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Floral plantings are often used in agriculture to attract pollinator communities, but they also play an important role in recruiting and establishing natural communities for natural pest control. Inconsistent effects of floral plantings for pest control may be a result of an absence of mechanistic insights and a reliance on the idea that simply increasing flower diversity will benefit these services. A more tailored set of flower species may be needed to benefit the natural enemies through provision of nectar and alternative prey. We used an outside pot experiment to investigate the effect of three flower plants (Fagopyrum esculentum, Vicia faba, and Trifolium pratense) on reducing aphid pests on four different plant cultivars of barley (Hordeum vulgare), over two years. We grew the four cultivars of barley alone, next to a single flower or next to a mixture of flowers, and observed aphid and natural enemy colonization across the growing season. Aphid population sizes were reduced on all barley cultivars grown next to a flower with stronger pest suppression when all flowers were present. Each flower species recruited a different community of non‐barley aphids that, in turn, varied in their ability to establish the natural enemy populations and subsequently the ability to reduce barley aphid populations. Overall, increased pest suppression in the mixed treatments was a result of numerous weaker interactions between different flower, aphid, and natural enemy species, rather than a few dominant interactions. Natural enemy communities could be enhanced by incorporating flower species that vary in their ability to attract and host alternative prey (i.e., non‐pest aphids) as well as suitable nectar provisioning. We can use our knowledge of ecological interactions to tailor floral plantings to increase the effectiveness of pest control services., Floral plantings are often used in agriculture to attract pollinator communities, but they also play an important role in recruiting and establishing natural communities for natural pest control. We used an outside pot experiment to investigate the effect of three flower plants (Fagopyrum esculentum, Vicia faba, and Trifolium pratense) on reducing aphid populations on potted barley plants (Hordeum vulgare), over two years. Each flower species recruited a different community of nonbarley aphids that, in turn, varied in their ability to establish the natural enemy populations and subsequently the ability to reduce barley aphid populations.

According to apparent competition theory, sharing a predator should cause indirect interactions among prey that can affect the structure and the dynamics of natural communities.Though shifts in prey dominance and predator resource use along environmental gradients are rather common, empirical evidence on the role of indirect prey–prey interactions through shared predation particularly with increasing productivity, is still scarce.In an 8‐week lake mesocosm experiment, we manipulated both the addition of inorganic nutrients and the presence of generalist fish predators (crucian carp, Carassius carassius L.), to test for the effects of indirect interactions through shared predation along a productivity gradient.We found that apparent mutualism (indirect positive interaction) between benthic and pelagic prey strongly affected short‐term responses of aquatic food webs to increasing productivity in the presence of a generalist fish. Increasing productivity favoured the relative abundance of benthic prey, following trends in natural productive lake systems. This led to a shift in fish selectivity from pelagic to benthic prey driven by changes in fish behaviour, which resulted in apparent mutualism due to the lower and delayed top‐down control of pelagic prey at increasing productivity.Our results show empirical evidence that the coupling of multiple production pathways can lead to strong indirect interactions through shared predation, whereby prey dynamics on short time‐scales are highly dependent on the foraging behaviour of generalist predators. This mechanism may play an important role in short‐term responses of food webs across environmental gradients., In an aquatic mesocosm study, the authors found that switching foraging in a generalist predator can lead to short‐term apparent mutualism between its benthic and pelagic prey. Such indirect prey–prey interactions had important consequences in prey community dynamics and strongly affected the food web response to increasing productivity.

Predicting coexistence patterns is a current challenge to understand diversity maintenance, especially in rich communities where these patterns' complexity is magnified through indirect interactions that prevent their approximation with classical experimental approaches. We explore cutting-edge Machine Learning techniques called Generative Artificial Intelligence (GenAI) to predict species coexistence patterns in vegetation patches, training generative adversarial networks (GAN) and variational AutoEncoders (VAE) that are then used to unravel some of the mechanisms behind community assemblage. The GAN accurately reproduces real patches' species composition and plant species' affinity to different soil types, and the VAE also reaches a high level of accuracy, above 99%. Using the artificially generated patches, we found that high-order interactions tend to suppress the positive effects of low-order interactions. Finally, by reconstructing successional trajectories, we could identify the pioneer species with larger potential to generate a high diversity of distinct patches in terms of species composition. Understanding the complexity of species coexistence patterns in diverse ecological communities requires new approaches beyond heuristic rules. Generative Artificial Intelligence can be a powerful tool to this end as it allows to overcome the inherent dimensionality of this challenge., The authors thank the Yesaires team for making the fieldwork of quantification of species gypsum affinity possible. R.S.-M. was supported by the Ministry of Science and Innovations (FPU grant FPU17/00629). Financial support was provided by the projects RTI2018-099672-J-I00 and PID2020-113157GB-I00 (funded by MCIN/AEI/10.13039/501100011033 and ‘ERDF A way of making Europe’).

