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Ecological effects on estimates of effective population size in an annual plant
Biological Conservation 143 (2010) 946–951
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Ecological effects on estimates of effective population size in an annual plant E.K. Espeland a,*, K.J. Rice b a b
USDA ARS Pest Management Research Unit, 1500 N. Central Avenue, Sidney, MT 59270, USA Ecology Graduate Group, Department of Plant Sciences, Mail Stop 1, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA
a r t i c l e
i n f o
Article history: Received 13 April 2009 Received in revised form 28 December 2009 Accepted 3 January 2010 Available online 25 January 2010 Keywords: Abiotic stress Competition Effective population size Genetic drift Plantago erecta Local adaptation Serpentine Conservation genetics
a b s t r a c t Effective population size (Ne) is a critical indicator of the vulnerability of a population to allele loss via genetic drift, and it can also be used to assess the evolutionary potential of a population. While some plant conservation plans have focused on outcrossing through cross-pollination as a way to increase estimated Ne, variance in reproductive output determined by ecological factors such as competition can also strongly affect estimated Ne. We examined the effects of intraspecific and interspecific competition, stressful soils, and local adaptation on estimates of Ne in an annual plant species. While ecological influences on plant growth rate variance have been predicted to influence estimates of Ne/N, we found a significant effect on the estimate of Ne/N, but no significant ecological effects on growth rate variance. Lower survivorship on stressful soil was the most important effect reducing estimates of Ne/N. If stochastic mortality is greater in environments that are abiotically stressful, then populations in these stressful environments may be slower to adapt because of lower census sizes and reduction of Ne /N. In populations of conservation concern, increasing survivorship may be of greater benefit for maximizing Ne than the reduction of variability in reproductive output among surviving adults. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Effective population size (Ne) can be used to assess the evolutionary potential of a population because it is an important indicator of the vulnerability of a population to allele loss through random processes (genetic drift). A population with a very low Ne is more susceptible to genetic drift and less able to respond to selection. This is because in small populations there is less genetic variation for natural selection to act upon, and there is a higher probability that beneficial alleles will not be maintained by selection and will instead be lost from the population because of random drift effects (Willi et al., 2007). Modeling population genetic processes in conservation has become relatively widespread (Halley and Manasee, 1993; Higgins and Lynch, 2001; Obioh and Isichei, 2007; Pertoldi et al., 2007; Palstra and Ruzzante, 2008), and when Ne is incorporated into minimum viable population size (MVP) models, MVP as censused may need to be substantially more than 5000 individuals (Obioh and Isichei, 2007). Rare and endangered plant populations in particular often have very low census sizes (N) and even lower estimated Ne (Chung et al., 2007; Zietsman et al., 2008). Plant and animal populations of conservation concern tend to have multiple factors acting to reduce Ne
* Corresponding author. Tel.: +1 406 433 9416; fax: +1 406 433 5038. E-mail addresses: [email protected] (E.K. Espeland), kjrice@ucdavis. edu (K.J. Rice). 0006-3207/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2010.01.003
(Lande, 1988; Frankham, 1995; Hedrick, 1995; Frankham et al., 2003), many of which are ecological in nature. Published estimates of Ne in plant populations have most often been used in a descriptive manner to characterize population subdivision and differentiation (Tremblay and Ackerman, 2001; Latta, 2008). Conservation biologists working with both plants and animals have used Ne estimates to explore the importance of population bottlenecks on genetic diversity (Frankham, 1995; Amos and Harwood, 1998). Other conservation uses of Ne estimates have focused on reconstructions of historical anthropogenic effects on gene flow and genetic diversity (Levy and Neal, 1999; Morris et al., 2002). Ne can be estimated in a number of ways, and there are three commonly-used types of Ne. Inbreeding Ne describes the probability of mating among relatives, therefore lowering genetic diversity of the population. Variance Ne describes the probability that individuals will pass on their genes to the next generation, with increased variance in offspring number leading to a decrease in Ne. Extinction or eigenvalue Ne describes the rate of loss of heterozygosity, with a calculation of an asymptotic Ne as the outcome. Although estimates of the three types of Ne often have the same result (Vitalis and Couvet, 2001), they can differ, so the choice of which type to calculate may revolve around the genetic process of interest (Caballero, 1994). Inbreeding Ne is often estimated via genetic methods by calculating the percent heterozygosity and comparing it to the expected heterozygosity in the population if
E.K. Espeland, K.J. Rice / Biological Conservation 143 (2010) 946–951
it were in equilibrium (Hartl and Clark, 2007). Inbreeding Ne can also be estimated as the geometric mean of population sizes through time (Caballero, 1994). Variance Ne is estimated as the variance in reproductive outputs among individuals in a population (Heywood, 1986). Published estimates of Ne /N for annual plants tend to be between 5% and 30%, with ratios most often below 10% (Heywood, 1986; Husband and Barrett, 1992; Goldringer et al., 2001; Siol et al., 2007). Recent work by Siol et al. (2007) suggests that relative reproductive variance must be taken into account in order to correctly estimate Ne. As understanding of the influence of reproductive variance on Ne estimation has increased, evidence has also accumulated to demonstrate that ecological factors can be extremely important in determining this variance (Van Kleunen et al., 2001, 2005). In fact, ecological models of density-dependence have already been theoretically extended to the estimation of Ne (Rice, 1990; Koide and Dickie, 2002; Van Kleunen et al., 2001, 2005). Other ecological factors such as herbivory (Doak, 1992) can affect the variance in reproductive outputs in plants and thus influence estimates of Ne/N. Although Ne is often estimated from the geometric mean of population sizes of above-ground plants, seed banks increase Ne above this estimate because the seed bank is often not part of the censused population size (Nunney, 2002). Thus, ecological factors that reduce the seed bank such as granivory and fungal infection will act to reduce Ne. The linkage between variation in reproductive output and estimates of Ne in annual plants was explored theoretically by Heywood (1986) and can be expressed by the following relationship:
Ne 1 ¼ ½ð1 þ FÞðr2 =l2 Þ þ 1 N
ð1Þ
where the inbreeding coefficient F ranges from 0 (complete outcrossing) to 1 (complete inbreeding), r2 = variance in reproductive output, and l = mean reproductive output. It follows that if an annual plant population with a large census size has a few individuals that produce the majority of the seed, reproductive variance is high and estimated Ne/N is small. In contrast, Ne is larger and estimated Ne/N is close to 1 in an annual plant population of large census size that has a variance in reproductive output that approximates a Poisson distribution. While the absolute value of Ne is more important than this ratio, we can use this ratio to determine the environmental influences that have the most important effect on estimates of Ne. Because N in plant populations is often determined by largerscale factors that we cannot influence, such as rainfall patterns (Zietsman et al., 2008), the estimated Ne /N ratio is more useful as a relative index to apply to the evaluation of conservation practices that may maximize Ne. In annual plant species, size is highly correlated with reproductive output, and competition is one of the main determinants of plant size (Harper, 1977). Models of competitive interactions not only predict relative plant sizes, but also the distribution of plant sizes in a population and thus the coefficient of variation in plant sizes. In annual plants, the coefficient of variation in plant sizes is directly related to the coefficient of variation in reproductive outputs (Heywood, 1986). A relevant model of above-ground asymmetric competition is where large plants shade small ones and the coefficient of variation (CV) of plant sizes increases with density (Weiner and Thomas, 1986). This larger CV of plant sizes in high density populations decreases estimated Ne/N (see Eq. (1)). A facilitative model of plant–plant interactions can also affect the CV of reproductive outputs. Resource sharing via common mycorrhizal networks can assist resource capture in smaller plants and increase their growth rates relative to larger plants (Shumway and Koide, 1995; Selosse et al., 2006). This type of equalizing
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resource distribution reduces the CV of plant sizes as densities increase (Koide and Dickie, 2002). This would lead to a relative increase in estimated Ne /N when seed output is positively correlated with plant size. In this experiment we used populations of the California native annual forb, Plantago erecta (E. Morris), to examine environmental and evolutionary influences on variance in reproductive output and estimated Ne. We designed the experiment to test the relative importance of density-dependent interactions and interspecific competition on reproductive hierarchies, and how stressful soils and local adaptation may alter estimates of Ne/N.
