Submitted to: Biological Conservation
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/3/2010
Publication Date: 1/1/2010
Publication URL: hdl.handle.net/10113/42229
Citation: Espeland, E.K., Rice, K.J. 2010. Ecological effects on effective population size in an annual plant. Biological Conservation. 143(4): 946-951. Interpretive Summary: There are many instances of plants evolving to do well on stressful soils: specialized plant communities growing on mine tailings is one example. Whether a plant population is capable of adapting to a stressful environment is partially based on an index called effective population size, or Ne. This index is linked to both the number of individuals in a population as well as the genetic diversity the population holds. Populations with low Ne can be vulnerable to drift, or random gene frequency change, change which is unrelated to agents of natural selection (such as stressful soil). One way that we can estimate Ne is to examine variance in reproductive output among individuals in a population: when only a few plants produce most of the seed in a population, the genetic diversity of the next generation will be lower compared to if most of the plants produce about the same amount of seed. Established models of plant-plant competitive interactions show that when plants compete for light, size variance is high. This is because large plants shade smaller plants, depressing their growth. Thus, small plants stay small and large plants get disproportionately large. When the plants are annuals (growing, setting seed, and dying within a single year), their seed output is directly correlated with their size. We expect a large amount of variance in seed output when soils are non-stressful, and competition is for light; a small amount of variance in seed output when soils are stressful, and competition is for below-ground nutrients. We predict that symmetrical competition in populations of Plantago erecta on harsh soil will lead to increased Ne due to low variation in seed outputs. We found evidence for competition for light on both soil types when seeds were planted very densely, and no evidence for competition for light when seeds were planted at natural densities. Variance in reproductive output was very high on stressful soil, not because of a lack of plant shading, but because of very high death rates on stressful soils. When few plants survive, concomitantly few plants set seed and variance in seed output is high. Not only did plants growing on serpentine soils have high death rates, but plants from serpentine soil had high death rates no matter where they were grown. We found that plant neighbors, usually seen as competitors, actually increased survivorship on both soil types. This indicates that neighboring plants have a positive effect on Ne, even though they have a negative effect on individual seed output. We found local adaptation: plants from stressful soil produced more seeds when planted on stressful soil and plants from non-stressful soils produced more seeds when planted on non-stressful soil. Adaptation to the stressful soil type appeared to be weaker than adaptation to the non-stressful soil type, which is consistent with our estimates of Ne (lower Ne on stressful soils due to increased mortality). This study confirms that stressful soil types can be selective agents, and that plants can adapt to these harsh conditions. We also show that stressful soil has another effect on evolutionary processes: it can reduce Ne and actually make the adaptation process less efficient.
Technical Abstract: Nutrient-limited soil can be a strong selective force on plant populations. In addition, ecological factors such as competitive interactions have been shown to have an effect on effective population size (Ne). Both Ne and selection are indicators of population evolutionary processes: selection can be a directional force for gene frequency change in a population, while populations with low Ne can be vulnerable to drift, or random gene frequency change. By linking established models of competitive interactions across stress gradients to population genetic models, we predict that symmetrical competition on harsh soil will lead to increased Ne. We experimentally examine the effect of soil type, competition, and local adaptation on estimates of Ne in populations of Plantago erecta, an annual plant growing on and off serpentine soils in California annual grasslands. Local adaptation was found, competition was symmetrical on both nutrient-limited and non-stressful soil, and intraspecific facilitation increased survivorship. All plants growing on serpentine soil had lower Ne estimates, regardless of population origin or competition treatment. Additionally, Ne was lower for plants from serpentine populations, regardless of growing soil type. Variation in plant competition did not significantly influence Ne. Instead, reduction in estimated Ne on serpentine was due to greater rates of mortality on serpentine that were unlinked to competitive environment. Results of this study indicate that soil type can have manifold effects on evolutionary processes within populations, beyond the simple effect of selection.