Do ecological niches differ between sexual and asexual lineages of an aphid species?

According to environmental-based theories on the maintenance of sexual reproduction, sexual and asexual populations may coexist if they occupy different ecological niches. The aphid Rhopalosiphum padi offers a good opportunity to test this hypothesis since sexual and asexual lineages show local coexistence during a large part of their respective life-cycles. Because these two reproductive variants are morphologically identical but genetically distinct, we first characterized them using genetic markers in populations of R. padi in areas where sexual and asexual lineages may occur in sympatry. We then inferred the natal host plant of sexual and asexual genotypes by analysing stable isotopic ratios and showed that sexual ones mostly originated from C₃ Poaceae while asexual ones originated from C₃ and C₄ plants, although the majority came from C₄ Poaceae. These findings indicate that ecological niches of sexual and asexual lineages of R. padi differ, offering a plausible explanation for the local coexistence of the two reproductive modes in this species through habitat specialisation.     

Local and regional abundance of exotic plant species on Mediterranean islands: are species traits important?

Aim We assess the importance of three relevant and readily obtainable life‐history traits (dispersal syndrome, stem height and growth form) and biogeographical origin (European vs. non‐European) on the local and regional abundance of over 400 exotic plant species across eight Mediterranean islands. Location The Mediterranean islands of Lesbos, Rhodes, Crete, Malta, Corsica, Sardinia, Majorca and Minorca. Methods We adopt two abundance criteria for each exotic species: the proportion of islands in which the species occurs (regional abundance), and a qualitative estimate of species abundance within each of five islands (local abundance). Subsequently, we assess the relationship between local and regional abundance, as well as the role of key life‐history traits on both regional and local abundance. These analyses were undertaken separately for the European exotics and the non‐European exotics. Results Only 10.9% of the species occur on more than four islands, and only four species are present on all eight islands. Both local and regional abundances were higher for the non‐European than the European species. Local and regional abundances were positively correlated, particularly for exotics with non‐European origins. Wind‐dispersed species tended to have higher regional abundance than species dispersed by other means but this trend only occurred for local abundance on two islands — Corsica and Majorca. Neither a species’ growth form nor its stem height explained trends in regional or local abundance. Conclusions Although wind‐dispersed exotics are more widespread in the Mediterranean, plant life‐history traits appear to play a lesser role in invasion success than area of biogeographical origin. In general, exotic species of non‐European origin were more abundant at both local and regional scales. Invasion patterns should be interpreted at both local and regional scales, but the stochastic nature of biological invasions may limit deterministic interpretations of invasion patterns, especially if islands are studied in isolation.     

Disturbance and stress: different meanings in ecological dynamics?

There is an increasing frequency of papers addressing disturbance and stress in ecology without clear delimitation of their meaning. Some authors use the terms disturbance and stress exclusively as impacts, while others use them for the entire process, including both causes and effects. In some studies, the disturbance is considered as a result of a temporary impact, which is positive for the ecosystem, while stress is a negative, debilitating impact. By developing and testing simple theoretical models, the authors propose to differentiate disturbance and stress by frequency. If the frequency of the event enables the variable to reach a dynamic equilibrium which might be exhibited without this event, then the event (plus its responses) is a disturbance for the system. If frequency prevents the variable’s return to similar pre-event dynamics and drives or shifts it to a new trajectory, then we are facing stress. The authors propose that changes triggered by the given stimuli can be evaluated on an absolute scale, therefore, direction of change of the variable must not be used to choose one term or the other, i.e. to choose between stress and disturbance.     

Demography of source—sink populations and the evolution of ecological niches

Summary The demography of populations living in variable environments is an important factor molding the evolution of ecological niches, for it determines the relative strength of selection pressures on adaptations to different habitats. Here I consider a coarse-grained environment consisting of two habitat types and investigate how the selection pressure on reproductive success in different habitats depends on their quality and frequency and the dispersal pattern. The results suggest that selection on adaptations to optimal habitats will usually be stronger than on adaptations to poor habitats and the ecological niche will thus tend to be an evolutionarily conservative character. It is because under the habitat choice or limited dispersal that seem to prevail in natural populations, more individuals encounter the better habitat than would be expected solely on the basis of its relative area. This bias results in reduced selection pressure on reproductive success in the poorer habitat. With habitat choice or limited dispersal, selection pressure on reproductive success in the poorer habitat may exceed that on reproductive success in the better habitat only if the poorer habitat is much more frequent in the environment than the better habitat and the difference in their quality is not large.     

