Grazing and aridity have contrasting effects on the functional and taxonomic diversity of ants

Ants are one of the most abundant and ubiquitous organisms on Earth and play critical roles in multiple ecosystem services such as seed dispersal and nutrient cycling. Despite this, the effects of climatic and land use stressors on particular species or groups of ants are poorly known. We conducted a regional field survey across 108 locations in south-eastern Australia, using correlation network analysis and structural equation modelling to identify how ants respond to environmental stressors. We found contrasting relationships amongst ants, and aridity, and vertebrate grazing intensity and history. Increasing aridity was associated with reduced ant richness, whereas increasing grazing intensity was associated with greater ant richness directly, and indirectly, via reductions in litter depth and perennial grass density. However, these taxonomically diverse groups of ant species still shared contrasting responses to increasing aridity and grazing intensity. We found strong associations between grazing, aridity and the abundance of Seed Harvesters, weak indirect relationships with Generalist Foragers, but no relationships for Predators or Sugar Feeders. Taken together, our work identifies contrasting relationships amongst grazing, aridity and ants (ant ‘winners’ or ‘losers’) across contrasting ecological contexts. Given that increasing aridity is generally associated with lower grazing intensity, our results suggest that locations with more arid sites will have lower ant richness with fewer Seed Harvesters, whereas more mesic sites with high grazing intensity might increase ant richness, and the abundance of specific ant species. Such knowledge is important if we are to maintain critical ant-mediated functions as Earth becomes drier and grazing intensity increases.     

Effects of contrasting omnivorous fish on submerged macrophyte biomass in temperate lakes: a mesocosm experiment

1. Freshwater fish can affect aquatic vegetation directly by consuming macrophytes or indirectly by changing water quality. However, most fish in the temperate climate zone have an omnivorous diet. The impact of fish as aquatic herbivores in temperate climates therefore remains unclear and depends on their dietary flexibility. 2. We tested the effects of a flexible omnivore and an herbivore on aquatic vegetation by comparing the effects of rudd (Scardinius erythrophthalmus, the most herbivorous fish in temperate climates) with grass carp (Ctenopharyngodon idella) in a mesocosm pond study. Exclosures distinguished herbivorous effects of fish on submerged macrophytes from indirect effects through changes in water quality, whereas stable isotope food‐web analysis provided information on fish diets. 3. We hypothesised that rudd, with its flexible diet and preference for animal food items, would only indirectly affect macrophytes, whereas grass carp, with its inflexible herbivorous diet, would directly affect macrophyte biomass. 4. Only grass carp significantly reduced macrophyte biomass through consumption. Rudd had no effect. Food‐web analysis indicated that rudd predominantly consumed animal prey, whereas grass carp included more plants in their diet, although they also consumed animal prey. Grass carp significantly affected water quality, resulting in lowered pH and increased N‐NH₄ concentrations, whereas more periphyton growth was observed in the presence of rudd. However, the indirect non‐herbivorous effects of both fish species had no effect on macrophyte biomass. 5. Both fish species should be considered as omnivores. Despite the fact that rudd is the most herbivorous fish in the western European climate zone, its effect on submerged macrophyte biomass is not substantial at natural densities and current temperatures.     

Host patch traits have scale-dependent effects on diversity in a stickleback parasite metacommunity

Many metacommunities are distributed across habitat patches that are themselves aggregated into groups. Perhaps the clearest example of this nested metacommunity structure comes from multi-species parasite assemblages, which occupy individual hosts that are aggregated into host populations. At both spatial scales, we expect parasite community diversity in a given patch (either individual host or population) to depend on patch characteristics that affect colonization rates and species sorting. But, are these patch effects consistent across spatial scales? Or, do different processes govern the distribution of parasite community diversity among individual hosts, versus among host patches? To answer these questions, we document the distribution of parasite richness among host individuals and among populations in a metapopulation of threespine stickleback Gasterosteus aculeatus. We find some host traits (host size, gape width) are associated with increased parasite richness at both spatial scales. Other patch characteristics affect parasite richness only among individuals (sex), or among populations (lake size, lake area, elevation and population mean heterozygosity). These results demonstrate that some rules governing parasite richness in this metacommunity are shared across scales, while others are scale-specific.     