Hyperparasitoids are some of the most diverse members of insect food webs. True hyperparasitoids parasitize the larvae of other parasitoids, reaching these larvae with their ovipositor through the herbivore that hosts the parasitoid larva. During pupation, primary parasitoids also may be attacked by pseudohyperparasitoids that lay their eggs on the parasitoid (pre)pupae. By attacking primary parasitoids, hyperparasitoids may affect herbivore population dynamics, and they have been identified as a major challenge in biological control. Over the past decades, research, especially on aphid- and caterpillar-associated hyperparasitoids, has revealed that hyperparasitoids challenge rules on nutrient use efficiency in trophic chains, account for herbivore outbreaks, or stabilize competitive interactions in lower trophic levels, and they may use cues derived from complex interaction networks to locate their hosts. This review focuses on the fascinating ecology of hyperparasitoids related to how they exploit and locate their often inconspicuous hosts and the insect community processes in which hyperparasitoids are prominent players. Expected final online publication date for the Annual Review of Entomology, Volume 67 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

It is known that biotic interactions are the key to species coexistence and maintenance of species diversity. Traditional studies focus overwhelmingly on pairwise interactions between organisms, ignoring complex higher-order interactions (HOIs). In this study, we present a novel method of calculating individual-level HOIs for trees, and use this method to test the importance of size- and distance-dependent individual-level HOIs to tree performance in a 25-ha temperate forest dynamic plot. We found that full HOI-inclusive models improved our ability to model and predict the survival and growth of trees, providing empirical evidence that HOIs strongly influence tree performance in this temperate forest. Specifically, assessed HOIs mitigate the competitive direct effects of neighbours on survival and growth of focal trees. Our study lays a foundation for future investigations of the prevalence and relative importance of HOIs in global forests and their impact on species diversity.

The magnet species hypothesis proposes that flowering plants that are attractive to pollinators can increase the relative pollination rates of neighbouring plants by acting as ‘magnets.’ Here, we test the hypothesis that insect-pollinated shrub species Larrea tridentata and wind-pollinated shrub species Ambrosia dumosa act as magnets for the pollinator visitation of understory annual plant species in an arid ecosystem. As an extension to the magnet species hypothesis, we propose the double magnet species hypothesis in which we further test for reciprocity by the floral island created in the understory of the benefactor shrubs as an additional pollinator magnet for the shrub itself. We used an annual plant placed near each shrub and the open to measure the effect of shrubs on annuals. The double magnet species hypothesis was tested using L. tridentata with and without surrounding annuals. We measured pollinator visitation and visit duration using video and in-situ observation techniques to test whether shrubs increase pollinator visitation to understory annual plants, if insect-pollinated shrubs act as better pollinator magnets than wind-pollinated shrubs (to determine the effects of the floral resource itself), and whether shrubs with annuals in their understory have higher pollinator visitation rates relative to shrubs without annuals. We found that insect-pollinated shrubs increased the visitation rate and duration of visits by pollinators to their understory plants and that wind-pollinated shrubs decreased the duration of visits of some insect visitors, but these relationships varied between years. While the presence of annuals did not change the visitation rate of all possible pollinators to L. tridentata flowers, they did decrease the visitation duration of specifically bees, indicating a negative reciprocal effect of the understory on pollination. Thus, the concentrated floral resources of flowers on insect-pollinated shrubs can act as a magnet that attract pollinators but that in turn provide a cost to pollination of the shrub. However, while wind-pollinated shrubs may provide other benefits, they may provide a cost to the pollination of their understory. These findings support the magnet species hypothesis as an additional mechanism of facilitation by insect-pollinated shrubs to other plant species within arid ecosystems.

According to classic theory, species interactions are less important than climatic variation for species’ population dynamics and distributions in climatically harsh regions compared with more climatically benign regions. In boreal ecosystems, the cyclic dynamics of rodents strongly affect many other species, including ground-nesting birds. According to the ‘alternative prey hypothesis’ (APH), high rodent densities positively affect the abundance of ground-nesting birds due to predator-mediated interactions. We analysed a spatiotemporal dataset of willow ptarmigan ( Lagopus lagopus ) and rodents to examine how the role of predator-mediated interactions changes along a climatic harshness gradient in comparison with climatic variation. Ptarmigan and rodent data were collected during a national program of line-transect sampling across Norway during 2007–2017. We build a hierarchical Bayesian model to estimate the sensitivity of ptarmigan populations to interannual variation in climate and rodent occurrence. Ptarmigan abundance was positively linked with rodent occurrence, consistent with the APH. Moreover, we found that both rodent dynamics and temporal climatic variation had stronger effects on ptarmigan in colder regions. Our study highlights how species interactions play an important role for the population dynamics of species at higher latitudes and suggest that they become even more important in the most climatically harsh regions.

