Plant community diversity and native plant abundance decline with increasing abundance of an exotic annual grass

Exotic plants are generally considered a serious problem in wildlands around the globe. However, some argue that the impacts of exotic plants have been exaggerated and that biodiversity and other important plant community characteristics are commonly improved with invasion. Thus, disagreement exists among ecologists as to the relationship of exotic plants with biodiversity and native plant communities. A better understanding of the relationships between exotic plants and native plant communities is needed to improve funding allocation and legislation regarding exotic plants, and justify and prioritize invasion management. To evaluate these relationships, 65 shrub–bunchgrass plant communities with varying densities of an exotic annual grass, Taeniatherum caput-medusae (L.) Nevski (medusahead), were sampled across 160,000 ha in southeastern Oregon, United States. Environmental factors were generally not correlated with plant community characteristics when exotic annual grass density was included in models. Plant diversity and species richness were negatively correlated with exotic annual grass density. Exotic annual grass density explained 62% of the variation in plant diversity. All native plant functional groups, except annual forbs, exhibited a negative relationship with T. caput-medusae. The results of this study suggest that T. caput-medusae invasions probably have substantial negative impacts on biodiversity and native plant communities. The strength of the relationships between plant community characteristics and T. caput-medusae density suggests that some exotic plants are a major force of change in plant communities and subsequently threaten ecosystem functions and processes. However, experimental studies are needed to fully substantiate that annual grass invasion is the cause of these observed correlations.     

HLA class I-restricted T-cell responses may contribute to the control of human immunodeficiency virus infection, but such responses are not always necessary for long-term virus control

A rare subset of human immunodeficiency virus (HIV)-infected individuals maintains undetectable HIV RNA levels without therapy ("elite controllers"). To clarify the role of T-cell responses in mediating virus control, we compared HLA class I polymorphisms and HIV-specific T-cell responses among a large cohort of elite controllers (HIV-RNA < 75 copies/ml), "viremic" controllers (low-level viremia without therapy), "noncontrollers" (high-level viremia), and "antiretroviral therapy suppressed" individuals (undetectable HIV-RNA levels on antiretroviral therapy). The proportion of CD4(+) and CD8(+) T cells that produce gamma interferon (IFN-gamma) and interleukin-2 (IL-2) in response to Gag and Pol peptides was highest in the elite and viremic controllers (P < 0.0001). Forty percent of the elite controllers were HLA-B*57 compared to twenty-three percent of viremic controllers and nine percent of noncontrollers (P < 0.001). Other HLA class I alleles more common in elite controllers included HLA-B*13, HLA-B*58, and HLA-B*81 (P < 0.05 for each). Within elite and viremic controller groups, those with protective class I alleles had higher frequencies of Gag-specific CD8(+) T cells than those without these alleles (P = 0.01). Noncontrollers, with or without protective alleles, had low-level CD8(+) responses. Thus, certain HLA class I alleles are enriched in HIV controllers and are associated with strong Gag-specific CD8(+)IFN-gamma(+)IL-2(+) T cells. However, the absence of evidence of T cell-mediated control in many controllers suggests the presence of alternative mechanisms for viral control in these individuals. Defining mechanisms for virus control in "non-T-cell controllers" might lead to insights into preventing HIV transmission or preventing virus replication.     

Litter species traits, but not richness, contribute to carbon and nitrogen dynamics in an alpine meadow on the Tibetan Plateau

Aims

Litter, as afterlife of plants, plays an important role in driving belowground decomposition processes. Here we tested effects of litter species identity and diversity on carbon (C) and nitrogen (N) dynamics during litter decomposition in N-limited alpine meadow soil from the Qinghai–Tibet Plateau.

Methods

We incubated litters of four meadow species, a sedge (“S”, Kobresia humilis), a grass (“G”, Elymus nutans), a herb (“H”, Saussurea superba), and a legume (“L”, Oxytropis falcata), in monoculture and in mixture with meadow soil. CO₂ release was measured 21 times during the incubation, and soil available N and microbial biomass C and N were measured before and after the experiment.

Results

The organic C decay rate did not differ much among soils amended with monocultures or mixtures of litter, except in the H, S, L, and S+H treatments, which had much higher decay rates. Potential decomposable C pools were lowest in the control, highest in the L treatment, and intermediate in the S treatment. Mineralized N was completely immobilized by soil microbes in all treatments except the control, S+L, and S+G+L treatments. Litter mixtures had both additive and non-additive effects on CO₂-C emission (mainly antagonistic effects), net N mineralization (mainly synergistic), and microbial biomass C and N (both). Overall, these parameters were not significantly correlated with litter species richness. Similarly, microbial C or N was not significantly correlated with litter N content or C/N. However, cumulative CO₂-C emission and net N mineralization were positively correlated with litter N content and negatively correlated with litter C/N.

