Species divergence and trait convergence in experimental plant community assembly

Despite decades of research, it remains controversial whether ecological communities converge towards a common structure determined by environmental conditions irrespective of assembly history. Here, we show experimentally that the answer depends on the level of community organization considered. In a 9‐year grassland experiment, we manipulated initial plant composition on abandoned arable land and subsequently allowed natural colonization. Initial compositional variation caused plant communities to remain divergent in species identities, even though these same communities converged strongly in species traits. This contrast between species divergence and trait convergence could not be explained by dispersal limitation or community neutrality alone. Our results show that the simultaneous operation of trait‐based assembly rules and species‐level priority effects drives community assembly, making it both deterministic and historically contingent, but at different levels of community organization.     

Historical precipitation predictably alters the shape and magnitude of microbial functional response to soil moisture

Soil moisture constrains the activity of decomposer soil microorganisms, and in turn the rate at which soil carbon returns to the atmosphere. While increases in soil moisture are generally associated with increased microbial activity, historical climate may constrain current microbial responses to moisture. However, it is not known if variation in the shape and magnitude of microbial functional responses to soil moisture can be predicted from historical climate at regional scales. To address this problem, we measured soil enzyme activity at 12 sites across a broad climate gradient spanning 442–887 mm mean annual precipitation. Measurements were made eight times over 21 months to maximize sampling during different moisture conditions. We then fit saturating functions of enzyme activity to soil moisture and extracted half saturation and maximum activity parameter values from model fits. We found that 50% of the variation in maximum activity parameters across sites could be predicted by 30‐year mean annual precipitation, an indicator of historical climate, and that the effect is independent of variation in temperature, soil texture, or soil carbon concentration. Based on this finding, we suggest that variation in the shape and magnitude of soil microbial response to soil moisture due to historical climate may be remarkably predictable at regional scales, and this approach may extend to other systems. If historical contingencies on microbial activities prove to be persistent in the face of environmental change, this approach also provides a framework for incorporating historical climate effects into biogeochemical models simulating future global change scenarios.     

Persistence and space preemption explain species‐specific founder effects on the organization of marine sessile communities

Community assembly may not follow predictable successional stages, with a large fraction of the species pool constituted by potential pioneering species and successful founders defined through lottery. In such systems, priority effects may be relevant in the determination of trajectories of developing communities and hence diversity and assemblage structure at later advanced states. In order to assess how different founder species may trigger variable community trajectories and structures, we conducted an experimental study using subtidal sessile assemblages as model. We manipulated the identity of functionally different founders and initial colony size (a proxy of the time lag before the arrival of later species), and followed trajectories. We did not observe any effects of colony size on response variables, suggesting that priority effects take place even when the time lag between the establishment of pioneering species and late colonizers is very short. Late community structure at experimental panels that started either with the colonial ascidian Botrylloides nigrum, or the arborescent bryozoan Bugula neritina, was similar to control panels allowed natural assembling. In spite of high potential for fast space domination, and hence negative priority effects, B. nigrum suffered high mortality and did not persist throughout succession. Bugula neritina provided complex physical microhabitats through conspecific clustering that have enhanced larval settlement of late species arrivals, but no apparent facilitation was observed. Differently, panels founded by the encrusting bryozoan Schizoporella errata led to different and less diverse communities compared to naturally assembled panels, evidencing strong negative priority effects through higher persistence and space preemption. Schizoporella errata founder colonies inhibited further conspecific settlement, which may greatly relax intraspecific competition, allowing resource allocation to colony growth and space domination, thus reducing the chances for the establishment of other species.     

Pavlovian backward conditioned inhibition in humans: summation and retardation tests

Two experiments using human participants investigated whether a Pavlovian backward inhibitory treatment (nonreinforced trials in phase 1 followed by reinforced trials in phase 2; i.e., AX- followed by A+) produces a stimulus which can pass summation and retardation tests for inhibition. The rationale for conducting these experiments was that previous demonstrations of Pavlovian backward inhibition informed participants about the nature of the outcome before starting the experiment. According to some theoretical views, this is a potential confound. In the present experiments we used a predictive task in which participants had no knowledge about the outcome until phase 2, when reinforcement occurred. The results of Experiment 1 (summation test) and Experiment 2 (retardation test) provide a clear demonstration of backward conditioned inhibition.     

山西历山国家级自然保护区千金榆群落种间分离研究

对山西历山国家级自然保护区普通沟和西峡的千金榆(Carpinus cordata)群落进行野外调查,共记录了40个样方,44个物种,组成946个种对.利用最近邻体法构造N×N最近邻体列联表,以x2检验和种间分离指数S作为区分指标,研究了千金榆群落的种间分离情况.结果表明:(1)946个种对中呈现随机毗邻的最多,有507个种对,占总数的53.59％;呈现正分离的有349个种对,占总数的36.89％;而呈现负分离种对最少,只有90对,占总数的9.52％.(2)群落的建群种或优势种因为有较强的生存能力和竞争能力,往往表现出正分离,例如千金榆、小叶鹅耳枥(Carpinus turczaninowii)、五角枫(Acer mono)、葛罗槭(Acer grosseri)等有较强生存能力的物种与大多数的物种发生正分离;而六道木(Abelia biflora)、东北茶镳子(Ribes mandshuricum)、小花溲疏(Deutziaparviflora),美蔷薇(Rosa bella)和紫花卫矛(Euonymus porphyries)等群落中的伴生种或小灌木,与部分物种形成负分离.其余物种之间发生随机毗邻的较多.(3)对千金榆群落44×44列联表进行全面分离,结果表明群落内的44个物种相互交错分布,不是全面分离.此外,种间分离的结果也揭示该群落处于演替初期,受岩石风化崩塌的影响较严重,人为干扰因素较轻.     

