Assessing the potential for indirect interactions between tropical tree species via shared insect seed predators

Natural enemies of plants have the potential to influence the dynamics of plant populations and the structure of plant communities. In diverse tropical forests, research on the effects of plant enemies has largely focused on the diversity-enhancing effects of highly specialized enemies, while the community-level effects of enemies with broader diets have rarely been considered. We investigated the community of insect seed predators interacting with seven tree species in the family Lauraceae on Barro Colorado Island (Panama). We present one of the first quantitative food webs for pre-dispersal insect seed predators and their host plants, and use the information in the web to assess the potential for indirect interactions between the tree species. Our data suggest that there is high potential for indirect interactions between Lauraceae species via their shared seed predators. The strength and direction of these interactions are largely unrelated to the phylogenetic distance and trait similarity between species but are likely governed by the volume of fruit produced by each tree species.     

Cascading trait‐mediation: disruption of a trait‐mediated mutualism by parasite‐induced behavioral modification

Trait‐mediated indirect interactions (TMII) are important driving‐forces causing trophic cascades in aquatic and terrestrial food webs. Furthermore, since most biological communities are not simple food chains but complex networks of interactions, one TMII within a community might easily be influenced by another TMII. In other words, TMII themselves can be cascades with potential implications for community dynamics. Here we report on one of such cascade, where a parasitic fly induces behavioral changes that disrupt a trait‐mediated ant–hemipteran mutualism. We show that during parasite‐induced low‐activity periods, the ant Azteca instabilis fails to protect its mutualistic scale‐insect partner against predatory ladybeetles. Thus, in the presence of the parasite, ladybeetles ate as many scales in ant‐patrolled plants as they did in ant‐free plants. These results demonstrate how, through a cascade of trait‐mediated interactions, associations between members of a community can be drastically altered.     

Linkages among trait-mediated indirect effects: a new framework for the indirect interaction web

Plants have diverse ways of responding to damage by herbivores, such as changes in allelochemistry, physiology, morphology, growth, and phenology. These responses form the mechanistic basis for trait-mediated indirect interactions (TMIIs) between organisms on the plants. There is a growing appreciation that such TMIIs form complex networks (i.e., indirect interaction webs) in terrestrial plant-associated arthropod communities. Almost all previous studies have had the same framework: examining trait-mediated indirect effects within a single interactive unit consisting of one initiator of herbivore, a host plant as a mediator, and one receiver [trait-mediated indirect interaction unit (TMIU)]. However, this framework is too simple to understand the dynamics of the indirect interaction web. Recent studies suggest that there is a wide variety of interactions among TMIUs within a community, which may largely affect the outcomes of indirect effects in each unit. Here, we review recent advance in studies of trait-mediated indirect effects in plant-associated arthropod communities and explore the mechanisms of linkages among TMIUs. Then, we argue the importance of examining linkages among TMIUs as a new framework for future studies on the indirect interaction web. Finally, we propose the hypothesis that linkages among TMIUs contribute to the maintenance of biodiversity.     

How habitat-modifying organisms structure the food web of two coastal ecosystems

The diversity and structure of ecosystems has been found to depend both on trophic interactions in food webs and on other species interactions such as habitat modification and mutualism that form non-trophic interaction networks. However, quantification of the dependencies between these two main interaction networks has remained elusive. In this study, we assessed how habitat-modifying organisms affect basic food web properties by conducting in-depth empirical investigations of two ecosystems: North American temperate fringing marshes and West African tropical seagrass meadows. Results reveal that habitat-modifying species, through non-trophic facilitation rather than their trophic role, enhance species richness across multiple trophic levels, increase the number of interactions per species (link density), but decrease the realized fraction of all possible links within the food web (connectance). Compared to the trophic role of the most highly connected species, we found this non-trophic effects to be more important for species richness and of more or similar importance for link density and connectance. Our findings demonstrate that food webs can be fundamentally shaped by interactions outside the trophic network, yet intrinsic to the species participating in it. Better integration of non-trophic interactions in food web analyses may therefore strongly contribute to their explanatory and predictive capacity.     

Complexity of multitrophic interactions in a grassland ecosystem depends on plant species diversity

