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Plant species bearing extrafloral nectaries (EFNs) may indirectly influence other plant species by sharing protective ants, potentially altering herbivory levels. However, the propagation of indirect effects in this type of mutualism has seldom been investigated.We investigated indirect effects via ant sharing among 21 EFN‐bearing plant species of the tribe Bignonieae distributed on 28 plots in the central Brazilian Amazon. We used an ecological network index to quantify potential indirect effects among plant species through shared dominant and subordinate ant species.Plant species that were more attractive to ants (promoter species) had the highest potential to indirectly influence ant visitation to less attractive plant species (receptor species) in the community, primarily through changes in the attraction of dominant ant species. However, these potential indirect effects did not result in significant differences in herbivory patterns among the studied plants. Ant attendance and herbivory levels were similar among promoter species, neighbouring plants, and non‐neighbouring plants. Unlike other mutualisms involving plants (e.g., pollination), the indirect effects among plant species that share protective ants had limited consequences. The low levels of herbivory and the small foraging areas of shared ant defenders could be responsible for the observed limited impact of indirect interactions among plant species in this system.Synthesis: This study demonstrates that plant species with EFNs can indirectly influence other plant species through ant sharing. The attractiveness of certain plants to ants enables them to affect the ant visitation patterns of less attractive plants in the community. However, these indirect effects did not significantly alter herbivory levels among plants. Overall, this study advances ecological understanding by showing that the sharing of protective ants among plant species can in some cases confer limited costs and benefits, leading indirect interactions to have minimal impact on herbivory patterns. [ABSTRACT FROM AUTHOR]

As exotic plants invade into a new range, they can escape from specialist enemies. However, they may support generalist enemies, including both native and introduced fungal pathogens, which creates the potential for spillover and apparent competition from exotic to native plants in communities.To assess the potential for spillover of putatively pathogenic, root‐associated fungi (hereafter, 'pathogens') in communities invaded by exotic plants, we conducted a two‐phase plant–soil feedback experiment: a monoculture experiment with native and exotic plants grown alone and a multi‐species, community‐level experiment that ranged in the extent of exotic dominance. We used next‐generation sequencing to characterise sharing of pathogens between native and exotic plants in communities.Exotic plants outperformed natives in communities, despite harbouring higher relative abundance of generalist pathogens. The higher generalism of pathogens supported by exotic plants made them more prone to be shared with natives. The proportion of pathogens shared between exotic and native plants in communities correlated with reduced competitive ability of native compared with exotic plants.Synthesis: These data suggest that exotic plants host more generalist pathogens that are shared with native plants, which may confer an indirect benefit to exotic over native plants through apparent competition. [ABSTRACT FROM AUTHOR]

Interactions among species are a fundamental aspect of biodiversity and drive ecosystem functioning and services. Species interactions include direct (pairwise) interactions among two species and indirect interactions that occur when a third species interacts and changes the pairwise direct interaction. In a three‐species interaction network, these interactions can be transitive (where one species outperforms all others) or intransitive (where each species outperforms another). Here, we investigate how direct and indirect interactions influence ecosystem functions in crop systems and how diversification and evolutionary adaptation can influence those interactions and therefore ecosystem functions.A common garden experiment was conducted with crop communities in monocultures, 2‐ and 3‐species mixtures that had either a common or no coexistence history (i.e. co‐adaptation) for the three previous years. Net, direct and indirect interaction intensities were estimated and compared between the diversity levels and coexistence histories. Furthermore, species interaction networks were inspected for transitive/intransitive interactions.We found evidence for less intense competition in mixtures and for reduced negative direct interaction intensity and enhanced facilitative effects upon co‐adaptation. We could further show that indirect interactions were generally less important for co‐adaptation than direct interactions. Additionally, we showed that co‐adaptation has the potential to shift interactions in the species interaction networks from competitive intransitive into pairwise competitive interactions where interactions occurred mainly between two species.Synthesis. Co‐adapted crop species with reduced negative interactions might have the potential to enhance productivity, especially in more diverse cropping systems. This supports the notion that intercropping is a vital part towards a more sustainable agriculture and one with further yield potential when developing cultivars optimised for growth in mixtures. [ABSTRACT FROM AUTHOR]

