore, the sampling variance around the estimate of the population morph frequency is smaller in small populations or even zero in some very small populations. In contrast, for large populations, our study actually overestimates the variance of morph frequencies around 0.5, because we sampled a much smaller fraction of their plants. Demographic stochasticity is random variation in the survival and fecundity of individuals in a population. Genetic stochasticity in its strict sense is random variation of gene frequencies arising from the sampling error when two gametes form a zygote. In small populations of P. veris, mortality of plants (a demographic process) will alter the morph ratio, i.e. a demographic parameter. However, because the genetic system is so simple, this will automatically map onto a
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changed gene frequency. We prefer to call this process demographic instead of genetic stochasticity, although the distinction is somewhat arbitrary in a distylous plant. In the following, we compare our findings with patterns found in tristylous and dioecious species and discuss some implications of our results. Stochastic processes including founder effects and population bottlenecks have been found to cause biased morph frequencies in several tristylous species (Morgan & Barrett 1988, Eckert & Barrett 1992, Husband & Barrett 1992ab, Eckert & Barrett 1995, Ågren & Ericson 1996, Eckert et al. 1996). An important difference between distylous and tristylous species is that populations of tristylous species that have lost one morph are viable (Eckert et al. 1996) while isolated, monomorphic populations of a distylous species with strict intra-morph incompatibility are doomed to extinction. Biased mating type frequencies in small populations have also been found in one dioecious Silene species (S. otites; Soldaat et al. 1997), but not in another (S. dioica, our analysis of data from Table 1 in Carlsson-Granér et al. 1998). Founder effects have repeatedly been put forward as an explanation of biased morph frequencies in tristylous species (Morgan & Barrett 1988, Eckert & Barrett 1995). It may be argued that they could have played a role in P. veris, too. If new populations were founded by unequal numbers of pins and thrums, morph frequencies may not yet have returned to equilibrium. However, the recent history of the habitat of P. veris in our study area makes such an explanation unlikely. Hardly any new suitable grasslands have been created recently, the fragmentation of P. veris habitats and therefore the distance to possible source populations has greatly increased and dispersal has become more difficult. As an anecdote, our finding of a monomorphic population of 10 thrum plants supports the hypothesis that population fragmentation was responsible for biased morph frequencies and not founder effects, because any reproduction in this population would have produced new pins. We found no morphological differences between the morphs in the field, in the common garden or in their reaction to experimental fertilizer treatment, nor were the morphs spatially segregated. This is consistent with the results of other studies of heterostylous plants (e.g. Husband & Barrett 1992c, Mal & Lovett-Doust 1997, but see Levin 1974, McCall 1996). The loss of an entire morph in a population is the most extreme form of a biased morph frequency. Theoretical models for tristylous species predict that large populations are more resistant to morph loss than small populations (Heuch 1980). The critical size of a population, above which morph loss becomes unlikely, may be quite small. In P. veris (this study), all
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monomorphic populations consisted of 10 or fewer plants, in P. vulgaris (Endels et al. 2002), of 8 or fewer, and in P. elatior (Jacquemyn et al. 2002), the only monomorphic population consisted of 4 plants (H. Jacquemyn, pers. comm.). In the tristylous Lythrum salicaria, morph loss occurred in populations with fewer than 8 (Eckert et al. 1996), 9 (Ågren 1996), 100 (Mal & Lovett-Doust 1997) and 230 plants (Ågren & Ericson 1996). Populations of intra-morph incompatible, distylous plants with a biased morph frequency may experience reduced reproduction and monomorphic populations may even completely fail to reproduce. There is support for this hypothesis in P. veris. From a study on the reproduction of P. veris in populations of different size (Kéry et al. 2000), we had data on the reproductive success in 16 of the 27 populations used to study morph differences in the field. There was a significant negative relationship between the absolute deviation from an even morph frequency and both the mean number of seeds per fruit (Fig 3a; r = – 0.72, p < 0.01) and the mean number of seeds per plant (Fig. 3b; r = –0.59, p < 0.05). Part of the pattern was probably caused by inbreeding depression or reduced pollinator activity in small populations (Kéry et al. 2000). However, these relationships suggest that insufficient mate availability in small populations resulted in pollen limitation and reproductive failure in the smallest populations (see Washitani et al. 1994). Distylous species with a heteromorphic incompatibility system with two mating types may only be a special case of the more general situation of a monomorphic incompatibility system with multiple mating types. For instance, most strictly outcrossing species in small isolated populations are likely to experience pollination limitation (Sih & Baltus 1987). DeMauro (1993) demonstrated the loss of rare mating types in small populations of the endangered plant Hymenoxys acaulis, and Byers (1995) identified only four incompatibility alleles in two populations of a rare monomorphic and outcrossing species. Similarly, in the endangered grassland herb Rutidosis leptorrhynchoides, Young et al. (2000) found a strong relationship between the number of mating types and population size. Demographic stochasticity of mating type frequencies of the kind suggested in this paper may thus exacerbate the problems of small populations not only of heterostylous species but of all those with multiple mating types. Acknowledgements. The authors wish to thank Jon Ågren, Giorgina Bernasconi, Markus Fischer, Benedikt Schmidt and the members of the Cosecha 2001 of the Institut für Umweltwissenschaften Journal Club for valuable comments on draft versions of this work. Hans-Heiner
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“John” Spillman provided us with additional data and with his experimental plants. The late Karl Altenbach and Stefan Peter saved some of our study populations from mowing. Josiane Rey was a great help in the field and also elsewhere. Many thanks to them all. This study was supported by the Swiss National Science Foundation, Priority Programme Environment (grants No. 5001-35231 to D. Matthies and B. Schmid and No. 5001-44626 to D. Matthies, B. Schmid and P. J. Edwards).