Resident plant diversity and introduced earthworms have contrasting effects on the success of invasive plants

Theoretical predictions and empirical studies suggest that resident species diversity is an important driver of community invasibility. Through trait-based processes, plants in communities with high resident species diversity occupy a wider range of ecological niches and are more productive than low diversity communities, potentially reducing the opportunities for invasion through niche preemption. In terrestrial plant communities, biotic ecosystem engineers such as earthworms can also affect invasibility by reducing leaf litter stocks and influencing soil conditions. In a greenhouse experiment, we simultaneously manipulated resident species diversity and earthworm presence to investigate independent and interactive effects of these two variables on the success of several invasive plants. Higher diversity of resident species was associated with lower biomass of invasives, with the effect mediated through resident species biomass. The presence of earthworms had a strong positive effect on the biomass of invasive species across all levels of resident species diversity and a weaker indirect negative effect via decreased soil moisture. Earthworms also weakened the positive correlation between resident species diversity and productivity. We did not observe any interactive effects of resident species biomass and earthworms on invasive species success. Partitioning the net biodiversity effect indicated that selection effects increased with resident species diversity whereas complementarity effects did not. Results suggest that managing for diverse forest communities may decrease the susceptibility of these communities to invasions. However, the presence of introduced earthworms in previously earthworm-free sites may undermine these efforts. Furthermore, future studies of plant community invasibility should account for the effects of introduced earthworms.     

Interaction between area effects and variation with habitat in Cepaea

Populations of Cepaea nemoralis in Warwickshire occupy habitats of considerable temporal stability, most being at least 250 years old, and some much older. As expected from earlier work, shell pattern polymorphisms in these populations show variation with habitat of a kind suggesting the operation of visual selection for crypsis. They also show patterns of microgeographical variation unrelated to habitat. Although of a less extreme character, this variation resembles the 'area effects' seen in downland populations of Cepaea , in the lack of coincidence of variation at different loci, and in the existence of stronger and larger scale patterns in banding than in colour morphs. A similar explanation is advanced for their occurrence: previous bottlenecks and colonization from small relicts with founder effects. The less marked character of the variation is expected from the greater habitat stability and continuity in Warwickshire compared with downland. A clear colonization effect is seen in the inverse relationship between age of habitat and frequency of yellow in woodland populations. The results suggest that population histories affect variation in Cepaea even in areas of relative habitat stability.     

Plastic and genetic responses of a common sedge to warming have contrasting effects on carbon cycle processes

Climate warming affects plant physiology through genetic adaptation and phenotypic plasticity, but little is known about how these mechanisms influence ecosystem processes. We used three elevation gradients and a reciprocal transplant experiment to show that temperature causes genetic change in the sedge Eriophorum vaginatum. We demonstrate that plants originating from warmer climate produce fewer secondary compounds, grow faster and accelerate carbon dioxide (CO₂) release to the atmosphere. However, warmer climate also caused plasticity in E. vaginatum, inhibiting nitrogen metabolism, photosynthesis and growth and slowing CO₂ release into the atmosphere. Genetic differentiation and plasticity in E. vaginatum thus had opposing effects on CO₂ fluxes, suggesting that warming over many generations may buffer, or reverse, the short‐term influence of this species over carbon cycle processes. Our findings demonstrate the capacity for plant evolution to impact ecosystem processes, and reveal a further mechanism through which plants will shape ecosystem responses to climate change.     

Long‐term influence of fallen logs on patch formation and their effects under contrasting grazing regimes

Patchiness is a critical functional feature of arid lands, increasing productivity and diversity, but there is little information on the long‐term dynamics of patch formation. While plant‐created heterogeneity is widespread, grazing reduces plant density, hence removing patchiness. Patchiness is also associated with fallen logs and it is particularly important to assess this association in grazed areas, as there are few patches which are not created by living plants. We studied differences in soil nutrient content and soil seed bank associated with the presence of logs in the long ungrazed Koonamore Vegetation Reserve and an adjacent heavily grazed paddock in chenopod shrublands of South Australia. We studied soil properties associated with fallen logs of unknown age, and others known to have persisted for up to 78 years. Logs acted as traps for soil nutrients and seeds. Organic carbon was higher in soils associated with logs, and also higher inside the reserve than in the grazed paddock. Propagule number and species richness were higher next to logs than in open spaces and viable seeds were in much higher abundance next to logs in the grazed paddock than any other site. Increased time in situ of a log had a relatively small effect on soil organic carbon and total nitrogen. Logs act as resource traps which may enhance the diversity of the system, and may be particularly important for maintaining patches of resources in areas of severe grazing damage. Hence, their preservation should be incorporated into management schemes. [Correction added on 3 March 2015, after online publication: The following parts in the abstract have been edited. ‘Organic carbon and total nitrogen were higher in soils associated with logs’ has been corrected to ‘Organic carbon was higher in soils associated with logs’, and ‘had a relatively small effect on soil organic carbon, total nitrogen and available potassium’ has been corrected to ‘had a relatively small effect on soil organic carbon and total nitrogen’.]     

