Maintenance of Positive Diversity-Stability Relations along a Gradient of Environmental Stress

Background

Environmental stress is widely considered to be an important factor in regulating whether changes in diversity will affect the functioning and stability of ecological communities.

Methodology/Principal Findings

We investigated the effects of a major environmental stressor (a decrease in water volume) on diversity-abundance and diversity-stability relations in laboratory microcosms composed of temperate multi-trophic rock pool communities to identify differences in community and functional group responses to increasing functional group richness along a gradient of environmental stress (low, medium, and high water volume). When a greater number of functional groups were present, communities were less temporally variable and achieved higher abundances. The stabilizing effect of increased functional group richness was observed regardless of the level of environmental stress the community was subjected too. Despite the strong consistent stabilizing effect of increased functional group richness on abundance, the way that individual functional groups were affected by functional group richness differed along the stress gradient. Under low stress, communities with more functional groups present were more productive and showed evidence of strong facilitative interactions. As stress increased, the positive effect of functional group richness on community abundance was no longer observed and compensatory responses became more common. Responses of individual functional groups to functional group richness became increasing heterogeneous are stress increased, prompting shifts from linear diversity-variability/abundance relations under low stress to a mix of linear and non-linear responses under medium and high stress. The strength of relations between functional group richness and both the abundances and temporal variability of functional groups also increased as stress increased.

Conclusions/Significance

While stress did not affect the relation between functional group richness and stability per se, the way in which functional groups responded to changes in functional group richness differed as stress increased. These differences, which include increases in the heterogeneity of responses of individual functional groups, increases in compensatory dynamics, and increases in the strength of richness-abundance and richness-variability relations, may be critical to maintaining stability under increasingly stressful environmental conditions.     

Abscisic acid, indole-3-acetic acid and mineral–nutrient changes induced by drought and salinity in longan (Dimocarpus longan Lour.) plants

Longan species (Dimocarpus longan Lour.) exhibit a high agronomic potential in many subtropical regions worldwide; however, little is known about its responses to abiotic stress conditions. Drought and salinity are the most environmental factors inducing negative effects on plant growth and development. In order to elucidate the responses of longan to drought and salinity, seedlings were grown under conditions of drought and salt stresses. Drought was imposed by suspending water supply leading to progressive soil dehydration, and salinity was induced using two concentrations of NaCl, 100 and 150 mM in water solution, for 64 days. Data showed that salt concentrations increased foliar abscisic acid (ABA) and only 150 mM NaCl reduced indole-3-acetic acid (IAA) and increased proline levels. NaCl treatments also increased Na⁺ and Cl^? content in plant organs proportionally to salt concentration. Drought increased leaf ABA but did not change IAA concentrations, and also increased proline synthesis. In addition, drought and salt stresses reduced the photosynthesis performance; however, only drought decreased leaf growth and relative leaf water content. Overall, data indicate that under severe salt stress, high ABA accumulation was accompanied by a reduction of IAA levels; however, drought strongly increased ABA but did not change IAA concentrations. Moreover, drought and high salinity similarly increased (or maintained) ion levels and proline synthesis. Data also suggest that ABA accumulation may mitigate the impact of salt stress through inducing stomatal closure and delaying water loss, but did not mediate the effects of long-term drought conditions probably because leaves reached a strong dehydration and the role of ABA at this stage was not effective to detain leaf injuries.     

Multilevel responses of emergent vegetation to environmental factors in a semiarid floodplain

Wetland emergent vegetation of Tablas de Daimiel National Park (Central Spain), mainly composed by Cladium mariscus, Phragmites australis and Typha domingensis, was studied to test if population responses to environmental factors were invariant to scaling-up conditions from the single plant to the entire wetland. While the significance of the main controlling, abiotic factors (wetland location, sedimentary and water nitrogen and phosphorus, water level, duration of flooding) was that of earlier studies, the importance of them changed along with the level of plant organization. Our study showed that multiple effects occurred in the responses of helophyte populations to abiotic factors, and that these responses appeared to depend upon the level of observation involved, showing positive (Typha biomass and sedimentary phosphorus), negative (Cladium biomass and sedimentary phosphorus, Cladium large patch growth and total phosphorus), delayed (landscape cover of Phragmites and Cladium and water level of the previous year), saturation (Cladium biomass and water level), threshold (small patch growth rate of Cladium and water level of the previous month) and non-linear (landscape cover of Phragmites and Cladium and total phosphorus in water) effects.     

