One of the greatest challenges in contemporary ecology is to understand how the homogenization of biodiversity at all levels of organization and spatial scales will influence the assembly of communities and the functioning of ecosystems. Such homogenization can occur through the gain of non-native species and the loss of native species. Here, we show that by disrupting a keystone mutualistic interaction, non-native ungulates indirectly impact foliar arthropod abundance and richness, but not soil properties (soil respiration, temperature and humidity), in a temperate forest of Patagonia. The results of this study show that the gain of non-native ungulates and the loss of a key interaction can trigger unnoticed cascading effects. Our findings highlight the importance of assessing biodiversity not only as the sum of different components but also through the direct and indirect interactions among them.

Plant defense against pathogens includes a range of mechanisms, including, but not limited to, genetic resistance, pathogen-antagonizing endophytes, and pathogen competitors. The relative importance of each mechanism can be expressed in a hierarchical view of defense. Several recent studies have shown that pathogen antagonism is inconsistently expressed within the plant defense hierarchy. Our hypothesis is that the hierarchy is governed by contingency rules that determine when and where antagonists reduce plant disease severity.Here, we investigated whether pathogen competition influences pathogen antagonism using Populus as a model system. In three independent field experiments, we asked whether competition for leaf mesophyll cells between a Melampsora rust pathogen and a microscopic, eriophyid mite affects rust pathogen antagonism by fungal leaf endophytes. The rust pathogen has an annual, phenological disadvantage in competition with the mite because the rust pathogen must infect its secondary host in spring before infecting Populus. We varied mite-rust competition by utilizing Populus genotypes characterized by differential genetic resistance to the two organisms. We inoculated plants with endophytes and allowed mites and rust to infect plants naturally.Two contingency rules emerged from the three field experiments: (a) Pathogen antagonism by endophytes can be preempted by host genes for resistance that suppress pathogen development, and (b) pathogen antagonism by endophytes can secondarily be preempted by competitive exclusion of the rust by the mite. Synthesis: Our results point to a Populus defense hierarchy with resistance genes on top, followed by pathogen competition, and finally pathogen antagonism by endophytes. We expect these rules will help to explain the variation in pathogen antagonism that is currently attributed to context dependency.

Intraspecific variation can be an important driver of ecological interactions in species‐rich communities. Predicting the effects of intraspecific variation in different environments, however, remains a major challenge. This is because we often do not quantify both the effects of functional traits on associated communities and the extent to which trait variation is due to genetics (genotype effects) vs. plasticity (environment effects). As a consequence, the relative importance of trait plasticity vs. genetic variation in structuring associated communities remains unclear. We sought to fill this gap by conducting common garden experiments with the plant Salix hookeriana across biotic (ant–aphid interactions) and abiotic (wind exposure) environmental gradients in a coastal dune ecosystem. In each experiment, we simultaneously measured plant traits and species richness of associated above‐ and below‐ground communities. We then used statistical models to quantify the relative importance of trait plasticity vs. genetic variation in structuring communities. Our major finding was that trait plasticity was more important than genetic variation in determining the number of species in associated communities. This result was consistent across different environmental contexts (experimental manipulations of ant–aphid interactions and wind exposure), multiple years, and for above‐ground arthropods and root microbes. This occurred because the traits that had the largest effect on species richness were also the most plastic. Synthesis. These results indicate that trait plasticity can be a dominant driver of above‐ and below‐ground biodiversity.

Summary Competition and herbivory are ubiquitous processes known to interactively shape plant performance, distribution and community assembly. Likewise, plant genetic variation and associated trait differences can impact both plant–plant and plant–herbivore interactions individually, yet few studies have explored these interactions simultaneously. Saltmarsh communities are an ideal system to study these questions, as they are dominated along the Atlantic and Gulf coasts of the United States by a foundation plant species, Spartina alterniflora, with high levels of genetically based trait variation. Furthermore, consumer pressure and plant competitors are known to influence both the distribution and production of Spartina. We manipulated Spartina genetic identity, neighbour identity (needlerush, Juncus roemerianus) and consumer presence (snail, Littoraria irrorata) in a 1‐year field experiment in St. Joseph Bay, FL, to test how the strength and direction of neighbour and consumer interactions vary by plant genotype. Consumer effects on Spartina were generally stronger and more variable than those of the plant neighbourhood, and these effects were generally consistent across Spartina genotypes. However, genotype‐specific variation in morphology, phenology and palatability significantly influenced both consumer and neighbour responses to Spartina: the consumer Littoraria was more likely to climb palatable Spartina genotypes, and neighbouring Juncus had shorter stems in the presence of highly productive Spartina genotypes. Synthesis. Our results add to the growing list of examples high‐lighting the role of intraspecific variation in species interactions and suggest that variation in these interactions may promote the maintenance of plant genetic variation and community composition.

