Parasitoid Food Webs Research Articles

Galling insect communities mediate the effects of fire on their associated parasitoid communities

Abstract Understanding fire effects on multitrophic levels is critical to the context of changes in fire regime and climate. Insects located at higher trophic levels, like parasitoids, are more vulnerable to habitat changes than insect herbivores because they need well‐established populations of their hosts to survive. Currently, fire effects on parasitoids and their interactions with their hosts are unknown. Our aim was to study the changes in abundance, richness and species composition, as well as the food web structure in a system involving parasitoids and galling insects under different fire scenarios. We asked whether potential changes in abundance, species richness and composition of parasitoid communities are explained by galled plant abundance, species richness and composition of galling insects, and how fire affects the structure of galling insect–parasitoid food webs. The highest parasitoid richness was found in the 9 years after fire scenario, whereas parasitoid abundance was not affected by fire. The parasitoid species composition in the 9 years after fire scenario was different from that in the unburned and 3 years after fire scenarios. Parasitoid communities were modulated by galled plant abundance, species richness and composition of galling insect communities. Vulnerability was significantly higher in 9 years after fire scenarios, but it increased with increasing network size. Fire affects the community of parasitoids through changes in their host communities. Differences in exclusivity to fire scenarios and diet specialisation of parasitoid species and galling insect richness may explain the patterns found. Our study supports evidence that fire creates habitats heterogeneous in the availability of hosts shaping the parasitoid communities, with a bottom‐up effect in food webs. Additionally, in burned areas were recorded unique galling insect–parasitoid interactions.

Composition and structure of winter aphid-parasitoid food webs along a latitudinal gradient in Chile.

All species interact in complex antagonistic or mutualistic networks that may be driven by turnover in species composition due to spatiotemporal environmental filtering. Therefore, studying differences in insect communities along environmental gradients may improve our understanding of the abiotic and biotic factors that shape the structure of trophic networks. Parasitoids are interesting models to do so, due to their intimate eco-evolutionary relationship with their hosts. We explored the differences in cereal aphid-parasitoid food webs during the winter among nine localities in Chilean central-south valley, along a gradient of 1200km from north (29° S) to south (40° S). We hypothesized that diapause incidence would increase in the coldest areas, resulting in a lower number of parasitoid species active during the winter. Consequently, network specialization, generality, and vulnerability indexes should increase with decreasing latitude, which implies fewer and more weakly connected links per parasitoid species through an increased fraction of basal host species. Based on the severity of winter, three areas along the explored gradient were distinguished, but clustering did not follow a clear north-south latitudinal gradient. Instead, few differences were observed in overwintering strategies, with very low levels of diapause in all localities, and no major differences in food-web composition. The major differences along the gradient were the relative abundances of the different aphid, parasitoid and hyperparasitoid species, with higher levels of spatial and temporal variation observed for the less abundant species. Our results provide a better understanding of the activity and abundance of aphid parasitoids during winter in relation to climatic conditions.

Specificity within bird–parasite–parasitoid food webs: A novel approach for evaluating potential biological control agents of the avian vampire fly

Abstract Quantitative food web analyses can provide insights into the specificity of consumers such as herbivores, parasites and parasitoids. Understanding such patterns can be useful in forecasting the potential benefits and risks of biological control agents being considered for introduction against invasive species. The avian vampire fly, Philornis downsi (Diptera: Muscidae), is a neotropical bird parasite that is invasive in the Galapagos Islands, where it is causing substantial mortality of endemic bird species. We used a novel in‐field experimental food web approach within the native range of P. downsi in Ecuador to test the hypotheses that pupal parasitoids known to attack P. downsi specialize on members of the genus Philornis, which occur only in bird nests. We deployed pupae of non‐Philornis fly species adjacent to bird nests to assess specificity of the parasitoids and used two indices to assess specificity: Resource Range (RR), which evaluates the breadth of host use, and Pair Difference Index (PDI), which evaluates interaction strength. The results revealed very strong compartmentalization within the guild of pupal fly parasitoids, with four species attacking only Philornis spp. Both specificity indices indicated significant levels of specificity towards the genus Philornis for two of these species: Conura annulifera and Trichopria sp. novus. We also assessed the specificity of the two dominant Philornis species attacking 11 bird species and preference of the two dominant parasitoid species for bird species. Although there was some significant preference for particular bird species by the Philornis spp., there was no indication that this drove specificity patterns by the parasitoids. Policy implications. Our results confirm previous laboratory studies indicating specificity by C. annulifera and support the hypothesis that this species would produce few, if any, nontarget impacts if released into Galapagos to suppress populations of the P. downsi. These results can inform an environmental risk assessment framework to guide governmental agencies in deliberation on potential field releases of parasitoids in the Galapagos Islands.

