Effects Of Species Interactions Research Articles

Evolutionary responses to global change in species-rich communities.

Evolution in nature occurs in the proverbial tangled bank. The species interactions characterizing this tangled bank can be strongly affected by global change and can also influence the fitness and selective effects of a global change on a focal population. As a result, species interactions can influence which traits will promote adaptation and the magnitude or direction of evolutionary responses to the global change. First, we provide a framework describing how species interactions may influence evolutionary responses to global change. Then, we highlight case studies that have explicitly manipulated both a global change and the presence or abundance of interacting species and used either experimental evolution or quantitative genetics approaches to test for the effects of species interactions on evolutionary responses to global change. Although still not frequently considered, we argue that species interactions commonly modulate the effects of global change on the evolution of plant and animal populations. As a result, predicting the evolutionary effects of the multitude of global changes facing natural populations requires both community ecology and evolutionary perspectives.

Adjustable temperature array for characterizing ecological and evolutionary effects on thermal physiology

Abstract To accurately characterize a species’ thermal niche and aid in predicting effects of climate change we must not only include information on thermal tolerances and physiological responses to changing temperatures, but also incorporate ecological effects and evolutionary processes that may shape a species’ niche. However, quickly and practically collecting data on key factors such as adaptation potential, behaviour, effects of species interactions, plasticity and thermal tolerances is logistically challenging. We have therefore created an adjustable temperature array (ATA) to assist with experimental ecology and evolution research. ATA’s are a row of independent temperature points controlled and set by the user and made from commercially available parts. This allows the user to create unique thermal landscapes relevant to their study organism(s) and question(s). Further, the option of using an enclosed cage allows the user to answer questions at the individual, population, or community level in the context of changing thermal environments. ATA’s are able to be user‐set to constant or dynamic temperature regimes and are designed for use on small animals (e.g. fruit flies, beetles, mosquitoes) or plants (e.g. germinating seeds). We have tested and confirmed the accuracy of the ATA to several thermal landscapes that would be useful for experimental ecology and evolution, including: (a) coarse resolution of a broad thermal niche ranging from 12° to 42°C in 2°C intervals (R2 = 0.998); (b) fine resolution of a narrow thermal niche ranging from 15° to 32°C in 1°C intervals (R2 = 0.997); (c) a pyramid‐shaped niche consisting of a gradient from 14° to 30°C in 2°C intervals (R2 = 0.997); and (d) a very narrow thermal niche with replicate thermal resources ranging from 26.5° to 34°C in 1.5°C intervals (R2 = 0.989). The equipment described here is an important tool for thermal niche studies and will aid in gathering information on effects of ecological and evolutionary processes to create a comprehensive picture of species responses to climate change.

Species interactions affect the spread of vector-borne plant pathogens independent of transmission mode.

Within food webs, vectors of plant pathogens interact with individuals of other species across multiple trophic levels, including predators, competitors, and mutualists. These interactions may in turn affect vector-borne pathogens by altering vector fitness and behavior. Predators, for example, consume vectors and reduce their abundance, but often spur movement of vectors as they seek to avoid predation. However, a general framework to predict how species interactions affect vectors of plant pathogens, and the resulting spread of vector-borne pathogens, is lacking. Here we developed a mathematical model to assess whether interactions such as predation, competition, and mutualism affected the spread of vector-borne plant pathogens with nonpersistent or persistent transmission modes. We considered transmission mode because interactions affecting vector-host encounter rates were expected to most strongly affect nonpersistent pathogens that are transmitted with short feeding bouts; interactions that affect vector feeding duration were expected to most strongly affect persistent pathogens that require long feeding bouts for transmission. Our results show that interactions that affected vector behavior (feeding duration, vector-host encounter rates) substantially altered rates of spread for vector-borne plant pathogens, whereas those affecting vector fitness (births, deaths) had relatively small effects. These effects of species interactions were largely independent of transmission mode, except when interactions affected vector-host encounter rates, where effects were strongest for nonpersistent pathogens. Our results suggest that a better understanding of how vectors interact with other species within food webs could enhance our understanding of disease ecology.

