Testing the cultivar vigor hypothesis: comparisons of the competitive ability of wild and cultivated populations of Pascopyrum smithii along a restoration chronosequence

Restoration practitioners often rely on seeds of widely available cultivars representing native species but nonlocal germplasm. Cultivation improves the supply of plant materials and minimizes revegetation costs, but can also favor agronomic traits, and resulting vigor may affect the competitive ability and long‐term persistence of cultivated genotypes at restoration sites. We compared cultivated, restored, and wild populations of Pascopyrum smithii (western wheatgrass) in a greenhouse study to test the extent to which cultivars outcompete local plants in biomass production, and to determine if morphological differences (including height and number of leaves) among cultivated and wild populations persist at restoration sites over time. We found evidence of vigor and greater competitive ability of cultivars in seed mass, growth rate, plant height, and biomass and this advantage occurred when plants were grown alone or in competition with other seed sources. Cultivar vigor persisted at restoration sites over 30 years, but restored populations more closely resembled wild, local populations when cultivars were planted in closer proximity to nearby undisturbed sites. This study supports the cultivar vigor hypothesis and provides evidence for the long‐term persistence of cultivated traits in the environment.     

Effects of distance to crop rows and to conspecific neighbours on the size of Brassica napus and Veronica persica weeds

We tested the hypothesis that local competitive conditions are a determinant of the size of individual weeds in cereal crops by investigating the relationship between individual weed size and (a) distance from the crop row and (b) distance to the nearest conspecific neighbour in cereal crops. There were significant but weak effects of distance to rows of summer and winter wheat (Triticum aestivum), and distance to conspecific individuals on individuals of two weed species, Brassica napus and Veronica persica. Our results suggest that local neighbourhood competitive conditions, although detectable, have only limited effects on weed growth. Size-asymmetric competition from the crop population and plasticity in weed growth reduce the importance of a weed individual's exact location relative to crop individuals and to other weed individuals. A static, two-dimensional view of space is not sufficient to describe competitive effects because the third dimension can be the most important in competition, and because many plants can change their locations through plastic growth.Wir überprüften die Hypothese, dass lokale Konkurrenzbedingungen ein Bestimmungsfaktor für die Größe einzelner Unkräuter in Getreidefeldern sind, indem wir die Beziehung zwischen der individuellen Unkrautgröße und (a) der Distanz zu den Pflanzenreihen und (b) der Distanz zu den nächsten, artgleichen Nachbarpflanzen in Getreidefeldern untersuchten. Es gab signifikante jedoch geringfügige Effekte der Distanz zu den Reihen von Sommer- und Winterweizen (Tritium aestivum) sowie der Distanz zu den artgleichen Individuen auf die Individuen von zwei Unkrautarten, Brassica napus und Veronica persica. Unsere Ergebnisse lassen vermuten, dass Konkurrenzbedingungen in der unmittelbaren Nachbarschaft nur einen begrenzten Effekt auf das Unkrautwachstum haben, auch wenn sie wahrnehmbar sind. Größenasymmetrische Konkurrenz seitens der Getreidepopulation und die Plastizität des Pflanzenwachstums reduzieren die Bedeutung der exakten Position einer einzelnen Unkrautpflanze in Beziehung zu einzelnen Getreide- oder anderen, einzelnen Unkrautwpflanzen. Eine statische, zweidimensionale Betrachtung des Raumes reicht nicht aus, um die Konkurrenzeffekte zu beschreiben, weil die dritte Dimension die wichtigste für die Konkurrenz sein kann und weil viele Pflanzen ihre Position durch plastisches Wachstum verändern können.     