Fluctuating temperatures have different effects on embryonic sensitivity to ABA in Sorghum varieties with contrasting pre-harvest sprouting susceptibility

The effect of fluctuating temperatures on the germination ofimmature caryopses of two Sorghum varieties presenting contrastingsusceptibility to pre-harvest sprouting was investigated. Fluctuatingtemperatures were able to stimulate germination of immaturecaryopses of both varieties from early stages of development(i.e. 15 d after pollination). Isolated embryos from both varietiesgerminated well in water irrespective of the thermal regimeof incubation. However, the ability of ABA to block germinationin Redland B2 (sproutingsusceptible) isolated embryos was significantlyreduced when embryos were incubated under fluctuating temperaturesfrom 23 DAP onwards. No such effect was found in IS 9530 (sprouting-resistant)embryos. No differences in the pattern with which embryonicABA content decreased during whole grain incubation were foundin 25 and 35 DAP grains from both varieties incubated underconstant or fluctuating temperatures. Therefore, these resultsindicate that alternating temperatures can promote germinationthrough different mechanisms. One of them is the decrease inembryo sensitivity to ABA inhibition which appears to be actingin Redland B2 caryopses from 23 DAP onwards; the other one seemsto be independent of ABA level and sensitivity and is activeat very early stages of development in one variety (RedlandB2) and throughout seed development in the other (IS 9530). Key words: Germination, dormancy, fluctuating temperatures, abscisic acid, seed development, Sorghum bicolor     

Fish induced macrophyte loss in shallow lakes: top–down and bottom–up processes in mesocosm experiments

SUMMARY 1. Macrophyte loss from Sites of Special Scientific Interest in England has become widespread over the last 20 years. One reason for this may be changing trends in angling, a multimillion pound industry that has an enormous impact on aquatic ecosystems. Stocking with cyprinid fish is a common angling management practice but the particular fish species and distribution of their biomass may be crucial to the ecosystem. 2. Carp (Cyprinus carpio), roach (Rutilus rutilus), bream (Abramis brama) and tench (Tinca tinca) at biomasses ranging from 0 to 800 kg ha^?1 and at various sizes were placed into experimental mesocosms in Little Mere, a shallow, fertile lake in Cheshire, U.K. The effects these treatments had on the aquatic ecosystem were studied over two summers. Specifically the effects of the treatments on macrophyte growth, benthic and macrophytic macro‐invertebrate populations, water chemistry, epiphyton production and plankton survival were investigated. 3. Carp had a greater detrimental effect on the macrophytes than bream, tench and in particular roach. A biomass of fish > 200 kg ha^?1 adversely affected the extent of macrophyte growth. 4. The decline in macrophyte growth was most likely as a result of increased epiphyton growth that probably reduced the amount of light and carbon dioxide available to the plant. There were no observed direct fish impacts on macrophytes. 5. The chemical data suggested that inorganic nitrogen levels were low and it is possible that release of nitrogen, from fish excreta, followed by immediate uptake, could have been a major factor stimulating epiphyton growth and subsequently macrophyte loss. Phosphorus concentrations increased even in the controls and substantial amounts were available. Phosphorus stimulation can therefore be discounted. Macrophyte‐associated macro‐invertebrates were positively correlated with epiphyton load but had no impact on the extent of epiphytic growth. Shading from disturbed sediment or phytoplankton was also unimportant.     

The introduced signal crayfish and native noble crayfish have different effects on sublittoral macroinvertebrate assemblages in boreal lakes

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Spatial explicit demography: The effects habitat patch isolation have on vole matrilines

Populations of many species are spatially structured in matrilines, and their dynamics may be determined by matriline specific demographic processes. We examined whether the isolation of habitat patches (i.e. interpatch distance) affected the demography of matrilines in 14 experimentally fragmented populations of the root vole. Matrilines inhabiting the most isolated patches decreased in size over the breeding season, while matrilines in less isolated patches increased. The survival rate of adult females was the main factor underlying the variation in growth rates among matrilines. Low survival when patches were isolated seemed to be due to long-distance interpatch movements exposing females to increased predation rate.
The differential success of matrilines in patchy populations with variable interpatch distances acted to decrease the matrilineal diversity at the population level. Furthermore, isolated patches may function as sinks. Thus spatially explicit landscape features may affect both population demography and genetics.     