Interspecific and intraspecific interactions between fiddler crabs Minuca pugnax (mud fiddler) and Leptuca pugilator (sand fiddler) influence species' burrowing behavior

Species occupying similar habitat and functional niches will necessarily have some external force driving differentiation between them, thus creating complementarity, or otherwise face high competition for resources. In order to fully understand the role of each species in an ecosystem and their contributions to ecosystem function, it is important to understand the nature of these interactions. However, for species that occur in the same spatial niche and appear to occupy redundant functional niches, it is often difficult to distinguish between complementarity and redundancy and therefore to designate the functional role each species plays. We used two co-occurring fiddler crab species that are presumed to be functionally similar, Leptuca pugilator (sand fiddler crab) and Minuca pugnax (mud fiddler crab), to explore how species interactions may influence burrowing behavior. We ran manipulative mesocosm experiments to assess potential effects of species interactions on the number and location of burrows as well as the burrowing behavior of the crabs. Overall, sand fiddler crabs dug more burrows on average than mud fiddlers across all experiments while mud fiddlers were choosier about the location and sediment type of their burrows, suggesting complementarity in burrowing between the two species. Burrow counts in heterospecific treatments suggest that neither species interfered with nor enhanced burrow creation by the other species. However, increased burrow occupancy by mud fiddlers in the presence of sand fiddlers and aggressive behavior by mud fiddlers toward sand fiddlers suggest that mud fiddler crabs may receive refuge benefits from sand fiddler crabs through antagonistic rather than facilitative interactions. Thus, species-specific habitat use and interspecific interactions are likely influencing the role each species plays as a burrower and marsh ecosystem engineer.

Species Mixing Effects on Height–Diameter and Basal Area Increment Models for Scots Pine and Maritime Pine

Models that incorporate known species-mixing effects on tree growth are essential tools to properly design silvicultural guidelines for mixed-species stands. Here, we developed generalized height–diameter (h-d) and basal area growth models for mixed stands of two main forest species in Spain: Scots pine (Pinus sylvestris L.) and Maritime pine (Pinus pinaster Ait.). Mixed-effects models were fitted from plot measurement and tree rings data from 726 Scots pine and 693 Maritime pine trees from mixed and pure stands in the Northern Iberian Range in Spain, with the primary objective of representing interactions between the species where they are interspersed in mixtures of varying proportions. An independent dataset was used to test the performance of the h-d models against models previously fitted for monospecific stands of both species. Basal area increment models were evaluated using a 10-fold block cross-validation procedure. We found that species mixing had contrasting effects on the species in both models. In h-d models, the species-mixing proportion determined the effect of species interactions. Basal area growth models showed that interspecific competition was influential only for Maritime pine; however, these effects differed depending on the mode of competition. For Scots pine, tree growth was not restricted by interspecies competition. The combination of mixed-effect models and the inclusion of parameters expressing species-mixing enhanced estimates of tree height and basal area growth compared with the available models previously developed for pure stands. Although the species-mixing effects were successfully represented in the fitted models, additional model components for accurately simulating the stand dynamics of mixtures with Scots pine and Maritime pine and other species mixtures require similar model refinements. Upon the completion of analyses required for these model refinements, the degree of improvement in simulating growth in species mixtures, including the effects of different management options, can be evaluated.

Regularized S‐map for inference and forecasting with noisy ecological time series