The biodiversity–ecosystem function literature provides a useful framework to examine many processes associated with species diversity in ecology. One such context is the maintenance of biodiversity by facilitation in arid ecosystems.Here, we examined the complex interactions between local plant species richness and the intensity of shrub facilitation for maintaining biodiversity in arid plant communities.A synthesis including a meta‐analysis was used to compile nearly 600 papers on positive interactions mediated by shrubs in dryland plant communities (search terms: shrub, positive, facilitat*) to examine whether interactions in these studies changed with reported local species richness. A total of 19 studies and 141 independent instances directly examined and reported facilitation of diversity measures in naturally assembled plant communities and provided estimates of local species richness.Synthesis. The net effect of increasing local plant species richness was negative and shifted the relative frequency of interactions with shrubs from positive to negative with increasing local species richness. This relationship suggests that increases in richness do not always enhance functions that maintain diversity in plants communities likely due to concurrent increases in the indirect negative interactions between species under shrubs or in changes in the local species pool. [ABSTRACT FROM AUTHOR]

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 plant 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 the effects of herbivory on plant flowering traits, interactions with flower visitors and plant fitness. [ABSTRACT FROM AUTHOR]

Classic theory holds that the main interaction within the herbivore guild is competition, based on research focused on co‐occurring, similarly sized species that reduce the quantity of shared plant resources. However, plant quality may also be crucial in mediating herbivore interspecific interactions. This is especially true when competition occurs between distantly related herbivore species, given that small terrestrial herbivores (e.g., insect herbivores) appear to be more sensitive to alterations of plant quality than plant quantity.In this study, we first tested in the field whether large vertebrate herbivores (cattle Bos taurus) exerted a negative effect on smaller insect herbivores (grasshopper Euchorthippusunicolor) through their overlapping foraging preferences for a dominant grass Leymus chinensis. We measured changes in grass quantity, grass quality, and microclimatic conditions in response to vertebrate grazing and conducted additional manipulative studies in the field and the laboratory to identify potential mechanisms underlying the interaction.Our results showed that grazing by large herbivores caused a significant decline in grasshopper population density and individual performance (survival, size, and weight of both female and male E. unicolor), despite a 38% increase in grass nitrogen (N) content in grazed plots. Experiments manipulating N levels of L. chinensis in the field and the laboratory confirmed that enriching plant N had a negative effect on grasshopper individual performance and population size. Therefore, enhanced quality (N content) of plant resources appears to be an important driver in mediating the negative effect of vertebrate grazing on grasshoppers.Synthesis. We document that phylogenetic relatedness and trait similarity can be poor predictors of interaction strength in some cases, since distantly related herbivores of disparate size can interact indirectly via changes in plant quality. Counter‐intuitively, the observed negative effect of cattle on grasshoppers was mediated, at least in part, by an increase in plant quality in cattle grazed areas. The implication is that light to moderate grazing, a common management strategy, may contribute to suppression of grasshoppers in the Eurasian steppe grassland system by altering plant nutrient supplies. Our results showed that grazing by large herbivores caused a significant decline in grasshopper population density and individual performance, despite an increase in grass nitrogen content in grazed plots. Experiments manipulating N levels of grass in the field and the laboratory confirmed that enriching plant N had a negative effect on grasshopper individual performance and population size. We document that phylogenetic relatedness and trait similarity can be poor predictors of interaction strength in some cases, since distantly related herbivores of disparate size can interact indirectly via changes in plant quality. [ABSTRACT FROM AUTHOR]