Plants show ontogenetic variation in growth-defence strategies to maximize reproductive output within a community context. Most work on plant ontogenetic variation in growth-defence trade-offs has focussed on interactions with antagonistic insect herbivores. Plants respond to herbivore attack with phenotypic changes. Despite the knowledge that plant responses to herbivory affect plant mutualistic interactions with pollinators required for reproduction, indirect interactions between herbivores and pollinators have not been included in the evaluation of how ontogenetic growth-defence trajectories affect plant fitness. In a common garden experiment with the annual Brassica nigra, we investigated whether exposure to various herbivore species on different plant ontogenetic stages (vegetative, bud, or flowering stage) affects plant flowering traits, interactions with flower visitors, and results in fitness consequences for the plant. Effects of herbivory on flowering plants traits and interactions with flower visitors depended on plant ontogeny. Plant exposure in the vegetative stage to the caterpillar Pieris brassicae and aphid Brevicoryne brassicae led to reduced flowering time and flower production, and resulted in reduced pollinator attraction, pollen beetle colonization, total seed production and seed weight. When plants had buds, infestation by most herbivore species tested reduced flower production and pollen beetle colonization. Pollinator attraction was either increased or reduced. Plants infested in the flowering stage with P. brassicae or Lipaphis erysimi flowered longer, while infestation by any of the herbivore species tested increased the number of flower visits by pollinators. Our results show that the outcome of herbivore - flower-visitor interactions in B. nigra is specific for the combination of herbivore species and plant ontogenetic stage. Consequences of herbivory for flowering traits and reproductive output were strongest when plants were attacked early in life. Such differences in selection pressures imposed by herbivores to specific plant ontogenetic stages may drive the evolution of distinct ontogenetic trajectories in growth-defence-reproduction strategies and include indirect interactions between herbivores and flower visitors. Synthesis. Plant ontogeny can define the direct and indirect consequences of herbivory. Our study shows that the ontogenetic stage of plant individuals determined effects of herbivory on plant flowering traits, interactions with flower visitors, and plant fitness.

Interactions between established and recruiting plants play an important role in species coexistence in natural plant communities. However, our knowledge on the particular ecological drivers of these interactions is still limited. We use spatial point pattern analysis to study the spatial patterns of recruitment and infection in two plant-pathogen systems, each involving a fungus with a different life cycle: the pair Quercus faginea-Trabutia quercina and the triad Crataegus monogyna-Gymnosporangium sp.-Juniperus oxycedrus. Our results show that T. quercina, an autoecious fungus, may act as a stabilizing mechanism in the population dynamics of Q. faginea. In turn, the effect of the heteroecious Gymnosporangium sp. on C. monogyna recruitment was more related to distance from the alternate host J. oxycedrus than to distance from conspecifics. These results demonstrate that the complexity of pathogen life cycle may impact recruitment and the development of interspecific plant-plant interactions in real plant communities., This study was conducted under the projects COEXMED (CGL 2012-36776) and COEXMED II (CGL 2015-69118-C2-1-P) of the Spanish Ministerio de Economía y Competitividad (MEC) and FEDER funds from the EU. AJP was supported by a FPI grant from the Spanish Ministerio de Ciencia, Innovación y Universidades (MCIU/BES-2016-077688) associated to COEXMED II project.

Food webs and meso-scale motifs allow us to understand the structure of ecological communities and define species roles within them. This species-level perspective on networks permits tests for relationships between species traits and their patterns of direct and indirect interactions. Such relationships could allow us to predict food-web structure based on more easily obtained trait information. Here, we calculated the roles of species (as vectors of motif position frequencies) in six well-resolved marine food webs and identified the motif positions associated with the greatest variation in species roles. We then tested whether the frequencies of these positions varied with species traits. Despite the coarse-grained traits we used, our approach identified several strong associations between traits and motifs. Feeding environment was a key trait in our models and may shape species roles by affecting encounter probabilities. Incorporating environment into future food-web models may improve predictions of an unknown network structure. Funding Agencies|Formas grant [942-2015-1262]