TESTS OF SEX ALLOCATION THEORY IN SIMULTANEOUSLY HERMAPHRODITIC ANIMALS

Sex allocation is a crucial life-history parameter in all sexual organisms. Over the last decades a body of evolutionary theory, sex allocation theory, was developed, which has yielded capital insight into the evolution of optimal sex allocation patterns and adaptive evolution in general. Most empirical work, however, has focused on species with separate sexes. Here I review sex allocation theory for simultaneous hermaphrodites and summarize over 50 empirical studies, which have aimed at evaluating this theory in a diversity of simultaneous hermaphrodites spanning nine animal phyla. These studies have yielded considerable qualitative support for several predictions of sex allocation theory, such as a female-biased sex allocation when the number of mates is limited, and a shift toward a more male-biased sex allocation with increasing numbers of mates. In contrast, many fundamental assumptions, such as the trade-off between male and female allocation, and numerous predictions, such as brooding limiting the returns from female allocation, are still poorly supported. Measuring sex allocation in simultaneously hermaphroditic animals remains experimentally demanding, which renders evaluation of more quantitative predictions a challenging task. I identify the main questions that need to be addressed and point to promising avenues for future research.     

Intersexual competition as an explanation for sex-ratio and dispersal biases in polygynous species

In polygynous mammals, it is commonly observed that both sex ratios at birth and dispersal are male biased. This has been interpreted as resulting from low female dispersal causing high female local resource competition, which would select for male-biased sex ratios. However, a female-biased sex ratio can be selected despite lower female than male-biased dispersal. This will occur if the low female dispersal is close to the optimal dispersal rate, while the male dispersal is not close to the optimal dispersal rate. The actual outcome depends on the joint evolution of sex-biased dispersal and sex ratio. Earlier analyses of joint evolution imply that there will be no sex-ratio nor dispersal biases at the joint evolutionarily stable strategy, thus they do not explain the data. However, these earlier analyses assume no intersexual competition for resources. Here, we show that when males and females compete with each other for access to resources, male-biased dispersal will be associated with male-biased birth sex ratio, as is commonly observed. A trend toward male-biased birth sex ratios is also expected if there is intersexual local resource competition and if birth sex ratio is constrained so that it cannot depart from balanced sex ratio.     

Negative density-dependent emigration of males in an increasing red deer population

In species with polygynous mating systems, females are regarded as food-limited, while males are limited by access to mates. When local density increases, forage availability declines, while mate access for males may increase due to an increasingly female-biased sex ratio. Density dependence in emigration rates may consequently differ between sexes. Here, we investigate emigration using mark-recovery data from 468 young red deer Cervus elaphus marked in Snillfjord, Norway over a 20-year period when the population size has increased sixfold. We demonstrate a strong negative density-dependent emigration rate in males, while female emigration rates were lower and independent of density. Emigrating males leaving the natal range settled in areas with lower density than expected by chance. Dispersing males moved 42 per cent longer at high density in 1997 (37 km) than at low density in 1977 (26 km), possibly caused by increasing saturation of deer in areas surrounding the marking sites. Our study highlights that pattern of density dependence in dispersal rates may differ markedly between sexes in highly polygynous species. Contrasting patterns reported in small-scale studies are suggestive that spatial scale of density variation may affect the pattern of temporal density dependence in emigration rates and distances.     

Sex-ratio adjustment when relatives interact: a test of constraints on adaptation

Studies of sex allocation offer excellent opportunities for examining the constraints and limits on adaptation. A major topic of debate within this field concerns the extent to which the ability of individuals to adaptively manipulate their offspring sex ratio is determined by constraints such as the method of sex determination. We address this problem by comparing the extent of sex-ratio adjustment across taxa with different methods of sex determination, under the common selective scenario of interactions between relatives. These interactions comprise the following: local resource competition (LRC), local mate competition (LMC), and local resource enhancement (LRE). We found that: (1) species with supposedly constraining methods of sex determination showed consistent sex-ratio adjustment in the predicted direction; (2) vertebrates with chromosomal sex determination (CSD) showed less adjustment then haplodiploid invertebrates; (3) invertebrates with possibly constraining sex-determination mechanisms (CSD and pseudo-arrhenotoky) did not show less adjustment then haplodiploid invertebrates; (4) greater sex-ratio adjustment was seen in response to LRC and LMC than LRE; (5) greater sex-ratio adjustment was seen in response to interactions between relatives (LRC, LMC, and LRE) compared to responses to other environmental factors. Our results also illustrate the problem that sex-determination mechanism and selective pressure are confounded across taxa because vertebrates with CSD are influenced primarily by LRE whereas invertebrates are influenced by LRC and LMC. Overall, our analyses suggest that sex-allocation theory needs to consider simultaneously the influence of variable selection pressures and variable constraints when applying general theory to specific cases.     