AbstractIt is well known that fluctuations of species abundances observed in ecological time series emerge from an interplay between deterministic nonlinear dynamics and stochastic forces. Importantly, nonlinearity and stochasticity introduce significant challenges to the analysis of ecological time series, such as the inference of the effect of species interactions on community dynamics and forecasting of species abundances.Local linear fits with state‐space‐dependent kernel functions, known as S‐maps, provide an efficient method to infer Jacobian coefficients (a proxy for the local effect of species interactions) and to make reliable forecasts from nonlinear time series. Yet, while it has been shown that the S‐map outperforms existing methods for nonparametric inference and forecasting, the methodology is sensitive to process noise. To overcome this limitation, we integrate the S‐map with different regularization schemes.To validate our approach, we test our methodology against different levels of noise and nonlinearity using three standard population dynamics models. We show that an appropriate choice of the regularization scheme, alongside an accurate choice of the kernel functions, can significantly improve the in‐sample inference of Jacobian coefficients and the out‐of‐sample forecast of species abundances in the presence of process noise. We further validate our methodology using two empirical time series of marine microbial communities.Our results illustrate that the regularized S‐map is an efficient method for nonparametric inference and forecasting from noisy, nonlinear, ecological time series. Yet, attention must be paid on the regularization scheme and the structure of the kernel for whether inference or forecasting is the ultimate goal of a research study.

How do mixing tree species and stand density affect seasonal radial growth during drought events?

Forecasted climate change impacts on temperate forest ecosystems include increased summer drought. Forest managers can increase the resistance of forest stands against summer drought by reducing stand density and favoring tree species mixtures. These strategies have been studied separately, but their combined effect on increasing forest stand resistance to summer drought is unknown.The main objective of our study was to quantify tree species interaction effects on radial growth during a water stress period and to determine whether these effects changed with different levels of competition reflected by stand density.The study was based in the Orleans state forest (Central France) at a long-term triplet experimental site (OPTMix) with pure and mixed stands of mature Quercus petraea and Pinus sylvestris. The experimental design comprised three repetitions of two densities (low and medium) in each composition (pure oak, mixed stands, pure pine). We monitored tree radial growth with 216 manual dendrometers placed throughout 18 plots, on small, medium and large trees. We analyzed three consecutive years with contrasted water stress: no water stress, a summer stress period, and a late summer stress period.We found that mixture did not improve tree growth of the either species during the summer water stress period. On the other hand, there was a mixture effect during the late summer water stress period but only in medium-density stands inversely for the two species studied. More growth occurred for oaks in mixtures while, inversely, more growth occurred for pines in monocultures. A density effect occurred only for oaks, which grew more in lower-density stands than in medium-density stands. Finally, tree size did not influence seasonal resistance to drought.

Effect of Species Complementarity on Financial Return in Mixed Stands of European Beech and Scots Pine in Northern Spain

The research on mixed-species forestry has rapidly increased in recent decades because there is a growing interest in these types of stands for environmental reasons. Their positive influence on ecosystem biodiversity, stability and resilience, as well as their role in the new challenge brought up by the adaptation to global change, have been the object of many research works. However, the economic implications of mixed-species forest management have not deserved the same attention. The objective of this work is to study the effect of species interactions on productivity, and to economically assess this effect. This research is focused on the analysis of financial return and risk in even aged mixed stands of Pinus sylvestris and Fagus sylvatica in Northern Spain. Growth and yield projections for monospecific and mixed stands of Scots pine and European beech were made by means of a previous model developed from a set of the Spanish National Forest Inventory plots in the region of Navarre. Data from yield tables for both species were used. The effect of species proportion on total stand yield was assessed and transgressive overyielding was found for some mixing ratios. A data series on average stumpage price for both species in Spain over a 29-year period was compiled and the joint probability distribution of price data was used to generate 500 price scenarios. Different management alternatives based on species proportion and rotation age were considered and evaluated in terms of profitability and risk. Some management recommendations can be derived from the results obtained, which point at an optimum mixing ratio from 30% to 40% Scots pine and 70% to 60% European beech.

Long-term dim light during nighttime changes activity patterns and space use in experimental small mammal populations

Artificial light at night (ALAN) is spreading worldwide and thereby is increasingly interfering with natural dark-light cycles. Meanwhile, effects of very low intensities of light pollution on animals have rarely been investigated. We explored the effects of low intensity ALAN over seven months in eight experimental bank vole (Myodes glareolus) populations in large grassland enclosures over winter and early breeding season, using LED garden lamps. Initial populations consisted of eight individuals (32 animals per hectare) in enclosures with or without ALAN. We found that bank voles under ALAN experienced changes in daily activity patterns and space use behavior, measured by automated radiotelemetry. There were no differences in survival and body mass, measured with live trapping, and none in levels of fecal glucocorticoid metabolites. Voles in the ALAN treatment showed higher activity at night during half moon, and had larger day ranges during new moon. Thus, even low levels of light pollution as experienced in remote areas or by sky glow can lead to changes in animal behavior and could have consequences for species interactions.