Background Plant reproduction is influenced by the net outcome of plant–herbivore and plant–pollinator interactions. While both herbivore impacts and pollinator impacts on plant reproduction have been widely studied, few studies examine them in concert. Methodology Here, we review the contemporary literature that examines the net outcomes of herbivory and pollination on plant reproduction and the impacts of herbivores on pollination through damage to shared host plants using systematic review tools. The direct or indirect effects of herbivores on floral tissue and reported mechanisms were compiled including the taxonomic breadth of herbivores, plants and pollinators. Results A total of 4,304 studies were examined producing 59 relevant studies for synthesis that reported both pollinator and herbivore measures. A total of 49% of studies examined the impact of direct damage to floral tissue through partial florivory while 36% of studies also examined the impact of vegetative damage on pollination through folivory, root herbivory, and stem damage. Only three studies examined the effects of both direct and indirect damage to pollination outcomes within the same study. Conclusions It is not unreasonable to assume that plants often sustain simultaneous forms of damage to different tissues and that the net effects can be assessed through differences in reproductive output. Further research that controls for other relative drivers of reproductive output but examines more than one pathway of damage simultaneously will inform our understanding of the mechanistic relevance of herbivore impacts on pollination and also highlight interactions between herbivores and pollinators through plants. It is clear that herbivory can impact plant fitness through pollination; however, the relative importance of direct and indirect damage to floral tissue on plant reproduction is still largely unknown.

Indirect interactions play an essential role in governing population, community and coevolutionary dynamics across a diverse range of ecological communities. Such communities are widely represented as bipartite networks: graphs depicting interactions between two groups of species, such as plants and pollinators or hosts and parasites. For over thirty years, studies have used indices, such as connectance and species degree, to characterise the structure of these networks and the roles of their constituent species. However, compressing a complex network into a single metric necessarily discards large amounts of information about indirect interactions. Given the large literature demonstrating the importance and ubiquity of indirect effects, many studies of network structure are likely missing a substantial piece of the ecological puzzle. Here we use the emerging concept of bipartite motifs to outline a new framework for bipartite networks that incorporates indirect interactions. While this framework is a significant departure from the current way of thinking about bipartite ecological networks, we show that this shift is supported by analyses of simulated and empirical data. We use simulations to show how consideration of indirect interactions can highlight differences missed by the current index paradigm that may be ecologically important. We extend this finding to empirical plant–pollinator communities, showing how two bee species, with similar direct interactions, differ in how specialised their competitors are. These examples underscore the need to not rely solely on network‐ and species‐level indices for characterising the structure of bipartite ecological networks.

Predators can affect prey through mechanisms other than predation; for example, redistributing resources could modify habitats favorably for other organisms. We examined Arctic fox (Vulpes lagopus) den use by lemmings, their primary prey, in winter during a year of low lemming densities. We found winter nests, which are built by lemmings under snow, on 69% of fox dens, whereas no control sites had nests. In August, dens had twice the vegetation cover and 50% greater nitrogen content in grass than controls, suggesting soil enrichment by foxes increased food quantity and quality for herbivores. Snow was ~4 times thicker in April on dens than controls, and 1.4 times thicker on dens with lemming nests than dens without, suggesting lemmings choose thicker snow for thermal insulation. Snow cover thickness was positively related to vegetation cover on dens, but not on control sites. Thus, Arctic foxes not only prey on lemmings but also engineer productive habitat that attracts lemmings. During winters with low lemming densities, when foxes often leave the denning area and predation risk is lower, fox dens may provide a refuge that could buffer the effects of deteriorating snow conditions with Arctic warming. This additional mechanism of predators interacting with their prey illustrates how ecosystem engineers potentially alter food web interactions and highlights the importance of integrating these bodies of theory in attempts to understand community dynamics.

Food web models parameterised using body size show promise to predict trophic interaction strengths (IS) and abundance dynamics. However, this remains to be rigorously tested in food webs beyond simple trophic modules, where indirect and intraguild interactions could be important and driven by traits other than body size. We systematically varied predator body size, guild composition and richness in microcosm insect webs and compared experimental outcomes with predictions of IS from models with allometrically scaled parameters. Body size was a strong predictor of IS in simple modules (r(2)=0.92), but with increasing complexity the predictive power decreased, with model IS being consistently overestimated. We quantify the strength of observed trophic interaction modifications, partition this into density-mediated vs. behaviour-mediated indirect effects and show that model shortcomings in predicting IS is related to the size of behaviour-mediated effects. Our findings encourage development of dynamical food web models explicitly including and exploring indirect mechanisms. CC BY 4.0

Herbivore-induced plant resistance and apparent competition are two indirect ways herbivores interact. If a less damaging herbivore indirectly suppresses the abundance of a more damaging herbivore via these mechanisms, then plants may ultimately benefit. Changes in herbivore density, however, can dictate the intensity of species interactions and may play a critical role in determining the outcome of plant- and predator-mediated herbivore interactions. We tested the effects of herbivore density on the strength of indirect interactions among phloem-feeding aphids and herbivorous caterpillars and the outcome of these interactions for their shared host plant, cotton (Gossypium hirsutum). We quantified the survival of caterpillars on host plants that were infested with varying densities of aphids in the presence and absence of predators (ladybeetles). We found that aphids induced defensive proteins in cotton plants and that caterpillar survival was negatively affected by induced resistance. Likewise, we found that the presence of aphids increased predation of caterpillars by ladybeetles, but that apparent competition between aphids and caterpillars was density dependent. Ladybeetles consumed relatively high numbers of small caterpillars at low to intermediate aphid densities, but essentially became aphid specialists at high aphid densities. Aphid induced defenses and apparent competition combined such that plant damage by caterpillars was lowest when predators were present at low aphid density (induced resistance + highest level of apparent competition). This suggests that herbivores can benefit plants, but the effect on host plants is mediated by herbivore density. Indirect herbivore-plant mutualisms may increase plant quality, plant fitness, and yield of crop plants and these interactions need to be considered in ecologically based pest management plans. In addition, these interactions likely alter arthropod community structure and natural selection on anti-herbivore defense traits in plants in natural systems.

