EVOLUTIONARILY STABLE MORPHOLOGIES IN PEA POPULATIONS

We investigated the hypothesis that plant form can dramatically affect plant competitive ability, and that forms with dense canopies can invade populations of plants with more open canopies regardless of initial relative frequencies. Under controlled field conditions, we examined the effects of plant form on growth rate, size variation, mortality, and reproduction in high-density monocultures and mixtures of two morphologically distinct varieties of peas. These two varieties differ genetically at only the afila locus. In high-density monocultures and mixtures, peas with finely dissected, minute leaflets (af/af) grew more slowly and produced fewer seeds than Af/—individuals with large leaflets that cast more shade on neighbors. After as few as four generations, mixtures begun with 10% Af/— peas would be expected to evolve to Af/— monocultures. We conclude that an increase in morphological complexity (e.g., virtually leafless to leafy) can have dramatic ecological and evolutionary impacts on plant population dynamics.     

Competition along a nutrient gradient: A case study with Daucus carota and Chenopodium album

A binary competition experiment between carrot (Daucus carota L.) and Chenopodium album L. was conducted in a greenhouse at seven population densities and eight nutrient concentrations to investigate the effects of a nutrient gradient on plant competition in both monocultures and mixtures. The patterns of carrot biomass allocation (measured as root : shoot ratio) in monocultures and mixtures with C. album were affected by both nutrient availability and population density. Chenopodium album had a broader response to nutrient concentrations than carrot. The maximum yield of carrot in both monocultures and mixtures occurred at fourfold the standard concentration of nutrients, while C. album in both monocultures and mixtures had the maximum yield at 16-fold the standard nutrient concentration. The yield–density relationship of carrot tended to be increasing or asymptotic at lower nutrient concentrations but parabolic at higher concentrations, whereas that of C. album was little affected by nutrient availability. Nutrient availability had a profound influence on the competitive relationships between the two species: at both low and high nutrient concentrations, C. album tended to be more competitive than carrot, while at intermediate levels of nutrients, carrot was more competitive than C. album. Our results suggest that in relation to competitive performance, the weed has a greater ability to adapt itself to varying environments than does the crop. Additionally, the relative merits of the quantitative measures of competitive ability are briefly discussed.     

More plant biomass results in more offspring production in annuals,or does it?

Competitive ability in plants has been previously measured almost exclusively in terms of traits related to growth (biomass) or plant size. In this study, however, we used a multi‐species competition experiment with six annuals to measure relative competitive ability in terms of reproductive output, i.e. the number of offspring produced for the next generation. Under greenhouse conditions, plants of each species were started in pots from germinating seeds and were grown singly (free of competition) and at high density in both monocultures and in mixtures with all study species. Several traits traditionally regarded as determinants of competitive ability in plants were recorded for each species grown singly, including: seed mass, germination time, early growth rate and potential plant size (biomass and height). Under competition, several traits were recorded as indicators of relative performance in both monocultures and mixtures, including: biomass of survivors, total number of survivors, number of reproductive survivors, and reproductive output (total seed production) of the survivors. As expected, species that grew to a larger biomass in isolation had higher seed production in isolation. However, none of the traditional plant growth/size‐related traits, measured either in isolation or under competition, could predict between species variation in reproductive output under competition in either monocultures or mixtures. In mixtures, 97% of this variation in reproductive output could be explained by between‐species variation in the number of reproductive survivors. The results indicate that traits measured on plants grown singly may be poor predictors of reproductive output under competition, and that species’ rank order of competitive ability in terms of the biomass of survivors may bear no relationship to their rank order in terms of the number of offspring produced by these survivors. This has important implications for the interpretation of mechanisms of species coexistence and community assembly within vegetation.     

Competitive ability of two Brassica varieties in relation to biomass allocation and morphological plasticity under varying nutrient availability