2. Methods and materials P. erecta is an annual plant with a native range extending from Baja California and Arizona north through the California Floristic Province to southern Oregon. Although completely self-compatible, some outcrossing is likely in this species (E. Espeland, unpublished data). Populations of P. erecta typically occur at high densities and are found in shallow, low fertility soils (e.g., serpentine outcrops, road cuts) as well as in deep, more fertile grassland soils. The experiment was conducted at McLaughlin Reserve (38.52°N, 122.24°W), located within the California North Coast Range (Hickman, 1993) and operated by the University of California Natural Reserve System. Seeds were collected from the field in spring 2004 from four serpentine populations and four non-serpentine populations at McLaughlin Reserve. Serpentine soil is a nutrient-limited, ultramafic soil type characterized by low calcium, very high levels of exchangeable magnesium (Jurjavcic et al., 2002), and toxic heavy metals: a stressful environment for plant growth (Kruckeberg, 1984). Serpentine areas are also often drier than non-serpentine areas (Macnair et al., 1989). Serpentine populations of P. erecta were identified by the presence of serpentine endemic plant species. Serpentine grasslands occur in a mosaic with loam grasslands at this site. Four populations on each soil type within a geographic range of three linear kilometers were selected to span a range of productivities (estimated by total above-ground biomass; E. Espeland, unpublished data) and were spatially blocked into serpentine/loam paired plots. Each population was about 0.5 ha in size. Seeds from 30 families from each population were bulked by soil type (serpentine or non-serpentine) before replanting into field collection locations. To increase germination, dry seeds were chilled at 4 °C for 10 days prior to sowing. The seeds were then planted into 16 circular 38.5 cm2 plots in a 1 m 1 m area at each of the eight locations on October 10 and 11, 2004. A factorial combination of two levels of interspecific competition, two sowing densities, and two seed sources (serpentine, non-serpentine) was replicated twice at each location. The 16 plots at each location comprised a randomized complete block design. For the high sowing density treatment, 15 seeds were affixed to a single 0.2 cm2 piece of tissue paper with water-soluble glue. This group of seeds was placed in the center of the plot and buried at a depth of 2 mm. For the low density sowing treatment, each seed was glued to a single toothpick and 10 toothpicks were inserted into the soil of each plot so that the seed was buried at a depth of 2 mm. The 10 toothpicks were evenly spaced over the plot area. Interspecific competition was manipulated experimentally. For half the plots, only P. erecta germinating from the resident seed bank was removed throughout the experiment. For the other half of the plots, all plants except for the planted P. erecta plants were removed throughout the fall and winter. This resulted in a range of biomass of plant species other than Plantago in the plots (hereafter referred to as ‘‘interspecific biomass”).
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Survivorship was tracked over the growing season as well as the height of each P. erecta plant in our plots. Plants were harvested prior to seed ripening at each site in spring 2005 (March 10, 2005 to June 7, 2005). Seed output was estimated by multiplying the number of flowers + buds by two, as there are two ovules per flower and P. erecta rarely aborts ovules (61% abortion rate, E. Espeland, unpublished data). P. erecta flowers synchronously at each location, much more synchronously than congeners found elsewhere such as P. aristata and P. patagonica (E. Espeland, personal observation). Because there is a relatively long period of time between flowering and seed ripening (3–5 weeks, E. Espeland, personal observation), we are confident that all plants that would have flowered that year had done so by harvest time. To ensure we did not underestimate the reproductive output of any plants that may have begun flowering later than the main cohort, we included unopened buds as well as open flower numbers in our reproductive output calculation. This is a measure of maternal fitness; we did not attempt to measure paternal fitness in this experiment. At harvest, all interspecific plant biomass was harvested from each plot, placed in a drying oven at 65 °C for 6 days, and then weighed. Because we measured emergence, not germination, in the field experiment, we conducted a growth chamber experiment to examine inherent differences in germination rates between serpentine and non-serpentine populations. Of the eight populations studied, seeds were collected from three serpentine and three non-serpentine populations during March 2008. These seed collections representing different seed source soil types were stored at room temperature until August 26, 2008. On that date, three Petri plates, fitted with moistened filter paper, and each containing 25 seeds each per population were placed in a dark germination chamber (12 h at 4 °C and 12 h at 16 °C for 14 days). Plates were scored semi-weekly for germination, and germinants were removed at each census.
2.1. Data analysis We sowed P. erecta at two extremely different densities to test if extremes in intraspecific density affect the response to the other experimental treatments. Effects of sowing density, interspecific biomass, location soil type (serpentine or non-serpentine soil at planting location), and seed source soil type on plant size CV and relative reproductive variance were assessed. The number of P. erecta that emerged in each plot was used as a covariate. CV was calculated as the coefficient of variation (standard deviation/mean) in plant sizes at the end of the experiment. This estimates the variance in plant growth rates within the population (Rice, 1990). Relative reproductive variance was calculated as the CV2 (variance/ mean2) of estimated seed output for plants that germinated in the plot, including zeros for plants that died. This calculation of relative reproductive variance relates directly to estimated Ne/N (see Eq. (1)). Because plant size is highly correlated with reproduction in this species (Espeland and Rice, 2007), the two dependent variables, CV and relative reproductive variance, should differ primarily by the inclusion of zeros in the relative reproductive variance calculation, thus partitioning the relative effect of survivorship on estimated Ne/N. We tested a subset of interactions of interest in our statistical model. Source soil by planting soil was tested to determine the effect of possible local adaptation. To determine the effect of soil type on inter- and intra-specific interactions, the interactions of (1) planting soil by interspecific biomass and (2) planting soil by sowing density were tested. To determine the effect of local adaptation on each of these two-way interactions, source soil was added to test the significance of three-way interactions. The data
were close to normality (Shapiro-Wilk >0.88), and no transformation of CV or relative reproductive variance was necessary. To test for fitness effects of local adaptation across life history stages, we examined the interaction of planting soil and source soil on the number of flowers produced per seed sown using a Tukey HSD test. Our seed collections were bulked across interspersed serpentine and non-serpentine sites, and we are therefore confident that any differences between serpentine and non-serpentine sources are due to the influence of soil type and not site specific factors. Because we used field collected seed, the influence of seed source on fitness may be due to maternal environmental effects, heritable genetic variation, or both. 3. Results Plants from non-serpentine sources produced significantly flowers per seed sown on serpentine soil (p < 0.05). Serpentine sources did not differ significantly (p > 0.05) in flower production between the two soil types, although the trend for serpentine sources was to produce more flowers than non-serpentine sources on serpentine soil and fewer flowers than non-serpentine sources on non-serpentine soil (Fig. 1). There were no significant treatment effects on the CV of plant sizes at the end of the growing season (Table 1, left side). Planting soil type was the only treatment factor that had a significant effect on relative reproductive variance (Table 1, right side). Plants growing on non-serpentine soil exhibited 14% lower relative reproductive variance than plants growing in serpentine soil (Table 2, left side). Germination totals and rates did not differ significantly between serpentine and non-serpentine collections in the 2008 germination trial (p > 0.7). Over the 14-day trial period, germination in non-serpentine sources averaged 92 ± 13% SD while germination for serpentine sources averaged 86 ± 13% SD. Using Eq. (1), we estimated Ne/N ratios for significant treatment effects on relative reproductive variance. Estimated Ne /N values were 8% smaller in plant populations growing on serpentine soil compared to populations growing on non-serpentine (Table 2, right side). Because the results for CV and relative reproductive variance differed, this indicated that survival may play a key role in estimates of Ne/N. Therefore, we performed an analysis of treatment effects on survivorship. We applied the same statistical model we used for CV and relative reproductive variance to arcsine transformed percent survivorship. Only the interaction between interspecific biomass and planting soil type was close to significant in determining survivorship (p = 0.07). Competition from interspecifics had no significant effect on survivorship on non-serpentine soil, but increased interspecific biomass decreased survivorship on serpentine soil (arcsine% survivorship = 1.25 0.55 g interspecific biomass, R2 = 0.08, p < 0.02). 4. Discussion and conclusions In our experiment, non-serpentine seed sources are more strongly adapted to the less stressful non-serpentine soil. Seeds from serpentine sources do not perform significantly differently on the two soil types. Although we found evidence for local adaptation, local adaptation does not appear to have an important influence on relative reproductive variance and estimated Ne/N. In addition, if ecological factors that influence variance in plant growth rates also influence relative reproductive variance, we would expect parallel results in our CV and relative reproductive variance analyses. However, plant growth rate variance (CV) was unaffected by our experimental treatments, and
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Number of flowers per seed sown
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2
A AB 1.5
AB B
1
0.5
0
Planting soil
Nonserpentine
Source soil
Serpentine
Nonserpentine
NONSERPENTINE
Serpentine
SERPENTINE
Fig. 1. Non-serpentine sources produce the most flowers per seed sown in home soil, suggesting local adaptation to soil type (bars with different letters are significantly different, p < 0.05). Serpentine sources show a trend towards local adaptation, but this difference is not significant.