Seed dispersal is a primary driver, via propagule pressure, of biological invasions. In animal‐dispersed plants, theory predicts that indirect ecological interactions at the neighbourhood scale may determine landscape‐scale patterns of seed dispersal and predation, and thus invasion.Indirect interactions in seed dispersal may arise between two plant species if shared seed dispersal agents adjust their foraging decisions to the relative frequencies of co‐occurring seeds. We used seed‐tracking experiments and empirically parameterized simulations to test the hypothesis that an indirect interaction between two plant species, mediated by scatterhoarding rodents that both consume and disperse seeds, affects recruitment of an invasive tree. We predicted that acorns of Quercus rubra (invasive in Central Europe) would experience higher caching (and therefore survival) rates when they co‐occurred with native Quercus petraea than when they occurred in isolation, that Q. petraea acorns would survive at lower rates with Q. rubra relative to isolation, and that this interaction would facilitate Q. rubra seedling recruitment in Q. petraea forests.Contrary to expectations, the presence of Q. petraea reduced caching of Q. rubra acorns, and co‐occurrence with Q. rubra increased caching of Q. petraea. Thus, a rodent‐mediated indirect interaction may be slowing rather than facilitating the Q. rubra invasion. However, our simulations indicate that the magnitude of this indirect interaction will change nonlinearly as Q. rubra becomes more dominant on the landscape, and that the pace of invasion will accelerate as the alien species is released from the rodent‐mediated effect of Q. petraea.Synthesis. The presence of native Quercus petraea decreases recruitment rates of Quercus rubra, and consequently, slows down the invasion process. The strong dependence of pairwise interactions on the presence of a third‐party species emphasizes the role of indirect interactions in the invasion processes. Seed dispersal is a primary driver, via propagule pressure, of biological invasions. We showed that "apparent predation" between native and invasive oak, mediated by scatterhoarding seed disperser, occurs in our system. The presence of native Quercus petrea decreases recruitment rates of Quercus rubra, and consequently, slows down the invasion process. However, strength of this interaction is predicted to gradually vanish as native plants are displaced from communities. [ABSTRACT FROM AUTHOR]

1. Interactions among neighbouring plants are often mediated by foraging choices of pollinators. For example, the presence of a conspicuous species may increase the number of pollinators attracted to its vicinity, indirectly increasing visitation rates also to neighbouring plants. Because pollinator choices are frequently density-dependent, the presence of a conspicuous species at high densities may also increase competition for pollination services. Additionally, models predict that plant density will interact with spatial distribution in manipulating the pollinator behaviour, yet experimental evidence for this effect is missing. 2. We performed a field experiment in which we introduced a highly conspicuous species in different densities and spatial configurations in a full-factorial manner into a species-rich meadow and studied its effect on neighbouring plants. 3. We showed that the highly conspicuous species strongly contributed to the attractiveness of its local patch and thus benefited its neighbours. However, because of the strong density effect, the conspicuous species changed its role and became a competitor for pollinators when its density increased. 4. We supported our theoretical assumptions and showed that when the introduced conspicuous species was regularly distributed among other plants in the patch, it increased visitation rate, and in some cases also seed set, to conspicuous neighbours relative to when it was aggregated, at least at low densities. 5. Synthesis. We suggest that complex interactions between density and spatial distribution of plant species at the patch scale are highly relevant for the interpretation of pollinator behaviour and therefore should be treated as factors of floral attractiveness in future studies. [ABSTRACT FROM AUTHOR]

