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1. Introduced species disrupt native communities and biodiversity worldwide. Parasitic infections (and at times, their absence) are thought to be a key component in the success and impact of biological invasions by plants and animals. They can facilitate or limit invasions, and positively or negatively impact native species. 2. Parasites have not only direct effects on their hosts, but also indirect effects on the species with which their hosts interact. Indirect effects include density-mediated effects (resulting from parasite-induced reduction in host reproduction and survival) as well as trait-mediated indirect effects (resulting from parasite-induced changes in host phenotype, behaviour or life history). These effects are not mutually exclusive but often interact. 3. The importance of these indirect interactions for invasion success, and the extent to which these effects ramify throughout communities and influence ecosystems undergoing biological invasion provide the focus of our review. Examples from the animal and plant literature illustrate the importance of parasites in mediating both competitive and consumer—resource interactions between native and invasive species. 4. Parasites are involved in indirect interactions at all trophic levels. Furthermore, the indirect effects of parasitic infection are important at a range of biological scales from within a host to the whole ecosystem in determining invasion success and impact. 5. To understand the importance of parasitic infection in invasion success and in the outcomes for invaded communities requires an interdisciplinary approach by ecologists and parasitologists, across animal and plant systems. Future research should develop a framework integrating community ecology, evolution and immunology to better understand and manage the spread of invasive species and their diseases.

1. 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. 2. 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 Euchorthippus unicolor) 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. 3. 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.a 4. 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.

Indirect species interactions are ubiquitous in nature, often outnumbering direct species interactions. Yet despite evidence that indirect interactions have strong ecological effects, relatively little is known about whether they can shape adaptive evolution by altering the strength and/or direction of natural selection. We tested whether indirect interactions affect the strength and direction of pollinator-mediated selection on floral traits of the bumble-bee pollinated wildflower Lobelia siphilitica. We estimated the indirect effects of two pollinator predators with contrasting hunting modes: dragonflies (Aeshnidae and Corduliidae) and ambush bugs (Phymata americana, Reduviidae). Because dragonflies are active pursuit predators, we hypothesized that they would strengthen pollinator-mediated selection by weakening plant–pollinator interactions (i.e., a density-mediated indirect effect). In contrast, because ambush bugs are sit-and-wait predators, we hypothesized that they would weaken or reverse the direction of pollinator-mediated selection by altering pollinator foraging behavior (i.e., a trait-mediated indirect effect). Specifically, if ambush bugs hunt from plants with traits that attract pollinators (i.e., prey), then pollinators will spend less time visiting those plants, weakening or reversing the direction of selection on attractive floral traits. We did not find evidence that high dragonfly abundance strengthened selection on floral traits via a density-mediated indirect effect: neither pollen limitation (a proxy for the strength of plant–pollinator interactions) nor directional selection on floral traits of L. siphilitica differed significantly between high- and low-dragonfly abundance treatments. In contrast, we did find evidence that ambush bug presence affected selection on floral traits via a trait-mediated indirect effect: ambush bugs hunted from L. siphilitica plants with larger daily floral displays, reversing the direction of pollinator-mediated selection on daily display size. These results suggest that indirect species interactions have the potential to shape adaptive evolution by altering natural selection.

1. Predators could reduce disease prevalence in prey populations by culling infected hosts and reducing host density. However, recently observed positive correlations between predator density and disease burdens in prey/hosts suggest that predators do not always ' keep the herds healthy'. Several possible mechanisms could explain this 'unhealthy herds' effect, including a predator-induced change in prey/host traits which enhances susceptibility or alters other epidemiologically important traits. 2. Here, we use an invertebrate predator, zooplankton host, yeast parasite system to demonstrate such trait-mediated indirect effects. We exposed ten genotypes of the prey/host Daphnia dentifera to infochemicals ('kairomones') produced by the invertebrate predator Chaoborus and to a yeast parasite. 3. We found that kairomone exposure induced larger and more susceptible D. dentifera. Clones that showed substantial increases in body length also yielded more spores upon death. However, exposure to kairomones did not alter reproduction from uninfected hosts. All of these results were captured with a dynamic energy budget model of parasitism. 4. Overall, our empirical and theoretical results show that predators can have strong indirect effects on host-parasite interactions that could produce positive correlations between predation intensity and disease burden.