水位波动对南美蟛蜞菊和蟛蜞菊种内及种间关系的影响

以外来入侵植物南美蟛蜞菊和本地近缘种蟛蜞菊为对象,通过温室模拟3种水位波动模式(水位无波动,水位波动模式分别为15 cm-0 cm-15 cm和0 cm-15 cm-0 cm)交叉5种定植模式(试验容器内分别为入侵种单株、本土种单株、入侵种6株、本土种6株以及2物种各3株混种)的试验,研究水位波动对入侵植物和本地近缘种生长繁殖性状及种内种间相互作用的影响.结果表明: 水位波动显著降低了南美蟛蜞菊和蟛蜞菊的总生物量、茎生物量、叶生物量、根生物量、茎长、节点数、叶片数及叶面积,对南美蟛蜞菊和蟛蜞菊种内及种间竞争系数的影响均显著.水位波动改变了南美蟛蜞菊的种内和种间竞争关系,说明入侵植物南美蟛蜞菊对水位波动更为敏感,对环境改变表现出更强的适应性.     

Social biology and sex ratios of the eusocial gall-inducing thrips Kladothrips hamiltoni

1. Thrips comprise the only order besides Hymenoptera where females are diploid and males are haploid. This makes them useful insects for studying the roles of kin selection and ecology in social evolution. 2. Kladothrips hamiltoni is one of six species of Australian gall-inducing thrips that have been identified as eusocial. Galls are initiated by a single foundress, who rears her brood and remains within the enclosed gall for life. The adults of both sexes of her first brood cohort are morphologically distinct from the second generation, comprising a nondispersing soldier caste. The foundress and sib-mated soldiers jointly produce a second, dispersing generation, approximately 60–80% of which are produced by the soldiers. Mean per capita egg production of female soldiers is less than 33% that of the foundress. 3. Adult eclosion of soldiers is protandrous but the overall sex ratio of the soldiers lacks bias (52% male). Protandry of soldiers increases the probability that female soldiers will be inseminated soon after their eclosion and therefore lay fertilised, female eggs. The lack of bias could be due to a balance between local resource competition and local mate competition. Gender-specific defensive behaviour of soldiers with their enemies may also be important in explaining this unexpected sex ratio. 4. The dispersing generation has an overall extreme female bias (5.6% male). Soldier incest increases relatedness between females more than between males, such that the foundress is more related to her granddaughters than her daughters, and female soldiers are more related to their daughters than their sons (assuming within-gall relatedness < 1). A female bias in the offspring of soldiers should be preferred by both the foundress and soldiers as they are more related to soldier-produced dispersing females than any other thrips in the gall. Female bias in the dispersing generation will also reduce local mate competition between males. Both soldier incest and local mate competition may therefore contribute to the extreme female bias in the dispersing generation. 5. Selection pressures for sociality in gall-inducing thrips appear to be more similar to those in gall-inducing aphids and naked mole rats than to those in Hymenoptera.     

Simultaneous failure of two sex-allocation invariants: implications for sex-ratio variation within and between populations

Local mate competition (LMC) occurs when male relatives compete for mating opportunities, and this may favour the evolution of female-biased sex allocation. LMC theory is among the most well developed and empirically supported topics in behavioural ecology, clarifies links between kin selection, group selection and game theory, and provides among the best quantitative evidence for Darwinian adaptation in the natural world. Two striking invariants arise from this body of work: the number of sons produced by each female is independent of both female fecundity and also the rate of female dispersal. Both of these invariants have stimulated a great deal of theoretical and empirical research. Here, we show that both of these invariants break down when variation in female fecundity and limited female dispersal are considered in conjunction. Specifically, limited dispersal of females following mating leads to local resource competition (LRC) between female relatives for breeding opportunities, and the daughters of high-fecundity mothers experience such LRC more strongly than do those of low-fecundity mothers. Accordingly, high-fecundity mothers are favoured to invest relatively more in sons, while low-fecundity mothers are favoured to invest relatively more in daughters, and the overall sex ratio of the population sex ratio becomes more female biased as a result.     