Food availability affects the strength of mutualistic host–microbiota interactions in Daphnia magna

The symbiotic gut microbial community is generally known to have a strong impact on the fitness of its host. Nevertheless, it is less clear how the impact of symbiotic interactions on the hosts'' fitness varies according to environmental circumstances such as changes in the diet. This study aims to get a better understanding of host–microbiota interactions under different levels of food availability. We conducted experiments with the invertebrate, experimental model organism Daphnia magna and compared growth, survival and reproduction of conventionalized symbiotic Daphnia with germ-free individuals given varying quantities of food. Our experiments revealed that the relative importance of the microbiota for the hosts'' fitness varied according to dietary conditions. The presence of the microbiota had strong positive effects on Daphnia when food was sufficient or abundant, but had weaker effects under food limitation. Our results indicate that the microbiota can be a potentially important factor in determining host responses to changes in dietary conditions. Characterization of the host-associated microbiota further showed that Aeromonas sp. was the most prevalent taxon in the digestive tract of Daphnia.     

Acclimation in plants – the Green Hub consortium

Acclimation is the capacity to adapt to environmental changes within the lifetime of an individual. This ability allows plants to cope with the continuous variation in ambient conditions to which they are exposed as sessile organisms. Because environmental changes and extremes are becoming even more pronounced due to the current period of climate change, enhancing the efficacy of plant acclimation is a promising strategy for mitigating the consequences of global warming on crop yields. At the cellular level, the chloroplast plays a central role in many acclimation responses, acting both as a sensor of environmental change and as a target of cellular acclimation responses. In this Perspective article, we outline the activities of the Green Hub consortium funded by the German Science Foundation. The main aim of this research collaboration is to understand and strategically modify the cellular networks that mediate plant acclimation to adverse environments, employing Arabidopsis, tobacco (Nicotiana tabacum) and Chlamydomonas as model organisms. These efforts will contribute to ‘smart breeding’ methods designed to create crop plants with improved acclimation properties. To this end, the model oilseed crop Camelina sativa is being used to test modulators of acclimation for their potential to enhance crop yield under adverse environmental conditions. Here we highlight the current state of research on the role of gene expression, metabolism and signalling in acclimation, with a focus on chloroplast-related processes. In addition, further approaches to uncovering acclimation mechanisms derived from systems and computational biology, as well as adaptive laboratory evolution with photosynthetic microbes, are highlighted.     

How do anthropogenic contaminants (ACs) affect behaviour? Multi-level analysis of the effects of copper on boldness in hermit crabs

Natural animal populations are increasingly exposed to human impacts on the environment, which could have consequences for their behaviour. Among these impacts is exposure to anthropogenic contaminants. Any environmental variable that influences internal state could impact behaviour across a number of levels: at the sample mean, at the level of among-individual differences in behaviour (‘animal personality’) and at the level of within-individual variation in behaviour (intra-individual variation, ‘IIV’). Here we examined the effect of exposure to seawater-borne copper on the startle response behaviour of European hermit crabs, Pagurus bernhardus across these levels. Copper exposure rapidly led to longer startle responses on average, but did not lead to any change in repeatability indicating that individual differences were present and equally consistent in the presence and absence of copper. There was no strong evidence that copper exposure led to changes in IIV. Our data show that exposure to copper for 1 week produces sample mean level changes in the behaviour of hermit crabs. However, there is no evidence that this exposure led to changes in repeatability through feedback loops.     