The ecology of restoration: historical links, emerging issues and unexplored realms

Restoration ecology is a young academic field, but one with enough history to judge it against past and current expectations of the science's potential. The practice of ecological restoration has been identified as providing ideal experimental settings for tests of ecological theory; restoration was to be the 'acid test' of our ecological understanding. Over the past decade, restoration science has gained a strong academic foothold, addressing problems faced by restoration practitioners, bringing new focus to existing ecological theory and fostering a handful of novel ecological ideas. In particular, recent advances in plant community ecology have been strongly linked with issues in ecological restoration. Evolving models of succession, assembly and state-transition are at the heart of both community ecology and ecological restoration. Recent research on seed and recruitment limitation, soil processes, and diversity–function relationships also share strong links to restoration. Further opportunities may lie ahead in the ecology of plant ontogeny, and on the effects of contingency, such as year effects and priority effects. Ecology may inform current restoration practice, but there is considerable room for greater integration between academic scientists and restoration practitioners.     

山西五鹿山自然保护区暴马丁香群落木本植物种间联结性分析

通过方差分析,χ²检验、Pearson相关系数和Spearman秩相关系数检验,采用2×2列联表对五鹿山暴马丁香林中频度较高的34个木本植物优势种,561个种对间的关联性进行了研究.分析结果表明: 34个优势种种间总体关联性呈现不显著负关联,表明该群落处于不稳定的顶级状态.χ²检验结果中,有216个种对呈正相关,345个种对呈负相关,正负关联比为0.626; Pearson相关系数检验结果为: 149个种对呈正相关,412个种对呈负相关,正负关联比为0.362; Spearman秩相关系数检验结果为: 206个种对呈正相关,365个种对呈负相关,正负关联比为0.564; Spearman秩相关系数比Pearson相关系数的检验方法灵敏度更高.     

chifish: a computer program testing for genetic heterogeneity at multiple loci using chi‐square and Fisher's exact test

chifish is a 32‐bit Windows/DOS program evaluating divergence at multiple gene loci. It tests the hypothesis of no difference at any locus both by means of Pearson's traditional chi‐square and by using Fisher's method of combining P values obtained by Fisher's exact test. Input data are read from a file formatted for genepop . Commonly used population genetics software do not perform chi‐square tests, and the simultaneous application of both techniques aids in situations where poor power of the ‘exact approach’ may prevent detection of true differentiation (e.g. few populations and few alleles per locus).     

Analysis of distribution indicates diverse functions of simple sequence repeats in Mycoplasma genomes

Simple sequence repeats (SSRs) composed of extensive tandem iterations of a single nucleotide or a short oligonucleotide are rare in most bacterial genomes, but they are common among Mycoplasma. Some of these repeats act as contingency loci in association with families of surface antigens. By contraction or expansion during replication, these SSRs increase genetic variance of the population and facilitate avoidance of the immune response of the host. Occurrence and distribution of SSRs are analyzed in complete genomes of 11 Mycoplasma and 3 related Mollicutes in order to gain insights into functional and evolutionary diversity of the SSRs in Mycoplasma. The results revealed an unexpected variety of SSRs with respect to their distribution and composition and suggest that it is unlikely that all SSRs function as contingency loci or recombination hot spots. Various types of SSRs are most abundant in Mycoplasma hyopneumoniae, whereas Mycoplasma penetrans, Mycoplasma mobile, and Mycoplasma synoviae do not contain unusually long SSRs. Mycoplasma hyopneumoniae and Mycoplasma pulmonis feature abundant short adenine and thymine runs periodically spaced at 11 and 12 bp, respectively, which likely affect the supercoiling propensities of the DNA molecule. Physiological roles of long adenine and thymine runs in M. hyopneumoniae appear independent of location upstream or downstream of genes, unlike contingency loci that are typically located in protein-coding regions or upstream regulatory regions. Comparisons among 3 M. hyopneumoniae strains suggest that the adenine and thymine runs are rarely involved in genome rearrangements. The results indicate that the SSRs in the Mycoplasma genomes play diverse roles, including modulating gene expression as contingency loci, facilitating genome rearrangements via recombination, affecting protein structure and possibly protein-protein interactions, and contributing to the organization of the DNA molecule in the cell.     

Effects of adaptation, chance, and history on the evolution of the toxic dinoflagellate Alexandrium minutum under selection of increased temperature and acidification

The roles of adaptation, chance, and history on evolution of the toxic dinoflagellate Alexandrium minutum Halim, under selective conditions simulating global change, have been addressed. Two toxic strains (AL1V and AL2V), previously acclimated for two years at pH 8.0 and 20°C, were transferred to selective conditions: pH 7.5 to simulate acidification and 25°C. Cultures under selective conditions were propagated until growth rate and toxin cell quota achieved an invariant mean value at 720 days (ca. 250 and ca. 180 generations for strains AL1V and AL2V, respectively). Historical contingencies strongly constrained the evolution of growth rate and toxin cell quota, but the forces involved in the evolution were not the same for both traits. Growth rate was 1.5-1.6 times higher than the one measured in ancestral conditions. Genetic adaptation explained two-thirds of total adaptation while one-third was a consequence of physiological adaptation. On the other hand, the evolution of toxin cell quota showed a pattern attributable to neutral mutations because the final variances were significantly higher than those measured at the start of the experiment. It has been hypothesized that harmful algal blooms will increase under the future scenario of global change. Although this study might be considered an oversimplification of the reality, it can be hypothesized that toxic blooms will increase but no predictions can be advanced about toxicity.