Plant genotype and nitrogen loading influence seagrass productivity, biochemistry, and plant-herbivore interactions

Genetic variation within and among key species can have significant ecological consequences at the population, community, and ecosystem levels. In order to understand ecological properties of systems based on habitat-forming clonal plants, it is crucial to clarify which traits vary among plant genotypes and how they influence ecological processes, and to assess their relative contribution to ecosystem functioning in comparison to other factors. Here we used a mesocosm experiment to examine the relative influence of genotypic identity and extreme levels of nitrogen loading on traits that affect ecological processes (at the population, community, and ecosystem levels) for Zostera marina, a widespread marine angiosperm that forms monospecific meadows throughout coastal areas in the Northern Hemisphere. We found effects of both genotype and nitrogen addition on many plant characteristics (e.g., aboveground and belowground biomass), and these were generally strong and similar in magnitude, whereas interactive effects were rare. Genotypes also strongly differed in susceptibility to herbivorous isopods, with isopod preference among genotypes generally matching their performance in terms of growth and survival. Chemical rather than structural differences among genotypes drove these differences in seagrass palatability. Nitrogen addition uniformly decreased plant palatability but did not greatly alter the relative preferences of herbivores among genotypes, indicating that genotype effects are strong. Our results highlight that differences in key traits among genotypes of habitat-forming species can have important consequences for the communities and ecosystems that depend on them and that such effects are not overwhelmed by known environmental stressors.     

Gpi genotypic effect on quantitative traits in the northern bay scallop,Argopecten irradians irradians

The relationships between glucose-6-phosphate isomerase (Gpi) genotype and quantitative variation related to reproduction and growth were explored over the lifespan of a single cohort of northern bay scallops, Argopecten irradians irradians (Lamarck), from September 1986 to April 1988 in the Niantic River estuary, Connecticut, USA. Analyses revealed that Gpi genotype explained an increasingly significant proportion of variation in scallop size (up to 15%) as the cohort aged. The genotype-specific effects were consistent across sampling dates and among measured traits. There was no evidence for heterosis with respect to size at this locus; however, rare genotypes contributed substantially to the relationship and showed some tendency to fall on the extremes of the phenotypic distribution. The strength and consistency of the genotypic effect on scallop size suggest that genetic variation for Gpi, or some locus in linkage disequilibrium with Gpi, may translate into biochemical and/or physiological variation and affect fitness in this species.     

Influence of Habitat Patchiness on Genetic Diversity and Spatial Structure of a Serpentine Endemic Plant

Abstract: Conservation of rare plant species often involves small, local populations dispersed within apparently suitable habitat. We used enzyme electrophoresis to study genotypic diversity in 32 populations of Calystegia collina , a self-incompatible clonal plant endemic to serpentine substrate in northern California's Coast Range. Genotypic diversity within a population was a good predictor of seed set by randomly marked flowers. Ramets from the most abundant genotypes in a population were most likely to produce flowers, but flowers from abundant genotypes were less likely to produce seed capsules than were flowers from rarer genotypes. These results are consistent with previous findings that reproductive success in C. collina is limited by the availability of compatible pollen. On small serpentine outcrops supporting only one or two populations, C. collina did not show reduced genotypic diversity or heterozygosity compared with populations on large outcrops supporting many populations. Instead, genotypic diversity and outcrop size had strong but independent influences on reproductive success. Although large serpentine outcrops contain more populations and provide better conditions for flower and seed production, significant diversity of unique genotypes clearly exists in isolated serpentine outcrops. Our findings suggest that plant conservation strategies must take into account the natural distribution of populations. The effects of habitat fragmentation on C. collina and other plant species that occur naturally in small, discrete patches may be unlike those that have been documented in more recently fragmented species.     