Conclusions

Litter N content and C/N rather than litter species richness drove the release of CO₂-C and net available N in this ecosystem. The antagonistic effects of litter mixtures contributed to a modest release of CO₂-C, but their synergistic effects enhanced net available N. We suggest that in alpine meadow communities, balancing species with high and low N contents will benefit soil carbon sequestration and plant competition for available N with soil microbes.     

Current plant diversity but not its soil legacy influences exotic plant invasion

植物多样性而非其土壤遗留效应影响外来植物入侵 植物多样性可以影响外来植物入侵,然而植物多样性的土壤遗留效应是否能够影响外来植物入侵目前仍不清楚。植物多样性能够改变土壤微生物群落和土壤理化性质,这种遗留效应可能会对该土壤中外来植物的生长产生影响。因此,我们假设植物多样性的土壤遗留效应会影响外来植物的入侵。为了检验该假说,我们开展了一个两阶段的植物-土壤反馈实验。在土壤驯化阶段,我们将12个植物物种(4种禾草植物、3种豆科植物和5种杂类草植物)分别单独种植,或者随机选择8个物种(包含3个功能型)混合种植在土壤中。在反馈阶段,我们将入侵植物三叶鬼针草(Bidens pilosa)分别与本地禾草荩草(Arthraxon hispidus)、本地杂类草翅果菊(Pterocypsela indica)或者同时与荩草和翅果菊种植在被驯化过的土壤中。研 究结果显示,三叶鬼针草相对于其本地竞争植物的生长取决于驯化植物和竞争植物物种的功能型。驯化植物的多样性对三叶鬼针草与其本地竞争植物之间的生长差异没有显著影响。然而,随着本地竞争植物物种多样性的增加,三叶鬼针草相对于其本地竞争植物的生长显著降低。这些结果表明,当前的植物多样性可以通过增加入侵植物和本地植物之间的生长不平衡性来减少外来植物的入侵。但是,植物多样性的土壤遗留效应对外来植物入侵的影响可能很小。     

Plastic and constant developmental traits contribute to adaptive differences in co-occurring Polygonum species

Adaptive differences among species are often thought to result from developmentally constant trait differences that enhance fitness in alternative environments. Species differences in patterns of individual phenotypic plasticity can also have ecological consequences. Indeed, functionally related constant and plastic traits may interact to determine the phenotype's adaptive value in particular conditions. We compared juvenile shade avoidance traits (height and its components, internode length and node number) across two field density treatments in Polygonumpersicaria and P. hydropiper, annual plant species that co‐occur in pastures comprised of a mosaic of plant densities. We used selection analyses to test trait contributions to fitness in alternative density treatments. Seedlings of both species expressed plasticity for internode elongation in response to density; P. persicaria plants increased internode length and consequently height significantly more in high density than did those of P. hydropiper. As predicted by the shade avoidance hypothesis, increased height was adaptive for both species in high density stands, so P. persicaria plants had higher fitness in this environment. By contrast, node numbers were relatively constant across density treatments in both species: P. hydropiper seedlings consistently produced more nodes than did those of P. persicaria. This constant trait difference contributed to P. hydropiper's greater relative fitness at low density, where more nodes and hence leaves enable plants to better exploit available light. Differences between species in these juvenile shade‐avoidance traits did not result from the evolutionary constraints of lack of heritable variation or costs of plasticity. We discuss how these interspecific trait differences may have been generated by divergent selective histories resulting from differences in herbivore resistance. These results illustrate how adaptive differences in both plastic and constantly expressed traits may jointly contribute to ecological distribution, including coexistence in patchy habitats.     

The grass may not always be greener: projected reductions in climatic suitability for exotic grasses under future climates in Australia

Climate change presents a new challenge for the management of invasive exotic species that threaten both biodiversity and agricultural productivity. The invasion of exotic perennial grasses throughout the globe is particularly problematic given their impacts on a broad range of native plant communities and livelihoods. As the climate continues to change, pre-emptive long-term management strategies for exotic grasses will become increasingly important. Using species distribution modelling we investigated potential changes to the location of climatically suitable habitat for some exotic perennial grass species currently in Australia, under a range of future climate scenarios for the decade centred around 2050. We focus on eleven species shortlisted or declared as the Weeds of National Significance or Alert List species in Australia, which have also become successful invaders in other parts of the world. Our results indicate that the extent of climatically suitable habitat available for all of the exotic grasses modelled is projected to decrease under climate scenarios for 2050. This reduction is most severe for the three species of Needle Grass (genus Nassella) that currently have infestations in the south-east of the continent. Combined with information on other aspects of establishment risk (e.g. demographic rates, human-use, propagule pressure), predictions of reduced climatic suitability provide justification for re-assessing which weeds are prioritised for intensive management as the climate changes.     