Cascading extinctions and ecosystem functioning: contrasting effects of diversity depending on food web structure

The consequences of species loss on cascading extinctions in food webs have been the focus of several recent theoretical studies, with differing results. Changes in ecosystem properties consecutive to cascading extinctions have received far less attention even though such dramatic events might strongly alter ecosystem functioning. Here we use various food web models to investigate the effects of species loss and diversity on both secondary extinctions and their associated changes in ecosystem properties. Our analysis shows that diversity has contrasting effects depending on the presence of self-limiting terms at consumer levels and, to a lower extent, on connectance and interspecific competition. Ecosystems that lose a high proportion of species through cascading extinctions exhibit the most important changes in ecosystem properties. Linking studies on cascading extinctions in food webs with studies that investigate the effects of biodiversity on ecosystem functioning appears crucial for a better understanding of the consequences of species extinctions.     

Membrane fusion promoters and inhibitors have contrasting effects on lipid bilayer structure and undulations

It has been established that the fusion of both biological membranes and phospholipid bilayers can be modulated by altering their lipid composition (Chernomordik et al., 1995 .J. Membr. Biol. 146:3). In particular, when added exogenously between apposing membranes, monomyristoylphosphatidylcholine (MMPC) inhibits membrane fusion, whereas glycerol monoleate (GMO), oleic acid (OA), and arachidonic acid (AA) promote fusion. This present study uses x-ray diffraction to investigate the effects of MMPC, GMO, OA, and AA on the bending and stability of lipid bilayers when bilayers are forced together with applied osmotic pressure. The addition of 10 and 30 mol% MMPC to egg phosphatidylcholine (EPC) bilayers maintains the bilayer structure, even when the interbilayer fluid spacing is reduced to approximately 3 A, and increases the repulsive pressure between bilayers so that the fluid spacing in excess water increases by 5 and 15 A, respectively. Thus MMPC increases the undulation pressure, implying that the addition of MMPC promotes out-of-plane bending and decreases the adhesion energy between bilayers. In contrast, the addition of GMO has minor effects on the undulation pressure; 10 and 50 mol% GMO increase the fluid spacing of EPC in excess water by 0 and 2 A, respectively. However, x-ray diffraction indicates that, at small interbilayer separations, GMO, OA, or AA converts the bilayer to a structure containing hexagonally packed scattering units approximately 50 A in diameter. Thus GMO, OA, or AA destabilizes bilayer structure as apposing bilayers are brought into contact, which could contribute to their role in promoting membrane fusion.     

Third-party mutualists have contrasting effects on host invasion under the enemy-release and biotic-resistance hypotheses

Plants engage in complex multipartite interactions with mutualists and antagonists, but these interactions are rarely included in studies that explore plant invasiveness. When considered in isolation, we know that beneficial microbes can enhance an exotic plant’s invasive ability and that herbivorous insects often decrease an exotic plant’s likeliness of success. However, the effect of these partners on plant fitness has not been well characterized when all three species coevolve. We use computational evolutionary modeling of a trait-based system to test how microbes and herbivores simultaneously coevolving with an invading plant affect the invaders’ probability of becoming established. Specifically, we designed a model that explores how a beneficial microbe would influence the outcome of an interaction between a plant and herbivore. To model novel interactions, we included a phenotypic trait shared by each species. Making this trait continuous and selectable allows us to explore how trait similarities between coevolving plants, herbivores and microbes affect fitness. Using this model, we answer the following questions: (1) Can a beneficial plant-microbe interaction influence the evolutionary outcome of antagonistic interactions between plants and herbivores? (2) How does the initial trait similarity between interacting organisms affect the likelihood of plant survival in novel locations? (3) Does the effect of tripartite interactions on the invasion success of a plant depend on whether organisms interact through trait similarity [Enemy Release Hypothesis (ERH)] or dissimilarity (Biotic Resistance Hypothesis)? We found that it was much more difficult for plants to invade under the ERH but that beneficial microbes increase the probability of plant survival in a novel range under both hypotheses. To our knowledge, this model is the first to use tripartite interactions to explore novel species introductions. It represents a step towards gaining a better understanding of the factors influencing establishment of exotic species to prevent future invasions.     