Bottom-up effects of plant genotype on aphids, ants, and predators

Theory predicts that bottom-up ecological forces can affect community dynamics, but whether this extends to the effects of heritable plant variation on tritrophic communities is poorly understood. In a field experiment, I contrasted the effects of plant genotype (28 genotypes; 1064 plants), aphid density, and the presence/absence of mutualistic ants in affecting the per capita population growth of a specialist aphid herbivore, as well as the effects of plant genotype on the third trophic level. Plant genotype strongly affected aphid population growth rate, explaining 29% of the total variation in growth rate, whereas aphid density and ant-aphid interactions explained substantially less variation (< 2%) in aphid population growth rate. Plant genotype also had direct and indirect effects on the third trophic level, affecting the abundance of aphid-tending ants and the richness of predators. Multiple regression identified several heritable plant traits that explained 49% of the variation in aphid growth rate and 30% of the variation in ant abundance among plant genotypes. These bottom-up effects of plant genotype on tritrophic interactions were independent of the effects of either initial aphid density or the presence/absence of mutualistic ants. This study shows that plant genotype can be one of the most important ecological factors shaping tritrophic communities.     

Host-plant genotypic diversity mediates the distribution of an ecosystem engineer

Ecosystem engineers affect ecological communities by physically modifying the environment. Understanding the factors determining the distribution of engineers offers a powerful predictive tool for community ecology. In this study, we examine whether the goldenrod bunch gall midge (Rhopalomyia solidaginis) functions as an ecosystem engineer in an old-field ecosystem by altering the composition of arthropod species associated with a dominant host plant, Solidago altissima. We also examine the suite of factors that could affect the distribution and abundance of this ecosystem engineer. The presence of bunch galls increased species richness and altered the structure of associated arthropod communities. The best predictors of gall abundance were host-plant genotype and plot-level genotypic diversity. We found positive, nonadditive effects of genotypic diversity on gall abundance. Our results indicate that incorporating a genetic component in studies of ecosystem engineers can help predict their distribution and abundance, and ultimately their effects on biodiversity.     

Spatial clustering of habitat structure effects patterns of community composition and diversity

Natural ecosystems often show highly productive habitats that are clustered in space. Environmental disturbances are also often nonrandomly distributed in space and are either intrinsically linked to habitat quality or independent in occurrence. Theoretical studies predict that configuration and aggregation of habitat patch quality and disturbances can affect metacommunity composition and diversity, but experimental evidence is largely lacking. In a metacommunity experiment, we tested the effects of spatially autocorrelated disturbance and spatial aggregation of patch quality on regional and local richness, among-community dissimilarity, and community composition. We found that spatial aggregation of patch quality generally increased among-community dissimilarity (based on two measures of beta diversity) of communities containing protozoa and rotifers in microcosms. There were significant interacting effects of landscape structure and location of disturbances on beta diversity, which depended in part on the specific beta diversity measures used. Effects of disturbance on composition and richness in aggregated landscapes were generally dependent on distance and connectivity among habitat patches of different types. Our results also show that effects of disturbances in single patches cannot directly be extrapolated to the landscape scale: the predictions may be correct when only species richness is considered, but important changes in beta diversity may be overlooked. There is a need for biodiversity and conservation studies to consider the spatial aggregation of habitat quality and disturbance, as well as connectivity among spatial aggregations.     

Effects of plant species traits on ecosystem processes: experiments in the Patagonian steppe

Several experiments have shown that aboveground net primary productivity increases with plant species richness. The main mechanism proposed to explain this relationship is niche complementarity, which is determined by differences in plant traits that affect resource use. We combined field and laboratory experiments using the most abundant species of the Patagonian steppe to identify which are the traits that determine niche complementarity in this ecosystem. We estimated traits that affect carbon, water, microclimate, and nitrogen dynamics. The most important traits distinguishing among species, from the standpoint of their effects on ecosystem functioning, were potential soil nitrification, rooting depth, and soil thermal amplitude. Additionally, we explored the relationship between trait diversity and aboveground net primary production (ANPP) using a manipulative field experiment. ANPP and the fraction of ANPP accounted for by trait diversity increased with number of traits. The effect of trait diversity decreased as the number of traits increased. Here, the use of traits gave us a mechanistic understanding of niche complementarity in the Patagonian steppe.     

