两种生境下野慈姑繁殖差异及其机制

植物的生长环境不但可直接作用于其可利用资源的多寡及其繁殖分配,而且可通过影响植物所在的群落以及传粉者的组成或行为而间接的导致繁殖差异。然而,直接与间接作用的方向或强度很少被同时关注,从而限制了从机制上理解环境对植物繁殖的影响。选取野慈姑(Sagittaria trifolia L.)为研究材料,将相同基因型组成的植株分别种植在光照差异明显的两个同质园(林荫区与日照区),于盛花期对野慈姑的开花数量、昆虫访花进行了观察,确定其主要访花昆虫、记录访花行为以及野慈姑的繁殖产出水平。研究表明,日照区野慈姑每天开放的花朵和植株数量显著高于林荫区。两个区域内野慈姑的主要访花者有四大类:蚜蝇类、蜂类、蝶类和蝇类;其中林荫区的主要传粉昆虫为蚜蝇类,而日照区为蜂类。日照区昆虫单位时间内的访花频率、昆虫每回合访问的雄花数、总花数和花序数均显著高于林荫区。日照区野慈姑的座果率显著高于林荫区,而单果种子数量与种子面积与林荫区相当。总体而言,日照区野慈姑的繁殖产出达到了林荫区的三倍以上,这是环境的直接作用与环境介导的传粉作用在同一方向上叠加的结果,且前者占主导地位。在关注植物、物理环境和生物因子三者之间互作的前提下,量化每个繁殖阶段的水平,特别是传粉昆虫在不同生境下的表现,阐述了环境异质性导致植物繁殖差异的根本原因。     

The influence of salt and nitrogen on herbivore abundance: direct and indirect effects

We report on the influence of experimentally increased interstitial salinity and plant nitrogen on the abundance of the delphacid planthopper, Prokelisia marginata (Van Duzee) (Homoptera: Delphacidae), which feeds on salt marsh cordgrass, Spartina alterniflora. We also report the effects of these treatments on parasitism of P. marginata eggs by the fairyfly parasitoid, Anagrus sophiae (Hymenoptera: Mymaridae). Soil salinity was significantly elevated following the addition of salt pellets broadcast over the ground and plant foliar nitrogen was significantly increased after the addition of fertilizer. The addition of fertilizer increased P. marginata densities on Spartina but addition of salt did not. Neither treatment significantly affected levels of egg parasitism by A. sophiae. In this system direct effects of plants on their herbivores via changes in plant chemistry appear more important than indirect effects of plants on herbivores via their natural enemies. Received: 1 August 1997 / Accepted: 29 September 1997     

Grazing in remnant wetland habitats in an arid region: direct and indirect effects on two specialist bird species

Capsule: Grazing by livestock can have complex effects on drivers of population change in the Clamorous Reed Warbler Acrocephalus stentoreus and Dead Sea Sparrow Passer moabiticus.

Aims: To investigate the effect on two specialist bird species on wetland degradation in the Jordan Valley.

Methods: The direct and indirect effects of grazing on the probability of occurrences of two specialist bird species, Clamorous Reed Warbler A. stentoreus and Dead Sea Sparrow P. moabiticus, were analysed during the breeding season at the patch scale, using path analysis.

Results: Tamarix shrub density was a strong predictor for the presence of both species. Grazing had a negative total effect on both; a significant indirect effect on Dead Sea Sparrow via its impact on the mean height of shrubs, and a significant, negative indirect effect on Clamorous Reed Warbler by reducing reed cover. Intensive grazing and browsing by livestock including goats, sheep and camels, apparently had a negative effect on the overall density of native Tamarix shrubs, while promoting encroachment by invasive Prosopis juliflora.

Conclusion: This may be part of a long-term cascade leading to an ecological transition and loss of important wetland habitats in the arid Jordan Valley.     

Transporting stochastic direct and indirect effects to new populations

Transported mediation effects may contribute to understanding how interventions work differently when applied to new populations. However, we are not aware of any estimators for such effects. Thus, we propose two doubly robust, efficient estimators of transported stochastic (also called randomized interventional) direct and indirect effects. We demonstrate their finite sample properties in a simulation study. We then apply the preferred substitution estimator to longitudinal data from the Moving to Opportunity Study, a large‐scale housing voucher experiment, to transport stochastic indirect effect estimates of voucher receipt in childhood on subsequent risk of mental health or substance use disorder mediated through parental employment across sites, thereby gaining understanding of drivers of the site differences.     

Estimation of separable direct and indirect effects in continuous time

Many research questions involve time-to-event outcomes that can be prevented from occurring due to competing events. In these settings, we must be careful about the causal interpretation of classical statistical estimands. In particular, estimands on the hazard scale, such as ratios of cause-specific or subdistribution hazards, are fundamentally hard to interpret causally. Estimands on the risk scale, such as contrasts of cumulative incidence functions, do have a clear causal interpretation, but they only capture the total effect of the treatment on the event of interest; that is, effects both through and outside of the competing event. To disentangle causal treatment effects on the event of interest and competing events, the separable direct and indirect effects were recently introduced. Here we provide new results on the estimation of direct and indirect separable effects in continuous time. In particular, we derive the nonparametric influence function in continuous time and use it to construct an estimator that has certain robustness properties. We also propose a simple estimator based on semiparametric models for the two cause-specific hazard functions. We describe the asymptotic properties of these estimators and present results from simulation studies, suggesting that the estimators behave satisfactorily in finite samples. Finally, we reanalyze the prostate cancer trial from Stensrud et al. (2020).     