Green cabbage (Brassica campestris, leafy variety) and turnip (Brassica campestris var. rapifera, rooty variety) were grown in both monocultures and mixtures at three nutrient levels to investigate their responses to nutrient availability with respect to biomass allocation, morphological plasticity and competitive ability. Their allocation parameters and leaf morphological traits were affected by both nutrient availability and developmental stage. Both of the varieties had a smaller biomass allocation to leaf blades, but a greater allocation to petioles at high nutrient levels. Root:shoot ratio (RSR) of green cabbage decreased with increasing nutrient availability, whereas that of turnip increased. Turnip had a smaller leaf blade weight ratio (LBWR) than cabbage, being compensated for by larger leaf area ratio (LAR) and specific leaf area (SLA). Leaf area ratio and SLA of both the varieties increased with increasing nutrient availability as did their mean dry weights. The mean dry weight of turnip was slightly greater than that of green cabbage in their respective monocultures, while that of green cabbage was greater than that of turnip in their 1:1 mixture. Therefore, green cabbage, having inherently greater biomass allocation to leaves, was generally more competitive than turnip with more biomass allocation to roots, especially at higher nutrient levels. However, within a variety, morphological plasticity (variation in LAR and SLA) was more important than the plasticity in biomass allocation (e.g. variation in RSR and LBWR) in determining competitive ability. The implication of our results is that competition models based on biomass allocation pattern alone may fail to predict competitive outcomes and that such models should also take morphological plasticity into full account.     

Limited effects of soil nutrient heterogeneity on populations of Abutilon theophrasti (Malvaceae)

An experiment was conducted to determine if spatial nutrient heterogeneity affects mean plant size or size hierarchies in experimental populations of the weedy annual Abutilon theophrasti Medic. (Malvaceae). Heterogeneity was imposed by alternating 8 × 8 × 10 cm blocks of low and high nutrient soil in a checkerboard design, while a homogeneous soil treatment consisted of a spatially uniform mixture of the two soil types (mixed soil). Populations were planted at three densities. The effect of soil type on the growth of individuals was determined through a bioassay experiment using potted plants. The high nutrient, low nutrient, and mixed soil differed in their ability to support plant growth as indicated by differences in growth rates and final aboveground biomass. Concentrations of N, K, P, and Mg, measured at the end of the growing season in the experimental plots, also differed among all three soil types. Nevertheless, nutrient heterogeneity had little effect at the population level. Mean maximum leaf width measured at midseason was greater for populations on heterogeneous soil, but soil treatment did not affect midseason measurements of plant height, total number of leaves per plant, or canopy width. Population density affected all these parameters except plant height. When aboveground biomass was harvested at the end of the growing season, soil treatment was found to have no main effect on mean plant biomass, total population biomass, the coefficient of variation in plant biomass, or the combined biomass of the five largest plants in the population, but mean plant biomass was greater for populations on heterogeneous soils at the intermediate planting density. Mean plant biomass, total population biomass, and the coefficient of variation in plant biomass all varied with planting density. Mortality was low overall but significantly higher on homogeneous soil across all three densities. Soil heterogeneity had its strongest effect on individuals. In heterogeneous treatments plant size depended on the location of the plant stem with respect to high and low nutrient patches. Thus, soil nutrient heterogeneity influenced whether particular individuals were destined to be dominant or subordinate within the population but had little effect on overall population structure.     

Soil nutrient heterogeneity and competitive ability of three grass species (<Emphasis Type="Italic">Festuca ovina,Arrhenatherum elatius</Emphasis> and <Emphasis Type="Italic">Calamagrostis epigejos</Emphasis>) in experimental conditions

We studied the effects of differences in root growth and nutrient pool on the competitive ability of Festuca ovina (short grass), Arrhenatherum elatius and Calamagrostis epigejos (tall grasses) grown in monocultures and in mixtures of homogeneous and heterogeneous environments during two growing seasons. Analysis of variance revealed a significant effect of plant species on nutrient concentrations in above-ground biomass and of substrate type on contents of N, K, Ca, Mg in biomass. The ANOVA also confirmed the significant effect of competitive environment on the concentration of N, K in above-ground biomass. In heterogeneous environments, both tall grasses (in competition with F. ovina) were able to produce more roots in the nutrient-rich patches and to accumulate more nitrogen in plant tissues, which was associated with higher yield of their above-ground biomass. Thus, the relative competitive ability for nutrients of both tall grasses was higher than that of F. ovina. This competitive ability of A. elatius to C. epigejos increased in heterogeneous treatments.     