1.?We studied the theoretical prediction that a loss of plant species richness has a strong impact on community interactions among all trophic levels and tested whether decreased plant species diversity results in a less complex structure and reduced interactions in ecological networks. 2.?Using plant species-specific biomass and arthropod abundance data from experimental grassland plots (Jena Experiment), we constructed multitrophic functional group interaction webs to compare communities based on 4 and 16 plant species. 427 insect and spider species were classified into 13 functional groups. These functional groups represent the nodes of ecological networks. Direct and indirect interactions among them were assessed using partial Mantel tests. Interaction web complexity was quantified using three measures of network structure: connectance, interaction diversity and interaction strength. 3.?Compared with high plant diversity plots, interaction webs based on low plant diversity plots showed reduced complexity in terms of total connectance, interaction diversity and mean interaction strength. Plant diversity effects obviously cascade up the food web and modify interactions across all trophic levels. The strongest effects occurred in interactions between adjacent trophic levels (i.e. predominantly trophic interactions), while significant interactions among plant and carnivore functional groups, as well as horizontal interactions (i.e. interactions between functional groups of the same trophic level), showed rather inconsistent responses and were generally rarer. 4.?Reduced interaction diversity has the potential to decrease and destabilize ecosystem processes. Therefore, we conclude that the loss of basal producer species leads to more simple structured, less and more loosely connected species assemblages, which in turn are very likely to decrease ecosystem functioning, community robustness and tolerance to disturbance. Our results suggest that the functioning of the entire ecological community is critically linked to the diversity of its component plants species.     

Effects of an invasive plant transcend ecosystem boundaries through a dragonfly-mediated trophic pathway

Trophic interactions can strongly influence the structure and function of terrestrial and aquatic communities through top-down and bottom-up processes. Species with life stages in both terrestrial and aquatic systems may be particularly likely to link the effects of trophic interactions across ecosystem boundaries. Using experimental wetlands planted with purple loosestrife (Lythrum salicaria), we tested the degree to which the bottom-up effects of floral density of this invasive plant could trigger a chain of interactions, changing the behavior of terrestrial flying insect prey and predators and ultimately cascading through top-down interactions to alter lower trophic levels in the aquatic community. The results of our experiment support the linkage of terrestrial and aquatic food webs through this hypothesized pathway, with high loosestrife floral density treatments attracting high levels of visiting insect pollinators and predatory adult dragonflies. High floral densities were also associated with increased adult dragonfly oviposition and subsequently high larval dragonfly abundance in the aquatic community. Finally, high-flower treatments were coupled with changes in zooplankton species richness and shifts in the composition of zooplankton communities. Through changes in animal behavior and trophic interactions in terrestrial and aquatic systems, this work illustrates the broad and potentially cryptic effects of invasive species, and provides additional compelling motivation for ecologists to conduct investigations that cross traditional ecosystem boundaries.     

The indirect consequences of a mutualism: comparing positive and negative components of the net interaction between honeydew-tending ants and host plants

1. In ecological webs, net indirect interactions between species are composed of interactions that vary in sign and magnitude. Most studies have focused on negative component interactions (e.g. predation, herbivory) without considering the relative importance of positive interactions (e.g. mutualism, facilitation) for determining net indirect effects. 2. In plant/arthropod communities, ants have multiple top-down effects via mutualisms with honeydew-producing herbivores and harassment of and predation on other herbivores; these ant effects provide opportunities for testing the relative importance of positive and negative interspecific interactions. We manipulated the presence of ants, honeydew-producing membracids and leaf-chewing beetles on perennial host plants in field experiments in Colorado to quantify the relative strength of these different types of interactions and their impact on the ant's net indirect effect on plants. 3. In 2007, we demonstrated that ants simultaneously had a positive effect on membracids and a negative effect on beetles, resulting in less beetle damage on plants hosting the mutualism. 4. In 2008, we used structural equation modelling to describe interaction strengths through the entire insect herbivore community on plants with and without ants. The ant's mutualism with membracids was the sole strong interaction contributing to the net indirect effect of ants on plants. Predation, herbivory and facilitation were weak, and the net effect of ants reduced plant reproduction. This net indirect effect was also partially because of behavioural changes of herbivores in the presence of ants. An additional membracid manipulation showed that the membracid's effect on ant activity was largely responsible for the ant's net effect on plants; ant workers were nearly ten times as abundant on plants with mutualists, and effects on other herbivores were similar to those in the ant manipulation experiment. 5. These results demonstrate that mutualisms can be strong relative to negative direct interspecific interactions and that positive interactions deserve attention as important components of ecological webs.     

Rewiring and indirect effects underpin modularity reshuffling in a marine food web under environmental shifts

Species are characterized by physiological and behavioral plasticity, which is part of their response to environmental shifts. Nonetheless, the collective response of ecological communities to environmental shifts cannot be predicted from the simple sum of individual species responses, since co‐existing species are deeply entangled in interaction networks, such as food webs. For these reasons, the relation between environmental forcing and the structure of food webs is an open problem in ecology. To this respect, one of the main problems in community ecology is defining the role each species plays in shaping community structure, such as by promoting the subdivision of food webs in modules—that is, aggregates composed of species that more frequently interact—which are reported as community stabilizers. In this study, we investigated the relationship between species roles and network modularity under environmental shifts in a highly resolved food web, that is, a “weighted” ecological network reproducing carbon flows among marine planktonic species. Measuring network properties and estimating weighted modularity, we show that species have distinct roles, which differentially affect modularity and mediate structural modifications, such as modules reconfiguration, induced by environmental shifts. Specifically, short‐term environmental changes impact the abundance of planktonic primary producers; this affects their consumers’ behavior and cascades into the overall rearrangement of trophic links. Food web re‐adjustments are both direct, through the rewiring of trophic‐interaction networks, and indirect, with the reconfiguration of trophic cascades. Through such “systemic behavior,” that is, the way the food web acts as a whole, defined by the interactions among its parts, the planktonic food web undergoes a substantial rewiring while keeping almost the same global flow to upper trophic levels, and energetic hierarchy is maintained despite environmental shifts. This behavior suggests the potentially high resilience of plankton networks, such as food webs, to dramatic environmental changes, such as those provoked by global change.     