Defensive Symbionts and the Evolution of Parasitoid Host Specialization

Insect host-parasitoid interactions abound in nature and are characterized by a high degree of host specialization. In addition to their behavioral and immune defenses, many host species rely on heritable bacterial endosymbionts for defense against parasitoids. Studies on aphids and flies show that resistance conferred by symbionts can be very strong and highly specific, possibly as a result of variation in symbiont-produced toxins. I argue that defensive symbionts are therefore an important source of diversifying selection, promoting the evolution of host specialization by parasitoids. This is likely to affect the structure of host-parasitoid food webs. I consider potential changes in terms of food web complexity, although the nature of these effects will also be influenced by whether maternally transmitted symbionts have some capacity for lateral transfer. This is discussed in the light of available evidence for horizontal transmission routes. Finally, I propose that defensive mutualisms other than microbial endosymbionts may also exert diversifying selection on insect parasitoids.

Species diversity and food web structure jointly shape natural biological control in agricultural landscapes

Land-use change and agricultural intensification concurrently impact natural enemy (e.g., parasitoid) communities and their associated ecosystem services (ESs), i.e., biological pest control. However, the extent to which (on-farm) parasitoid diversity and food webs mediate landscape-level influences on biological control remains poorly understood. Here, drawing upon a 3-year study of quantitative parasitoid-hyperparasitoid trophic networks from 25 different agro-landscapes, we assess the cascading effects of landscape composition, species diversity and trophic network structure on ecosystem functionality (i.e., parasitism, hyperparasitism). Path analysis further reveals cascaded effects leading to biological control of a resident crop pest, i.e., Aphis gossypii. Functionality is dictated by (hyper)parasitoid diversity, with its effects modulated by food web generality and vulnerability. Non-crop habitat cover directly benefits biological control, whereas secondary crop cover indirectly lowers hyperparasitism. Our work underscores a need to simultaneously account for on-farm biodiversity and trophic interactions when investigating ESs within dynamic agro-landscapes.

Molecular analyses reveal consistent food web structure with elevation in rainforest Drosophila – parasitoid communities

The analysis of interaction networks across spatial environmental gradients is a powerful approach to investigate the responses of communities to global change. Using a combination of DNA metabarcoding and traditional molecular methods we built bipartite Drosophila – parasitoid food webs from six Australian rainforest sites across gradients spanning 850 m in elevation and 5°C in mean temperature. Our cost‐effective hierarchical approach to network reconstruction separated the determination of host frequencies from the detection and quantification of interactions. The food webs comprised 5–9 host and 5–11 parasitoid species at each site, and showed a lower incidence of parasitism at high elevation. Despite considerable turnover in the relative abundance of host Drosophila species, and contrary to some previous results, we did not detect significant changes to fundamental metrics of network structure including nestedness and specialisation with elevation. Advances in community ecology depend on data from a combination of methodological approaches. It is therefore especially valuable to develop model study systems for sets of closely‐interacting species that are diverse enough to be representative, yet still amenable to field and laboratory experiments.