Host-plant genetic variation affects the diversity and composition of associated above and belowground communities. Most evidence supporting this view is derived from studies within a single common garden, thereby constraining the range of biotic and abiotic environmental conditions that might directly or indirectly (via phenotypic plasticity) affect communities. If natural variability in the environment renders host-plant genetic effects on associated communities unimportant, then studying the community-level consequences of genetic variation may not be warranted. We addressed this knowledge gap by planting a series of common gardens consisting of 10 different clones (genotypes) of the willow Salix hookeriana in a coastal dune ecosystem and manipulated natural variation in ant-aphid interactions (biotic) and wind exposure (abiotic) in two separate experiments. We then quantified the responses of associated species assemblages both above (foliar arthropods) and belowground (rhizosphere fungi and bacteria). In addition, we quantified plant phenotypic responses (plant growth, leaf quality, and root quality) to tease apart the effects of genetic variation, phenotypic plasticity, and direct environmental effects on associated communities. In the ant-aphid experiment, we found that willow genotype explained more variation in foliar arthropod communities than aphid additions and proximity to aphid-tending ant mounds. However, aphid additions modified willow genetic effects on arthropod community composition by attracting other aphid species to certain willow genotypes. In the wind experiment, wind exposure explained more variation than willow genotype in structuring communities of foliar arthropods and rhizosphere bacteria. Still, willow genotype had strong effect sizes on several community properties of arthropods and fungi, indicating that host-plant genetic variation remains important. Across both experiments, genetic variation in plant traits was more important than phenotypic plasticity in structuring associated communities. The relative importance of genetic variation vs. direct environmental effects though depended on the type of environmental gradient (G > E-aphid, but E-wind > G). Taken together, our results suggest that host-plant genetic variation is an important driver of above and belowground biodiversity, despite natural variation in the biotic and abiotic environment.

Many host-plants exhibit genetic variation in resistance to pathogens; however, little is known about the extent to which genetic variation in pathogen resistance influences other members of the host-plant community, especially arthropods at higher trophic levels. We addressed this knowledge gap by using a common garden experiment to examine whether genotypes of Populus trichocarpa varied in resistance to a leaf-blistering pathogen, Taphrina sp., and in the density of web-building spiders, the dominant group of predatory arthropods. In addition, we examined whether variation in spider density was explained by variation in the density and size of leaf blisters caused by Taphrina. We found that P. trichocarpa genotypes exhibited strong differences in their resistance to Taphrina and that P. trichocarpa genotypes that were more susceptible to Taphrina supported more web-building spiders, the dominant group of predatory arthropods. We suspect that this result is caused by blisters increasing the availability of suitable habitat for predators, and not due to variation in herbivores because including herbivore density as a covariate did not affect our models. Our study highlights a novel pathway by which genetic variation in pathogen resistance may affect higher trophic levels in arthropod communities.

Summary Phenology, or the timing of life cycle events, is a key trait of organisms that has significance for how communities are assembled and ecosystems function. Although variation in phenology in plants has received increased attention over the past decade as a result of changing climate, we are only beginning to understand the role of genetic variation in these phenological traits on ecological interactions and ecosystem‐level processes. The influence of tree species on riparian environments presents an interesting system for understanding the effects of phenology in terrestrial species on aquatic ecosystems. Here, we used a dominant riparian tree ( Populus trichocarpa : Salicaceae) and tested intraspecific genetic variation in the phenological timing of leaf drop, which influenced leaf‐litter inputs into our experimental aquatic ecosystems. Our empirical results found that genotypic differences in P. trichocarpa explained much of the variation both in leaf‐litter decomposition and aquatic invertebrate species richness within our experimental ponds. Moreover, our results showed that variation in the timing of leaf‐litter inputs outweighed the effects of variation in leaf‐litter quality among P. trichocarpa genotypes on aquatic invertebrate species richness. Taken together, our results suggest that genetic variation in the timing of litter inputs from dominant plant species is likely to be a strong underlying mechanism driving litter decomposition and invertebrate communities in aquatic ecosystems. This emphasises that studies disregarding phenology may significantly underestimate an important and variable component in communities and ecosystems.

Community genetics was originally proposed as a novel approach to identifying links between genes and ecosystems, and merging ecological and evolutional perspectives. The dozen years since the birth of community genetics have seen many empirical studies and common garden experiments, as well as the rise of eco-evolutionary dynamics research and a general shift in ecology to incorporate intraspecific variation. So what have we learned from community genetics? Can individual genes affect entire ecosystems? Are there interesting questions left to be answered, or has community genetics run its course? This perspective makes a series of key points about the general patterns that have emerged and calls attention to gaps in our understanding to be addressed in the coming years.