Plastic inducible morphologies are not always adaptive: The importance of time delays in a stochastic environment

Summary We present a mathematical model for predicting the expected fitness of phenotypically plastic organisms experiencing a variable environment. We assume that individuals experience two discrete environments probabilistically in time (as a Markov process) and that there are two different phenotypic states, each yielding the highest fitness in one of the two environments. We compare the expected fitness of a phenotypically fixed individual to that of an individual whose phenotype is induced to produce the better phenotype in each environment with a time lag between experiencing a new environment and realization of the new phenotype. Such time lags are common in organisms where phenotypically plastic, inducible traits have been documented. We find that although plasticity is generally adaptive when time lags are short (relative to the time scale of environmental variability), plasticity can be disadvantageous for longer lag times. Asymmetries in environmental change probabilities and/or the relative fitnesses of each phenotype strongly influence whether plasticity is favoured. In contrast to other models, our model does not require costs for plasticity to be disadvantageous; costs affect the results quantitatively, not qualitatively.     

Variation in climatic tolerance,but not stomatal traits,partially explains Pooideae grass species distributions

Background and AimsGrasses in subfamily Pooideae live in some of the world’s harshest terrestrial environments, from frigid boreal zones to the arid windswept steppe. It is hypothesized that the climate distribution of species within this group is driven by differences in climatic tolerance, and that tolerance can be partially explained by variation in stomatal traits.MethodsWe determined the aridity index (AI) and minimum temperature of the coldest month (MTCM) for 22 diverse Pooideae accessions and one outgroup, and used comparative methods to assess predicted relationships for climate traits versus fitness traits, stomatal diffusive conductance to water (g_w) and speed of stomatal closure following drought and/or cold.Key ResultsResults demonstrate that AI and MTCM predict variation in survival/regreening following drought/cold, and g_w under drought/cold is positively correlated with δ ¹³C-measured water use efficiency (WUE). However, the relationship between climate traits and fitness under drought/cold was not explained by g_w or speed of stomatal closure.ConclusionsThese findings suggest that Pooideae distributions are at least partly determined by tolerance to aridity and above-freezing cold, but that variation in tolerance is not uniformly explained by variation in stomatal traits.     

Geographic variations in phenotypic traits of the exotic herb Solidago altissima and abundance of recent established exotic herbivorous insects

Abstract

Many invasive plants increase aggressiveness after introduction. Since evolutionary forces such as herbivore pressure may change over different time scales, understanding the changes in biotic interactions in invasive plants through time can clarify the mechanism of their evolution in aggressiveness. In this study, we examined the geographic variation in phenotypic traits of Solidago altissima and the abundance of two exotic herbivorous insect species (the aphid, Uroleucon nigrotuberculatum and the lacebug, Corythucha marmorata), which are recently expanding their habitat on S. altissima populations over Japan. The two exotic insects were present at high density on S. altissima throughout their range. No differences in growth traits (plant height and number of leaves) were found among populations, and all plants examined appear to be exclusively hexaploid. Future studies on population genetics and common garden experiments are necessary to evaluate the potential evolutionary dynamics of the S. altissima after introduction.     

Dialects of an invasive songbird are preserved in its invaded but not native source range

Biological invasions are not only events with substantial environmental and socioeconomic impacts but are also interesting natural experiments, allowing the study of phenomena such as the cultural evolution of bird song following introduction. We took an excellent opportunity to compare the distribution of dialects of the yellowhammer Emberiza citrinella, a small Eurasian passerine, in its native source region (Great Britain) and invaded range (New Zealand) more than hundred years after relocation. Recent field recordings (including those provided by volunteers within a citizen science project) were complemented by those from archives, each assigned to appropriate dialect by visual inspection of a sonogram, and the resulting spatial patterns of dialect distribution were interpreted using historical data on the yellowhammer invasion. The two countries differ markedly in the composition and distribution of dialects. New Zealand populations sing a greater number of different dialects, seven in total, five of which were not detected in the current British population, but have been reported by previous studies from the continental Europe. Two identified localities of capture (Brighton, Sussex, UK) and release (Dunedin, Otago, NZ) differ even more strikingly, having no dialects in common. The largely sedentary nature of yellowhammers allows for two mutually exclusive explanations for European dialects being detected in New Zealand but not in Great Britain: 1) the corresponding song types have emerged de novo in New Zealand, through convergent cultural evolution; 2) the dialects have disappeared from Great Britain, while being preserved in New Zealand. Indirect evidence from the widespread occurrence of these dialects in continental Europe and the reported stability of yellowhammer song, supports the latter explanation. We suggest that the yellowhammer dialect system is an avian equivalent of a phenomenon already noted in human languages, in which ancient words or structures are retained in expatriate communities.     