14-3-3 proteins and the response to abiotic and biotic stress

14-3-3 proteins function as regulators of a wide range of target proteins in all eukaryotes by effecting direct protein-protein interactions. Primarily, interactions between 14-3-3 proteins and their targets are mediated by phosphorylation at specific sites on the target protein. Hence, interactions with 14-3-3s are subject to environmental control through signalling pathways which impact on 14-3-3 binding sites. Because 14-3-3 proteins regulate the activities of many proteins involved in signal transduction, there are multiple levels at which 14-3-3 proteins may play roles in stress responses in higher plants. In this article, we review evidence which implicates 14-3-3 proteins in responses to environmental, metabolic and nutritional stresses, as well as in defence responses to wounding and pathogen attack. This evidence includes stress-inducible changes in 14-3-3 gene expression, interactions between 14-3-3 proteins and signalling proteins and interactions between 14-3-3 proteins and proteins with defensive functions.     

Focus on Chromatin/Epigenetics: Flowering Locus C’s Lessons: Conserved Chromatin Switches Underpinning Developmental Timing and Adaptation

Analysis of how seasonal cues influence the timing of the floral transition has revealed many important principles for how epigenetic regulation can integrate a variety of environmental cues with developmental signals. The study of the pathways that necessitate overwintering in plants and their ability to respond to prolonged cold (the vernalization requirement and response pathways) has elaborated different chromatin regulatory pathways and the involvement of noncoding RNAs. The major target of these vernalization pathways in Arabidopsis (Arabidopsis thaliana) is Flowering Locus C (FLC). A relatively simple picture of FLC regulation is emerging of a few core complexes and mechanisms that antagonize each other’s actions. This balance provides a fine degree of control that has nevertheless permitted evolution of a wide range of natural variation in vernalization in Arabidopsis. Similar simple routes of adaptation may underlie life history variation between species.The time at which different species flower is an important marker of seasonal and climatic changes and is ecologically and economically important. The sessile nature of plants means that they experience the full range of environmental changes over the seasons. Flowering time control in many species is highly responsive to environmental cues and therefore very sensitive to local climate conditions. The impact of this on many ecosystem and agricultural processes has made understanding flowering time control an important objective.Many genetic pathways influence flowering time, either as part of seasonal (photoperiod and past and present temperature), developmental (developmental phase and age), or stress response (overcrowding and nutrient stress). Despite the variety of competing inputs, these many and various mechanisms are integrated at the action of a small number of nodes in Arabidopsis (Arabidopsis thaliana) termed floral pathway integrators (Simpson and Dean, 2002). Analyses of the genes identified by flowering time mutants have shown many have roles as chromatin modifiers (Andrés and Coupland, 2012; Pajoro et al., 2014). Timing of flowering seems particularly sensitive to chromatin regulation, potentially due to the necessity for long-term storage of seasonal information.In this review, we focus on vernalization in Arabidopsis and summarize our understanding of how chromatin modifiers interact with other proteins and noncoding RNAs to integrate developmental and temperature cues into chromatin changes at the key integrating locus Flowering Locus C (FLC). Further, we explore how changes in these mechanisms underlie different life history strategies and vernalization responses in different accessions and species. A key observation we wish to convey is that the complexity of these systems at the molecular level belies simplicity in balancing forces that enable a fine degree of control and adaptive responses at the phenotypic level.     

Effects of water level on waterbird abundance and diversity along the middle section of the Danube River