References Ågren J (1996) Population size, pollinator limitation, and seed set in the self-compatible herb Lythrum salicaria. Ecology 77: 1779–1790. Ågren J, Ericson L (1996) Population structure and morphspecific fitness differences in tristylous Lythrum salicaria. Evolution 50: 126–139. Andersson S (1994) Unequal morph frequencies in populations of tristylous Lythrum salicaria (Lythraceae) from southern Sweden. Heredity 72: 81–85. Barrett SCH (1992) Heterostylous genetic polymorphisms: Model systems for evolutionary analysis. In: Barrett SCH (ed) Evolution and function of heterostyly. Springer-Verlag, Berlin, pp 1–29. Byers DL (1995) Pollen quantity and quality as explanations for low seed set in small populations exemplified by Eupatorium (Asteraceae). American Journal of Botany 82: 1000–1006. Bierzychudek P, Eckhart V (1988) Spatial segregation of the sexes of dioecious plants. American Naturalist 132: 34–43. Carlsson-Granér U, Elmqvist T, Ågren J, Gardfjell H, Ingvarsson P (1998) Floral sex ratios, disease and seed set in dioecious Silene dioica. Journal of Ecology 86: 79–91. Casper BB, Charnov EL (1982) Sex allocation in heterostylous plants. Journal of Theoretical Biology 96: 143–149. DeMauro MM (1993) Relationship of breeding system to rarity in the lakeside daisy (Hymenoxys acaulis var. glabra). Conservation Biology 7: 542–550. Eckert CG, Barrett SCH (1992) Stochastic loss of style morphs from populations of tristylous Lythrum salicaria and Decodon verticillatus (Lythraceae). Evolution 46: 1014–1029. Eckert CG, Barrett SCH (1995) Style morph ratios in tristylous Decodon verticillatus (Lythraceae): Selection vs. historical contingency. Ecology 76: 1051–1066. Eckert CG, Manicacci D, Barrett SCH (1996) Genetic drift and founder effect in native versus introduced populations of an invading plant, Lythrum salicaria (Lythraceae). Evolution 50: 1512–1519. Ellenberg H, Weber HE, Düll R, Wirth V, Werner W, Paulissen D (1992) Zeigerwerte von Pflanzen in Mitteleuropa. 2. Aufl. Scripta Geobotanica 18: 1–258. Endels P, Jacquemyn H, Brys R, Hermy M, de Blust G (2002) Temporal changes (1986–1999) in populations of primrose (Primula vulgaris Huds.) in an agricultural landscape and implications for conservation. Biological Conservation 105: 11–25.
Basic Appl. Ecol. 4, 3 (2003)
206
M. Kéry et al.
Fisher RA (1930) The genetical theory of natural selection. Dover, New York. Fuller E (2001) Extinct birds. Oxford University Press, Oxford. Gabriel W, Burger R (1992) Survival of small populations under demographic stochasticity. Theoretical Population Biology 41: 44–71. Goodman D (1987) The demography of chance extinction. In: Soulé ME (ed) Viable populations for conservation. Cambridge University Press, Cambridge, pp 11–34. Harper JL (1977) Population Biology of Plants. Academic Press, London. Hegi G (1927) Illustrierte Flora von Mitteleuropa. Lehmanns Verlag, München. Heuch I (1979) Equilibrium populations of heterostylous plants. Theoretical Population Biology 15: 43–57. Heuch I (1980) Loss of incompatibility types in finite populations of the heterostylous plant Lythrum salicaria. Hereditas 92: 53–57. Husband BC, Barrett SCH (1992a) Effective population size and genetic drift in tristylous Eichhornia paniculata (Pontederiaceae). Evolution 46: 1875–1890. Husband BC, Barrett SCH (1992b) Genetic drift and the maintenance of style length polymorphism in tristylous populations of Eichhornia paniculata (Pontederiaceae). Heredity 69: 440–449. Husband BC, Barrett SCH (1992c) Pollinator visitation in populations of tristylous Eichhornia paniculata in northeastern Brazil. Oecologia 89: 365–371. Jacquemyn H, Brys R, Hermy M (2002) Patch occupancy, population size and reproductive success of a forest herb (Primula elatior) in fragmented landscape. Oecologia 130: 617–625. Kéry M, Matthies D, Spillmann H-H (2000) Reduced fecundity and offspring performance in small populations of the declining grassland plants Primula veris and Gentiana lutea. Journal of Ecology 88: 17–30. Keymer RJ, Leach SJ (1987) Calcareous grassland – a limited resource