The study of species co-occurrences has been central in community ecology since the foundation of the discipline. Co-occurrence data are, nevertheless, a neglected source of information to model species distributions and biogeographers are still debating about the impact of biotic interactions on species distributions across geographical scales. We argue that a theory of species co-occurrence in ecological networks is needed to better inform interpretation of co-occurrence data, to formulate hypotheses for different community assembly mechanisms, and to extend the analysis of species distributions currently focused on the relationship between occurrences and abiotic factors. The main objective of this paper is to provide the first building blocks of a general theory for species co-occurrences. We formalize the problem with definitions of the different probabilities that are studied in the context of co-occurrence analyses. We analyze three species interactions modules and conduct multi-species simulations in order to document five principles influencing the associations between species within an ecological network: (i) direct interactions impact pairwise co-occurrence, (ii) indirect interactions impact pairwise co-occurrence, (iii) pairwise co-occurrence rarely are symmetric, (iv) the strength of an association decreases with the length of the shortest path between two species, and (v) the strength of an association decreases with the number of interactions a species is experiencing. Our analyses reveal the difficulty of the interpretation of species interactions from co-occurrence data. We discuss whether the inference of the structure of interaction networks is feasible from co-occurrence data. We also argue that species distributions models could benefit from incorporating conditional probabilities of interactions within the models as an attempt to take into account the contribution of biotic interactions to shaping individual distributions of species.

Indirect interactions between plant pathogens and phytophagous pests can occur when infestation by an attacker alters the shared host in a way that affects another attacker that could be spatially or temporally separated from the first one. The most commonly invoked scenario involves the induction of plant defence reactions which primarily depend on characteristics of both pathogens and herbivores considered. We carried out experiments under controlled conditions on young rose plants in order to assess the impact of such indirect interactions on life history traits of two pests on rose: a necrotrophic fungus, Botrytis cinerea, and an aphid, Rhodobium porosum. We highlighted (1) an indirect two-way negative interaction between B. cinerea and R. porosum which is expressed by a reduction of aphid growth rate and a decrease in growth speed of fungal lesions on the plants, and (2) a negative indirect effect of B. cinerea on insect's behaviour, as aphid preferred B. cinerea-free plants than B. cinerea-infested plants when given a choice. The implications of results in Integrated Pest Management on rose and mechanisms involved in the interactions are both discussed.

Facilitation of recruitment by 'nurse' plants can play a major role in harsh environments. Yet the extent to which consumer-mediated apparent competition from habitat-forming plants may counteract facilitative interactions remains largely unexplored. We examined whether seedling predation by tenebrionid beetles seeking refuge under nurse shrubs may prevent tree recruitment facilitation in a Patagonian steppe-woodland ecotone. Newly emerged seedlings of Austrocedrus chilensis were planted in shrub canopy, off-shrub shelter and bare soil microsites, and monitored for causes of early mortality and for overall survival under ambient and elevated beetle densities. Most seedlings in open microsites died from abiotic stress, whereas shrub cover and artificial shelters decreased desiccation mortality. Herbivory was the main cause of mortality in shrub microsites. Beetle addition increased predation beneath shrubs and in off-shrub shelters, indicating that apparent competition 'spilled over' from shrubs with high insect densities. Litter removal from shrubs prevented seedling predation suggesting that nurse plants altered recruitment by providing food as well as shelter to insects. Herbivory rates did not depend on seedling patch density but decreased with seedling age, with 1-week old plants being most vulnerable to beetle predation. Synthesis. Apparent competition from nurse plants can strongly reduce recruitment facilitation in stressful environments, although weak herbivore density dependence and seedling growth into ontogenetic refuges may limit the strength of apparent competition. An explicit consideration of negative, as well as positive, herbivore-mediated indirect effects from habitat-forming plants would broaden the understanding of community dynamics along stress gradients. © 2010 The Authors. Journal compilation © 2010 British Ecological Society. Fil: Chaneton, Enrique Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía; Argentina Fil: Mazía, Cristina Noemí. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Animal; Argentina Fil: Kitzberger, Thomas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones en Biodiversidad y Medioambiente. Universidad Nacional del Comahue. Centro Regional Universidad Bariloche. Instituto de Investigaciones en Biodiversidad y Medioambiente; Argentina