An increase in predation risk triggers a trait response of prey, which alters the interactions between the prey and other species, ultimately affecting other species in the ecosystem. Such predator‐driven trait‐mediated indirect effects (TMIEs) may have been shaped by long‐term evolutionary processes involving the organisms involved, but learning may also be important, especially in contemporary ecosystems experiencing repeated biological invasions. The apple snail Pomacea canaliculata is an important introduced pest of rice, Oryza sativa. Recently, the carrion crow Corvus corone has been found to prey on this species only in some areas, suggesting that learning is involved in this predation. In addition, apple snails can learn to escape from predators and exhibit predator‐specific responses. Thus, the “chain of learning” by the crow and the snail may shape novel TMIEs in the rice ecosystem. We conducted field and mesocosm experiments to test this hypothesis. In the field experiment, we simulated predation by crows in rice fields and investigated the behavior of apple snails. The snails exhibited escape behaviors in response to the simulated predation, and both the proportion of individuals showing the escape response and the degree of escape response were greater in fields with predation by crows than those without predation. In the mesocosm experiment, apple snails from fields with and without predation by crows were separately introduced into mesocosms simulating rice fields, and the behaviors of the snails and the number of remaining rice plants were recorded for 16 days at three levels of predation risk (daily, every 4 days, or no predation). Both the presence/absence of predation in the collection fields and simulated predation affected the escape responses of the snails. Moreover, damage to rice was more severe in mesocosms containing snails from fields without predation than those containing snails from fields with predation. These results suggest that the “chain of learning” causes TMIEs in agroecosystems.

Predation theory and empirical evidence suggest that top predators benefit the survival of resource prey through the suppression of mesopredators. However, whether such behavioural suppression can also affect the physiology of resource prey has yet to be examined. Using a three‐tier reef fish food web and intermittent‐flow respirometry, our study examined changes in the metabolic rate of resource prey exposed to combinations of mesopredator and top predator cues. Under experimental conditions, the mesopredator (dottyback, Pseudochromis fuscus) continuously foraged and attacked resource prey (juveniles of the damselfish Pomacentrus amboinensis) triggering an increase in prey O₂ uptake by 38 ± 12·9% (mean ± SE). The visual stimulus of a top predator (coral trout, Plectropomus leopardus) restricted the foraging activity of the mesopredator, indirectly allowing resource prey to minimize stress and maintain routine O₂ uptake. Although not as strong as the effect of the top predator, the sight of a large non‐predator species (thicklip wrasse, Hemigymnus melapterus) also reduced the impact of the mesopredator on prey metabolic rate. We conclude that lower trophic‐level species can benefit physiologically from the presence of top predators through the behavioural suppression that top predators impose on mesopredators. By minimizing the energy spent on mesopredator avoidance and the associated stress response to mesopredator attacks, prey may be able to invest more energy in foraging and growth, highlighting the importance of the indirect, non‐consumptive effects of top predators in marine food webs.

Parasitic plants can serve as critical intermediaries between their hosts and other organisms; however these relationships are not well understood. To investigate the relative importance of plant traits in such interactions, we studied the role of the root hemiparasite, Castilleja levisecta (Orobanchaceae), as a mediator of interactions between the host plants it parasitizes and the lepidopteran herbivore Euphydryas editha (Nymphalidae), whose caterpillars feed on Castilleja and sequester iridoid glycosides from it. We tested whether the hemiparasite’s size, leaf N concentration, and iridoid glycoside concentrations were influenced by the identity of its host plant, and then whether these traits influenced outcomes for the herbivore. We found that the hemiparasite’s size and leaf N depended on the host it parasitized, and these traits in turn affected outcomes for E. editha. Specifically, Euphydryas editha survival increased with hemiparasite size and caterpillar mass increased with leaf N; caterpillars with greater mass were more likely to survive during diapause. We also found preliminary evidence that host identity influenced iridoid glycoside sequestration by the herbivore. Mean iridoid glycoside concentrations in caterpillars ranged from 1–12% depending on the host being parasitized by Castilleja. This study demonstrates that root parasitism can result in strong indirect effects on higher trophic levels, influencing organisms’ survival, growth, and chemical interactions.