Changes in the Contribution of Termites to Mass Loss of Dead Wood among Three Tree Species during 23 Months in a Lowland Tropical Rainforest

This study investigated the contribution of termites to mass loss of dead wood (Macaranga bancana, Elateriospermum tapos, and Dillenia beccariana) in a lowland tropical rainforest, Brunei Darussalam. Mesh bag method was used to exclude termites, and the mass remaining was monitored after 3, 7, 13, and 23 months. C/N ratio of the samples was analyzed after 13 and 23 months. Initial wood density was 0.63, 0.92, and 1.02 g/cm3 for M. bancana, E. tapos, and D. beccariana, respectively, and the termite contribution to mass loss was an average (range) of 13.05±5.68 (4.17-29.59%), 3.48±1.13 (2.20-6.49), and 3.40±1.92% (0.74-10.78), respectively. Until 7 months, termites contributed highly to mass loss, given the low initial wood density, and interaction effect of species and treatment was significant. After 7 months, the contribution decreased in M. bancana and E. tapos, whereas it increased consistently in D. beccariana. The interaction effect was not significant, whereas differences in C/N ratio among the species were significant, with a lower C/N ratio in M. bancana and E. tapos than in D. beccariana. After 23 months, the differences in C/N ratio were not significant, and ants were present at 40% of control samples in M. bancana and E. tapos. Our results suggest that the contribution of termites to mass loss varies by dead wood species and is temporally variable. Initial wood traits could affect the termite feeding in the beginning, however, termites thereafter could forage in response to the varying C/N ratio among species and predators.

Dome-shaped transition between positive and negative interactions maintains higher persistence and biomass in more complex ecological networks

Positive relationship between diversity and biomass was often observed by empirical experiments at the community level, but the effects of species interactions on community total biomass have been rarely explored from a network perspective. Weak interaction and specific non-monotonic interactions have been proposed to be stabilizing mechanisms in maintaining persistence in more complex ecosystems, but it is unclear how they contribute to the high level of biomass or productivity of these systems. In this study, we examined the effects of various interactions, specifically dome-shaped (shifting from positive to negative effect with increase of density) and satiated interactions, on biomass and biomass flow with the increase of complexity in theoretical networks. Our results indicated that, as compared to linear or satiated interactions, dome-shaped interactions maintained both higher persistence and biomass or biomass flow in more complex networks, but resulted in larger variations of species biomass. However, variation of network biomass was much smaller than that of species biomass. Our results suggest that species interaction shifting between mutualism at low density and competition or predation at high density could be a driving force for maintaining high levels of persistence, diversity, biomass and biomass flow in natural ecosystems.

An a posteriori species clustering for quantifying the effects of species interactions on ecosystem functioning

Abstract Quantifying the effects of species interactions is key to understanding the relationships between biodiversity and ecosystem functioning but remains elusive due to combinatorics issues. Functional groups have been commonly used to capture the diversity of forms and functions and thus simplify the reality. However, the explicit incorporation of species interactions is still lacking in functional group‐based approaches. Here, we propose a new approach based on an a posteriori clustering of species to quantify the effects of species interactions on ecosystem functioning. We first decompose the observed ecosystem function using null models, in which species diversity does not affect ecosystem function, to separate the effects of species interactions and species composition. This allows the identification of a posteriori functional groups that have contrasting diversity effects on ecosystem functioning. We then develop a formal combinatorial model of species interactions in which an ecosystem is described as a combination of co‐occurring functional groups, which we call an assembly motif. Each assembly motif corresponds to a particular biotic environment. We demonstrate the relevance of our approach using datasets from a microbial experiment and the long‐term Cedar Creek Biodiversity II experiment. We show that our a posteriori approach is more accurate, more efficient and more parsimonious than a priori approaches. The discrepancy between a priori and a posteriori approaches results from the way each clustering is set up: a priori approaches are based on ecosystem or species properties, such as ecosystem size (number of species or functional groups) or species’ functional traits, whereas our a posteriori approach is based only on the observed interaction and composition effects on ecosystem functioning. Our findings demonstrate that an a posteriori approach is highly explanatory: it identifies who interacts with whom, and quantifies the effects of species interactions on ecosystem functioning. They also highlight that a combinatorial modelling of ecosystem functioning can predict the functioning of an ecosystem without any hypothesis about the biotic or environmental determinants or any information on species functional traits. It only requires the species composition of the ecosystem and the observed functioning of others that share the same assembly motif.