Table 1 Statistical tables of (a) treatment and covariate effects on coefficient of variation and on relative reproductive variance, (b) growing soil and source soil on number of flowers per seed sown, p < 0.05 in bold. Source
(a) #P. erecta emerged Block Sowing density (D) Interspecific biomass (B) Soil Source Soil source Soil B Source soil D Source soil D
Coefficient of variation
Relative reproductive variance
DF
SS
F-ratio
P>F
SS
F-ratio
P>F
1 3 1 1 1 1 1 1 1 1
0.423 0.816 0.300 0.052 0.306 0.204 0.274 0.002 0.047 0.423
3.29 2.11 2.33 0.41 2.39 1.58 2.13 0.02 0.37 3.29
0.072 0.103 0.130 0.524 0.125 0.211 0.147 0.896 0.546 0.072
0.072 0.587 0.028 0.010 0.274 0.062 0.152 0.009 0.0005 0.036
1.28 3.44 0.498 0.178 4.83 1.10 2.68 0.16 0.01 0.64
0.261 0.019 0.482 0.674 0.030 0.297 0.105 0.687 0.922 0.427
Source
DF
SS
F-ratio
P>F
(b) Soil Source Soil source
1 1 1
1.064 0.141 4.441
1.591 0.212 6.641
0.210 0.646 0.011
Table 2 Average for relative reproductive variance (RRV) ± one standard deviation, plus estimated Ne/N by location soil.
Non-serpentine Serpentine
RRV
Ne/N
0.59 ± 0.20 0.69 ± 0.29
0.74 0.68
relative reproductive variance was significantly influenced only by planting soil type and block. Stressful serpentine soil increased relative reproductive variance compared to non-stressful soil. Although we would expect below-ground competition on stressful soil types to decrease plant size hierarchies and decrease plant growth rate variance, this was not the case. Plant growth rate variance was not affected by planting soil, and relative reproductive variance was higher, not lower, in populations on the stressful soil type. The lack of parallel between CV and relative reproductive variance indicates that survivorship plays a significant role in the observed relationship between soil type and estimated Ne/N. When survivorship is the main driver of estimated Ne/N, stressful environments may intrinsically have lower estimated Ne /N as well as lower census sizes. Low census sizes in plant populations mean that the accumulation of new variation through mutation
is unlikely, and that inbreeding will decrease heterozygosity. The census size of plant populations is positively correlated with fitness measures (Reed, 2005), and in a recent meta-analysis Leimu and Fischer (2008) showed that populations of small census size are less likely to be locally adapted. This meta-analysis is supported by individual studies showing a positive ability for larger populations to adapt to new environments (Willi and Hoffman, 2009). When lowered survival not only makes census sizes small but also makes the estimated Ne /N ratio small, this suggests that the population will also be less likely to retain any beneficial genetic variation because of genetic drift. Plant populations growing on stressful soil types have a twofold obstacle to local adaptation: reduced population growth rates and reduced genetic variation (low Ne /N) less able to respond to selection (Willi and Hoffman, 2009). Plants and other sessile organisms that are limited in their ability to sample the environment may thus suffer doubly in stressful environments. Plant populations then face a more difficult evolutionary hurdle in adapting to stressful environments compared to non-stressful environments. In contrast, populations growing under less stressful abiotic conditions, with larger census sizes and larger estimated Ne /N, may both generate and harbor more genetic variation. Greater evolutionary potential in populations with larger estimated Ne /N should allow them to more effectively respond to natural selection
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and adapt to local conditions. While stress-adapted species and populations are commonly documented (Chapin et al., 1993), adaptation to stressful environments may be less common than expected because lower census sizes and lower estimated Ne/N may slow the adaptation process. Within stressful environments, the balance between strong selection regimes and the evolutionary capacity of populations to respond to selection is an important consideration in the management of populations of conservation concern. In small populations of conservation concern, factors that reduce estimated Ne will also decrease the ability of a population to retain beneficial alleles. These factors may be ameliorated with appropriate management practices. In this case, with a common plant species, estimated Ne /N was well above 0.5, indicating that reproductive variance is not the main influence on genetic drift processes in these populations (Heywood, 1986). Ecological influences on survivorship, not growth rates, were the main factor determining estimated Ne/N in P. erecta. No studies that we know of have examined the effect of differential survival on estimated Ne /N estimates in plants, however, survivorship is an extremely important factor in estimating Ne /N in marine species (Waples, 2002). Maximizing survivorship within semelparous species of conservation concern may be more important than reducing the variability among surviving breeding adults in these species. To ensure that plant populations have the potential to adapt to changing environmental conditions, we need to further investigate ecological drivers of relative reproductive variance that include survivorship, within plant populations in order to limit the effects of genetic drift within these populations. Although some plant conservation plans have focused on promoting cross-pollination as a way to increase estimated Ne (Ingvarsson, 2002; Porcher et al., 2004), variance in reproductive output can often have a much greater effect on estimated Ne than breeding system (Heywood, 1986). Plant conservators are beginning to prioritize reducing reproductive variance in order to maintain genetic diversity (Yonezawa et al., 1996; Vencovsky and Crossa, 2003). Our results show that mortality may strongly influence reproductive variance, thus reducing estimated Ne/N. The source of mortality in our experiment appeared to be fungal infection at the seedling stage. In some cases plant conservationists may be able to increase survivorship in populations of concern, for example by excluding predators in select years (Espeland et al., 2005). Effective population size may need to be managed very differently for plants and animals. Colonizing species such as annual plants produce large numbers of progeny with the same genetic background, and not all of these progeny will survive. In these colonizing species, mortality may play a larger role in genetic drift processes compared to species whose population growth rates are determined by intraspecific competition, such as large herbivores (Waples, 2002). Environmental factors can very strongly affect Ne /N estimates and thus environmental factors may be primary determinants of a population’s ability to retain genetic variation through time.
Acknowledgements This work was performed at the McLaughlin Reserve of the University of California Natural Reserve System (UCNRS) and partially funded by a Mildred E. Mathias Grant for work at the UCNRS to EKE and a Packard Foundation Interdisciplinary Science Grant (200001607) to both K.J.R. and E.K.E. Thanks to S. Mueller for field assistance, to M. O’Mara for lab assistance, and to S. Harrison, K. Moore, S. Elmendorf, M. Schlesinger, K. Jones, J. Harding, the Big Science Lab, D. Grace, J. Weiner, S. Sultan, A. Caballero, J. Gaskin, and E. Leger for comments on early versions of the manuscript.