We studied the effects of water level in the Danube River on waterbird species abundance and species assemblages along 83?km from the border of Hungary and Slovakia. 651,622 birds of 62 species were counted between 1992 and 2009. The dominant species was Mallard Anas platyrhynchos contributing 436,198 individuals (66.9% of all observations). Throughout the year total numbers of individuals and species richness were negatively correlated with local water level registered on the same day throughout the year. Shannon diversity indices were generally positively correlated with water levels. During floods, dabbling ducks, especially the dominant Anas platyrhynchos, dispersed to peripheral waters, where they are less likely to be counted, and diving ducks (Bucephala clangula, Aythya fuligula and Mergus spp.) left the area entirely, because fast flowing, highly turbid waters reduced local feeding efficiency. Abundance of most waterbird species decreased with elevated water levels. High water levels remove distinctive microhabitats (gravel banks, paved riverbanks and shoals), and create unfavourable conditions of high water velocity and turbidity. Retention of high water levels as a result of damming of the Danube creates long-term conditions similar to natural flooding effects. In our opinion, further manipulation of Danube water levels is likely to reduce waterbird richness and abundance and should be subject to appropriate environmental impact assessment.     

The impact of slow stomatal kinetics on photosynthesis and water use efficiency under fluctuating light

Dynamic light conditions require continuous adjustments of stomatal aperture. The kinetics of stomatal conductance (g_s) is hypothesized to be key to plant productivity and water use efficiency (WUE). Using step-changes in light intensity, we studied the diversity of light-induced g_s kinetics in relation to stomatal anatomy in five banana genotypes (Musa spp.) and modeled the impact of both diffusional and biochemical limitations on photosynthesis (A). The dominant A limiting factor was the diffusional limitation associated with g_s kinetics. All genotypes exhibited a strong limitation of A by g_s, indicating a priority for water saving. Moreover, significant genotypic differences in g_s kinetics and g_s limitations of A were observed. For two contrasting genotypes, the impact of differential g_s kinetics was further investigated under realistic diurnally fluctuating light conditions and at the whole-plant level. Genotype-specific stomatal kinetics observed at the leaf level was corroborated at whole-plant level by transpiration dynamics, validating that genotype-specific responses are still maintained despite differences in g_s control at different locations in the leaf and across leaves. However, under diurnally fluctuating light conditions the impact of g_s speediness on A and intrinsic (_iWUE) depended on time of day. During the afternoon there was a setback in kinetics: absolute g_s and g_s responses to light were damped, strongly limiting A and impacting diurnal _iWUE. We conclude the impact of differential g_s kinetics depended on target light intensity, magnitude of change, g_s prior to the change in light intensity, and particularly time of day.     

Cooperative Adaptive Responses in Gene Regulatory Networks with Many Degrees of Freedom

Cells generally adapt to environmental changes by first exhibiting an immediate response and then gradually returning to their original state to achieve homeostasis. Although simple network motifs consisting of a few genes have been shown to exhibit such adaptive dynamics, they do not reflect the complexity of real cells, where the expression of a large number of genes activates or represses other genes, permitting adaptive behaviors. Here, we investigated the responses of gene regulatory networks containing many genes that have undergone numerical evolution to achieve high fitness due to the adaptive response of only a single target gene; this single target gene responds to changes in external inputs and later returns to basal levels. Despite setting a single target, most genes showed adaptive responses after evolution. Such adaptive dynamics were not due to common motifs within a few genes; even without such motifs, almost all genes showed adaptation, albeit sometimes partial adaptation, in the sense that expression levels did not always return to original levels. The genes split into two groups: genes in the first group exhibited an initial increase in expression and then returned to basal levels, while genes in the second group exhibited the opposite changes in expression. From this model, genes in the first group received positive input from other genes within the first group, but negative input from genes in the second group, and vice versa. Thus, the adaptation dynamics of genes from both groups were consolidated. This cooperative adaptive behavior was commonly observed if the number of genes involved was larger than the order of ten. These results have implications in the collective responses of gene expression networks in microarray measurements of yeast Saccharomyces cerevisiae and the significance to the biological homeostasis of systems with many components.     

Do Amplitudes of Water Level Fluctuations Affect the Growth and Community Structure of Submerged Macrophytes?