Edge effects on insect–plant food webs: assessing the influence of geographical orientation and microclimatic conditions

1. Geographical orientation affects the magnitude of microclimatic edge effects, mainly by altering the exposure to solar radiation. There is considerable evidence available concerning the influence of microclimatic edge effects on organisms at the population level, although little is known about effects at the community level, and variations with edge geographical orientation have barely been addressed. The present study investigated the influence of microclimatic conditions at the forest interior and edges with different geographical orientation on plant–leafminer–parasitoid food webs in the fragmented Chaco Serrano forest located in Central Argentina.2. Microclimatic variables were measured in three locations: interior, north, and south‐facing edges of six forest fragments. In each location, plant resource availability was assessed and mined leaves were collected to investigate plant–leafminer–parasitoid communities by rearing leafminer and parasitoid adults.3. The results obtained revealed changes in the specific composition of plant, leafminer and parasitoid communities among locations, although they failed to detect location influences on most variables linked to abundance (except for leafminers), richness or food web structure. Leafminer abundance was negatively related to temperature, whereas their richness was related, also negatively, to moisture. Instead, parasitoids were favoured by higher temperatures and light levels, stronger winds, and lower humidity.4. The results of the present study indicate that the effects of microclimatic conditions may depend on the trophic level considered. Moreover, the findings suggest that possible trade‐offs between direct and indirect effects (such as plant quality, floral resources, etc.) of microclimatic conditions could influence plant and insect responses to location.

Facultative bacterial endosymbionts shape parasitoid food webs in natural host populations: A correlative analysis

Facultative bacterial endosymbionts can protect their aphid hosts from natural enemies such as hymenopteran parasitoids. As such, they have the capability to modulate interactions between aphids, parasitoids and hyperparasitoids. However, the magnitude of these effects in natural aphid populations and their associated parasitoid communities is currently unknown. Moreover, environmental factors such as plant fertilization and landscape complexity are known to affect aphid–parasitoid interactions but it remains unclear how such environmental factors affect the interplay between aphids, parasitoids and endosymbionts.Here, we tested whether facultative endosymbionts confer protection to parasitoids in natural populations of the English grain aphid, Sitobion avenae, and if this is affected by plant fertilization and landscape complexity. Furthermore, we examined whether the effects of facultative endosymbionts can cascade up to the hyperparasitoid level and increase primary‐hyperparasitoid food web specialization.Living aphids and mummies were collected in fertilized and unfertilized plots within 13 wheat fields in Central Germany. We assessed the occurrence of primary parasitoid, hyperparasitoid and endosymbiont species in aphids and mummies using a newly established molecular approach.Facultative endosymbiont infection rates were high across fields (~80%), independent of whether aphids were parasitized or unparasitized. Aphid mummies exhibited a significantly lower share of facultative endosymbiont infection (~38%). These findings suggest that facultative endosymbionts do not affect parasitoid oviposition behaviour, but decrease parasitoid survival in the host. Facultative endosymbiont infection rates were lower in mummies collected from fertilized compared to unfertilized plants, indicating that plant fertilization boosts the facultative endosymbiont protective effect. Furthermore, we found strong evidence for species‐specific and negative cascading effects of facultative endosymbionts on primary and hyperparasitoids, respectively. Facultative endosymbionts impacted parasitoid assemblages and increased the specialization of primary‐hyperparasitoid food webs: these effects were independent from and much stronger than other environmental factors.The current findings strongly suggest that facultative endosymbionts act as a driving force in aphid–parasitoid–hyperparasitoid networks: they shape insect community composition at different trophic levels and modulate, directly and indirectly, the interactions between aphids, parasitoids and their environment.