References Amos, W., Harwood, J., 1998. Factors affecting levels of genetic diversity in natural populations. Philosophical Transactions of the Royal Society of London Series B – Biological Sciences 353, 177–186. Caballero, A., 1994. Developments in the prediction of effective population size. Heredity 73, 657–679. Chapin, F.S., Autumn, K., Pugnaire, F., 1993. Evolution of suites of traits in response to environmental stress. American Naturalist 142, S78–S92. Chung, M.Y., Park, C.W., Chung, M.G., 2007. Extremely low levels of allozyme variation in southern Korean populations of the two rare and endangered lithophytic or epiphytic Bulbophyllum drymoglossum and Sarcanthus scolopendrifolius (Orchidaceae): implications for conservation. Biodiversity and Conservation 16, 775–786. Doak, D.F., 1992. Lifetime impacts of herbivory for a perennial plant. Ecology 73, 2086–2099. Espeland, E.K., Carlsen, T.M., MacQueen, D., 2005. Fire and dynamics of granivory on a rare California grassland forb. Biodiversity and Conservation 14, 267–280. Espeland, E.K., Rice, K.J., 2007. Facilitation across stress gradients: the importance of local adaptation. Ecology 88, 2404–2409. Frankham, R., 1995. Effective population size/adult population size ratios in wildlife: a review. Genetics Research 66, 95–107. Frankham, R., Ballou, J.D., Brisco, D.A., 2003. Introduction to Conservation Genetics. Cambridge University Press, Cambridge, UK. Goldringer, I., Enjalbert, J., Raquin, A.-L., Brabant, P., 2001. Strong selection in wheat populations during ten generations of dynamic management. Genetics Selection Evolution 33, S441–S463. Halley, J.M., Manasee, R.S., 1993. A population-dynamics model for annual plants subject to inbreeding depression. Evolutionary Ecology 7, 15–24. Harper, J.L., 1977. Population Biology of Plants. Academic Press, New York, NY. Hartl, D.L., Clark, A.G., 2007. Principles of Population Genetics. Sinauer Associates, Sunderland, MA. Hedrick, P.W., 1995. Elephant seals and the estimation of a population bottleneck. Journal of Heredity 86, 232–235. Heywood, J.S., 1986. The effect of plant size variation on genetic drift in populations of annuals. The American Naturalist 127, 851–861. Hickman, J.C., 1993. The Jepson Manual: Higher Plants of California. University of California Press, Berkeley, CA. Higgins, K., Lynch, M., 2001. Metapopulation extinction caused by mutation accumulation. Proceedings of the National Academy of Sciences 98, 2928–2933. Husband, B.C., Barrett, S.C.H., 1992. Effective population-size and genetic drift in tristylous Eichhornia paniculata (Pontederiaceae). Evolution 46, 1875–1890. Ingvarsson, P.K., 2002. A metapopulation perspective on genetic diversity and differentiation in partially self-fertilizing plants. Evolution 56, 2368–2373. Jurjavcic, N.L., Harrison, S., Wolf, A.T., 2002. Abiotic stress, competition and the distribution of the native annual grass Vulpia microstachys in a mosaic environment. Oecologia 130, 555–562. Koide, R.T., Dickie, I.A., 2002. Effects of mycorrhizal fungi on plant populations. Plant and Soil 244, 307–317. Kruckeberg, A.R., 1984. California Serpentines: Flora, Vegetation, Geology, Soils and Management Problems. Univ. of California Press, Berkeley. Lande, R., 1988. Genetics and demography in biological conservation. Science 241, 1455–1460. Latta, R.G., 2008. Conservation genetics as applied evolution: from genetic pattern to evolutionary process. Evolutionary Applications 1, 84–94. Leimu, R., Fischer, M., 2008. A meta-analysis of local adaptation in plants. PLoS ONE 3, e4010. doi:10.1371/journal.pone.0004010. Levy, F., Neal, C.L., 1999. Spatial and temporal genetic structure in chloroplast and allozyme markers in Phacelia dubia implicate genetic drift. Heredity 82, 422–431. Macnair, M.R., Macnair, V.E., Martin, B.E., 1989. Adaptive speciation in Mimulus – an ecological comparison of Mimulus cupriphilus with its presumed progenitor Mimulus guttatus. New Phytologist 112, 269–279. Morris, A.B., Baucom, R.S., Cruzan, M.B., 2002. Stratified analysis of the soil seed bank in the cedar glade endemic Astragalus bibullatus: evidence for historical changes in genetic structure. American Journal of Botany 89, 29–36. Nunney, L., 2002. The effective size of annual plant populations: The interaction of a seed bank with fluctuating population size in maintaining genetic variation. The American Naturalist 160, 195–204. Obioh, G.I.B., Isichei, A.O., 2007. A population viability analysis of serendipity berry (Dioscoreophyllum cumminsii) in a semi-deciduous forest in Nigeria. Ecological Modelling 201, 558–562. Palstra, F.P., Ruzzante, D.E., 2008. Genetic estimates of contemporary effective population size: what can they tell us about the importance of genetic stochasticity for wild population persistence? Molecular Ecology 17, 3428– 3447. Pertoldi, C., Bijlsma, R., Loeschcke, V., 2007. Conservation genetics in a globally changing environment: present problems, paradoxes and future challenges. Biodiversity and Conservation 16, 4147–4163. Porcher, E., Gouyon, P.H., Lavigne, C., 2004. Dynamic management of genetic resources: maintenance of outcrossing in experimental metapopulations of a predominantly inbreeding species. Conservation Genetics 5, 259–269. Reed, D.H., 2005. Relationship between population size and fitness. Conservation Biology 19, 563–568. Rice, K.J., 1990. Reproductive hierarchies in Erodium – effects of variation in plantdensity and rainfall distribution. Ecology 71, 1316–1322.
E.K. Espeland, K.J. Rice / Biological Conservation 143 (2010) 946–951 Selosse, M., Richard, F., He, X., Simard, S.W., 2006. Mycorrhizal networks: des liaisons dangereuses? Trends in Ecology and Evolution 21, 621–628. Shumway, D.L., Koide, R.T., 1995. Size and reproductive inequality in mycorrhizal and nonmycorrhizal populations of Abutilon-theophrasti. Journal of Ecology 83, 613–620. Siol, M., Bonnin, I., Oliveri, I., Prosperi, J.M., Ronfort, J., 2007. Effective population size associated with self-fertilization: lessons from temporal changes in allele frequencies in the selfing annual Medicago truncatula. Journal of Evolutionary Biology 20, 2349–2360. Tremblay, R.L., Ackerman, J.D., 2001. Gene flow and effective population size in Lepanthes (Orchidaceae): a case for genetic drift. Biological Journal of the Linnean Society 72, 47–62. Van Kleunen, M., Fischer, M., Schmid, B., 2001. Effects of intraspecific competition on size variation and reproductive allocation in a clonal plant. Oikos 94, 515– 524. Van Kleunen, M., Fischer, M., Schmid, B., 2005. Three generations of low versus high neighborhood density affect the life history of a clonal plant through differential selection and genetic drift. Oikos 108, 573–581. Vencovsky, R., Crossa, J., 2003. Measurements of representativeness used in genetic resources conservation and plant breeding. Crop Science 43, 1912–1921.
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Vitalis, R., Couvet, D., 2001. Estimation of effective population size and migration rate from one- and two-locus identity measures. Genetics 157, 911–925. Waples, R.S., 2002. Evaluating the effect of stage-specific survivorship on the Ne/N ratio. Molecular Ecology 11, 1029–1037. Weiner, J., Thomas, S.C., 1986. Size variability and competition in plant monocultures. Oikos 47, 211–222. Willi, Y., Van Buskirk, J., Schmid, B., Fischer, M., 2007. Genetic isolation of fragmented populations is exacerbated by drift and selection. Journal of Evolutionary Biology 20, 534–542. Willi, Y., Hoffman, A.A., 2009. Demographic factors and genetic variation influence population persistence under environmental change. Journal of Evolutionary Biology 22, 124–133. Yonezawa, K., Ishii, T., Nomura, T., Morishima, H., 1996. Effectiveness of some management procedures for seed regeneration of plant genetic resources accessions. Genetic Research in Crop Evolution 43, 517–524. Zietsman, J., Dreyer, L.L., Esler, K.J., 2008. Reproductive biology and ecology of selected rare and endangered Oxalis L. (Oxalidaceae) plant species. Biological Conservation 141, 1475–1483.