Fertilization affects the establishment ability of species differing in seed mass via direct nutrient addition and indirect competition effects

Fertilization causes species loss and species dominance changes in plant communities worldwide. However, it still remains unclear how fertilization acts upon species functional traits, e.g. seed mass. Seed mass is a key trait of the regeneration strategy of plants, which influences a range of processes during the seedling establishment phase. Fertilization may select upon seed mass, either directly by increased nutrient availability or indirectly by increased competition. Since previous research has mainly analyzed the indirect effects of fertilization, we disentangled the direct and indirect effects to examine how nutrient availability and competition influence the seed mass relationships on four key components during seedling establishment: seedling emergence, time of seedling emergence, seedling survival and seedling growth. We conducted a common garden experiment with 22 dry grassland species with a two‐way full factorial design that simulated additional nutrient supply and increased competition. While we found no evidence that fertilization either directly by additional nutrient supply or indirectly by increased competition alters the relationship between seed mass and (time of) seedling emergence, we revealed that large seed mass is beneficial under nutrient‐poor conditions (seedlings have greater chances of survival, particularly in nutrient‐poor soils) as well as under competition (large‐seeded species produced larger seedlings, which suffered less from competition than small‐seeded species). Based on these findings, we argue that both factors, i.e. nutrient availability and competition intensity, ought to be considered to understand how fertilization influences seedling establishment and species composition with respect to seed mass in natural communities. We propose a simple conceptual model, in which seed mass in natural communities is determined by competition intensity and nutrient availability. Here, we hypothesize that seed mass shows a U‐shaped pattern along gradients of soil fertility, which may explain the contrasting soil fertility‐seed mass relationships found in the recent literature.     

Separation and measurement of direct and indirect effects of light on stomata

Conductance for water vapor, assimilation of CO₂, and intercellular CO₂ concentration of leaves of five species were determined at various irradiances and ambient CO₂ concentrations. Conductance and assimilation were then plotted as functions of irradiance and intercellular CO₂ concentration. The slopes of these curves allowed us to estimate infinitesimal changes in conductance (and assimilation) that occurred when irradiance changed and intercellular CO₂ concentration was constant, and when CO₂ concentration changed and irradiance was constant. On leaves of Xanthium strumarium L., Gossypium hirsutum L., Phaseolus vulgaris L., and Perilla frutescens (L.), Britt., the stomatal response to light was determined to be mainly a direct response to light and to a small extent only a response to changes in intercellular CO₂ concentration. This was also true for stomata of Zea mays L., except at irradiances < 150 watts per square meter, when stomata responded primarily to the depletion of the intercellular spaces of CO₂ which in turn was caused by changes in the assimilation of CO₂.     

Toxicity of DNA-incorporated iodine-125: quantifying the direct and indirect effects

To understand the biophysical mechanism(s) underlying the induction of cell death by the decay of the Auger electron emitter iodine-125 in DNA, Chinese hamster V79 lung fibroblasts were labeled with 5-[(125)I]iodo-2'-deoxyuridine ((125)IdU) for two doubling times and frozen and stored at -135 degrees C in the presence of 0.26-3.0 M dimethyl sulfoxide (DMSO), which acts simultaneously as a cryoprotector and a hydroxyl radical scavenger. After the accumulation of (125)I decays, the cells were defrosted and their survival was determined. Within the range of the number of decays examined (up to 470 disintegrations per cell), the survival curves are exponential. The dependence of the D(37) on DMSO concentration is triphasic and seems to reach a plateau at approximately 1.3 M. By extrapolating to infinite DMSO concentration, we estimate the D(37) for maximal hydroxyl radical scavenging to be 411 +/- 36 disintegrations per cell. To determine the D(37) in the absence of DMSO, we extrapolate the D(37) curve to zero concentration, and a D(37) of 54 +/- 5 disintegrations per cell is obtained. The maximal dose modification factor, calculated as the ratio of the D(37) at infinite DMSO concentration (i.e. direct effects only) to the D(37) at zero DMSO concentration (i.e. direct and indirect effects), is 7.6 +/- 1.0. By inference, approximately 90% of the radiotoxic effects of DNA-incorporated (125)I are due to indirect mechanisms.     