Benthic grazing strongly controls periphyton biomass. The question therefore arises whether benthic grazing could be used as a tool to reduce excessive growth of periphyton in nutrient-enriched rivers. Although benthic invertebrate grazers reduce the growth of periphyton, this is highly context dependent. Here we assessed whether the only obligate herbivorous fish in European rivers, the common nase (Chondrostoma nasus L.), is able to reduce periphyton biomass in a eutrophic river. We conducted three consecutive in situ experiments at low, intermediate and high densities of nase in the river using standard tiles on the river bottom naturally covered with periphyton that were accessible to fish and tiles that excluded fish foraging with electric exclosures. The biomass of benthic invertebrate grazers was very low relative to nase. We hypothesised that nase would reduce periphyton biomass on accessible tiles and therefore expected higher periphyton biomass on the exclosure tiles, at least at intermediate and high densities of nase in the river. Contrary to our expectation, the impact of fish grazing was low even at high fish density, as judged by the significantly lower chlorophyll a concentration on exclosure tiles even though the ash-free dry mass on accessible and exclosure tiles did not differ. The lower chlorophyll a concentrations on exclosure tiles might be explained by a higher biomass of invertebrate grazers on the exclosure tiles, which would indicate that the effect of invertebrate grazers was stronger than that of herbivorous fish grazers. The high biomass of invertebrate grazers on exclosure tiles likely arose from the exclusion of zoobenthivorous fish, which occur in the river at high densities. The results of our small-scale experiments suggested that cascading top-down effects of zoobenthivorous fish have a higher impact on periphyton biomass than direct effects of herbivorous nase.

Given the generalist tendencies of most predatory arthropods, it is widely thought that their impact on a particular prey species in a given habitat (e.g., an insect pest in a crop) will depend frequently on the local availability of other prey (which for omnivorous predators, can include plant resources, such as fruit and pollen). Thus, from a slightly different perspective, aphids, other herbivorous insects, and plants often may interact indirectly by sharing natural enemies. Such interactions may be either negative or positive, as in the concepts of apparent competition and apparent mutualism, and may therefore have variable impact on the herbi- vores' host plants as well. I examine the different mechanisms for such indirect effects among herbivores as explored in the experi- mental literature. An impressive collective effort by numerous researchers recently has expanded considerably our empirical base of support for a variety of hypothesized mechanisms; aphids stand out as the most commonly studied subjects in research on these mechanisms. I therefore focus especially on the recent literature of cases involving aphids interacting indirectly with other prey for generalist predators. My remarks are organized by considering how the availability of alternate prey may alter functional and numerical (aggregative and reproductive) responses of predators to focal prey density. Although the distinctions among these dif- ferent classes of predator responses and the associated indirect effects are often blurred and scale-dependent, this classification remains useful for organizing the diverse ways in which aphids have been found to participate in indirect interactions among prey as mediated by predators. Collectively, the results of the numerous studies reviewed here suggest that many such indirect interactions likely occur frequently in natural settings, with consequences ultimately for host plant performance.

7 páginas, 4 figuras, We studied possible interactions among invertebrate species and algal assemblages in a shallow sublittoral community of the Mediterranean Sea using a 2-step approach. First, we analysed the general pattern by correlation analysis. Thereafter, we experimentally assessed cause–effect relationships between the species or assemblages that were clearly related in the correlation study. The abundance of Cliona viridis (Schmidt 1862) was positively correlated with that of sea urchins and negatively correlated with fleshy algal cover. These relationships were confirmed by field experiments: treatments without fleshy algae, owing to both natural and simulated sea urchin grazing, promoted C. viridis growth significantly more than controls. Our results support the hypothesis that sea urchin grazing on seaweeds increases light availability at the basal stratum, which favours the primary production of the symbiotic zooxanthellae and thus C. viridis growth through nutrient transfer. We found a network of interactions with several signs, directions and strengths: a strong positive indirect interaction (‘facilitation’) between sea urchins and C. viridis, a negative direct interaction between sea urchins and seaweeds (‘predation’), a negative direct interaction between seaweeds and C. viridis (‘shading-interference’) and a weaker but significant direct interaction between C. viridis and Pione vastifica (‘space competition’). The multiple interactions observed suggest a cascade that involves 4 trophic levels: primary producers, herbivorous, carnivorous, and filterfeeders. This cascading process can have negative cryptic implications on the environment, as the excavating sponge C. viridis is a bio-eroder that may strongly impact the shallow sublittoral landscape by producing substrate weakening and instability. The knowledge acquired on the sign and strength of these multi-species interactions is useful to model and predict the responses of shallow benthic communities to anthropogenic disturbances (i.e. overfishing and eutrophication)., This research was partially funded by grants from the European Community (SPONGES project COOP-CT-205-017800) and the CICYT (Spain) (INTERGEN, CTM2004-05265-CO2/MAR) to M.J.U.