The long‐term impacts of predators on prey: inducible defenses,population dynamics,and indirect effects

Despite the amount of research on the inducible defenses of prey against predators, our understanding of the long‐term significance of non‐lethal predators on prey phenotypes, prey population dynamics, and community structure has rarely been explored. Our objectives were to assess the effects of predators on prey defenses, prey population dynamics, and the relative magnitude of density‐ versus trait‐mediated indirect interactions (DMIIs and TMIIs) over multiple prey generations. Using a freshwater snail and three common snail predators, we constructed a series of community treatments with pond mesocosms that manipulated trophic structure, the identity of the top predator, and whether predators were caged or uncaged. We quantified snail phenotypes, snail population size, and resource abundance over multiple snail generations. We found that snails were expressing inducible defenses in our system although the magnitude of the responses varied over time and across predator species. Despite the expression of inducible defenses, caged predators did not reduce snail population size. There also was no evidence of TMIIs throughout the experiment suggesting that TMIIs have a minimal role in the long‐term structure of our communities. The absence of TMIIs was largely driven by the lack of predator‐induced reductions in resource consumption and the lack of consistent reductions in population size with predator cues. In contrast, we detected strong DMIIs associated with lethal predators suggesting that DMIIs are the dominant long‐term mechanism influencing community structure. Our results demonstrate that although predators can have significant effects on prey phenotypes and sometimes cause short‐term TMIIs, there may be few long‐term consequences of these responses on population dynamics and indirect interactions, at least within simple food webs. Research directed towards addressing the long‐term consequences of predator–prey interactions within communities will help to reveal whether the conclusions and predictions generated from short‐term experiments are applicable over ecological and evolutionary timescales.     

The predators of insects

Abstract. 1. Insect–insectivore trophic relations were reviewed using presence–absence data from sixty-one invertebrate-dominated food webs and fifteen food webs from Briand's (1983) original forty web collection. From counts of prey links in higher taxa (orders, classes, phyla), six phyla and thirteen classes of non-insect insectivores and fourteen orders of insect predators and prey were found. 2. Detritus-based habitats (phytotelmata, felled logs, carcasses, dungpads) harboured fewer orders of insects, that interact with other insects, than webs from grazer-based (host plants, some galls) and mixed-based systems (aquatic webs). Consumer–resource networks of higher insect taxa in these webs shared several features found in some species-level biological networks: the trend was towards few pairs of strong asymmetrical links, several weak links and many null interactions. 3. From counts of insect predator–insect prey links, hymenopterans as terrestrial predators and parasitoids interacted with the most number of higher insect taxa. Hymenopterans were also linked as prey more often than other terrestrial insects. In freshwater habitats, plecopterans were linked as predators more often than other aquatic taxa, whereas dipterans were listed as prey more often than other insects. 4. Dipterans were linked in the diets of non-insect insectivores from seven of eight common taxonomic classes. Arachnids were identified as insect predators by food web researchers in the largest number of webs, followed by passerine birds and cyprinodont fishes. From analysis of prey links at the ordinal level, predaceous insects were less polyphagous than other predators (other ectotherms and endotherms). 5. Analysis of chain lengths, as expected, showed that insect prey occupied mostly lowermost trophic levels, non-insect insectivores were found mostly at uppermost trophic levels, and predaceous insects were found mostly at intermediate trophic levels across most habitats. 6. This analysis offers evidence that insects are not just occupying intermediate trophic levels in some communities. Indeed, some taxa feed at the upper ends of long food chains, for example eupelmids in galls, staphylinids in carcasses, and perlid plecopterans in streams.     