The simpler the better: When decreasing landscape complexity increases community stability

Herbivores and their predators are affected by changes in land-use and habitat fragmentation. Past studies of tri-trophic herbivore communities have found that increasing land-use intensity leads to declines in community stability. The majority of these studies analysed community stability in highly fragmented ecosystems characterised by intensive agriculture. In this study we considered how landscape configuration and composition affected habitat networks and parasitoid food webs under moderate but increasing land use. We used gall wasp communities as models to test the effects of landscape change on multi-species hierarchical communities of plants and animals. We investigated characteristics of networks formed by rose bushes and quantitative webs of rose gall parasitoids along a gradient of land-use intensity. We found that link density and compartmentalisation of rose bush networks, and local extinction within parasitoid webs increased with increasing landscape homogenization. Because these network and web characteristics are linked with resilience, our results suggest that stability of these communities can increase as landscapes become less complex. This is an intriguing aspect of landscape homogenisation effects on biological communities that contrasts with most expectations and the majority of the relevant literature, where decreasing community stability is usually associated with landscape homogenization.

Unravelling mummies: cryptic diversity, host specificity, trophic and coevolutionary interactions in psyllid \u2013 parasitoid food webs

BackgroundParasitoids are hyperdiverse and can contain morphologically and functionally cryptic species, making them challenging to study. Parasitoid speciation can arise from specialisation on niches or diverging hosts. However, which process dominates is unclear because cospeciation across multiple parasitoid and host species has rarely been tested. Host specificity and trophic interactions of the parasitoids of psyllids (Hemiptera) remain mostly unknown, but these factors are fundamentally important for understanding of species diversity, and have important applied implications for biological control.ResultsWe sampled diverse parasitoid communities from eight Eucalyptus-feeding psyllid species in the genera Cardiaspina and Spondyliaspis, and characterised their phylogenetic and trophic relationships using a novel approach that forensically linked emerging parasitoids with the presence of their DNA in post-emergence insect mummies. We also tested whether parasitoids have cospeciated with their psyllid hosts. The parasitoid communities included three Psyllaephagus morphospecies (two primary and, unexpectedly, one heteronomous hyperparasitoid that uses different host species for male and female development), and the hyperparasitoid, Coccidoctonus psyllae. However, the number of genetically delimited Psyllaephagus species was three times higher than the number of recognisable morphospecies, while the hyperparasitoid formed a single generalist species. In spite of this, cophylogenetic analysis revealed unprecedented codivergence of this hyperparasitoid with its primary parasitoid host, suggesting that this single hyperparasitoid species is possibly diverging into host-specific species. Overall, parasitoid and hyperparasitoid diversification was characterised by functional conservation of morphospecies, high host specificity and some host switching between sympatric psyllid hosts.ConclusionsWe conclude that host specialisation, host codivergence and host switching are important factors driving the species diversity of endoparasitoid communities of specialist host herbivores. Specialisation in parasitoids can also result in heteronomous life histories that may be more common than appreciated. A host generalist strategy may be rare in endoparasitoids of specialist herbivores despite the high conservation of morphology and trophic roles, and endoparasitoid species richness is likely to be much higher than previously estimated. This also implies that the success of biological control requires detailed investigation to enable accurate identification of parasitoid-host interactions before candidate parasitoid species are selected as biological control agents for target pests.

Complexity of leaf miner–parasitoid food webs declines with canopy height in Patagonian beech forests

1. Consumer–resource species interactions form complex, dynamic networks, which may exhibit structural heterogeneity at various scales. This study set out to address whether host–parasitoid food web size and topology vary across forest canopy strata, and to what extent foliar resources and species abundances account for vertical patterns in network structure.2. The vertical stratification of leaf miner–parasitoid food webs was examined in two monotypic beech (Nothofagus pumilio) forests in northern Patagonia, Argentina. Quantitative food webs were constructed for separate canopy layers by sampling foliage from three tree‐height classes at 0.5–1, 2–3 and 5–6 m above ground.3. Leaf miner abundance per unit leaf mass and foliar damage (%) did not differ across strata, although foliage quality and quantity increased from the understorey to the upper canopy. Parasitism rates and food web complexity decreased with canopy height, as reflected by reduced linkage richness, linkage density, mean interaction strength, and host vulnerability.4. Null model analyses revealed that food web metrics, especially in the upper canopy, were often lower than expected when compared with randomly structured networks. Overall, these patterns held for two forests differing in vertical structure and in dominant miner morphotype and parasitoid species.5. These results suggest that vertical declines in network complexity may be driven by the parasitoids' limited functional response to host abundance and dispersal from pupation sites in the forest floor. A broader constraint on food web structure seemed to be imposed by host–parasitoid trait matching, a reflection of large‐scale assembly processes.