Co-flowering plant species commonly share flower visitors, and thus have the potential to influence each other's pollination. In this study we analysed 750 quantitative plant–pollinator networks from 28 studies representing diverse biomes worldwide. We show that the potential for one plant species to influence another indirectly via shared pollinators was greater for plants whose resources were more abundant (higher floral unit number and nectar sugar content) and more accessible. The potential indirect influence was also stronger between phylogenetically closer plant species and was independent of plant geographic origin (native vs. non-native). The positive effect of nectar sugar content and phylogenetic proximity was much more accentuated for bees than for other groups. Consequently, the impact of these factors depends on the pollination mode of plants, e.g. bee or fly pollinated. Our findings may help predict which plant species have the greatest importance in the functioning of plant–pollination networks

Specialist herbivores are often thought to benefit the larger plant community, because they prevent their host species from becoming competitively dominant. In contrast, specialist enemies are not generally expected to have negative impacts on non-hosts. However, we describe a cascade of indirect interactions whereby a specialist sooty mold (Scorias spongiosa) colonizes the honeydew from a specialist beech aphid (Grylloprociphilus imbricator), ultimately decreasing the survival of seedlings beneath American beech trees (Fagus grandifolia). A common garden experiment indicated that this mortality resulted from moldy honeydew impairing leaf function rather than from chemical or microbial changes to the soil. In addition, aphids consistently colonized the same beech trees regardless of host density, suggesting that seedling-depauperate islands may form beneath these trees. Thus this highly specialized three-way beech-aphid-fungus interaction has the potential to impact local forest regeneration via a cascade of indirect effects.

Using an experimental approach, this study addresses the following two questions for the perennial herb Paeonia broteroi (Paeoniaceae) at a location in southeastern Spain. (1) What are the relative magnitudes of the effects of pollinators and herbivores (invertebrates and vertebrates feeding on flowers and developing fruits) on maternal fe- cundity (total seed production per plant)? (2) Are the effects of pollinators and herbivores on fecundity additive, or is there some significant interaction between them? A two-factor (‘‘Pollinators’’ and ‘‘Herbivores,’’ each with two levels, ‘‘present’’ and ‘‘excluded’’) ex- perimental design was used, with individual plants being treated as experimental units and maternal fecundity as the response variable. On average, the effects of pollinators and herbivores were of opposite sign and roughly similar absolute magnitude (—7.5 seeds per plant), thus approximately canceling each other. A significant interaction (non-additivity) between factors did exist, with plants exposed to pollinators experiencing a disproportion- ately higher incidence of herbivores than those from which pollinators had been excluded. This was mainly due to mammals browsing only on the larger fruits from flowers that had been exposed to pollinators. Only in the absence of herbivores did pollinators account for a significant amount of between-plant variance in maternal fecundity. It is concluded that the interaction between pollinators and herbivores will lead to variable, herbivore-dependent ‘‘opportunity for selection’’ (sensu Arnold and Wade 1984) on P. broteroi by its pollinators. Regional variation in herbivore incidence experienced by P. broteroi populations will pre- sumably generate local variation in the degree of adaptedness of the plant to its pollinators.

Nutrient availability and soil quality influence herbivores through changes in plant traits and can have cascading effects on herbivore interactions. In complex systems, with many positive and negative interactions, the consequences of these bottom-up effects are still not well established. We carried out a set of studies to determine the impact of soil quality (organic compost amendments) on a hemipteran herbivore (Coccus viridis), two ant mutualists, predators, pathogens, parasitoids of C. viridis, and an arboreal arthropod community on coffee (Coffea arabica). We also determined the impact of Azteca instabilis ants on the arthropod community with an exclusion experiment. In an observational study, the carbon to nitrogen ratio (C:N) of leaf tissue correlated negatively with C. viridis density, however caffeine content did not correlate with C. viridis. In a field experiment with coffee seedlings, both C. viridis and total arthropod abundance were greater on high-quality plants than on low-quality plants. Excluding A. instabilis resulted in higher C. viridis abundance and parasitism rate, and higher spider and total arthropod abundance. Although A. instabilis attendance of C. viridis only marginally differed across soil quality treatments, in a second experiment, Pheidole synanthropica ants recruited more workers per C. viridis individual on highrelative to low-quality plants. Soil quality treatments did not impact predator abundance or fungal pathogen prevalence. These results suggest soil quality impacts C. viridis herbivores, P. synanthropica ants, and total abundance of arthropods on coffee, but did not impact the third trophic level. Thus this study provides a complex case study of pathways in which bottom-up effects influence arthropod interaction webs. © 2013 Gonthier et al.