Cascading effects of belowground predators on plant communities are density‐dependent

Soil food webs comprise a multitude of trophic interactions that can affect the composition and productivity of plant communities. Belowground predators feeding on microbial grazers like Collembola could decelerate nutrient mineralization by reducing microbial turnover in the soil, which in turn could negatively influence plant growth. However, empirical evidences for the ecological significance of belowground predators on nutrient cycling and plant communities are scarce. Here, we manipulated predator density (Hypoaspis aculeifer: predatory mite) with equal densities of three Collembola species as a prey in four functionally dissimilar plant communities in experimental microcosms: grass monoculture (Poa pratensis), herb monoculture (Rumex acetosa), legume monoculture (Trifolium pratense), and all three species as a mixed plant community. Density manipulation of predators allowed us to test for density‐mediated effects of belowground predators on Collembola and lower trophic groups. We hypothesized that predator density will reduce Collembola population causing a decrease in nutrient mineralization and hence detrimentally affect plant growth. First, we found a density‐dependent population change in predators, that is, an increase in low‐density treatments, but a decrease in high‐density treatments. Second, prey suppression was lower at high predator density, which caused a shift in the soil microbial community by increasing the fungal: bacterial biomass ratio, and an increase of nitrification rates, particularly in legume monocultures. Despite the increase in nutrient mineralization, legume monocultures performed worse at high predator density. Further, individual grass shoot biomass decreased in monocultures, while it increased in mixed plant communities with increasing predator density, which coincided with elevated soil N uptake by grasses. As a consequence, high predator density significantly increased plant complementarity effects indicating a decrease in interspecific plant competition. These results highlight that belowground predators can relax interspecific plant competition by increasing nutrient mineralization through their density‐dependent cascading effects on detritivore and soil microbial communities.     

Plant growth and nutrient removal in constructed monoculture and mixed wetlands related to stubble attributes

The aim of this study is to test the hypothesis that it depends on plant species used in the wetlands and their stubble growth attributes, as to whether monoculture or mixed wetland is superior in plant growth and nutrient removal. Monoculture and mixed wetland microcosms of five wetland plant species were studied. Significant differences in growth and aboveground biomass were found in the monoculture wetlands. Species that showed faster growth and larger biomass in monoculture wetland were also dominant in the mixed wetland. The mixed wetland exhibited similar biomass and root growth to the averages of five monocultures. ANOVA showed that there were very significant differences among the wetlands in removal rates of all the nutrients studied except nitrate nitrogen (NO₃-N) and chemical oxygen demand (COD). The removal rates from the mixed wetland were generally comparable to the highest removal rates from the monocultures. The species exhibited different stubble growth attributes, with some species showing increasing stubble growth and removal rates, while other species showing decreasing stubble growth and removal rates. The results indicated that in both monocultures and mixed constructed wetlands, growth and nutrient removal rates depended on plant species, and attributes of plant stubble growth affected overall growth and nutrient removal capabilities.     

Positive interactions can increase size inequality in plant populations

1.  Large variation in the size of individuals is a ubiquitous feature of natural plant populations. While the role of competition in generating this variation has been studied extensively, the potential effects of positive interactions among plants, which are common in high-stress environments, have not been investigated.
2.  Using an individual-based 'zone-of-influence' model, we investigate the effects of competition, abiotic stress and facilitation on size inequality in plant monocultures. In the model, stress reduces the growth rate of plants, and facilitation ameliorates the effects of stress. Both facilitation and competition occur in overlapping zones of influence. We tested some of the model's predictions with a field experiment using the clonal grass Elymus nutans in an alpine meadow.
3.  Facilitation increased the size inequality of model populations when there was no density-dependent mortality. This effect decreased with density as competition overwhelmed facilitation. The lowest size inequality was found at intermediate densities both with the model and in the field.
4.  When density-dependent mortality was included in the model, stress delayed its onset and reduced its rate by reducing growth rates, so the number of survivors at any point in time was higher under harsh than under more benign conditions. Facilitation increased size inequality during self-thinning.
5.   Synthesis . Our results demonstrate that facilitation interacts with abiotic stress and competition to influence the degree of size inequality in plant populations. Facilitation increased size inequality at low to intermediate densities and during self-thinning.     

Effects of larval crowding on quantitative variation for development time and viability in Drosophila melanogaster

Competition between individuals belonging to the same species is a universal feature of natural populations and is the process underpinning organismal adaptation. Despite its importance, still comparatively little is known about the genetic variation responsible for competitive traits. Here, we measured the phenotypic variation and quantitative genetics parameters for two fitness‐related traits—egg‐to‐adult viability and development time—across a panel of Drosophila strains under varying larval densities. Both traits exhibited substantial genetic variation at all larval densities, as well as significant genotype‐by‐environment interactions (GEIs). GEI was attributable to changes in the rank order of reaction norms for both traits, and additionally to differences in the between‐line variance for development time. The coefficient of genetic variation increased under stress conditions for development time, while it was higher at both high and low densities for viability. While development time also correlated negatively with fitness at high larval densities—meaning that fast developers have high fitness—there was no correlation with fitness at low density. This result suggests that GEI may be a common feature of fitness‐related genetic variation and, further, that trait values under noncompetitive conditions could be poor indicators of individual fitness. The latter point could have significant implications for animal and plant breeding programs, as well as for conservation genetics.     