From metamorphosis to maturity in complex life cycles: equal performance of different juvenile life history pathways

Performance in one stage of a complex life cycle may affect performance in the subsequent stage. Animals that start a new stage at a smaller size than conspecifics may either always remain smaller or they may be able to "catch up" through plasticity, usually elevated growth rates. We study how size at and date of metamorphosis affected subsequent performance in the terrestrial juvenile stage and lifetime fitness of spadefoot toads (Pelobates fuscus). We analyzed capture-recapture data of > 3000 individuals sampled during nine years with mark-recapture models to estimate first-year juvenile survival probabilities and age-specific first-time breeding probabilities of toads, followed by model selection to assess whether these probabilities were correlated with size at and date of metamorphosis. Males attained maturity after two years, whereas females reached maturity 2-4 years after metamorphosis. Age at maturity was weakly correlated with metamorphic traits. In both sexes, first-year juvenile survival depended positively on date of metamorphosis and, in males, also negatively on size at metamorphosis. In males, toads that metamorphosed early at a small size had the highest probability to reach maturity. However, because very few toadlets metamorphosed early, the vast majority of male metamorphs had a very similar probability to reach maturity. A matrix projection model constructed for females showed that different juvenile life history pathways resulted in similar lifetime fitness. We found that the effects of date of and size at metamorphosis on different juvenile traits cancelled each other out such that toads that were small or large at metamorphosis had equal performance. Because the costs and benefits of juvenile life history pathways may also depend on population fluctuations, ample phenotypic variation in life history traits may be maintained.     

The relative importance of host-plant genetic diversity in structuring the associated herbivore community

Recent studies suggest that intraspecific genetic diversity in one species may leave a substantial imprint on the surrounding community and ecosystem. Here, we test the hypothesis that genetic diversity within host-plant patches translates into consistent and ecologically important changes in the associated herbivore community. More specifically, we use potted, grafted oak saplings to construct 41 patches of four saplings each, with one, two, or four tree genotypes represented among the host plants. These patches were divided among two common gardens. Focusing first at the level of individual trees, we assess how tree-specific genotypic identity, patch-level genetic diversity, garden-level environmental variation, and their interactions affect the structure of the herbivore community. At the level of host-plant patches, we analyze whether the joint responses of herbivore species to environmental variation and genetic diversity result in differences in species diversity among tree quartets. Strikingly, both species-specific abundances and species diversity varied substantially among host-tree genotypes, among common gardens, and among specific locations within individual gardens. In contrast, the genetic diversity of the patch left a detectable imprint on local abundances of only two herbivore taxa. In both cases, the effect of genetic diversity was inconsistent among gardens and among host-plant genotypes. While the insect community differed significantly among individual host-plant genotypes, there were no interactive effects of the number of different genotypes within the patch. Overall, additive effects of intraspecific genetic diversity of the host plant explained a similar or lower proportion (7-10%) of variation in herbivore species diversity than did variation among common gardens. Combined with the few previous studies published to date, our study suggests that the impact of host-plant genetic diversity on the herbivore community can range from none to nonadditive, is generally low, and reaches its most pronounced impact at small spatial scales. Overall, our findings strengthen the emerging view that the impacts of genetic diversity are system, scale, and context dependent. As the next step in community genetics, we should then start asking not only whether genetic diversity matters, but under what circumstances its imprint is accentuated.     

Meta-analysis of the differential effects of habitat fragmentation and degradation on plant genetic diversity