8 páginas, 1 figura., Background. Plant facilitation occurs when the presence of a plant (i.e. a nurse plant) modifies the environment, making it more favourable for the establishment and survival of other species (i.e. facilitated plants), which can germinate and grow nearby. Facilitative associations can be maintained or turned into competition as the facilitated seedling grows. According to the competition-relatedness hypothesis that suggests that closely related species tend to compete more, facilitation turns into competition between phylogenetically close species. However, some examples of facilitation between congeneric species, which are supposed to be closely related species, have been found in nature., Scope. In this work, some examples of congeneric facilitation and subsequent coexistence are reviewed and an attempt is made to explain those exceptions to the competition-relatedness hypothesis., Conclusions. Two mechanisms are proposed that can switch the facilitation–competition balance: trait divergence and indirect interactions. When traits have diverged within the genus, the niche overlap is reduced and competition relaxed, thus allowing the coexistence of congeneric species. The presence of third interplayers (mycorrhizal fungi, seed dispersers, pollinators or pathogens) participating in the interaction between plants can alleviate the competition or enhance the reproduction and allow the coexistence of species that could not coexist in their absence., We thank Alicia Montesinos for her helpful comments on the manuscript. This work was funded by AECID (Projects A017475/08, A023461/09), MICINN (CGL2011-29585-C02- 01), DGAPA-UNAM (Projects IN-224808-3 and IN-202811-3) and CYTED (Acción 409AC0369).

Fil: Mazía, Noemí C. Universidad de Buenos Aires. Facultad de Agronomía; Argentina. Fil: Chaneton, Enrique J. Universidad de Buenos Aires (UBA). Facultad de Agronomía. IFEVA- CONICET; Argentina. Fil: Kitzberger, Thomas. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche. Laboratorio Ecotono-CONICET; Argentina. Fil: Garibaldi, Lucas Alejandro. Universidad Nacional del Comahue. Centro Regional Universitario Bariloche. Laboratorio Ecotono; Argentina. Fil: Garibaldi, Lucas Alejandro. Universidad de Buenos Aires. Facultad de Agronomía; Argentina. Predators are thought to play a key role in controlling herbivory, thus having positive indirect effects on plants. However, evidence for terrestrial trophic cascades is still fragmentary, perhaps due to variation in top-down forces created by environmental heterogeneity. We examined the magnitude of predation effects on foliar damage by chewing insects and mean leaf size, by excluding birds from saplings in ‘dry’ and ‘wet’ Nothofagus pumilio forests in the northern Patagonian Andes, Argentina.The experiment lasted 2 years encompassing a severe drought during the La Niña phase of a strong El Niño/Southern Oscillation event, which was followed by unusually high background folivory levels. Insect damage was consistently higher in wet than in dry forest saplings. In the drought year (1999), bird exclusion increased folivory rates in both forests but did not affect tree leaf size. In the ensuing season (2000), leaf damage was generally twice as high as in the drought year. As a result, bird exclusion not only increased the extent of folivory but also significantly decreased sapling leaf size.The latter effect was stronger in the wet forest, suggesting compensation of leaf area loss by dry forest saplings. Overall, the magnitude of predator indirect effects depended on the response variable measured. Insectivorous birds were more effective at reducing folivory than at facilitating leaf area growth. Our results indicate that bird-initiated trophic cascades protect N. pumilio saplings from insect damage even during years with above-normal herbivory, and also support the view that large-scale climatic events influence the strength of trophic cascades.

Identifying the determinants of biological interactions in mutualistic networks is key to understanding the rules that govern the organization of biodiversity. We used structural equation modeling and dissimilarities in nine ecological variables to investigate community processes underlying the turnover of species and their interaction frequencies (interaction pattern) among highly resolved plant-pollinator networks. Floral and pollinator community composition, i.e., species identities and their abundances, were strong determinants of the microstructure of pairwise interactions among the networks, explaining almost 69% of their variation. Flower and pollinator traits were directly related to interaction patterns, but were partly masked in the model by shared variance with community composition. Time of year and geographic location, floral and pollinator abundances independent of species identity, and relative abundance of exotic flowers had indirect and relatively weak effects on interaction patterns. Our analyses lead to precise predictions about the processes behind the interaction patterns in mutualistic networks. Future understanding of these processes will be aided by studies that evaluate these predictions experimentally at the network level. Fil: Kaiser Bunbury, Christopher N.. Ecological Networks; Alemania Fil: Vazquez, Diego P.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Provincia de Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Universidad Nacional de Cuyo. Instituto Argentino de Investigaciones de las Zonas Áridas; Argentina Fil: Stang, Martina. Leiden University; Países Bajos Fil: Ghazoul, Jaboury. Institute of Terrestrial Ecosystems; Suiza