Biotic interchange has structured Western Hemisphere mammal communities

AbstractAimMany hypotheses posit that species‐rich tropical communities are dominated by species–species interactions, apparent as competitive exclusion or character displacement, whereas species‐poor temperate communities are dominated by species–environment interactions. Recent studies demonstrate a strong influence of macroevolutionary and biogeographical factors. We simultaneously test for the effects of species interactions, climate and biotic interchange on Western Hemisphere mammal communities using a phylogenetic and functional diversity approach.LocationWestern Hemisphere.Time periodModern.Major taxa studiedMammalia.MethodsUsing Western Hemisphere mammal distributional and body mass data, we calculate body mass dispersion, phylogenetic diversity (Net Relatedness Index) and assemblage‐averaged rates of co‐occurrence (Checkerboard scores) in 100 km × 100 km grid cells under an equal area projection. We model body mass dispersion as a function of phylogenetic diversity, co‐occurrence rates and species richness, as well as mean annual temperature and precipitation. We infer rates of dispersal among the temperate and tropical zones of the Western Hemisphere using phylogenetic methods.ResultsThe dispersion of northern temperate mammal body masses is higher than null communities and shows correlated change with climate, consistent with resource competition and environmental filtering. Conversely, the dispersions of tropical and southern temperate mammal body masses are lower than and not differentiable from null expectations, respectively, suggesting a limited role of species–species and species–environment interactions at the grain of our analysis. Low tropical body mass dispersion and phylogenetic evenness are best explained by the high rates of faunal mixing. High rates of dispersal might also explain the similarity in community structure between the southern temperate and tropical zones.Main conclusionsMammal community assembly processes differ among the temperate and tropical zones of the Western Hemisphere, and faunal mixture during dispersal events, such as the Great American Biotic Interchange (Pliocene c. 3 Ma), may have been important in structuring Western Hemisphere mammal communities.

Direct and context‐dependent effects of light, temperature, and phytoplankton shape bacterial community composition

AbstractSpecies interactions, environmental conditions, and stochastic processes work in concert to bring about changes in community structure. However, the relative importance of specific factors and how their combined influence affects community composition remain largely unclear. We conducted a multi‐factorial experiment to (1) disentangle the direct and interaction‐mediated effects of environmental conditions and (2) augment our understanding of how environmental context modulates species interactions. We focus on a planktonic system where interactions with phytoplankton effect changes in the composition of bacterial communities. Further, light and temperature conditions can influence bacteria directly as well as through their interactions with phytoplankton. Epilimnetic bacteria from two humic lakes were combined with phytoplankton assemblages from each lake (home or away) or a no‐phytoplankton control and incubated for 5 d under all combinations of light (surface, ~25% surface irradiance) and temperature (five levels from 10° to 25°C). Observed light effects were primarily direct, while phytoplankton and temperature effects on bacterial community composition were highly interdependent. The influence of temperature on aquatic bacteria was consistently mediated by phytoplankton and most pronounced for bacteria incubated with “away” phytoplankton treatments, likely due to the availability of novel phytoplankton‐derived resources. The effects of phytoplankton on bacterial community composition were generally increased at higher temperatures. Incorporating mechanisms underlying the observed interdependent effects of species interactions and environmental conditions into modeling frameworks may improve our ability to forecast ecological responses to environmental change.