Genetic diversity is a key factor for population survival and evolution. However, anthropogenic habitat disturbance can erode it, making populations more prone to extinction. Aiming to assess the global effects of habitat disturbance on plant genetic variation, we conducted a meta-analysis based on 92 case studies obtained from published literature. We compared the effects of habitat fragmentation and degradation on plant allelic richness and gene diversity (equivalent to expected heterozygosity) and tested whether such changes are sensitive to different life-forms, life spans, mating systems, and commonness. Anthropogenic disturbance had a negative effect on allelic richness, but not on gene diversity. Habitat fragmentation had a negative effect on genetic variation, whereas habitat degradation had no effect. When we examined the individual effects in fragmented habitats, allelic richness and gene diversity decreased, but this decrease was strongly dependent on certain plant traits. Specifically, common long-lived trees and self-incompatible species were more susceptible to allelic richness loss. Conversely, gene diversity decreased in common short-lived species (herbs) with self-compatible reproduction. In a wider geographical context, tropical plant communities were more sensitive to allelic richness loss, whereas temperate plant communities were more sensitive to gene diversity loss. Our synthesis showed complex responses to habitat disturbance among plant species. In many cases, the absence of effects could be the result of the time elapsed since the disturbance event or reproductive systems favoring self-pollination, but attention must be paid to those plant species that are more susceptible to losing genetic diversity, and appropriate conservation should be actions taken.     

Does interspecific competition influence relationships between heterozygosity and fitness-related behaviors in juvenile Atlantic salmon (<Emphasis Type="Italic">Salmo salar</Emphasis>)?

Very few studies have investigated the effect of genetic diversity on the behavioral and phenotypic traits linked to the competitive ability of individuals. In this study, we reared juvenile Atlantic salmon (Salmo salar) alone or with the competitive rainbow trout (Oncorhynchus mykiss) in order to: (1) to assess correlations between heterozygosity and traits related to individual competitive ability [i.e., heterozygosity–fitness correlations (HFCs)] in Atlantic salmon, and (2) to evaluate the effect of the competitive rainbow trout on any such HFCs. We also tested whether a few loci had a disproportionately large effect (i.e., the local effect hypothesis) or, on the contrary, if all loci contributed equally (i.e., the global effect hypothesis) in explaining the observed HFCs. We found significant HFCs for phenotypic traits related to the competitive ability of juvenile Atlantic salmon, i.e., the growth rate and the distance to the feeding source. Some HFCs were nonlinear, suggesting that individuals with intermediate levels of heterozygosity were favored. In addition, we found that the competition exerted by rainbow trout only weakly modified these HFCs as the relationships were highly consistent across treatments. We demonstrated that the local-effect hypothesis best explained both linear and nonlinear HFCs. Overall, our results illustrated the importance of genetic diversity in explaining the behavioral variability observed within populations. Moreover, we provide evidence that, even if a competitive species can have strong ecological effects, the relationships between genetic diversity and fitness-related traits in juvenile Atlantic salmon were not influenced by such effects.     

The role of conjugation in the gene-individual-population relationships in increasing eco-exergy

The genotypic and phenotypic processes were incorporated into one system in the gene-individual-population relationships under the framework of Individual based models (IBMs). The gene types addressing different degrees of metabolic efficiency and toxin susceptibility were provided as attributes in the individuals. Subsequently ecological processes such as food competition and movement were allowed concurrently on the 2-D space to determine the suitable species adapted to the system. The integrative gene-individual-population model accordingly responded to gene exchanges between the neighboring individuals through conjugation. At a substantially low level of gene exchange, system heterogeneity increased to produce high levels of eco-exergy, being presented by species diversity and total population size in the system. The issues related to genetic and ecological effects in the integrative gene-individual-population relationships were further discussed.     

A direct comparison of the consequences of plant genotypic and species diversity on communities and ecosystem function

Biodiversity loss is proceeding at an unprecedented rate, yet we lack a thorough understanding of the consequences of losing diversity at different scales. While species diversity is known to impact community and ecosystem processes, genotypic diversity is assumed to have relatively smaller effects. Nonetheless, a few recent studies suggest that genotypic diversity may have quantitatively similar ecological consequences compared to species diversity. Here we show that increasing either genotypic diversity of common evening primrose (Oenothera biennis) or species diversity of old-field plant species resulted in nearly equivalent increases (approximately 17%) in aboveground primary production. The predominant mechanism explaining this effect, niche complementarity, was similar for each type of diversity. Arthropod species richness also increased with both types of plant diversity, but the mechanisms leading to this effect differed: abundance-driven accumulation of arthropod species was important in plant genotypic polycultures, whereas resource specialization was important in plant species polycultures. Thus, similar increases in primary productivity differentially impacted higher trophic levels in response to each type of plant diversity. These results highlight important ecological similarities and differences between genotypic and species diversity and suggest that genotypic diversity may play a larger role in community and ecosystem processes than previously realized.     