Tracking the elusive history of diversification in plant-herbivorous insect-parasitoid food webs: insights from figs and fig wasps.

The food webs consisting of plants, herbivorous insects and their insect parasitoids are a major component of terrestrial biodiversity. They play a central role in the functioning of all terrestrial ecosystems, and the number of species involved is mind-blowing (Nyman et al. 2015). Nevertheless, our understanding of the evolutionary and ecological determinants of their diversity is still in its infancy. In this issue of Molecular Ecology, Sutton et al. (2016) open a window into the comparative analysis of spatial genetic structuring in a set of comparable multitrophic models, involving highly species-specific interactions: figs and fig wasps. This is the first study to compare genetic structure using population genetics tools in a fig-pollinating wasp (Pleistodontes imperialis sp1) and its main parasitoid (Sycoscapter sp.A). The fig-pollinating wasp has a discontinuous spatial distribution that correlates with genetic differentiation, while the parasitoid bridges the discontinuity by parasitizing other pollinator species on the same host fig tree and presents basically no spatial genetic structure. The full implications of these results for our general understanding of plant-herbivorous insect-insect parasitoids diversification become apparent when envisioned within the framework of recent advances in fig and fig wasp biology.

Food Web Designer: a flexible tool to visualize interaction networks.

Species are embedded in complex networks of ecological interactions and assessing these networks provides a powerful approach to understand what the consequences of these interactions are for ecosystem functioning and services. This is mandatory to develop and evaluate strategies for the management and control of pests. Graphical representations of networks can help recognize patterns that might be overlooked otherwise. However, there is a lack of software which allows visualizing these complex interaction networks. Food Web Designer is a stand-alone, highly flexible and user friendly software tool to quantitatively visualize trophic and other types of bipartite and tripartite interaction networks. It is offered free of charge for use on Microsoft Windows platforms. Food Web Designer is easy to use without the need to learn a specific syntax due to its graphical user interface. Up to three (trophic) levels can be connected using links cascading from or pointing towards the taxa within each level to illustrate top-down and bottom-up connections. Link width/strength and abundance of taxa can be quantified, allowing generating fully quantitative networks. Network datasets can be imported, saved for later adjustment and the interaction webs can be exported as pictures for graphical display in different file formats. We show how Food Web Designer can be used to draw predator–prey and host-parasitoid food webs, demonstrating that this software is a simple and straightforward tool to graphically display interaction networks for assessing pest control or any other type of interaction in both managed and natural ecosystems from an ecological network perspective.Electronic supplementary materialThe online version of this article (doi:10.1007/s10340-015-0686-7) contains supplementary material, which is available to authorized users.

Complementarity and redundancy of interactions enhance attack rates and spatial stability in host-parasitoid food webs.

Complementary resource use and redundancy of species that fulfill the same ecological role are two mechanisms that can respectively increase and stabilize process rates in ecosystems. For example, predator complementarity and redundancy can determine prey consumption rates and their stability, yet few studies take into account the multiple predator species attacking multiple prey at different rates in natural communities. Thus, it remains unclear whether these biodiversity mechanisms are important determinants of consumption in entire predator-prey assemblages, such that food-web interaction structure determines community-wide consumption and stability. Here, we use empirical quantitative food webs to study the community-wide effects of functional complementarity and redundancy of consumers (parasitoids) on herbivore control in temperate forests. We find that complementarity in host resource use by parasitoids was a strong predictor of absolute parasitism rates at the community level and that redundancy in host-use patterns stabilized community-wide parasitism rates in space, but not through time. These effects can potentially explain previous contradictory results from predator diversity research. Phylogenetic diversity (measured using taxonomic distance) did not explain functional complementarity or parasitism rates, so could not serve as a surrogate measure for functional complementarity. Our study shows that known mechanisms underpinning predator diversity effects on both functioning and stability can easily be extended to link food webs to ecosystem functioning.