Community context can alter the likelihood of interactions among community members and thus exert critical ecological effects with potential evolutionary implications. For instance, plant-animal mutualisms can be exploited by third species that usurp the resources and/or service that the mutualists offer, while delivering limited or no benefits in return., We experimentally revealed for the first time how exploiters of plant-disperser mutualisms (i.e. pulp feeders) alter the frequency of plant interactions with subsequent mutualistic (seed dispersers) and antagonistic (seed predators) animal associates. In doing so, we chose to study the endozoochore Pyrus bourgaeana, which interacts with a diverse assemblage of frugivores including exploiters (pulp-feeding rabbits), legitimate seed dispersers (mammalian carnivores) and seed and fruit predators (rodents and deer, respectively). We hypothesized that pulp feeders would render fruit barely rewarding, affecting subsequent tree-animal interactions., As predicted, pulp removal lessened tree dispersal success (i.e. lowered interaction frequency with seed dispersers) causing an indirect negative effect on its fitness. Furthermore, pulp feeders facilitated foraging by seed-eating rodents, leading to a negative indirect effect on seed survival. Nonetheless, these negative effects of pulp removal on tree fitness were partly counterbalanced by a noticeable decrease in fruit predation by deer. Because both seed dispersers and seed predators preferred large fruits, they exerted selection pressures on fruit size in opposite directions; thus, the net selection regime on fruit size experienced by the tree appeared largely contingent on community composition., Synthesis. Our results illustrate how interactions among functionally distinct frugivores can act synergistically or antagonistically and thus alter their ecological outcomes in ways that differ from those predicted by pairwise interactions. Further research on the relationships between fruiting plants and their consumers will certainly further our understanding of how community context can modify ecological and evolutionary outcomes of complex multispecies interactions. [ABSTRACT FROM AUTHOR]

Invasive alien plant species pose a severe threat to native plant communities world-wide, especially on islands. While many studies focus on the direct impact of alien plants on native systems, indirect effects of plant invaders on co-flowering natives, for example through competition for pollination services, are less well studied and the results are variable. We used six temporally and taxonomically highly resolved plant-pollinator networks from the island of Mahé, Seychelles, to investigate the indirect impact of invasive alien plant species on remnant native plant communities mediated by shared pollinators. We employed fully quantitative network parameters and information on plant reproductive success, and pollinator diversity and behaviour, to detect changes in plant-pollinator networks along an invasion gradient. The number of visits to and fruit set of native plants did not change with invasion intensity. Weighted plant linkage and interaction evenness, however, was lower at invaded sites than at less invaded sites. These patterns were primarily driven by shifts in interactions of the most common pollinator, the introduced honey bee Apis mellifera, while weak interactions and strong native interactions remained unchanged. Synthesis. The implications of these findings are twofold: first, quantitative network parameters are important tools for detecting underlying biological patterns. Secondly, alien plants and pollinators may play a greater role in shaping network structure at high than low levels of invasion. We could not show, however, whether the presence of invasive plants result in a simplification of plant-pollinator networks that is detrimental to native plants and pollinators alike. [ABSTRACT FROM AUTHOR]

1. 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. 2. 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. 3. 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. 4. 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. 5. 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. [ABSTRACT FROM AUTHOR]

1. We investigated the role of a native generalist soil pathogen through which a non-native invasive plant species may suppress naturalized/native plant species. 2. We found that rhizosphere soils of Chromolaena odorata, one of the world's most destructive tropical invasive weeds, accumulate high concentrations of the generalist soil borne fungi, Fusarium (tentatively identified as F. semitectum), thus creating a negative feedback for native plant species. 3. Soils collected beneath Chromolaena in the Western Ghats of India inhibited naturalized/native species and contained over 25 times more spores of the pathogenic fungi Fusarium semitectum than soils collected at the same locations beneath neighbouring native species that were at least 20 m from any Chromolaena plant. Sterilization of these soils eliminated their inhibitory effect. Chromolaena root leachate experimentally added to uninvaded soils increased Fusarium spore density by over an order of magnitude, and increased the inhibitory effect of the soils. 4. The positive effect of Chromolaena root leachates on Fusarium spores was attenuated by activated carbon, suggesting a biochemical basis for how the invader stimulated the pathogen. 5. Synthesis. Invasive plants have been shown to escape inhibitory soil biota in their native range and to inhibit soil biota in their invaded range, but our results indicate that the impacts of Chromolaena are due to the exacerbation of biotic interactions among native plants and native soil biota, which is to our knowledge a new invasive pathway. [ABSTRACT FROM AUTHOR]