Staged invasions across disparate grasslands: effects of seed provenance,consumers and disturbance on productivity and species richness

Exotic plant invasions are thought to alter productivity and species richness, yet these patterns are typically correlative. Few studies have experimentally invaded sites and asked how addition of novel species influences ecosystem function and community structure and examined the role of competitors and/or consumers in mediating these patterns. We invaded disturbed and undisturbed subplots in and out of rodent exclosures with seeds of native or exotic species in grasslands in Montana, California and Germany. Seed addition enhanced aboveground biomass and species richness compared with no‐seeds‐added controls, with exotics having disproportionate effects on productivity compared with natives. Disturbance enhanced the effects of seed addition on productivity and species richness, whereas rodents reduced productivity, but only in Germany and California. Our results demonstrate that experimental introduction of novel species can alter ecosystem function and community structure, but that local filters such as competition and herbivory influence the magnitude of these impacts.     

Competition components along productivity gradients – revisiting a classic dispute in ecology

Competition is ubiquitous in plant communities with various effects on plant fitness and community structure. A long-standing debate about different approaches to explain competition is the controversy between David Tilman and Philip Grime. Grime stated that the importance of competition relative to the impact of the environment increases along a productivity gradient, while Tilman argued that the intensity of competition is independent of productivity. To revisit this controversy, we assumed that the effects of plant–plant interactions are additive and applied the new competition indices by Díaz-Sierra et al. (2017) in a field experiment along a productivity gradient in S-Germany, using the rare arable plant Arnoseris minima as a study species. The ‘target technique' was applied, to separate the effects of root and shoot competition. The study plants were exposed to five competition treatments with three replicates in 18 sites, respectively. We investigated the expectation that root competition is more intense in unproductive sites than shoot competition. Additionally, we predicted survival to be less affected by competition than growth-related plant parameters. Using the biomass of individuals without competition as a proxy for site productivity there was a positive relationship with competition importance but no relationship with competition intensity when plants experienced full competition. Survival of the target plants was unaffected by competition. Root competition was the main mechanism determining the performance of the target plants, whereas the effect of shoot competition was relatively low albeit increasing with productivity. We conclude that when considering plant–plant interactions additive both Grime's and Tilman's theories can be supported.     

Evaluating reasons for biased sex ratios in Crustacea

ABSTRACT

While equal ratios of males and females are normal, crustacean sex ratios are frequently biased. Here I review potential reasons for sex ratio biases in animals, and evaluate how likely they apply to crustaceans. I introduce crustacean examples and highlight promising crustacean taxa for future studies where examples are lacking. I conclude that ecological differences between the sexes appear to be the dominant reason for sex ratio bias in crustaceans. Major life history patterns limit the importance of evolutionary drivers to sedentary taxa with limited dispersal. Still largely unresolved is the question whether females can bias their offspring sex ratio, and given the diversity of sex determining systems, answers will vary from case to case.

Abbreviations: LMC: local mate competition; LSC: local sperm competition.     

Experimental evidence for root competition effects on community evenness in one of two phytometer species

Aims Plant–plant interactions, being positive or negative, are recognized to be key factors in structuring plant communities. However, it is thought that root competition may be less important than shoot competition due to greater size symmetry belowground. Because direct experimental tests on the importance of root competition are scarce, we aim at elucidating whether root competition may have direct or indirect effects on community structure. Indirect effects may occur by altering the overall size asymmetry of competition through root–shoot competitive interactions.     