Applying community ecological theory to maximize productivity of cultivated biocrusts.

Degraded rangelands around the world may benefit from the reestablishment of lost biological soil crusts (biocrusts, soil surface cryptogamic-microbial communities). Cultivation of biocrust organisms is the first step in this process, and may benefit from harnessing species interactions. Species interactions are a dominant force structuring ecological communities. One key element of community structure, species richness, is itself important because it can promote the productivity of the entire community. Here, we use biological soil crusts as a model to test the effects of species interactions on production of biocrust materials for use in ecosystem rehabilitation. We screened eight different moss and lichen species from semiarid rangelands of Montana, USA, for growth potential under two watering regimes. Mosses generally grew well, but we were unable to cultivate the selected lichen species. We produced a >400% increase in the biomass of one species (Ceratodon purpureus). We tested whether a parasite-host relationship between two lichens could be used to enhance productivity of the parasite species, but this also resulted in no net gain of lichen productivity. Finally, we constructed all possible community combinations from a pool of five moss species to test for overyielding (community productivity exceeding that expected from the growth of community members in monoculture), and to determine both if, and the mode in which, species richness increases productivity. Polycultures yielded more than would be expected based upon the production of community constituents in monoculture. Using structural equationmodels, we determined that there was a modest effect of species richness on community productivity (r=0.24-0.25), which was independent of a stronger effect of the identity of species in the community (r=0.41-0.50). These results will contribute to the optimization of biocrust cultivation, promoting the development of this emerging ecological rehabilitation technology.

Effects of crown architecture and stand structure on light absorption in mixed and monospecificFagus sylvaticaandPinus sylvestrisforests along a productivity and climate gradient through Europe

AbstractWhen tree‐species mixtures are more productive than monocultures, higher light absorption is often suggested as a cause. However, few studies have quantified this effect and even fewer have examined which light‐related interactions are most important, such as the effects of species interactions on tree allometric relationships and crown architecture, differences in vertical or horizontal canopy structure, phenology of deciduous species or the mixing effects on tree size and stand density.In this study, measurements of tree sizes and stand structures were combined with a detailed tree‐level light model (Maestra) to examine the contribution of each light‐related interaction on tree‐ and stand‐level light absorption at 21 sites, each of which contained a triplet of plots including a mixture and monocultures ofFagus sylvaticaandPinus sylvestris(63 plots). These sites were distributed across the current distribution of these species within Europe.Averaged across all sites, the light absorption of mixtures was 14% higher than the mean of the monocultures. At the whole community level, this positive effect of mixing on light absorption increased as canopy volume or site productivity increased, but was unrelated to climate. At the species population or individual tree levels, the mixing effect on light absorption resulted from light‐related interactions involving vertical canopy structure, stand density, the presence of a deciduous species (F. sylvatica), as well as the effects of mixing on tree size and allometric relationships between diameter and height, crown diameter and crown length.The mixing effects on light absorption were only correlated with the mixing effects on growth forP. sylvestris, suggesting that the mixing effects on this species were driven by the light‐related interactions, whereas mixing effects onF. sylvaticaor whole community growth were probably driven by non‐light‐related interactions.Synthesis. The overall positive effect of mixing on light absorption was the result of a range of light‐related interactions. However, the relative importance of these interactions varied between sites and is likely to vary between other species combinations and as stands develop.

Heat tolerance predicts the importance of species interaction effects as the climate changes.