Variation in tolerance to drought among Scandinavian populations of <Emphasis Type="Italic">Arabidopsis lyrata</Emphasis>

The ability to cope with water limitation influences plant distributions, and several plant traits have been interpreted as adaptations to drought stress. In Scandinavia, the perennial herb Arabidopsis lyrata occurs in open habitats that differ widely in climate and water availability in summer, suggesting differential selection on drought-related traits. We conducted two greenhouse experiments to examine differentiation in drought response traits among six Scandinavian populations, and to determine whether leaf trichomes confer protection against drought. We quantified tolerance to drought as fitness (survival and biomass of survivors) when exposed to drought relative to fitness under non-drought conditions. Two Swedish populations from shores along the Bothnian Bay had higher tolerance to drought than four riverbed populations from Norway. Under conditions of drought, the shore populations experienced less leaf damage compared to the riverbed populations, and their survival and biomass were less reduced relative to non-drought conditions. Across populations, tolerance to drought was positively related to leaf mass per area and negatively related to flowering propensity and proportion roots, but not related to plant size at the initiation of the drought treatment. In populations polymorphic for trichome production, trichome-producing plants were more tolerant to drought than glabrous plants. The results suggest that both leaf morphology and life-history traits contribute to differential drought response in natural populations of A. lyrata, and that this system offers excellent opportunities for examining the adaptive value and genetic basis of drought-related traits.     

Effects of positive interactions, size symmetry of competition and abiotic stress on self-thinning in simulated plant populations

Background and Aims

Competition drives self-thinning (density-dependent mortality) in crowded plant populations. Facilitative interactions have been shown to affect many processes in plant populations and communities, but their effects on self-thinning trajectories have not been investigated.

Methods

Using an individual-based ‘zone-of-influence’ model, we studied the potential effects of the size symmetry of competition, abiotic stress and facilitation on self-thinning trajectories in plant monocultures. In the model, abiotic stress reduced the growth of all individuals and facilitation ameliorated the effects of stress on interacting individuals.

Key Results

Abiotic stress made the log biomass – log density relationship during self-thinning steeper, but this effect was reduced by positive interactions among individuals. Size-asymmetric competition also influenced the self-thinning slope.

Conclusions

Although competition drives self-thinning, its course can be affected by abiotic stress, facilitation and competitive symmetry.     

Benefits and potential trade‐offs associated with yeast‐like symbionts during virulence adaptation in a phloem‐feeding planthopper

Insect herbivores form symbioses with a diversity of prokaryotic and eukaryotic microorganisms. A role for endosymbionts during host feeding on nutrient‐poor diets – including phloem – is now supported by a large body of evidence. Furthermore, symbiont‐herbivore associations have been implicated in feeding preferences by host races (mainly aphids) on multiple plant species. However, the role of symbionts in mediating herbivore preferences between varieties of the same plant species has received little research attention despite the implications for virulence adaptation to resistant crops. This study investigates the role of yeast‐like symbionts (YLS) in virulence adaptation and host plant switching among populations of the brown planthopper, Nilaparvata lugens (Stål) (Hemiptera: Delphacidae), that were selected on various rice [Oryza sativa L. (Poaceae)] lines differing in their resistance against herbivores. Planthopper fitness (nymph weight) declined where YLS densities were depleted through heat treatment. However, compared to normal symbiotic planthoppers, the depletion of symbionts did not generally change the relative fitness of planthoppers (each ‘adapted’ to a single natal host) when switched to feed on a range of rice lines (exposed hosts). In some cases, this occurred despite differences in YLS density responses to the various hosts. Furthermore, we detected no fitness costs associated with YLS in adapted populations. Therefore, the results of this study suggest that, whereas YLS are essential for planthopper nutrition, changes in YLS density play little role during virulence adaptation and host plant switching by the brown planthopper.     