Submerged macrophytes are subjected to potential mechanical stresses associated with fluctuating water levels in natural conditions. However, few experimental studies have been conducted to further understand the effects of water level fluctuating amplitude on submerged macrophyte species and their assemblages or communities. We designed a controlled experiment to investigate the responses of three submerged macrophyte species (Hydrilla verticillata, Ceratophyllum demersum and Elodea nuttallii) and their combinations in communities to three amplitudes (static, ± 30 cm, ± 60 cm) of water level fluctuations. Results showed that water level fluctuating amplitude had little effects on the community performance and the three tested species responded differently. H. verticillata exhibited more growth in static water and it was negatively affected by either of the water level fluctuations amplitude, however, growth parameters of H. verticillata in two fluctuating water level treatments (i.e., ± 30 cm, ± 60 cm) were not significantly different. On the other hand, the growth of C. demersum was not significantly correlated with different amplitude treatments. However, it became more abundant when water levels fluctuated. E. nuttallii was inhibited by the two fluctuating water level treatments, and was less in growth parameters compared to the other species especially in water level fluctuating conditions. The inherent differences in the adaptive capabilities of the tested species indicate that C. demersum or other species with similar responses may be dominant species to restore submerged macrophyte communities with great fluctuating water levels. Otherwise, H. verticillata, E. nuttallii or other species with similar responses could be considered for constructing the community in static water conditions.     

Role of AP2/EREBP transcription factors in gene regulation during abiotic stress.

Crop plants are exposed to many types of abiotic stress during their life cycle. Water deficit derived from drought, low temperature or high salt concentration in the soil, is one of the most common environmental stresses that affects growth and development of plants through alterations in metabolism and gene expression. Adaptation to these conditions may involve passive tolerance or active homeostatic mechanisms for maintaining water balance. Active responses occur at different levels in the plant and may represent a concomitant protection against other types of stress such as pathogen attack. Many morphological and physiological adaptations to water stress are under the control of the plant hormone abscisic acid and involve specific activation of target genes that in one way or another protect cells against water deficit or participate in the regulation of the drought response. Here, we discuss recent advances in our understanding of drought adaptation mediated by specific changes in gene expression and the role of AP2/EREBP nuclear factors in these processes.     

Spatial patterns of historical growth changes in Norway spruce across western European mountains and the key effect of climate warming

Key message

Productivity changes in Norway spruce show important regional and local spatial variations, highlighting their context dependence at different spatial scales. These variations suggest the enhancing role of climate warming, and interplay with local water and nutrient limitations.

Abstract

While forest growth changes have been observed in many places worldwide, their spatial variation and environmental origin remain poorly documented. Analysis of these historical changes in contrasted regional contexts, and their mapping over continuous environmental gradients, may help uncover their environmental causes. The approach was tested on Norway spruce (Picea abies) in three western European mountain contexts (Massif Central, Alps and Jura), using National Forest Inventory (NFI) data. We explored the environmental factors influencing stand basal area increment (BAI) in each context. We then estimated and compared mean regional changes in BAI and related these to the regional environmental limitations evidenced. Within each region, we further mapped local BAI trends using a geographically weighted regression (GWR) approach. In each region, local estimates of BAI changes were finally correlated to environmental indicators. We found an increase in BAI in the three regions over 1980–2005, greater in the Massif Central (+71 %) than in the Alps (+19 %) and the Jura Mountains (+21 %). Inter-regional differences in BAI changes suggested the release of a thermal constraint—found more important in the Massif Central—by the strong temperature increase over the period, and a limitation by water availability in the Jura and the Alps Mountains. Spatial patterns of BAI change revealed significant local variations in the Massif Central and the Alps. From the correlation analysis, these were again found consistent with the hypothesis of an enhancing effect of climate warming in these mountain ranges. They were also related to local soil nutritional status in the two regions, and negatively related to nitrogen deposition level in the Massif Central. As a main outcome, a strong context and spatial scale dependence of productivity changes is emphasized. In addition, the enhancing effect of climate warming on productivity is suggested, with local modulation by climatic and nutritional conditions.     