Predators affect community structure by influencing prey density and traits, but the importance of these effects often is difficult to predict. We measured the strength of blue crab predator effects on mud crab prey consumption of juvenile oysters across a flow gradient that inflicts both physical and sensory stress to determine how the relative importance of top predator density‐mediated indirect effects (DMIEs) and trait‐mediated indirect effects (TMIEs) change within systems. Overall, TMIEs dominated in relatively benign flow conditions where blue crab predator cues increased oyster survivorship by reducing mud crab–oyster consumption. Blue crab DMIEs became more important in high sensory stress conditions, which impaired mud crab perception of blue crab chemical cues. At high physical stress, the environment benefitted oyster survival by physically constraining mud crabs. Thus, factors that structure communities may be predicted based on an understanding of how physical and sensory performances change across environmental stress gradients. We examined the relative importance of predator blue crab effects on intermediate prey mud crab consumption of basal resource oysters across a flow gradient which imposes physical and sensory stress. We found that the environment simultaneously imposes physical and sensory stressors that modify the relative strengths of predator density‐ and trait‐mediated indirect effects through distinct processes.

Phenotypic selection that is sustained over time underlies both anagenesis and cladogenesis, but the conditions that lead to such selection and what causes variation in selection are not well known. We measured the selection exerted by three species of predispersal seed predators of lodgepole pine (Pinus contorta latifolia) in the South Hills, Idaho, and found that net selection on different cone and seed traits exerted by red crossbills (Loxia curvirostra) and cone borer moths (Eucosma recissoriana) over 10 years of seed crops was similar to that measured in another mountain range. We also found that the strength of selection increased as seed predation increased, which provides a mechanism for the correlation between the escalation of seed defenses and the density of seed predators. Red crossbills consume the most seeds and selection they exert accounts for much of the selection experienced by lodgepole pine, providing additional support for a coevolutionary arms race between crossbills and lodgepole pine in the South Hills. The third seed predator, hairy woodpeckers (Picoides villosus), consumed less than one-sixth as many seeds as crossbills. Across the northern Rocky Mountains, woodpecker abundance and therefore selective impact appears limited by the elevated seed defenses of lodgepole pine.

1. The sublethal impact of parasites on host behaviour and the mechanism linking them to population level effects remain largely unquantified. On the Great Barrier Reef, juvenile gnathiid isopods (mobile ectoparasites) are one of the most common ectoparasites of fishes. Previous laboratory studies on damselfishes suggest that a single gnathiid can kill settlement-stage larvae and very young juveniles, while repeated attacks affect the growth of a juvenile damselfish. Nothing, however, is known of how gnathiids affect the performance, and the survival of settlement stage fishes in the wild. 2. We sampled juveniles of the damselfish Pomacentrus amboinensis for gnathiids and tested the effect of a single gnathiid (Gnathia auresmaculosa) on juvenile survival in the laboratory. We also tested the effect of this gnathiid on the swimming performance, oxygen consumption, and successful establishment in the wild of settlement-stage larvae. 3. Of the juvenile fish sampled at dawn, 3.5% had a gnathiid attached; at other times of the day, fish had no gnathiids. In the laboratory, most gnathiids (79%) remained attached to juvenile fish for up to 6 h and all fish survived exposure to one gnathiid. 4. When tested in pairs in a double-lane swim chamber, fish that had previously been fed on by one gnathiid ceased swimming first in 77% of the trials and had a lower critical swimming speed compared to fish not exposed to a gnathiid. Previously parasitized fish had a 35% higher oxygen consumption rate than did unexposed fish. When tagged fish were placed in pairs on dead coral patches in the wild and monitored, the previously parasitized fish disappeared from the reef first in 67% of the trials. 5. Our analysis indicates that a single ectoparasitic gnathiid isopod significantly decreases the performance of young P. amboinensis and their persistence on the reef. Gnathiids, by affecting fish performance, may therefore indirectly affect the successful establishment of fishes on the reef at settlement, a critical transitional stage for most coral reef fishes. Unsuccessful establishment on the reef most likely increases the interactions of settling fish with predators and thus gnathiids may contribute to the high mortality observed at this time.