Contents lists available at ScienceDirect
Biological Conservation journal homepage: www.elsevier.com/locate/biocon
Ecological effects on estimates of effective population size in an annual plant E.K. Espeland a,*, K.J. Rice b a b
USDA ARS Pest Management Research Unit, 1500 N. Central Avenue, Sidney, MT 59270, USA Ecology Graduate Group, Department of Plant Sciences, Mail Stop 1, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA
a r t i c l e
i n f o
Article history: Received 13 April 2009 Received in revised form 28 December 2009 Accepted 3 January 2010 Available online 25 January 2010 Keywords: Abiotic stress Competition Effective population size Genetic drift Plantago erecta Local adaptation Serpentine Conservation genetics
a b s t r a c t Effective population size (Ne) is a critical indicator of the vulnerability of a population to allele loss via genetic drift, and it can also be used to assess the evolutionary potential of a population. While some plant conservation plans have focused on outcrossing through cross-pollination as a way to increase estimated Ne, variance in reproductive output determined by ecological factors such as competition can also strongly affect estimated Ne. We examined the effects of intraspecific and interspecific competition, stressful soils, and local adaptation on estimates of Ne in an annual plant species. While ecological influences on plant growth rate variance have been predicted to influence estimates of Ne/N, we found a significant effect on the estimate of Ne/N, but no significant ecological effects on growth rate variance. Lower survivorship on stressful soil was the most important effect reducing estimates of Ne/N. If stochastic mortality is greater in environments that are abiotically stressful, then populations in these stressful environments may be slower to adapt because of lower census sizes and reduction of Ne /N. In populations of conservation concern, increasing survivorship may be of greater benefit for maximizing Ne than the reduction of variability in reproductive output among surviving adults. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Effective population size (Ne) can be used to assess the evolutionary potential of a population because it is an important indicator of the vulnerability of a population to allele loss through random processes (genetic drift). A population with a very low Ne is more susceptible to genetic drift and less able to respond to selection. This is because in small populations there is less genetic variation for natural selection to act upon, and there is a higher probability that beneficial alleles will not be maintained by selection and will instead be lost from the population because of random drift effects (Willi et al., 2007). Modeling population genetic processes in conservation has become relatively widespread (Halley and Manasee, 1993; Higgins and Lynch, 2001; Obioh and Isichei, 2007; Pertoldi et al., 2007; Palstra and Ruzzante, 2008), and when Ne is incorporated into minimum viable population size (MVP) models, MVP as censused may need to be substantially more than 5000 individuals (Obioh and Isichei, 2007). Rare and endangered plant populations in particular often have very low census sizes (N) and even lower estimated Ne (Chung et al., 2007; Zietsman et al., 2008). Plant and animal populations of conservation concern tend to have multiple factors acting to reduce Ne
* Corresponding author. Tel.: +1 406 433 9416; fax: +1 406 433 5038. E-mail addresses: [email protected] (E.K. Espeland), kjrice@ucdavis. edu (K.J. Rice). 0006-3207/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2010.01.003
(Lande, 1988; Frankham, 1995; Hedrick, 1995; Frankham et al., 2003), many of which are ecological in nature. Published estimates of Ne in plant populations have most often been used in a descriptive manner to characterize population subdivision and differentiation (Tremblay and Ackerman, 2001; Latta, 2008). Conservation biologists working with both plants and animals have used Ne estimates to explore the importance of population bottlenecks on genetic diversity (Frankham, 1995; Amos and Harwood, 1998). Other conservation uses of Ne estimates have focused on reconstructions of historical anthropogenic effects on gene flow and genetic diversity (Levy and Neal, 1999; Morris et al., 2002). Ne can be estimated in a number of ways, and there are three commonly-used types of Ne. Inbreeding Ne describes the probability of mating among relatives, therefore lowering genetic diversity of the population. Variance Ne describes the probability that individuals will pass on their genes to the next generation, with increased variance in offspring number leading to a decrease in Ne. Extinction or eigenvalue Ne describes the rate of loss of heterozygosity, with a calculation of an asymptotic Ne as the outcome. Although estimates of the three types of Ne often have the same result (Vitalis and Couvet, 2001), they can differ, so the choice of which type to calculate may revolve around the genetic process of interest (Caballero, 1994). Inbreeding Ne is often estimated via genetic methods by calculating the percent heterozygosity and comparing it to the expected heterozygosity in the population if
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it were in equilibrium (Hartl and Clark, 2007). Inbreeding Ne can also be estimated as the geometric mean of population sizes through time (Caballero, 1994). Variance Ne is estimated as the variance in reproductive outputs among individuals in a population (Heywood, 1986). Published estimates of Ne /N for annual plants tend to be between 5% and 30%, with ratios most often below 10% (Heywood, 1986; Husband and Barrett, 1992; Goldringer et al., 2001; Siol et al., 2007). Recent work by Siol et al. (2007) suggests that relative reproductive variance must be taken into account in order to correctly estimate Ne. As understanding of the influence of reproductive variance on Ne estimation has increased, evidence has also accumulated to demonstrate that ecological factors can be extremely important in determining this variance (Van Kleunen et al., 2001, 2005). In fact, ecological models of density-dependence have already been theoretically extended to the estimation of Ne (Rice, 1990; Koide and Dickie, 2002; Van Kleunen et al., 2001, 2005). Other ecological factors such as herbivory (Doak, 1992) can affect the variance in reproductive outputs in plants and thus influence estimates of Ne/N. Although Ne is often estimated from the geometric mean of population sizes of above-ground plants, seed banks increase Ne above this estimate because the seed bank is often not part of the censused population size (Nunney, 2002). Thus, ecological factors that reduce the seed bank such as granivory and fungal infection will act to reduce Ne. The linkage between variation in reproductive output and estimates of Ne in annual plants was explored theoretically by Heywood (1986) and can be expressed by the following relationship:
Ne 1 ¼ ½ð1 þ FÞðr2 =l2 Þ þ 1 N
ð1Þ
where the inbreeding coefficient F ranges from 0 (complete outcrossing) to 1 (complete inbreeding), r2 = variance in reproductive output, and l = mean reproductive output. It follows that if an annual plant population with a large census size has a few individuals that produce the majority of the seed, reproductive variance is high and estimated Ne/N is small. In contrast, Ne is larger and estimated Ne/N is close to 1 in an annual plant population of large census size that has a variance in reproductive output that approximates a Poisson distribution. While the absolute value of Ne is more important than this ratio, we can use this ratio to determine the environmental influences that have the most important effect on estimates of Ne. Because N in plant populations is often determined by largerscale factors that we cannot influence, such as rainfall patterns (Zietsman et al., 2008), the estimated Ne /N ratio is more useful as a relative index to apply to the evaluation of conservation practices that may maximize Ne. In annual plant species, size is highly correlated with reproductive output, and competition is one of the main determinants of plant size (Harper, 1977). Models of competitive interactions not only predict relative plant sizes, but also the distribution of plant sizes in a population and thus the coefficient of variation in plant sizes. In annual plants, the coefficient of variation in plant sizes is directly related to the coefficient of variation in reproductive outputs (Heywood, 1986). A relevant model of above-ground asymmetric competition is where large plants shade small ones and the coefficient of variation (CV) of plant sizes increases with density (Weiner and Thomas, 1986). This larger CV of plant sizes in high density populations decreases estimated Ne/N (see Eq. (1)). A facilitative model of plant–plant interactions can also affect the CV of reproductive outputs. Resource sharing via common mycorrhizal networks can assist resource capture in smaller plants and increase their growth rates relative to larger plants (Shumway and Koide, 1995; Selosse et al., 2006). This type of equalizing
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resource distribution reduces the CV of plant sizes as densities increase (Koide and Dickie, 2002). This would lead to a relative increase in estimated Ne /N when seed output is positively correlated with plant size. In this experiment we used populations of the California native annual forb, Plantago erecta (E. Morris), to examine environmental and evolutionary influences on variance in reproductive output and estimated Ne. We designed the experiment to test the relative importance of density-dependent interactions and interspecific competition on reproductive hierarchies, and how stressful soils and local adaptation may alter estimates of Ne/N.