Few studies have quantified the relative importance of direct effects of climate change on communities versus indirect effects that are mediated thorough species interactions, and the limited evidence is conflicting. Trait-based approaches have been popular in studies of climate change, but can they be used to estimate direct versus indirect effects? At the species level, thermal tolerance is a trait that is often used to predict winners and losers under scenarios of climate change. But thermal tolerance might also inform when species interactions are likely to be important because only subsets of species will be able to exploit the available warmer climatic niche space, and competition may intensify in the remaining, compressed cooler climatic niche space. Here, we explore the relative roles of the direct effects of temperature change and indirect effects of species interactions on forest ant communities that were heated as part of a large-scale climate manipulation at high- and low-latitude sites in eastern North America. Overall, we found mixed support for the importance of negative species interactions (competition), but found that the magnitude of these interaction effects was predictable based on the heat tolerance of the focal species. Forager abundance and nest site occupancy of heat-intolerant species were more often influenced by negative interactions with other species than by direct effects of temperature. Our findings suggest that measures of species-specific heat tolerance may roughly predict when species interactions will influence responses to global climate change.

The effects of genome duplications in a community context

Contents 57 I. 57 II. 59 III. 59 IV. 63 V. 64 VI. 64 VII. 66 66 References 66 SUMMARY: Whole-genome duplication (WGD), or polyploidy, has important effects on the genotype and phenotype of plants, potentially altering ecological interactions with other organisms. Even though the connections between polyploidy and species interactions have been recognized for some time, we are only just beginning to test whether WGD affects community context. Here I review the sparse information on polyploidy and community context and then present a set of hypotheses for future work. Thus far, community-level studies of polyploids suggest an array of outcomes, from no changes in community context to shifts in the abundance and composition of interacting species. I propose a number of mechanisms for how WGD could alter community context and how the emergence of polyploids in populations could also alter the community context of parental diploids and other plant species. Resolving how and when these changes are expected to occur will require a deeper understanding of the connections among WGD, phenotypic changes, and the direct and indirect effects of species interactions.

Interaction between stress induced by competition, predation, and an insecticide on the response of aquatic invertebrates.

The present study investigated the effects of species interactions like competition and (intraguild) predation on the sensitivity of aquatic organisms to the insecticide chlorpyrifos. In the first experiment, combined effects of chlorpyrifos and different levels of intraspecific and interspecific interaction were assessed on Gammarus pulex survival using Asellus aquaticus as an interacting species. Intraspecific and interspecific interactions increased the time to extinction of G. pulex up to a factor of 2, most likely because of the cannibalistic nature of G. pulex and its intraguild predation on A. aquaticus under stress conditions. In the second experiment, combined effects of chlorpyrifos and intraspecific and interspecific interaction were assessed on Daphnia pulex abundance using Brachionus calyciflorus as a competing species and Chaoborus sp. larvae as a predator. Intraspecific and interspecific interactions significantly affected the D. pulex population structure, but they did not influence the total population size. Predation decimated D. pulex abundance; however, interacting effects of predation and chlorpyrifos exposure were less noticeable at high exposure concentrations because of the reduced predatory efficiency of Chaoborus sp. larvae. The present study shows that species interactions do not always increase the vulnerability of aquatic populations to chemical stress and that some interactions (e.g., cannibalism and intraguild predation) or reduced predator grazing pressure can alleviate competition and predation stress on population-level insecticide effects under food-limiting conditions. Environ Toxicol Chem 2017;36:2485-2492. © 2017 SETAC.

Aphid density and community composition differentially affect apterous aphid movement and plant virus transmission

1. Although many vector-borne pathogens are transmitted by an array of vector species, most studies do not account for the potential effects of species interactions. 2. By manipulating conspecific and heterospecific vector density in small experimental mesocosms, this study disentangled the impact of vector density and community composition on vector movement and plant virus transmission in the potato virus Y system. 3. The following predictions were tested: (i) increasing aphid density will increase aphid movement and virus transmission; (ii) adding low-efficiency vectors and thereby decreasing the average transmission efficiency of the vector assemblage will decrease virus transmission; and (iii) aphid movement and the average vector transmission efficiency will mediate the effect of aphid density and community composition on virus transmission. 4. It was found that initial density positively affected aphid movement, but had no effect on virus transmission, and that conspecific density was more important than heterospecific density. Conversely, community composition affected both aphid movement and virus transmission. These effects were driven by species identity, rather than species richness per se. 5. The results of this study emphasise the importance of accounting for vector behaviour, and analysing it within the context of the wider vector assemblage.