Extreme weather events and plant–plant interactions: shifts between competition and facilitation among grassland species in the face of drought and heavy rainfall

Biotic interactions play an important role in ecosystem function and structure in the face of global climate change. We tested how plant–plant interactions, namely competition and facilitation among grassland species, respond to extreme drought and heavy rainfall events. We also examined how the functional composition (grasses, forbs, legumes) of grassland communities influenced the competition intensity for grass species when facing extreme events. We exposed experimental grassland communities of different functional compositions to either an extreme single drought event or to a prolonged heavy rainfall event. Relative neighbour effect, relative crowding and interaction strength were calculated for five widespread European grassland species to quantify competition. Single climatic extremes caused species specific shifts in plant–plant interactions from facilitation to competition or vice versa but the nature of the shifts varied depending on the community composition. Facilitation by neighbouring plants was observed for Arrhenatherum elatius when subjected to drought. Contrarily, the facilitative effect of neighbours on Lotus corniculatus was transformed into competition. Heavy rainfall increased the competitive effect of neighbours on Holcus lanatus and Lotus corniculatus in communities composed of three functional groups. Competitive pressure on Geranium pratense and Plantago lanceolata was not affected by extreme weather events. Neither heavy rainfall nor extreme drought altered the overall productivity of the grassland communities. The complementary responses in competition intensity experienced by grassland species under drought suggest biotic interactions as one stabilizing mechanism for overall community performance. Understanding competitive dynamics under fluctuating resources is important for assessing plant community shifts and degree of stability of ecosystem functions.     

Intraspecific competition replaces interspecific facilitation as abiotic stress decreases: The shifting nature of plant–plant interactions

Plant–plant interactions change depending on environmental conditions, shifting from competition to facilitation when the stress is high. In addition to these changes, the relevance of intraspecific compared to interspecific interactions may also shift as abiotic stress does. We inferred intra- and interspecific plant–plant interactions of the cushion plant Hormathophylla spinosa as related to the dominant shrub Juniperus sabina in two sites with contrasting abiotic conditions (a slope with high-stress conditions vs. a valley bottom with milder conditions) in a Mediterranean high mountain. Specifically, we studied the spatial patterns and several variables related to plant performance (plant size and form, non-structural carbohydrate – NSC – concentrations and radial growth) of H. spinosa.The spatial pattern varied depending on site conditions. H. spinosa plants were positively associated with juniper in the high-stress slope site, probably through higher establishment rates due to the amelioration of soil conditions. In contrast, in the milder valley site H. spinosa establishment occurred mostly in open areas. Age structure, inferred from annual rings, reflected a massive establishment event in the whole study area which occurred 30–50 years ago. Canopy variables and radial growth were density dependent: both were negatively affected by the high density of H. spinosa individuals in the valley, but favoured by junipers on the slope. Interestingly, NSCs showed the opposite pattern, suggesting lower investment in growth by H. spinosa plants in the valley than on the slope.Our results reinforce the strong links existing between intra- and interspecific relationships and the need to include both when studying the influence of abiotic conditions on plant–plant interactions. This approach enabled us to detect that the direction and intensity of plant–plant interactions may shift at different ecological levels. Particularly interesting was the finding that optimal sites at the population level may not necessarily be the sites showing maximum individual performance.     

Keystone species and vulnerable species in ecological communities: strong or weak interactors?

The loss of a species from an ecological community can trigger a cascade of secondary extinctions. The probability of secondary extinction to take place and the number of secondary extinctions are likely to depend on the characteristics of the species that is lost--the strength of its interactions with other species--as well as on the distribution of interaction strengths in the whole community. Analysing the effects of species loss in model communities we found that removal of the following species categories triggered, on average, the largest number of secondary extinctions: (a) rare species interacting strongly with many consumers, (b) abundant basal species interacting weakly with their consumers and (c) abundant intermediate species interacting strongly with many resources. We also found that the keystone status of a species with given characteristics was context dependent, that is, dependent on the structure of the community where it was embedded. Species vulnerable to secondary extinctions were mainly species interacting weakly with their resources and species interacting strongly with their consumers.     