Movement between host plants during the growing season is a common behaviour among insect herbivores, although the mechanisms promoting these movements are poorly understood for many systems. Two possible reasons why insect herbivores relocate include compensating for host plant quantity and/or quality changes and the avoidance of natural enemies. The Arctic caterpillar (Gynaephora groenlandica (Wocke); Lepidoptera: Lymantriidae) moves several metres each day, feeds on its patchily distributed host plant, Arctic willow (Salix arctica Pallas; Salicaceae), and has two main natural enemies, the parasitoids Exorista thula Wood (Diptera: Tachinidae) and Hyposoter diechmanni (Nielsen) (Hymenoptera: Ichneumonidae). We physically moved caterpillars between Arctic willows and restricted other caterpillar individuals each to a single willow throughout the active period of Arctic caterpillars. We found that growth rate, herbivory rate, and the proportion of available leaf fascicles eaten were higher for experimentally moved caterpillars. Parasitoid abundances were low and did not differ between experimentally moved and stationary caterpillars. Taken together, our study addresses the bottom-up and top-down controls on insect herbivore movement during the short duration of the growing season in the Arctic. Our results suggest that caterpillars are likely moving to new willow shrubs to access high quality resources.

Theory predicts that intraspecific genetic variation can increase the complexity of an ecological network. To date, however, we are lacking empirical knowledge of the extent to which genetic variation determines the assembly of ecological networks, as well as how the gain or loss of genetic variation will affect network structure. To address this knowledge gap, we used a common garden experiment to quantify the extent to which heritable trait variation in a host plant determines the assembly of its associated insect food web (network of trophic interactions). We then used a resampling procedure to simulate the additive effects of genetic variation on overall food-web complexity. We found that trait variation among host-plant genotypes was associated with resistance to insect herbivores, which indirectly affected interactions between herbivores and their insect parasitoids. Direct and indirect genetic effects resulted in distinct compositions of trophic interactions associated with each host-plant genotype. Moreover, our simulations suggest that food-web complexity would increase by 20% over the range of genetic variation in the experimental population of host plants. Taken together, our results indicate that intraspecific genetic variation can play a key role in structuring ecological networks, which may in turn affect network persistence.

Aims While a growing number of studies have demonstrated the importance of intraspecific differences within plant species on associated arthropod communities, little is known regarding the relative strength of these effects compared to environmental factors. In this study, we examined whether intraspecific plant differences and nutrient fertilization interact to shape the arthropod community of a dominant coastal shrub, Baccharis pilularis (coyote bush). Methods We overlaid a fertilization treatment on a 12-year-old common garden experiment planted with erect and prostrate architectural morphs of Baccharis in California, USA. To collect the associated arthropod community, we vacuum sampled the crown of each Baccharis and identified individuals to species or morphospecies. Important Findings We found that arthropod richness and abundance were 2- to 3-fold greater on prostrate Baccharis than on erect morphs, but observed no main effects of fertilizer addition on the overall arthropod communities. Predators responded as strongly as herbivores to plant morph, and both were unaffected by nutrient additions. Only the specialist stem galler, Gnorimoschema baccharisella, showed an interactive response to plant morph and fertilization. Nitrogen, phosphorous and potassium addition had opposite effects on the two morphs, increasing stem gall abundance by 50% on prostrate morphs, but reducing galling by 20% on erect morphs. The architectural complexity of prostrate morphs could be the driving mechanism of differences in arthropod assemblages. Overall, our results demonstrate that community-level consequences of intraspecific differences in plants are strong, rather than being context dependent, and are generally maintained under different resource environments. The growing number of studies showing strong genotype than nutrient effects on associated arthropod communities suggests that this might be a generalized pattern.

Ecological theory, developed largely from ungulates and grassland systems, predicts that herbivory accelerates nutrient cycling more in productive than unproductive systems. This prediction may be important for understanding patterns of ecosystem change over time and space, but its applicability to other ecosystems and types of herbivore remain uncertain. We estimated fluxes of nitrogen (N) and phosphorus (P) from herbivory of a common tree species (Betula pubescens) by a common species of herbivorous insect along a -5000-yr boreal chronosequence. Contrary to established theory, fluxes of N and P via herbivory increased along the chronosequence despite a decline in plant productivity. The herbivore-mediated N and P fluxes to the soil are comparable to the main alternative pathway for these nutrients via tree leaf litterfall. We conclude that insect herbivores can make large contributions to nutrient cycling even in unproductive systems, and influence the rate and pattern of ecosystem development, particularly in systems with low external nutrient inputs.