2. Methods and materials P. erecta is an annual plant with a native range extending from Baja California and Arizona north through the California Floristic Province to southern Oregon. Although completely self-compatible, some outcrossing is likely in this species (E. Espeland, unpublished data). Populations of P. erecta typically occur at high densities and are found in shallow, low fertility soils (e.g., serpentine outcrops, road cuts) as well as in deep, more fertile grassland soils. The experiment was conducted at McLaughlin Reserve (38.52°N, 122.24°W), located within the California North Coast Range (Hickman, 1993) and operated by the University of California Natural Reserve System. Seeds were collected from the field in spring 2004 from four serpentine populations and four non-serpentine populations at McLaughlin Reserve. Serpentine soil is a nutrient-limited, ultramafic soil type characterized by low calcium, very high levels of exchangeable magnesium (Jurjavcic et al., 2002), and toxic heavy metals: a stressful environment for plant growth (Kruckeberg, 1984). Serpentine areas are also often drier than non-serpentine areas (Macnair et al., 1989). Serpentine populations of P. erecta were identified by the presence of serpentine endemic plant species. Serpentine grasslands occur in a mosaic with loam grasslands at this site. Four populations on each soil type within a geographic range of three linear kilometers were selected to span a range of productivities (estimated by total above-ground biomass; E. Espeland, unpublished data) and were spatially blocked into serpentine/loam paired plots. Each population was about 0.5 ha in size. Seeds from 30 families from each population were bulked by soil type (serpentine or non-serpentine) before replanting into field collection locations. To increase germination, dry seeds were chilled at 4 °C for 10 days prior to sowing. The seeds were then planted into 16 circular 38.5 cm2 plots in a 1 m 1 m area at each of the eight locations on October 10 and 11, 2004. A factorial combination of two levels of interspecific competition, two sowing densities, and two seed sources (serpentine, non-serpentine) was replicated twice at each location. The 16 plots at each location comprised a randomized complete block design. For the high sowing density treatment, 15 seeds were affixed to a single 0.2 cm2 piece of tissue paper with water-soluble glue. This group of seeds was placed in the center of the plot and buried at a depth of 2 mm. For the low density sowing treatment, each seed was glued to a single toothpick and 10 toothpicks were inserted into the soil of each plot so that the seed was buried at a depth of 2 mm. The 10 toothpicks were evenly spaced over the plot area. Interspecific competition was manipulated experimentally. For half the plots, only P. erecta germinating from the resident seed bank was removed throughout the experiment. For the other half of the plots, all plants except for the planted P. erecta plants were removed throughout the fall and winter. This resulted in a range of biomass of plant species other than Plantago in the plots (hereafter referred to as ‘‘interspecific biomass”).
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Survivorship was tracked over the growing season as well as the height of each P. erecta plant in our plots. Plants were harvested prior to seed ripening at each site in spring 2005 (March 10, 2005 to June 7, 2005). Seed output was estimated by multiplying the number of flowers + buds by two, as there are two ovules per flower and P. erecta rarely aborts ovules (61% abortion rate, E. Espeland, unpublished data). P. erecta flowers synchronously at each location, much more synchronously than congeners found elsewhere such as P. aristata and P. patagonica (E. Espeland, personal observation). Because there is a relatively long period of time between flowering and seed ripening (3–5 weeks, E. Espeland, personal observation), we are confident that all plants that would have flowered that year had done so by harvest time. To ensure we did not underestimate the reproductive output of any plants that may have begun flowering later than the main cohort, we included unopened buds as well as open flower numbers in our reproductive output calculation. This is a measure of maternal fitness; we did not attempt to measure paternal fitness in this experiment. At harvest, all interspecific plant biomass was harvested from each plot, placed in a drying oven at 65 °C for 6 days, and then weighed. Because we measured emergence, not germination, in the field experiment, we conducted a growth chamber experiment to examine inherent differences in germination rates between serpentine and non-serpentine populations. Of the eight populations studied, seeds were collected from three serpentine and three non-serpentine populations during March 2008. These seed collections representing different seed source soil types were stored at room temperature until August 26, 2008. On that date, three Petri plates, fitted with moistened filter paper, and each containing 25 seeds each per population were placed in a dark germination chamber (12 h at 4 °C and 12 h at 16 °C for 14 days). Plates were scored semi-weekly for germination, and germinants were removed at each census.
2.1. Data analysis We sowed P. erecta at two extremely different densities to test if extremes in intraspecific density affect the response to the other experimental treatments. Effects of sowing density, interspecific biomass, location soil type (serpentine or non-serpentine soil at planting location), and seed source soil type on plant size CV and relative reproductive variance were assessed. The number of P. erecta that emerged in each plot was used as a covariate. CV was calculated as the coefficient of variation (standard deviation/mean) in plant sizes at the end of the experiment. This estimates the variance in plant growth rates within the population (Rice, 1990). Relative reproductive variance was calculated as the CV2 (variance/ mean2) of estimated seed output for plants that germinated in the plot, including zeros for plants that died. This calculation of relative reproductive variance relates directly to estimated Ne/N (see Eq. (1)). Because plant size is highly correlated with reproduction in this species (Espeland and Rice, 2007), the two dependent variables, CV and relative reproductive variance, should differ primarily by the inclusion of zeros in the relative reproductive variance calculation, thus partitioning the relative effect of survivorship on estimated Ne/N. We tested a subset of interactions of interest in our statistical model. Source soil by planting soil was tested to determine the effect of possible local adaptation. To determine the effect of soil type on inter- and intra-specific interactions, the interactions of (1) planting soil by interspecific biomass and (2) planting soil by sowing density were tested. To determine the effect of local adaptation on each of these two-way interactions, source soil was added to test the significance of three-way interactions. The data
were close to normality (Shapiro-Wilk >0.88), and no transformation of CV or relative reproductive variance was necessary. To test for fitness effects of local adaptation across life history stages, we examined the interaction of planting soil and source soil on the number of flowers produced per seed sown using a Tukey HSD test. Our seed collections were bulked across interspersed serpentine and non-serpentine sites, and we are therefore confident that any differences between serpentine and non-serpentine sources are due to the influence of soil type and not site specific factors. Because we used field collected seed, the influence of seed source on fitness may be due to maternal environmental effects, heritable genetic variation, or both. 3. Results Plants from non-serpentine sources produced significantly flowers per seed sown on serpentine soil (p < 0.05). Serpentine sources did not differ significantly (p > 0.05) in flower production between the two soil types, although the trend for serpentine sources was to produce more flowers than non-serpentine sources on serpentine soil and fewer flowers than non-serpentine sources on non-serpentine soil (Fig. 1). There were no significant treatment effects on the CV of plant sizes at the end of the growing season (Table 1, left side). Planting soil type was the only treatment factor that had a significant effect on relative reproductive variance (Table 1, right side). Plants growing on non-serpentine soil exhibited 14% lower relative reproductive variance than plants growing in serpentine soil (Table 2, left side). Germination totals and rates did not differ significantly between serpentine and non-serpentine collections in the 2008 germination trial (p > 0.7). Over the 14-day trial period, germination in non-serpentine sources averaged 92 ± 13% SD while germination for serpentine sources averaged 86 ± 13% SD. Using Eq. (1), we estimated Ne/N ratios for significant treatment effects on relative reproductive variance. Estimated Ne /N values were 8% smaller in plant populations growing on serpentine soil compared to populations growing on non-serpentine (Table 2, right side). Because the results for CV and relative reproductive variance differed, this indicated that survival may play a key role in estimates of Ne/N. Therefore, we performed an analysis of treatment effects on survivorship. We applied the same statistical model we used for CV and relative reproductive variance to arcsine transformed percent survivorship. Only the interaction between interspecific biomass and planting soil type was close to significant in determining survivorship (p = 0.07). Competition from interspecifics had no significant effect on survivorship on non-serpentine soil, but increased interspecific biomass decreased survivorship on serpentine soil (arcsine% survivorship = 1.25 0.55 g interspecific biomass, R2 = 0.08, p < 0.02). 4. Discussion and conclusions In our experiment, non-serpentine seed sources are more strongly adapted to the less stressful non-serpentine soil. Seeds from serpentine sources do not perform significantly differently on the two soil types. Although we found evidence for local adaptation, local adaptation does not appear to have an important influence on relative reproductive variance and estimated Ne/N. In addition, if ecological factors that influence variance in plant growth rates also influence relative reproductive variance, we would expect parallel results in our CV and relative reproductive variance analyses. However, plant growth rate variance (CV) was unaffected by our experimental treatments, and
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Number of flowers per seed sown
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2
A AB 1.5
AB B
1
0.5
0
Planting soil
Nonserpentine
Source soil
Serpentine
Nonserpentine
NONSERPENTINE
Serpentine
SERPENTINE
Fig. 1. Non-serpentine sources produce the most flowers per seed sown in home soil, suggesting local adaptation to soil type (bars with different letters are significantly different, p < 0.05). Serpentine sources show a trend towards local adaptation, but this difference is not significant.