Influence of growth on reproductive traits and its effect on fertility and gene diversity in a clonal seed orchard of scots pine, Pinus Sylvestris L

This study was carried out in a clonal seed orchard of scots pine (Pinus sylvestris L.), to determine the difference and interaction for reproductive and growth characters among clones and its impact on fertility variation and gene diversity Numbers of female and male strobili, and height and diameter at breast height were studied on six grafts chosen randomly in each of the 27 clones for the purpose. One-way analysis of variance revealed large differences in both reproductive and growth characters among clones. The differences were higher in growth characters than in reproductive traits. There was significant phenotypic correlation among growth and reproductive characters. So, growth characters had a greater effect on male and female fertility Estimates of total fertility variation (Sibling coefficient = 1.012), status number (26.8) and relative gene diversity (0.981) were computed. Fertility variation among clones was low, which caused a high relative population size (99% of census number). The positive phenotypic correlation between growth and reproductive characters showed that enhanced growth rate could be effective in improving fertility and gene diversity of seed orchard crop. The results of the study have implications in breeding and selection of plus tree and populations, establishment and thinning of seed orchards of the species.     

Potato Diversity in the Andean Center of Crop Domestication

The diversity and population structure of potato landraces ( Solanum spp.) within their center of domestication was studied using isozyme surveys of four polymorphic loci. The objective in assessing the distribution of genetic diversity was to assist in planning conservation strategies of crop genetic resources that are threatened by genetic erosion. In situ conservation methods depend on this type of analysts. Research was conducted in the region of Cusco, Peru. Eight fields spread among two microregions were randomly sampled, and 610 tubers were studied from this sample. In addition, 503 tubers were collected from markets in seven different meso-regions (provinces) surrounding the regional center of Cusco. Thirty genotypes were identified in the field sample and 82 in the regional sample. The frequency and distribution of genotypes and alleles are described. A high degree of genotype endemism was found at both the field and regional levels. Genotypes were unevenly distributed, and most of the genotypic diversity was between rather than within populations. At the allele level, however, we found that a very high percentage of the diversity was within rather than between populations. The genotype is the key unit for maintaining the population of potato landraces. Our findings suggest that collections need to be both geographically extensive and intensive. Because farmers are able to maintain most alleles on relatively small portions of their land, in situ conservation is a viable strategy.     

Genetic Structure of Fragmented Populations of the Endangered Daisy Rutidosis leptorrhynchoides

Abstract: The endangered grassland daisy Rutidosis leptorrhynchoides has been subject to severe habitat destruction and fragmentation over the past century. Using allozyme markers, we examined the genetic diversity and structure of 16 fragmented populations. The species had high genetic variation compared to other plant species, and both polymorphism and allelic richness showed strong positive relationships with log reproductive population size, reflecting a loss of rare alleles (frequency of q < 0.1) in smaller populations. Fixation coefficients were positively related to size, due either to a lack of rare homozygotes in small populations or to Wahlund effects (owing to spatial genetic structure) in large ones. Neither gene diversity nor heterozygosity was related to population size, and other population parameters such as density, spatial contagion, and isolation had no apparent effect on genetic variation. Genetic divergence among populations was low , despite a large north-to-south break in the species' current distribution. To preserve maximum genetic variation, conservation strategies should aim to maintain the five populations larger than 5000 reproductive plants, all of which occur in the north of the range, as well as the largest southern population of 626 plants at Truganina. Only one of these is currently under formal protection. High heterozygosity in smaller populations suggests that they are unlikely to be suffering from inbreeding depression and so are also valuable for conservation. Erosion of allelic richness at self-incompatibility loci, however, may limit the reproductive capacity of populations numbering less than 20 flowering plants.     