A classic pattern in biogeography is the decline in species richness from lower to higher latitudes. Communities, however, can also vary with other geographical patterns, such as the abiotic gradients that occur from coastal to interior habitats. In this study, we surveyed arthropod communities and herbivore pressure on populations of a dominant shrub, Baccharis pilularis , across a 2000 km latitudinal transect to determine whether coastal versus interior location mediates arthropod responses to latitude. We found that arthropod species richness and abundance declined with increasing latitude. We also found significant coastal‐interior shifts in community composition and trophic structure. Specifically, predator and scavenger richness were two and three fold greater at coastal sites compared to interior sites, and were three‐ and six‐fold more abundant on the coast than in the interior. Herbivore pressure displayed a similar pattern, with greater abundance at lower latitudes and at coastal sites. Our results corroborate the general macroecological pattern that diversity declines with increasing latitude, and that coastal versus interior location can also shape community assemblages. We did not, however, find any interaction between latitude and location suggesting the effect of latitude on arthropod communities remains consistent inland compared to more constant coastal conditions.

The impacts of invasive alien species are greatest when they become dominant members of a community, introduce novel traits, and displace native species. Invasions by alien mollusks represent a novel context by which to compare trait differences between generalist native and introduced herbivores in terrestrial ecosystems. Here, we determined the abundance, habitat, feeding preferences, as well as the metabolic rate of the native Pacific banana slug (Ariolimax columbianus) and the alien black slug (Arion rufus) in the coastal forests of British Columbia, Canada. Through a series of observational and experimental studies, we found that alien slugs are more abundant, differ in their habitat preferences, and consumed more fungi (mushrooms) than native banana slugs. Conversely, in an enclosures experiment we found that herbivory damage by native slugs was higher compared to enclosures with alien only and control enclosures. Finally, metabolic rates were similar for both slug species. These results suggest that alien black slugs possess a suite of traits that make them functionally different from native banana slugs.

Research in eco-evolutionary dynamics and community genetics has demonstrated that variation within a species can have strong impacts on associated communities and ecosystem processes. Yet, these studies have centred around individual focal species and at single trophic levels, ignoring the role of phenotypic variation in multiple taxa within an ecosystem. Given the ubiquitous nature of local adaptation, and thus intraspecific variation, we sought to understand how combinations of intraspecific variation in multiple species within an ecosystem impacts its ecology. Using two species that co-occur and demonstrate adaptation to their natal environments, black cottonwood (Populus trichocarpa) and three-spined stickleback (Gasterosteus aculeatus), we investigated the effects of intraspecific phenotypic variation on both top-down and bottom-up forces using a large-scale aquatic mesocosm experiment. Black cottonwood genotypes exhibit genetic variation in their productivity and consequently their leaf litter subsidies to the aquatic system, which mediates the strength of top-down effects from stickleback on prey abundances. Abundances of four common invertebrate prey species and available phosphorous, the most critically limiting nutrient in freshwater systems, are dictated by the interaction between genetic variation in cottonwood productivity and stickleback morphology. These interactive effects fit with ecological theory on the relationship between productivity and top-down control and are comparable in strength to the effects of predator addition. Our results illustrate that intraspecific variation, which can evolve rapidly, is an under-appreciated driver of community structure and ecosystem function, demonstrating that a multi-trophic perspective is essential to understanding the role of evolution in structuring ecological patterns.

Summary Community genetics research has posited a genetic basis to the assembly of ecological communities. For arthropod herbivores in particular, there is strong support that genetic variation in host plants is a key factor shaping their diversity and composition. However, the specific plant phenotypes underlying herbivore responses remain poorly explored for most systems. We address this knowledge gap by examining the influence of both genetic and phenotypic variation in a dominant host‐plant species, Salix hookeriana, on its associated arthropod herbivore community in a common garden experiment. Specifically, we surveyed herbivore responses among five different arthropod feeding guilds to 26 distinct S. hookeriana genotypes. Moreover, we quantified the heritability of a suite of plant traits that determine leaf quality (e.g. phenolic compounds, trichomes, specific leaf area, C : N) and whole‐plant architecture, to identify which traits best accounted for herbivore community responses to S. hookeriana genotype. We found that total herbivore abundance and community composition differed considerably among S. hookeriana genotypes, with strong and independent responses of several species and feeding guilds driving these patterns. We also found that leaf phenolic chemistry displayed extensive heritable variation, whereas leaf physiology and plant architecture tended to be less heritable. Of these traits, herbivore responses were primarily associated with leaf phenolics and plant architecture; however, different herbivore species and feeding guilds were associated with different sets of traits. Despite our thorough trait survey, plant genotype remained a significant predictor of herbivore responses in most trait association analyses, suggesting that unmeasured host‐plant characteristics and/or interspecific interactions were also contributing factors. Taken together, our results support that the genetic basis of herbivore community assembly occurs through a suite of plant traits for different herbivore species and feeding guilds. Still, identifying these phenotypic mechanisms requires measuring a broad range of plant traits and likely further consideration of how these traits affect interspecific interactions.