Table 1 Statistical tables of (a) treatment and covariate effects on coefficient of variation and on relative reproductive variance, (b) growing soil and source soil on number of flowers per seed sown, p < 0.05 in bold. Source
(a) #P. erecta emerged Block Sowing density (D) Interspecific biomass (B) Soil Source Soil source Soil B Source soil D Source soil D
Coefficient of variation
Relative reproductive variance
DF
SS
F-ratio
P>F
SS
F-ratio
P>F
1 3 1 1 1 1 1 1 1 1
0.423 0.816 0.300 0.052 0.306 0.204 0.274 0.002 0.047 0.423
3.29 2.11 2.33 0.41 2.39 1.58 2.13 0.02 0.37 3.29
0.072 0.103 0.130 0.524 0.125 0.211 0.147 0.896 0.546 0.072
0.072 0.587 0.028 0.010 0.274 0.062 0.152 0.009 0.0005 0.036
1.28 3.44 0.498 0.178 4.83 1.10 2.68 0.16 0.01 0.64
0.261 0.019 0.482 0.674 0.030 0.297 0.105 0.687 0.922 0.427
Source
DF
SS
F-ratio
P>F
(b) Soil Source Soil source
1 1 1
1.064 0.141 4.441
1.591 0.212 6.641
0.210 0.646 0.011
Table 2 Average for relative reproductive variance (RRV) ± one standard deviation, plus estimated Ne/N by location soil.
Non-serpentine Serpentine
RRV
Ne/N
0.59 ± 0.20 0.69 ± 0.29
0.74 0.68
relative reproductive variance was significantly influenced only by planting soil type and block. Stressful serpentine soil increased relative reproductive variance compared to non-stressful soil. Although we would expect below-ground competition on stressful soil types to decrease plant size hierarchies and decrease plant growth rate variance, this was not the case. Plant growth rate variance was not affected by planting soil, and relative reproductive variance was higher, not lower, in populations on the stressful soil type. The lack of parallel between CV and relative reproductive variance indicates that survivorship plays a significant role in the observed relationship between soil type and estimated Ne/N. When survivorship is the main driver of estimated Ne/N, stressful environments may intrinsically have lower estimated Ne /N as well as lower census sizes. Low census sizes in plant populations mean that the accumulation of new variation through mutation
is unlikely, and that inbreeding will decrease heterozygosity. The census size of plant populations is positively correlated with fitness measures (Reed, 2005), and in a recent meta-analysis Leimu and Fischer (2008) showed that populations of small census size are less likely to be locally adapted. This meta-analysis is supported by individual studies showing a positive ability for larger populations to adapt to new environments (Willi and Hoffman, 2009). When lowered survival not only makes census sizes small but also makes the estimated Ne /N ratio small, this suggests that the population will also be less likely to retain any beneficial genetic variation because of genetic drift. Plant populations growing on stressful soil types have a twofold obstacle to local adaptation: reduced population growth rates and reduced genetic variation (low Ne /N) less able to respond to selection (Willi and Hoffman, 2009). Plants and other sessile organisms that are limited in their ability to sample the environment may thus suffer doubly in stressful environments. Plant populations then face a more difficult evolutionary hurdle in adapting to stressful environments compared to non-stressful environments. In contrast, populations growing under less stressful abiotic conditions, with larger census sizes and larger estimated Ne /N, may both generate and harbor more genetic variation. Greater evolutionary potential in populations with larger estimated Ne /N should allow them to more effectively respond to natural selection
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and adapt to local conditions. While stress-adapted species and populations are commonly documented (Chapin et al., 1993), adaptation to stressful environments may be less common than expected because lower census sizes and lower estimated Ne/N may slow the adaptation process. Within stressful environments, the balance between strong selection regimes and the evolutionary capacity of populations to respond to selection is an important consideration in the management of populations of conservation concern. In small populations of conservation concern, factors that reduce estimated Ne will also decrease the ability of a population to retain beneficial alleles. These factors may be ameliorated with appropriate management practices. In this case, with a common plant species, estimated Ne /N was well above 0.5, indicating that reproductive variance is not the main influence on genetic drift processes in these populations (Heywood, 1986). Ecological influences on survivorship, not growth rates, were the main factor determining estimated Ne/N in P. erecta. No studies that we know of have examined the effect of differential survival on estimated Ne /N estimates in plants, however, survivorship is an extremely important factor in estimating Ne /N in marine species (Waples, 2002). Maximizing survivorship within semelparous species of conservation concern may be more important than reducing the variability among surviving breeding adults in these species. To ensure that plant populations have the potential to adapt to changing environmental conditions, we need to further investigate ecological drivers of relative reproductive variance that include survivorship, within plant populations in order to limit the effects of genetic drift within these populations. Although some plant conservation plans have focused on promoting cross-pollination as a way to increase estimated Ne (Ingvarsson, 2002; Porcher et al., 2004), variance in reproductive output can often have a much greater effect on estimated Ne than breeding system (Heywood, 1986). Plant conservators are beginning to prioritize reducing reproductive variance in order to maintain genetic diversity (Yonezawa et al., 1996; Vencovsky and Crossa, 2003). Our results show that mortality may strongly influence reproductive variance, thus reducing estimated Ne/N. The source of mortality in our experiment appeared to be fungal infection at the seedling stage. In some cases plant conservationists may be able to increase survivorship in populations of concern, for example by excluding predators in select years (Espeland et al., 2005). Effective population size may need to be managed very differently for plants and animals. Colonizing species such as annual plants produce large numbers of progeny with the same genetic background, and not all of these progeny will survive. In these colonizing species, mortality may play a larger role in genetic drift processes compared to species whose population growth rates are determined by intraspecific competition, such as large herbivores (Waples, 2002). Environmental factors can very strongly affect Ne /N estimates and thus environmental factors may be primary determinants of a population’s ability to retain genetic variation through time.
Acknowledgements This work was performed at the McLaughlin Reserve of the University of California Natural Reserve System (UCNRS) and partially funded by a Mildred E. Mathias Grant for work at the UCNRS to EKE and a Packard Foundation Interdisciplinary Science Grant (200001607) to both K.J.R. and E.K.E. Thanks to S. Mueller for field assistance, to M. O’Mara for lab assistance, and to S. Harrison, K. Moore, S. Elmendorf, M. Schlesinger, K. Jones, J. Harding, the Big Science Lab, D. Grace, J. Weiner, S. Sultan, A. Caballero, J. Gaskin, and E. Leger for comments on early versions of the manuscript.