Interaction effects between sender and receiver processes in indirect transmission of Campylobacter jejuni between broilers

ABSTRACT: BACKGROUND: Infectious diseases in plants, animals and humans are often transmitted indirectly between hosts (or between groups of hosts), i.e. via some route through the environment instead of via direct contacts between these hosts. Here we study indirect transmission experimentally, using transmission of Campylobacter jejuni (C. jejuni) between spatially separated broilers as a model system. We distinguish three stages in the process of indirect transmission; (1) an infectious "sender" excretes the agent, after which (2) the agent is transported via some route to a susceptible "receiver", and subsequently (3) the receiver becomes colonised by the agent. The role of the sender and receiver side (stage 1 and stage 3) was studied here by using acidification of the drinking water as a modulation mechanism. RESULTS: In the experiment one control group and three treatment groups were monitored for the presence of C. jejuni by taking daily cloacal swabs. The three treatments consisted of acidification of the drinking water of the inoculated animals (the senders), acidification of the drinking water of the susceptible animals (the receivers) or acidification of the drinking water of both inoculated and susceptible animals. In the control group 12 animals got colonised out of a possible 40, in each treatment groups 3 animals out of a possible 40 were found colonised with C. jejuni. CONCLUSIONS: The results of the experiments show a significant decrease in transmission rate (beta) between the control group and treatments group (p < 0.01 for all groups) but not between different treatments; there is a significant negative interaction effect when both the sender and the receiver group receive acidified drinking water (p = 0.01). This negative interaction effect could be due to selection of bacteria already at the sender side thereby diminishing the effect of acidification at the receiver side.     

Plant diversity and insect herbivores: effects of environmental change in contrasting model systems

There is increasing concern over the potential impact of anthropogenic factors (e.g. increasing nutrient inputs, global climate change) on the rate of loss of diversity in ecosystems. Such losses may affect ecosystem processes. In addition, a change in diversity of one group of organisms may influence the diversity of species of the next trophic level. We examined the extent to which plant species richness influences that of insect herbivores in two systems: a long‐term field experiment on heather moorland and a model community in the Ecotron controlled environment facility. We examined the response of these two plant communities to environmental change, specifically increased levels of nutrients, grazing and atmospheric CO₂. We measured the indirect effects of changes in these factors on insect herbivores, both above‐ and below‐ground. In the moorland system, grazing was the largest influence on plant community structure. The community was dominated by one species, Calluna vulgaris, and loss of cover under heavy grazing allowed competing species to invade. However, grazing regime was not a major influence on the species richness of the insect herbivore community. Site was more important: there were a greater number of Hemiptera species on sites with more mineral soils than on peat sites, possibly because a greater variety of grass and herb species was present on the former sites. In the Ecotron, below‐ground factors were also important drivers of community change: elevated CO₂ increased carbon availability in the soil and there were simultaneous changes in the community composition of soil biota. Above‐ground, some plant species increased in abundance and others decreased, leading to interaction‐specific effects on the insect herbivores. In two very different studies of the effects of environmental change on the interactions between plants and their herbivores, several similar conclusions can be drawn: (1) effects are likely to be site‐ and interaction‐specific; (2) outcomes are likely to be strongly dependent on the initial state and the dominant species of the plant community; and (3) indirect effects, often mediated by below‐ground factors, may have a bigger influence on insect‐plant interactions than more direct effects of above‐ground factors.     

Predicting immigration of two species in contrasting landscapes: effects of scale, patch size and isolation

Migration is a key process for spatially structured populations. We examined how a variety of patch based metrics commonly used to predict the number of immigrants to a habitat patch performed based on data from three different years, in two distinct insect systems. The first system was an herbivorous beetle inhabiting patches of its host plant within a 'typical' patch network. In this system there were numerous patches located relatively close to one another, given the beetle's dispersal ability. The second system consisted of a butterfly inhabiting a series of 17 subalpine meadows. Here, the patches were arranged in a linear fashion and were more distant from each other. Overall, we found that the best models incorporating aspects of patch size and/or isolation explained a large (30–40%) amount of deviance in immigration, but there were considerable differences between the systems. For the first system, we found that metrics including the size of the target patch explained the highest proportion of deviance in immigrant numbers, while metrics based only on interpatch distances explained very little deviance. The situation was reversed for the second system. Metrics including the size of the target patch explained little deviance, while metrics based on the distance between patches explained the bulk of deviance in the number of immigrants. The results of our study show that the effects of patch size and isolation on the number of immigrants are highly important, but dependent on spatial scale, the organism studied, and how it responds to the spatial arrangement of patches. Correspondingly, there will be no single generalized metric to predict immigration for all cases. Given the dependency of the results on the system studied, we recommend that future studies provide explicit data on habitat areas and dispersal distance relative to interpatch distance to allow for meaningful comparison among organisms and systems.