Computer simulations support a core prediction of a contentious plant model

? Premise of the study: An overarching but vigorously debated plant model proposed by the West, Brown, Enquist (WBE) theory predicts the scaling relationships for numerous botanical phenomena. However, few studies have evaluated this model's basic assumptions, one of which is that natural selection has resulted in hierarchal networks that minimize the energy required to distribute nutrients internally and have thus produced highly efficient organisms. ? Methods: If these core assumptions are correct, an "idealized" plant complying with all of the scaling relationships emerging from the WBE plant model should rapidly outcompete other plants, even those that differ slightly from it. To test this reasoning, a computer model was used to simulate competition between an idealized WBE plant, a generic "average" angiosperm (GA), and one of seven variants of the idealized WBE plant, each being similar to the GA in one of the GA's scaling parameters. ? Key results: Replicate simulations show that the idealized WBE plant rapidly outcompetes all other plants under light-shade and open-field conditions. However, changing only one of the WBE's scaling parameters results in death or in the coexistence of WBE and GA plants. ? Conclusions: These simulations support a core assumption of the WBE plant model and suggest why this idealized plant has not evolved.     

UNEXPLAINED SPLIT SEX RATIOS IN THE NEOTROPICAL PLANT-ANT, ALLOMERUS OCTOARTICULATUS VAR. DEMERARAE (MYRMICINAE): A TEST OF HYPOTHESES

We investigated sex allocation in the Neotropical ant Allomerus octoarticulatus var. demerarae . Because Allomerus is a plant symbiont, we could make geographically extensive collections of complete colonies and of foundresses in saplings, allowing us to estimate not only population- and colony-level sex allocation but also colony resource levels and the relatednesses of competing ant foundresses. This species exhibits a strongly split sex ratio, with 80% of mature colonies producing ≥90% of one sex or the other. Our genetic analyses (DNA microsatellites) reveal that Allomerus has a breeding system characterized by almost complete monogyny and a low frequency of polyandry. Contrary to theoretical explanations, we find no difference in worker relatedness asymmetries between female- and male-specialist colonies. Furthermore, no clear link was found between colony sex allocation and life history traits such as the number of mates per queen, or colony size, resource level, or fecundity. We also failed to find significant support for male production by workers, infection by Wolbachia , local resource competition, or local mate competition. We are left with the possibility that Allomerus exhibits split sex ratios because of the evolution of alternative biasing strategies in queens or workers, as recently proposed in the literature.     

植物竞争研究综述

概述了竞争的概念、理论 (包括最大生长率理论、最小资源需求理论和资源动态两阶段说 )、特性 (主要有竞争不对称性和环境相关性 )及其影响因素 ,竞争与群落组成、结构和动态的关系。并对近几十年来生态学上关于植物竞争的争论焦点作了论述 ,即竞争对个体生长、繁殖和存活及群落演替的相对重要性 ,主要介绍了Grime、Newman&Tilman和Deborah&Ariel的 3种观点 ,其中Grime认为竞争不重要 ,相反Newman&Tilman认为竞争比较重要 ,Deborah&Ariel则提出了资源动态两阶段说 ,认为竞争重要与否取决于两个条件 :一为植物对土壤资源的吸收利用与非生物因素两者分别在多大程度上决定了资源可利用性的渐次降低 ;二为资源正波动期因竞争引起的生长量减少在多大程度上决定了资源负波动期个体的存活率     

木本植物邻体干扰研究进展

植物种间关系一直是植物生态学研究的核心问题之一。密度和平均生物量之间的相关性研究在一定程度上揭示了植物之间的相互作用。对于单纯种群,已有３／２稀疏定律,但这种关系可能掩盖了植物生长和发育方面的重大变化,因为植物所具有的固着性及其表型的可塑性,使得局地...     

沙区植物种的一类种间竞争模型及应用

以沙坡头人工固沙植物油蒿和柠条为研究对象,针对水分是沙坡头植物生r长的限制因子的现实,研究了在水分条件限制下两种植物的种间关系,建立了混播油蒿、柠条种间竞争模型,验证了油蒿、柠条既竞争又稳定共存的关系,其目的为充分有效利用水资源、稳定人工生态系统提供科学依据。