Belowground interactions shift the relative importance of direct and indirect genetic effects

Intraspecific genetic variation can affect decomposition, nutrient cycling, and interactions between plants and their associated belowground communities. However, the effects of genetic variation on ecosystems can also be indirect, meaning that genes in a focal plant may affect ecosystems by altering the phenotype of interacting (i.e., neighboring) individuals. We manipulated genotype identity, species identity, and the possibility of belowground interactions between neighboring Solidago plants. We hypothesized that, because our plants were nitrogen (N) limited, the most important interactions between focal and neighbor plants would occur belowground. More specifically, we hypothesized that the genotypic identity of a plant's neighbor would have a larger effect on belowground biomass than on aboveground biomass, but only when neighboring plants were allowed to interact belowground. We detected species‐ and genotype‐level variation for aboveground biomass and ramet production. We also found that belowground biomass and ramet production depended on the interaction of neighbor genotype identity and the presence or absence of belowground interactions. Additionally, we found that interspecific indirect genetic effects (IIGEs; changes in focal plant traits due to the genotype identity of a heterospecific neighbor) had a greater effect size on belowground biomass than did focal genotype; however, this effect only held in pots that allowed belowground interactions. These results expand the types of natural processes that can be attributed to genotypes by showing that, under certain conditions, a plant's phenotype can be strongly determined by the expression of genes in its neighbor. By showing that IIGEs are dependent upon plants being able to interact belowground, our results also provide a first step for thinking about how genotype‐based, belowground interactions influence the evolutionary outcomes of plant‐neighbor interactions.     

Decoupling the direct and indirect effects of nitrogen deposition on ecosystem function

Elevated nitrogen (N) inputs into terrestrial ecosystems are causing major changes to the composition and functioning of ecosystems. Understanding these changes is challenging because there are complex interactions between 'direct' effects of N on plant physiology and soil biogeochemistry, and 'indirect' effects caused by changes in plant species composition. By planting high N and low N plant community compositions into high and low N deposition model terrestrial ecosystems we experimentally decoupled direct and indirect effects and quantified their contribution to changes in carbon, N and water cycling. Our results show that direct effects on plant growth dominate ecosystem response to N deposition, although long-term carbon storage is reduced under high N plant-species composition. These findings suggest that direct effects of N deposition on ecosystem function could be relatively strong in comparison with the indirect effects of plant community change.     

Ocean acidification boosts reproduction in fish via indirect effects

Ocean acidification affects species populations and biodiversity through direct negative effects on physiology and behaviour. The indirect effects of elevated CO₂ are less well known and can sometimes be counterintuitive. Reproduction lies at the crux of species population replenishment, but we do not know how ocean acidification affects reproduction in the wild. Here, we use natural CO₂ vents at a temperate rocky reef and show that even though ocean acidification acts as a direct stressor, it can indirectly increase energy budgets of fish to stimulate reproduction at no cost to physiological homeostasis. Female fish maintained energy levels by compensation: They reduced activity (foraging and aggression) to increase reproduction. In male fish, increased reproductive investment was linked to increased energy intake as mediated by intensified foraging on more abundant prey. Greater biomass of prey at the vents was linked to greater biomass of algae, as mediated by a fertilisation effect of elevated CO₂ on primary production. Additionally, the abundance and aggression of paternal carers were elevated at the CO₂ vents, which may further boost reproductive success. These positive indirect effects of elevated CO₂ were only observed for the species of fish that was generalistic and competitively dominant, but not for 3 species of subordinate and more specialised fishes. Hence, species that capitalise on future resource enrichment can accelerate their reproduction and increase their populations, thereby altering species communities in a future ocean.

Ocean acidification affects species populations and diversity through direct negative effects on physiology and behavior, but the indirect effects are less clear. Using volcanic carbon dioxide vents as natural analogues of future ocean acidification, this study shows that elevated CO2 can stimulate fish reproduction in the wild through increased food abundance, leading to increased energy budgets at no cost to physiological homeostasis.     

Analysis of protein-protein interactions by mutagenesis: direct versus indirect effects

Site-directed mutagenesis, including double-mutant cycles, is used routinely for studying protein-protein interactions. We now present a case analysis of chymotrypsin inhibitor 2 (CI2) and subtilisin BPN' using (i) a residue in CI2 that is known to interact directly with subtilisin (Tyr42) and (ii) two CI2 residues that do not have direct contacts with subtilisin (Arg46 and Arg48). We find that there are similar changes in binding energy on mutation of these two sets of residues. It can thus be difficult to interpret mutagenesis data in the absence of structural information.     

Evolution of direct and indirect reciprocity

Indirect reciprocity (IR) occurs when individuals help those who help others. It is important as a potential explanation for why people might develop cooperative reputations. However, previous models of IR are based on the assumption that individuals never meet again. Yet humans and other animals often interact repeatedly within groups, thereby violating the fundamental basis of these models. Whenever re-meeting can occur, discriminating reciprocators can decide whether to help those who helped others (IR) or those who helped them (direct reciprocity, DR). Here I used simulation models to investigate the conditions in which we can expect the different forms of reciprocity to predominate. I show that IR through image scoring becomes unstable with respect to DR by experience scoring as the probability of re-meeting increases. However, using the standing strategy, which takes into account the context of observed defections, IR can be stable with respect to DR even when individuals interact with few partners many times. The findings are important in showing that IR cannot explain a concern for reputation in typical societies unless reputations provide as reliable a guide to cooperative behaviour as does experience.