Molecular analysis reveals high compartmentalization in aphid–primary parasitoid networks and low parasitoid sharing between crop and noncrop habitats

The ecosystem service of insect pest regulation by natural enemies, such as primary parasitoids, may be enhanced by the presence of uncultivated, semi-natural habitats within agro-ecosystems, although quantifying such host-parasitoid interactions is difficult. Here, we use rRNA 16S gene sequencing to assess both the level of parasitism by Aphidiinae primary parasitoids and parasitoid identity on a large sample of aphids collected in cultivated and uncultivated agricultural habitats in Western France. We used these data to construct ecological networks to assess the level of compartmentalization between aphid and parasitoid food webs of cultivated and uncultivated habitats. We evaluated the extent to which uncultivated margins provided a resource for parasitoids shared between pest and nonpest aphids. We compared the observed quantitative ecological network described by our molecular approach to an empirical qualitative network based on aphid-parasitoid interactions from traditional rearing data found in the literature. We found that the molecular network was highly compartmentalized and that parasitoid sharing is relatively rare between aphids, especially between crop and noncrop compartments. Moreover, the few cases of putative shared generalist parasitoids were questionable and could be due to the lack of discrimination of cryptic species or from intraspecific host specialization. Our results suggest that apparent competition mediated by Aphidiinae parasitoids is probably rare in agricultural areas and that the contribution of field margins as a source of these biocontrol agents is much more limited than expected. Further large-scale (spatial and temporal) studies on other crops and noncrop habitats are needed to confirm this.

Freezing cold yet diverse: dissecting a high-Arctic parasitoid community associated with Lepidoptera hosts

AbstractDespite increasing worldwide interest in host–parasitoid food webs, the parasitoid communities of the high Arctic remain poorly explored. To allow analyses of global patterns, and to detect the effects of ongoing climate change, such data are urgently needed. In this paper, we describe a systematic effort to characterise the high-Arctic Hymenoptera and Diptera parasitoid community associated with Lepidoptera hosts of the Zackenberg Valley (74°30′N, 21°00′W), northeast Greenland. Here, we first sampled adult parasitoids by a combination of Malaise traps, pitfall traps, and, less extensively, yellow pan traps and sweep netting. We then identified the host use of individual parasitoid taxa by rearing a large number of host individuals and species across multiple years. We now describe our preliminary findings on the species diversity of the target community, on trophic links between hosts and parasitoids, and on the sampling effort and techniques needed to characterise the community. We report on 30 local parasitoid taxa representing four families, three of which are species new to Greenland. In describing the community, we make a specific effort to summarise what is known about the taxonomy, phenology, and host use of the component species, to the benefit of future research in the area.

ECOLOGICAL VERSUS PHYLOGENETIC DETERMINANTS OF TROPHIC ASSOCIATIONS IN A PLANT-LEAFMINER-PARASITOID FOOD WEB

Specialized trophic interactions in plant-herbivore-parasitoid food webs can spur "bottom-up" diversification if speciation in plants leads to host-shift driven divergence in insect herbivores, and if the effect then cascades up to the third trophic level. Conversely, parasitoids that search for victims on certain plant taxa may trigger "top-down" diversification by pushing herbivores into "enemy-free space" on novel hosts. We used phylogenetic regression methods to compare the relative importance of ecology versus phylogeny on associations between Heterarthrinae leafmining sawflies and their parasitoids. We found that: (1) the origin of leafmining led to escape from most parasitoids attacking external-feeding sawflies; (2) the current enemies mainly consist of generalists that are shared with other leafmining taxa, and of more specialized lineages that may have diversified by shifting among heterarthrines; and (3) parasitoid-leafminer associations are influenced more by the phylogeny of the miners' host plants than by relationships among miner species. Our results suggest that vertical diversifying forces have a significant-but not ubiquitous-role in speciation: many of the parasitoids have remained polyphagous despite niche diversification in the miners, and heterarthrine host shifts also seem to be strongly affected by host availability.