Impact of land use intensity and temperature on the reproductive performance of <Emphasis Type="Italic">Dactylis glomerata</Emphasis> populations in the southeastern Alps

An understanding of the processes and environmental conditions governing spatial variation in reproductive performance of plants can provide important information about the factors characterizing plant community structure and influencing fitness in natural plant populations, especially in the context of climate and land use change. In this study, 60 mountain populations of Dactylis glomerata distributed along a fertilization regime in varying grassland hay meadows were evaluated. Variations in field management, climate, soil fertility, vegetation structure, population density and species richness on reproductive performance were examined. The results indicated that field management and soil nutrient availability are the main variables influencing population density and reproductive output of D. glomerata. Moreover, the results show the effect of temperature on seed mass and resource investment in reproduction. Climate and soil change suggest a morphological differentiation of reproductive traits: (i) individuals grown on sites with higher soil nutrient availability or nutrient supply have larger inflorescences with a greater number and heavier seeds; (ii) individuals grown on warmer sites have heavier seeds. We conclude that if the climate warms and increases land use intensification in hay meadows in the Alps, this will have a pronounced positive impact on the reproductive performance of D. glomerata. Moreover, it can be hypothesized that the migration potential of D. glomerata towards higher altitudes may be likely in the near future in response to accelerated climate change.     

Interactions between seed source,planting arrangement,and soil treatment in determining plant size and root allocation in Phlox drummondii

Summary Two varieties of the annual plant Phlox drummondii were grown in pots in the greenhouse. Pots differed in the mixture of varieties, total number of plants, spatial clustering of plants, and soil treatment.The shoot biomass of centrally located target plants was significantly influenced by the varietal composition within a pot, and the pattern of this genetic effect varied with both plant density and soil treatment. Density and soil treatment interacted strongly, with the negative effect of neighbors on shoot growth being significantly greater when soil nutrient concentrations were lower.The varieties differed significantly in their relative allocation of biomass to roots. Relative root allocation was increased in response to both the presence of neighbors and a decrease in soil nutrient availability. These factors did not interact significantly in their effects.Although both plant size and relative allocation to roots were influenced by varietal composition, these genetic effects were small relative to the effects of density and soil treatment.The strong interactions between plant location, plant identity, and soil treatment suggest that relations between neighbors will be very complex in natural populations occupying microgeographically heterogeneous habitats. A more complete understanding of these interactions will be required before realistic dynamic models of natural populations can be constructed.     

Competition between grassland plants of different initial sizes

Summary Four species of grassland plant, Plantago lanceolata, Holcus lanatus, Lolium perenne and Rumex acetosa, were grown as monocultures and mixtures in pots of nutrient poor soil in a glasshouse for 8 months. There were four plants per pot and these were arranged in two competition modes: either root and shoot interactions were permitted, or only roots allowed to interact by using above-ground partitions. Time of introduction of seedlings was varied to give a range of plant size ratios at the start of the experiment. The factorial design catered for all combinations of species, competition modes and planting times, replicated in four blocks. The shoots were clipped at a fixed height at each of five harvests. Rumex grew badly and was mostly omitted from analysis of the data.By (i) following the change in the relationship of clip dry weights against planting time with successive harvests, (ii) plotting the change in the logarithm of the ratio of cumulative clip dry weights with time and (iii) the use of de Wit logarithmic ratio plots it was demonstrated that each monoculture and mixture combination's ratios of plant weights converged towards stable equilibrium values. Three hypotheses are put forward to explain why in monocultures a smaller plant was at a competitive advantage relative to a larger neighbour and was not suppressed in its growth by the latter. In mixtures this plant size effect was superimposed to different extents on the relative aggressiveness of the species considered. It was concluded that in a nutrient poor soil, when competition for light was low, root interactions can promote the co-existence of neighbouring plant species.     

Analysis of the relationship between allozyme heterozygosity and fitness in the rare Gentiana pneumonanthe L.

Especially for rare species occurring in small populations, which are prone to loss of genetic variation and inbreeding, detailed knowledge of the relationship between heterozygosity and fitness is generally lacking. After reporting on allozyme variation and fitness in relation to population size in the rare plant Gentiana pneumonanthe, we present a more detailed analysis of the association between heterozygosity and individual fitness. The aim of this study was to test whether increased fitness of more heterozygous individuals is explained best by the ‘inbreeding’ hypothesis or by the ‘overdominance’ hypothesis. Individual fitness was measured during 8 months of growth in the greenhouse as the performance for six life-history parameters. PCA reduced these parameters to four main Fitness Components. Individual heterozygosity was scored for seven polymorphic allozyme loci. For some of these loci (e.g. Aat3, Pgm1 and 6Pgdh2) heterozygotes showed a significantly higher relative fitness than homozygotes. To test the inbreeding model, regression analyses were performed between each Fitness Component and the number of heterozygous loci per individual. Multiple regressions with the adaptive distance of five loci as independent variables were used to test the overdominance model. Only the inbreeding model was a statistically significant explanation for the relationship between heterozygosity and fitness in G. pneumonanthe. The number of heterozygous loci was significantly negatively correlated with the coefficients of variation of three of the six initially measured fitness parameters. This suggests a lower developmental stability among more homozygous plants and may explain the higher phenotypic variation in small populations of the species observed earlier. The importance of the results for conservation biology is discussed.     