References Amos, W., Harwood, J., 1998. Factors affecting levels of genetic diversity in natural populations. Philosophical Transactions of the Royal Society of London Series B – Biological Sciences 353, 177–186. Caballero, A., 1994. Developments in the prediction of effective population size. Heredity 73, 657–679. Chapin, F.S., Autumn, K., Pugnaire, F., 1993. Evolution of suites of traits in response to environmental stress. American Naturalist 142, S78–S92. Chung, M.Y., Park, C.W., Chung, M.G., 2007. Extremely low levels of allozyme variation in southern Korean populations of the two rare and endangered lithophytic or epiphytic Bulbophyllum drymoglossum and Sarcanthus scolopendrifolius (Orchidaceae): implications for conservation. Biodiversity and Conservation 16, 775–786. Doak, D.F., 1992. Lifetime impacts of herbivory for a perennial plant. Ecology 73, 2086–2099. Espeland, E.K., Carlsen, T.M., MacQueen, D., 2005. Fire and dynamics of granivory on a rare California grassland forb. Biodiversity and Conservation 14, 267–280. Espeland, E.K., Rice, K.J., 2007. Facilitation across stress gradients: the importance of local adaptation. Ecology 88, 2404–2409. Frankham, R., 1995. Effective population size/adult population size ratios in wildlife: a review. Genetics Research 66, 95–107. Frankham, R., Ballou, J.D., Brisco, D.A., 2003. Introduction to Conservation Genetics. Cambridge University Press, Cambridge, UK. Goldringer, I., Enjalbert, J., Raquin, A.-L., Brabant, P., 2001. Strong selection in wheat populations during ten generations of dynamic management. Genetics Selection Evolution 33, S441–S463. Halley, J.M., Manasee, R.S., 1993. A population-dynamics model for annual plants subject to inbreeding depression. Evolutionary Ecology 7, 15–24. Harper, J.L., 1977. Population Biology of Plants. Academic Press, New York, NY. Hartl, D.L., Clark, A.G., 2007. Principles of Population Genetics. Sinauer Associates, Sunderland, MA. Hedrick, P.W., 1995. Elephant seals and the estimation of a population bottleneck. Journal of Heredity 86, 232–235. Heywood, J.S., 1986. The effect of plant size variation on genetic drift in populations of annuals. The American Naturalist 127, 851–861. Hickman, J.C., 1993. The Jepson Manual: Higher Plants of California. University of California Press, Berkeley, CA. Higgins, K., Lynch, M., 2001. Metapopulation extinction caused by mutation accumulation. Proceedings of the National Academy of Sciences 98, 2928–2933. Husband, B.C., Barrett, S.C.H., 1992. Effective population-size and genetic drift in tristylous Eichhornia paniculata (Pontederiaceae). Evolution 46, 1875–1890. Ingvarsson, P.K., 2002. A metapopulation perspective on genetic diversity and differentiation in partially self-fertilizing plants. Evolution 56, 2368–2373. Jurjavcic, N.L., Harrison, S., Wolf, A.T., 2002. Abiotic stress, competition and the distribution of the native annual grass Vulpia microstachys in a mosaic environment. Oecologia 130, 555–562. Koide, R.T., Dickie, I.A., 2002. Effects of mycorrhizal fungi on plant populations. Plant and Soil 244, 307–317. Kruckeberg, A.R., 1984. California Serpentines: Flora, Vegetation, Geology, Soils and Management Problems. Univ. of California Press, Berkeley. Lande, R., 1988. Genetics and demography in biological conservation. Science 241, 1455–1460. Latta, R.G., 2008. Conservation genetics as applied evolution: from genetic pattern to evolutionary process. Evolutionary Applications 1, 84–94. Leimu, R., Fischer, M., 2008. A meta-analysis of local adaptation in plants. PLoS ONE 3, e4010. doi:10.1371/journal.pone.0004010. Levy, F., Neal, C.L., 1999. Spatial and temporal genetic structure in chloroplast and allozyme markers in Phacelia dubia implicate genetic drift. Heredity 82, 422–431. Macnair, M.R., Macnair, V.E., Martin, B.E., 1989. Adaptive speciation in Mimulus – an ecological comparison of Mimulus cupriphilus with its presumed progenitor Mimulus guttatus. New Phytologist 112, 269–279. Morris, A.B., Baucom, R.S., Cruzan, M.B., 2002. Stratified analysis of the soil seed bank in the cedar glade endemic Astragalus bibullatus: evidence for historical changes in genetic structure. American Journal of Botany 89, 29–36. Nunney, L., 2002. The effective size of annual plant populations: The interaction of a seed bank with fluctuating population size in maintaining genetic variation. The American Naturalist 160, 195–204. Obioh, G.I.B., Isichei, A.O., 2007. A population viability analysis of serendipity berry (Dioscoreophyllum cumminsii) in a semi-deciduous forest in Nigeria. Ecological Modelling 201, 558–562. Palstra, F.P., Ruzzante, D.E., 2008. Genetic estimates of contemporary effective population size: what can they tell us about the importance of genetic stochasticity for wild population persistence? Molecular Ecology 17, 3428– 3447. Pertoldi, C., Bijlsma, R., Loeschcke, V., 2007. Conservation genetics in a globally changing environment: present problems, paradoxes and future challenges. Biodiversity and Conservation 16, 4147–4163. Porcher, E., Gouyon, P.H., Lavigne, C., 2004. Dynamic management of genetic resources: maintenance of outcrossing in experimental metapopulations of a predominantly inbreeding species. Conservation Genetics 5, 259–269. Reed, D.H., 2005. Relationship between population size and fitness. Conservation Biology 19, 563–568. Rice, K.J., 1990. Reproductive hierarchies in Erodium – effects of variation in plantdensity and rainfall distribution. Ecology 71, 1316–1322.
E.K. Espeland, K.J. Rice / Biological Conservation 143 (2010) 946–951 Selosse, M., Richard, F., He, X., Simard, S.W., 2006. Mycorrhizal networks: des liaisons dangereuses? Trends in Ecology and Evolution 21, 621–628. Shumway, D.L., Koide, R.T., 1995. Size and reproductive inequality in mycorrhizal and nonmycorrhizal populations of Abutilon-theophrasti. Journal of Ecology 83, 613–620. Siol, M., Bonnin, I., Oliveri, I., Prosperi, J.M., Ronfort, J., 2007. Effective population size associated with self-fertilization: lessons from temporal changes in allele frequencies in the selfing annual Medicago truncatula. Journal of Evolutionary Biology 20, 2349–2360. Tremblay, R.L., Ackerman, J.D., 2001. Gene flow and effective population size in Lepanthes (Orchidaceae): a case for genetic drift. Biological Journal of the Linnean Society 72, 47–62. Van Kleunen, M., Fischer, M., Schmid, B., 2001. Effects of intraspecific competition on size variation and reproductive allocation in a clonal plant. Oikos 94, 515– 524. Van Kleunen, M., Fischer, M., Schmid, B., 2005. Three generations of low versus high neighborhood density affect the life history of a clonal plant through differential selection and genetic drift. Oikos 108, 573–581. Vencovsky, R., Crossa, J., 2003. Measurements of representativeness used in genetic resources conservation and plant breeding. Crop Science 43, 1912–1921.
951
Vitalis, R., Couvet, D., 2001. Estimation of effective population size and migration rate from one- and two-locus identity measures. Genetics 157, 911–925. Waples, R.S., 2002. Evaluating the effect of stage-specific survivorship on the Ne/N ratio. Molecular Ecology 11, 1029–1037. Weiner, J., Thomas, S.C., 1986. Size variability and competition in plant monocultures. Oikos 47, 211–222. Willi, Y., Van Buskirk, J., Schmid, B., Fischer, M., 2007. Genetic isolation of fragmented populations is exacerbated by drift and selection. Journal of Evolutionary Biology 20, 534–542. Willi, Y., Hoffman, A.A., 2009. Demographic factors and genetic variation influence population persistence under environmental change. Journal of Evolutionary Biology 22, 124–133. Yonezawa, K., Ishii, T., Nomura, T., Morishima, H., 1996. Effectiveness of some management procedures for seed regeneration of plant genetic resources accessions. Genetic Research in Crop Evolution 43, 517–524. Zietsman, J., Dreyer, L.L., Esler, K.J., 2008. Reproductive biology and ecology of selected rare and endangered Oxalis L. (Oxalidaceae) plant species. Biological Conservation 141, 1475–1483.