Impacts of drought and predicted effects of climate change on fish growth in temperate Australian lakes

Climate change is expected to negatively impact many freshwater environments due to reductions in stream‐flow and increases in temperature. These conditions, however, can already be found today in areas experiencing significant drought; current observations of species' responses to droughts can be used to make predictions about their future responses to climate change. Using otolith analysis, we recreated golden perch (Macquaria ambigua) growth chronologies from two temperate lake populations in southeastern Australia over a 15‐year period pre‐ and during a supraseasonal drought. We related interannual growth variation to landscape‐scale changes in temperature and hydrological regimes: fish growth declined as water levels in the lakes dropped during the drought, but this effect was offset by increased growth in warmer years. We hypothesize that golden perch are responding to fluctuations in food availability and intraspecific competition related to water level and to an optimization of physiological growth conditions related to increases in growing season length. Based on our analyses, we made predictions of future growth under a number of climate change scenarios that incorporate forecast deviations in stream‐flows and air temperature. Despite climatic models predicting significant declines in future water availability, fish growth may increase due to a disproportionate lengthening of the growing season. As the two lakes are at the limit of the southerly range of golden perch, our results are consistent with previous findings of climate‐change driven latitudinal range shifts in a poleward direction. We discuss assumptions concerning the constancy of ecological interactions into the future that warrant further study. Our research provides a novel application of biochronological analysis that could be used elsewhere to further our knowledge of species responses to changing environments.     

Initiation and abolition of the action potential of a single node of Ranvier

1. Using single node preparations of the bull frog or the toad, observations were made on the variation of the voltage across the nodal membrane under various experimental conditions. 2. The time constant of the variation in the membrane voltage caused by a long subthreshold rectangular pulse was of the order of 0.1 msec. 3. The action potential was initiated when the potential inside the node was raised stimulating pulses above a threshold level of approximately 15 mv. for a node in normal Ringer; it was greater in a relatively refractory node and in a partially narcotized node. 4. The variation of the membrane voltage caused by long stimulating pulses of subrheobasic strengths was in general proportional to the strength of the applied pulse. A non-linear behavior of the membrane voltage was observed with barely subthreshold stimulating pulses. 5. The early portion of the action potential of a node was not modified by a direct current which was strong enough to produce measurable potential changes (IR drops) across the resting membrane. 6. A strong pulse of inward current applied to the node during activity abolished the portion of the action potential following the pulse in all-or-none manner. 7. There was no refractory period after a response abolished in its early phase. Following a response abolished later, the recovery in the spike height started from the level of the action potential at the time of abolition. 8. Initiation and abolition of action potentials at a single node are interpreted as "transitions" between the two "equilibrium potential levels" at the node.     

Summer and fall ants have different physiological responses to food macronutrient content

Seasonally, long-lived animals exhibit changes in behavior and physiology in response to shifts in environmental conditions, including food abundance and nutritional quality. Ants are long-lived arthropods that, at the colony level, experience such seasonal shifts in their food resources. Previously we reported summer- and fall-collected ants practiced distinct food collection behavior and nutrient intake regulation strategies in response to variable food protein and carbohydrate content, despite being reared in the lab under identical environmental conditions and dietary regimes. Seasonally distinct responses were observed for both no-choice and choice dietary experiments. Using data from these same experiments, our objective here is to examine colony and individual-level physiological traits, colony mortality and growth, food processing, and worker lipid mass, and how these traits change in response to variable food protein–carbohydrate content. For both experiments we found that seasonality per se exerted strong effects on colony and individual level traits. Colonies collected in the summer maintained total worker mass despite high mortality. In contrast, colonies collected in the fall lived longer, and accumulated lipids, including when reared on protein-biased diets. Food macronutrient content had mainly transient effects on physiological responses. Extremes in food carbohydrate content however, elicited a compensatory response in summer worker ants, which processed more protein-biased foods and contained elevated lipid levels. Our study, combined with our previously published work, strongly suggests that underlying physiological phenotypes driving behaviors of summer and fall ants are likely fixed seasonally, and change circannually.