Do species evenness and plant density influence the magnitude of selection and complementarity effects in annual plant species mixtures?

Abstract Plant species richness influences primary productivity via mechanisms that (1) favour species with particular traits (selection effect) and (2) promote niche differentiation between species (complementarity). Influences of species evenness, plant density and other properties of plant communities on productivity are poorly defined, but may depend on whether selection or complementarity prevails in species mixtures. We predicted that selection effects are insensitive to species evenness but increase with plant density, and that the converse is true for complementarity. To test predictions, we grew three species of annuals in monocultures and in three‐species mixtures in which evenness of established plants was varied at each of three plant densities in a cultivated field in Texas, USA. Above‐ground biomass was smaller in mixtures than expected from monocultures because of negative ‘complementarity’ and a negative selection effect. Neither selection nor complementarity varied with species evenness, but selection effects increased at the greatest plant density as predicted.     

Geographic patterns of plant–herbivore interactions are driven by soil fertility

AimsGeographic patterns of the intensity of plant herbivory in relation to climate factors have garnered little general support and appear to be species specific. However, plant–herbivore interactions are also driven by resource availability, such as soil nutrient content, and it remains unclear whether broad-scale variation in soil factors is reflected in herbivore consumption rates across species’ ranges. Additionally, we know little of how intraspecific variation in tissue quality associates with edaphic and climatic factors, and how this variation controls herbivore consumption. The resource availability hypothesis (RAH) predicts that plant individuals growing in low-resource environments will have lower leaf nutritional quality and more constitutive defenses, which will result in lower rates of leaf consumption.     

Temporal shifts on tree species niches: how do they affect species dynamics and community diversity?

Plant Ecology - Species–habitat associations can be used as a proxy for species niches. Previous research has shown that niche plasticity may increase diversity in plant communities, and that...     

Functionally and structurally relevant residues of enzymes: are they segregated or overlapping?

There is a delicate balance between stability and flexibility needed for enzyme function. To avoid undesirable alteration of the functional properties during the evolutionary optimization of the structural stability under certain circumstances, and vice versa, to avoid unwanted changes of stability during the optimization of the functional properties of proteins, common sense would suggest that parts of the protein structure responsible for stability and parts responsible for function developed and evolved separately. This study shows that nature did not follow this anthropomorphic logic: the set of residues involved in function and those involved in structural stabilization of enzymes are rather overlapping than segregated.     

Philip Grime's fourth corner: are there plant species adapted to high disturbance and low productivity?

Grime's CSR species life‐strategy theory (competition–stress–ruderality) provides a conceptual framework to classify species into competitive (growing under high productivity, low disturbance), stress‐tolerant (low productivity, low disturbance) and ruderal (high productivity, high disturbance). Importantly, this classification is based on the assumption that the niche space of disturbance and productivity is filled unevenly: while in productive habitats species can adapt to different disturbance regimes, species of low‐productivity and disturbed habitats do not exist, resulting in a triangular distribution of species optima along axes of disturbance and productivity. This assumption has often been criticised, but it has not yet been put under a rigorous test. Here we use existing data on niche positions of central European plant species to test this hypothesis, namely its prediction that species adapted to jointly stressed (low‐productive) and disturbed habitats do not exist. We use Ellenberg indicator values and newly developed indicator values for disturbance as proxies of species positions in the space of productivity and disturbance. We found that positions of species optima along the gradients of productivity and disturbance severity are not independent of each other, with very few species adapted to low‐productive and severely disturbed habitats. In contrast, there is no relationship between productivity and disturbance frequency; a number of species occur in low‐productive and frequently disturbed habitats. The relationship between productivity and disturbance severity can be either due to tradeoffs between life history traits responsible for response to disturbance and productivity (as originally assumed by Grime) or due to historical rarity of severely disturbed habitats in unproductive conditions and consequent absence of evolution of species adapted to them. Our data are based on one specific flora, shaped by glaciations and early introduction of agriculture, but the question of what causes this pattern can be resolved by future analyses of floras with different evolutionary and ecological histories.