SURVIVAL OF HYBRIDS IN A MOSAIC HYBRID ZONE

The ground crickets Allonemobius fasciatus and A. socius meet in a mosaic hybrid zone that stretches from New Jersey at least as far west as Illinois. Within mixed populations from the contact zone, “pure” species individuals predominate. To determine whether hybrids are less viable than pure-species individuals, and to assess whether the high proportion of pure-species individuals in mixed populations results from hybrid inviability, we performed a cohort analysis. In this study, five mixed populations from the hybrid zone were each sampled three to five times from the fall of 1986 to the spring of 1988. Individuals were assigned to one of three categories based on their genotypes: A. socius (individuals harboring only alleles unique to A. socius), hybrid (individuals with alleles unique to both species), and A. fasciatus (individuals harboring only alleles unique to A. fasciatus). This sampling and measurement scheme permitted monitoring of the proportion of hybrid individuals in a population over time. The results were fairly consistent. The relative survival of A. socius was greater than the relative survival of members of the other two groups in four of the five populations. The relative viability of A. fasciatus was greater than that of hybrids in one population, approximately equal to that of hybrids in two populations, and less than that of hybrids in two populations. These results not only shed light on an important component of fitness, viability from hatching to adulthood, but they demonstrate that loss of hybrid individuals during the course of the field season will not explain deviations from random mating expectations present in mixed populations in late summer. The deviations must be due to assortative mating or to a reproductive barrier operating prior to egg hatch.     

High predictability of direct competition between marine diatoms under different temperatures and nutrient states

The distribution of marine phytoplankton will shift alongside changes in marine environments, leading to altered species frequencies and community composition. An understanding of the response of mixed populations to abiotic changes is required to adequately predict how environmental change may affect the future composition of phytoplankton communities. This study investigated the growth and competitive ability of two marine diatoms, Phaeodactylum tricornutum and Thalassiosira pseudonana, along a temperature gradient (9–35°C) spanning the thermal niches of both species under both high‐nitrogen nutrient‐replete and low‐nitrogen nutrient‐limited conditions. Across this temperature gradient, the competitive outcome under both nutrient conditions at any assay temperature, and the critical temperature at which competitive advantage shifted from one species to the other, was well predicted by the temperature dependencies of the growth rates of the two species measured in monocultures. The temperature at which the competitive advantage switched from P. tricornutum to T. pseudonana increased from 18.8°C under replete conditions to 25.3°C under nutrient‐limited conditions. Thus, P. tricornutum was a better competitor over a wider temperature range in a low N environment. Being able to determine the competitive outcomes from physiological responses of single species to environmental changes has the potential to significantly improve the predictive power of phytoplankton spatial distribution and community composition models.     

THE COMPARATIVE DEMOGRAPHY OF RECIPROCALLY SOWN POPULATIONS OF PHLOX DRUMMONDII HOOK. I. SURVIVORSHIPS,FECUNDITIES, AND FINITE RATES OF INCREASE

Comparative demography of reciprocally sown populations for all stages of the life cycle of the winter annual, Phlox drummondii, was recorded for two seasons in natural sites. The sites investigated covered the natural range of the species and were separated by linear distances ranging from 10 to over 500 kilometers. Interpopulational variation was observed in all stages of the life cycle. Prereproductive survivorship ranged from 0 to 92 percent. Fecundity per plant ranged from 0 to 81 seeds. Finite rates of increase ranged from 0 to 34.1 yr^?1. The experimental populations with low values were growing in sites that received little rainfall or had experienced an insect or fungal infestation. The populations with high growth rates occurred in the northern sites which received greater rainfall. The relative fitness of individuals from the alien populations was compared to the local populations for all stages of the life cycle. The alien relative fitness for survivorship averaged 0.72. The average relative fitness of aliens for fecundity was 0.71 and that for finite rate of increase was 0.57. The life history parameters found in Phlox populations differ from one part of the range to another so that aliens have lower relative fitnesses than individuals indigenous to the site.