Warming and nitrogen affect size structuring and density dependence in a host–parasitoid food web

Body size is a major factor constraining the trophic structure and functioning of ecological communities. Food webs are known to respond to changes in basal resource abundance, and climate change can initiate compounding bottom-up effects on food-web structure through altered resource availability and quality. However, the effects of climate and co-occurring global changes, such as nitrogen deposition, on the density and size relationships between resources and consumers are unknown, particularly in host-parasitoid food webs, where size structuring is less apparent. We use a Bayesian modelling approach to explore the role of consumer and resource density and body size on host-parasitoid food webs assembled from a field experiment with factorial warming and nitrogen treatments. We show that the treatments increased resource (host) availability and quality (size), leading to measureable changes in parasitoid feeding behaviour. Parasitoids interacted less evenly within their host range and increasingly focused on abundant and high-quality (i.e. larger) hosts. In summary, we present evidence that climate-mediated bottom-up effects can significantly alter food-web structure through both density- and trait-mediated effects.

Agricultural intensification and cereal aphid–parasitoid–hyperparasitoid food webs: network complexity, temporal variability and parasitism rates

Agricultural intensification (AI) is currently a major driver of biodiversity loss and related ecosystem functioning decline. However, spatio-temporal changes in community structure induced by AI, and their relation to ecosystem functioning, remain largely unexplored. Here, we analysed 16 quantitative cereal aphid–parasitoid and parasitoid–hyperparasitoid food webs, replicated four times during the season, under contrasting AI regimes (organic farming in complex landscapes vs. conventional farming in simple landscapes). High AI increased food web complexity but also temporal variability in aphid–parasitoid food webs and in the dominant parasitoid species identity. Enhanced complexity and variability appeared to be controlled bottom-up by changes in aphid dominance structure and evenness. Contrary to the common expectations of positive biodiversity–ecosystem functioning relationships, community complexity (food-web complexity, species richness and evenness) was negatively related to primary parasitism rates. However, this relationship was positive for secondary parasitoids. Despite differences in community structures among different trophic levels, ecosystem services (parasitism rates) and disservices (aphid abundances and hyperparasitism rates) were always higher in fields with low AI. Hence, community structure and ecosystem functioning appear to be differently influenced by AI, and change differently over time and among trophic levels. In conclusion, intensified agriculture can support diverse albeit highly variable parasitoid–host communities, but ecosystem functioning might not be easy to predict from observed changes in community structure and composition.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-012-2366-0) contains supplementary material, which is available to authorized users.

Food web structure and biocontrol in a four-trophic level system across a landscape complexity gradient

Decline in landscape complexity owing to agricultural intensification may affect biodiversity, food web complexity and associated ecological processes such as biological control, but such relationships are poorly understood. Here, we analysed food webs of cereal aphids, their primary parasitoids and hyperparasitoids in 18 agricultural landscapes differing in structural complexity (42-93% arable land). Despite little variation in the richness of each trophic group, we found considerable changes in trophic link properties across the landscape complexity gradient. Unexpectedly, aphid-parasitoid food webs exhibited a lower complexity (lower linkage density, interaction diversity and generality) in structurally complex landscapes, which was related to the dominance of one aphid species in complex landscapes. Nevertheless, primary parasitism, as well as hyperparasitism, was higher in complex landscapes, with primary parasitism reaching levels for potentially successful biological control. In conclusion, landscape complexity appeared to foster higher parasitism rates, but simpler food webs, thereby casting doubt on the general importance of food web complexity for ecosystem functioning.