Density-dependent biodiversity effects on physical habitat modification by freshwater bivalves

Several decades of research have shown that biodiversity affects ecosystem processes associated with resource capture and the production of biomass within trophic levels. Although there are good reasons to expect that biodiversity influences non-trophic ecosystem processes, such as the physical creation or modification of habitat, studies investigating the role of biodiversity on physical processes are scarce. Here we report the results of a study using artificial streams to test the influence of freshwater mussel biodiversity on gravel erosion during high flows while manipulating mussel abundance. Mussel species vary in traits that should influence their effects on erosion, such as size, shell morphology, and burrowing behavior. We found that mussel species richness was associated with an increase in erosion at both low and high densities. Planned contrasts showed that the erosion observed in species mixtures was purely additive at low density, indicating that erosion in a species polyculture could routinely be predicted by the performance of monocultures. However, at high density certain combinations of species showed nonadditive effects on erosion, suggesting that organism abundance can fundamentally alter biodiversity effects. Although this may have been a result of altered species interactions at high density, our study design cannot confirm this.     

Benefits of Conservation of Plant Genetic Diversity to Arthropod Diversity

Abstract:  We argue that the genetic diversity of a dominant plant is important to the associated dependent community because dependent species such as herbivores are restricted to a subset of genotypes in the host-plant population. For plants that function as habitat, we predicted that greater genetic diversity in the plant population would be associated with greater diversity in the dependent arthropod community. Using naturally hybridizing cottonwoods (  Populus spp.) in western North America as a model system, we tested the general hypothesis that arthropod alpha (within cross-type richness) and beta (among cross-type composition) diversities are correlated with cottonwood cross types from local to regional scales. In common garden experiments and field surveys, leaf-modifying arthropod richness was significantly greater on either the F₁ (1.54 times) or backcross (1.46 times) hybrid cross types than on the pure broadleaf cross type (  P. deltoides Marshall or P. fremontii Watson). Composition was significantly different among three cross types of cottonwoods at all scales. Within a river system, cottonwood hybrid zones had 1.49 times greater richness than the broadleaf zone, and community composition was significantly different between each parental zone and the hybrid zone, demonstrating a hierarchical concentration of diversity. Overall, the habitats with the highest cottonwood cross-type diversity also had the highest arthropod diversity. These data show that the genetics of habitat is an important conservation concept and should be a component of conservation theory.     

全尺度人工湿地中植物多样性对生产力与多样性效应的影响

在中国东南部的全尺度复合垂直流人工湿地中开展2年的植物多样性实验,以研究植物多样性（包括植物物种丰富度和植物组成）对群落生产力与多样性效应（即互补效应、选择效应和净多样性效应）的影响及其产生机制。结果表明,2007年物种丰富度与群落生产力呈线形正相关,而2008年显著的单峰格局,其关系式为：y=-0.213x2＋3.455x＋15.192（R=0.215）。2008年物种丰富度与互补效应呈显著地线形负相关,而2007年呈单峰格局,其关系式为：y=-0.389x2＋6.974x-10.707（R=0.247）,而且2007年与2008年的互补效应与生产力都呈显著的正相关,表明互补效应对生产力的提高有重要作用。然而,2007年与2008年物种丰富度与选择效应之间均没有显著相关性,且选择效应与群落生产力之间也没有显著相关性,表明选择效应对生产力的提高作用不显著。2007年与2008年中物种组成对生产力、互补效应、选择效应与净多样性效应均有显著影响,说明人工湿地的植物配置对其生态系统功能的维持尤为重要。2008年物种丰富度与净多样性效应呈极显著地线形负相关,而2007年呈显著单峰格局,其关系式为：y=-0.329 x2＋5.968 x-12.659（R=0.234）,这种趋势主要是由于植物多样性-生态系统功能关系的影响因素（如物种的竞争力和生态位）在2年中有所变化。同时,2007年与2008年的多样性净效应与生产力都呈显著正相关关系,表明生产力与多样性净效应的变化趋势是同步的。与抽样效应假说不同的是,本实验中单种最高产物种（芦竹）在混种时没有表现出高产,主要是由于生长的分配、资源的竞争力与环境的变化等。