in Britain. In: Hillier SH, Walton DWH, Wells DA (eds) Calcareous grasslands – ecology and management. Bluntisham Books, Bluntisham, pp 11–17. Landolt E (1977) Ökologische Zeigerwerte zur Schweizer Flora. Veröffentlichungen des Geobotanischen Institutes der ETH, Stiftung Rübel, Zürich. Landolt E (1991) Gefährdung der Farn- und Blütenpflanzen in der Schweiz. Bundesamt für Umwelt, Wald und Landschaft (BUWAL), Bern. Levin DA (1974) Spatial segregation of pins and thrums in populations of Hedyotis nigricans. Evolution 28: 648–655. Lewis D, Jones DA (1992) The genetics of heterostyly. In: Barrett SCH (ed) Evolution and function of heterostyly. Springer-Verlag, Berlin, pp 129–150. Lovett-Doust J, Laporte G, (1991) Population sex ratios, population mixtures and fecundity in a clonal dioecious macrophyte Vallisneria americana. Journal of Ecology 79: 477–490. Mal KM, Lovett-Doust J (1997) Morph frequencies and floral variation in a heterostylous colonizing weed, Lythrum salicaria. Canadian Journal of Botany 75: 1034–1045. McCall C (1996) Gender specialization and distyly in Hoary Puccoon, Lithospermum croceum (Boraginaceae). American Journal of Botany 83: 162–168.
Basic Appl. Ecol. 4, 3 (2003)
McCullagh P, Nelder JA (1989) Generalized linear models. Chapman and Hall, London. Morgan MT, Barrett SCH (1988) Historical factors and anisoplethic population structure in tristylous Pontederia cordata: A reassessment. Evolution 42: 496–504. Onyekwelu SS, Harper JL (1979) Sex ratio and niche differentiation in spinach (Spinacia oleracea L.). Nature 282: 609–611. Payne RW, Lane PW, Digby PGN, Harding SA, Leech PK, Morgan GW, Todd AD, Thompson R, Tunnicliffe Wilson G, Welham SJ, White RP (1993) Genstat 5, Release 3, Reference Manual. Clarendon Press, Oxford. Persson S (1981) Ecological indicator values as an aid in the interpretation of ordination diagrams. Journal of Ecology 69: 71–84. Putwain PD, Harper JL (1972) Studies in the dynamics of plant populations. V. Mechanisms governing the sex ratio in Rumex acetosa and R. acetosella. Journal of Ecology 60: 113–129. Richards JH, Ibrahim HB (1982) The breeding system in Primula veris L. II. Pollen tube growth and seed-set. New Phytologist 90: 305–314. Shaffer ML (1987) Minimum viable populations: coping with uncertainty. In: Soulé ME (ed) Viable populations for conservation. Cambridge University Press, Cambridge, pp 69–86. Sih A, Baltus M-S (1987) Patch size, pollination behavior, and pollinator limitation in Catnip. Ecology 68: 1679–1690. Soldaat LL, Vetter B, Klotz S (1997) Sex ratio in populations of Silene otites in relation to vegetation cover, population size and fungal infection. Journal of Vegetation Science 8: 697–702. Spillmann H-H (1998) Studies on the population biology of Primula veris L. s.str. and Gentiana lutea L. Diploma thesis, University of Zürich. Tamm CO (1972) Survival and flowering of perennial herbs III: The behaviour of Primula veris on permanent plots. Oikos 23: 159–166. ter Braak CJF, Smilauer P (1998) CANOCO Reference Manual and User’s Guide to Canoco for Windows: Software for Canonical Community Ordination (version 4). Microcomputer Power, Ithaca, NY. Washitani I, Osawa R, Namai H, Niwa M (1994) Patterns of female fertility in heterostylous Primula sieboldii under severe pollinator limitation. Journal of Ecology 82: 571–579. Wedderburn F, Richards AJ (1990) Variation in within-morph incompatibility inhibition sites in heteromorphic Primula L. New Phytologist 116: 149–162. Weller SG (1976) Breeding system polymorphism in a heterostylous species. Evolution 30: 442–454. Young AG, Brown ADH, Murray BG, Miller CH (2000) Genetic erosion, restricted mating and reduced viability in fragmented populations of the endangered grassland herb Rutidosis leptorrhynchoides. In: Young AG, Clarke GM (eds) Genetics, demography and viability of fragmented populations. Cambridge University Press, Cambridge, pp 335–359. Zoller H, Wagner C (1986) Rückgang und Gefährdung von Mesobromion-Arten im Schweizer Jura. Veröffentlichungen des Geobotanischen Institutes der ETH 87: 239–259.