Seed 1-cysteine peroxiredoxin antioxidants are not involved in dormancy, but contribute to inhibition of germination during stress

Peroxiredoxins (Prx) are thiol-dependent antioxidants containing one (1-cysteine [-Cys]) or two (2-Cys) conserved Cys residues that protect lipids, enzymes, and DNA against reactive oxygen species. In plants, the 1-Cys Prxs are highly expressed during late seed development, and the expression pattern is dormancy related in mature seeds. We have expressed the Arabidopsis 1-Cys Prx AtPER1 in Escherichia coli and show that this protein has antioxidant activity in vitro and protects E. coli in vivo against the toxic oxidant cumene hydroperoxide. Although some 1-Cys Prxs are targeted to the nucleus, a green fluorescent protein-AtPER1 fusion protein was also localized to the cytoplasm in an onion epidermis subcellular localization assay. It has been proposed that seed Prxs are involved in maintenance of dormancy and/or protect the embryo and aleurone layer surviving desiccation against damage caused by reactive oxygen species. These hypotheses were tested using transgenic Arabidopsis lines overexpressing the barley (Hordeum vulgare) 1-Cys PER1 protein and lines with reduced levels of AtPER1 due to antisensing or RNA interference. We found no correlation between Prx levels and the duration of the after-ripening period required before germination. Thus, Prxs are unlikely to contribute to maintenance of dormancy. RNA interference lines almost devoid of AtPER1 protein developed and germinated normally under standard growth room conditions. However, seeds from lines overexpressing PER1 were less inclined to germinate than wild-type seeds in the presence of NaCl, mannitol, and methyl viologen, suggesting that Prx can sense harsh environmental surroundings and play a part in the inhibition of germination under unfavorable conditions.     

I feel how you feel but not always: the empathic brain and its modulation

The ability to share the other's feelings, known as empathy, has recently become the focus of social neuroscience studies. We review converging evidence that empathy with, for example, the pain of another person, activates part of the neural pain network of the empathizer, without first hand pain stimulation to the empathizer's body. The amplitude of empathic brain responses is modulated by the intensity of the displayed emotion, the appraisal of the situation, characteristics of the suffering person such as perceived fairness, and features of the empathizer such as gender or previous experience with pain-inflicting situations. Future studies in the field should address inter-individual differences in empathy, development and plasticity of the empathic brain over the life span, and the link between empathy, compassionate motivation, and prosocial behavior.     

Variation for phenotypic plasticity among populations of an invasive exotic grass

Phenotypic plasticity is a common feature of plant invaders, but little is known about variation in plasticity among invading populations. Variation in plasticity of ecologically important traits could facilitate the evolution of greater plasticity and invasiveness. We examined plasticity among invasive populations of Microstegium vimineum (Japanese stiltgrass), a widespread and often dominant grass of forests in the eastern U.S. with two separate experiments. First, we exposed seven Microstegium populations to a drought treatment in growth chambers and monitored growth and physiological responses. Then, we established a greenhouse experiment using a subset of the populations; two that exhibited the most divergent responses and one intermediate population. In the greenhouse, we manipulated drought and shade and evaluated biomass production and specific leaf area (SLA). Microstegium exhibited plasticity for biomass production and SLA in the greenhouse experiment, and populations significantly varied in the degree of plasticity under drought and shade treatments. Two populations significantly increased biomass production under favorable conditions, unlike the third population. The most productive populations also responded to shade stress via greater SLA, possibly allowing for greater utilization of available light, while the third population did not. These results show that Microstegium can exhibit plastic responses to environmental conditions. Moreover, variation for plasticity among populations provides the potential for further evolution of plasticity. Future studies should focus on the relative importance of plasticity for the success of Microstegium and other plant invaders and evaluate post-introduction evolution of plasticity.     

High abundance of an exotic amphipod indicates disturbance in tropical rainforests

Previous studies have proposed terrestrial amphipods as potential bioindicators of forest condition. In order to investigate the response of the exotic terrestrial amphipod Talitroides topitotum (Crustacea: Amphipoda: Talitridae) to anthropogenic disturbances and its potential as a bioindicator, we compared its abundance among three forest reserves in southeastern Brazil, under different types and intensities of disturbance. We observed significantly higher abundances in disturbed sites compared to undisturbed sites in two of the reserves sampled, corroborating previous studies. In the third reserve, in which both forest disturbance and the abundance of amphipods were much lower than in the other two reserves, there was no significant difference between the sampling sites. We also speculate about the potential use of terrestrial amphipods as global indicators of forest disturbance.     

Experimental test of the Invasional Meltdown Hypothesis: an exotic herbivore facilitates an exotic plant,but the plant does not reciprocally facilitate the herbivore

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