Eco-evolutionary dynamics in herbivorous insect communities mediated by induced plant responses

It is increasingly recognized that the ecology of communities and evolution of species within communities are interdependent, and researchers have been paying attention to this rapidly emerging field of research, i.e., through studies on eco-evolutionary dynamics. Most of the studies on eco-evolutionary dynamics have been concerned with direct trophic interactions. However, community ecologists have shown that trait-mediated indirect effects play an important role in shaping the structure of natural communities. In particular, in terrestrial plant–insect systems, indirect effects mediated through herbivore-induced plant responses are common and have a great impact on the structure of herbivore communities. This review describes eco-evolutionary dynamics in herbivorous insect communities, and specifically focuses on the key role of herbivore-induced plant responses in eco-evolutionary dynamics. First, I review studies on the evolution of herbivore traits relevant to plant induction and discuss evolution in a community context mediated by induced plant responses. Second, I highlight how intraspecific genetic variation or evolution in herbivore traits can influence herbivore community structure. Finally, I propose the hypothetical model that induced plant responses supports eco-evolutionary feedback in herbivore communities. In this review, I argue that the application of the indirect interaction web approaches into studies on eco-evolutionary will provide profound insights into understanding of mechanisms of the generation and maintenance of biodiversity.     

Top‐down and bottom‐up diversity cascades in detrital vs. living food webs

Apex predators and plant resources are both critical for maintaining diversity in biotic communities, but the indirect (‘cascading’) effects of top‐down and bottom‐up forces on diversity at different trophic levels are not well resolved in terrestrial systems. Manipulations of predators or resources can cause direct changes of diversity at one trophic level, which in turn can affect diversity at other trophic levels. The indirect diversity effects of resource and consumer variation should be strongest in aquatic systems, moderate in terrestrial systems, and weakest in decomposer food webs. We measured effects of top predators and plant resources on the diversity of endophytic animals in an understorey shrub Piper cenocladum (Piperaceae). Predators and resource availability had significant direct and indirect effects on the diversity of the endophytic animal community, but the effects were not interactive, nor were they consistent between living vs. detrital food webs. The addition of fourth trophic level beetle predators increased diversity of consumers supported by living plant tissue, whereas balanced plant resources (light and nutrients) increased the diversity of primary through tertiary consumers in the detrital resources food web. These results support the hypotheses that top‐down and bottom‐up diversity cascades occur in terrestrial systems, and that diversity is affected by different factors in living vs. detrital food webs.     

Emerging aquatic insects as predators in terrestrial systems across a gradient of stream temperature in North and South America

1. Empirical and theoretical research over the past decade has demonstrated the widespread importance of aquatic subsidies to terrestrial food webs. In particular, adult aquatic insects that emerge from streams and lakes are prey for terrestrial predators. While variation in the magnitude of this subsidy is clearly important, the potential top‐down effects of the predatory adults of some aquatic insects in terrestrial food webs are largely unknown. 2. I used published data on benthic insect density (as a proxy for emergence) in North and South America to explore how the proportion of benthic insects that are predatory as adults varies across a gradient of mean annual stream temperature. 3. The proportion of benthic insects that are predatory as adults varied widely across sites (0–12% by abundance; 0–86% by biomass). There was a positive relationship between mean annual stream temperature and the proportion of predatory adults across all sites, driven largely by the greater abundance/biomass of predatory taxa (e.g. odonates), relative to non‐predators (e.g. midges, mayflies, caddisflies), in tropical than in temperate streams. 4. The ‘trophic structure’ (i.e. the proportion of predators) of emerging adult aquatic insects is an understudied source of variation in aquatic–terrestrial interactions. Incorporation of trophic structure in future studies is needed to understand how future modification of fresh waters may affect adjacent terrestrial food webs through both bottom‐up and top‐down effects.     

Terrestrial trophic cascades: how much do they trickle?

Although more consensus is now emerging on the magnitude and frequency of cascading trophic effects in aquatic communities, the debate over their terrestrial counterparts continues. We used meta-analysis to analyze field experiments on trophic cascades in terrestrial arthropod-dominated food webs to evaluate the overall magnitude of trophic cascades and conditions affecting their occurrence and strength. We found extensive support for the presence of trophic cascades in terrestrial communities. In the majority of experiments, predator removal led to increased densities of herbivorous insects and higher levels of plant damage. Cascades in which removing predators led to decreased herbivory also were detected but were less frequent and weaker, suggesting a predominantly three-trophic-level behavior of arthropod-dominated terrestrial food webs. Despite the clear evidence that cascades often decreased plant damage, residual effects of predation produced either no or only minimal changes in overall plant biomass. Agricultural systems and natural communities exhibited similarly strong effects of predation on herbivore abundance. However, resulting effects on plant damage and community-wide effects of trophic cascades on plant biomass usually were highly variable, and only in the managed agricultural systems did predators occasionally have strong indirect effects on plant biomass. Our meta-analysis suggests that the effects of trophic cascades on the biomass of primary producers are weaker in terrestrial than aquatic food webs.     

A cross-ecosystem comparison of the strength of trophic cascades

Although trophic cascades (indirect effects of predators on plants via herbivores) occur in a wide variety of food webs, the magnitudes of their effects are often quite variable. We compared the responses of herbivore and plant communities to predator manipulations in 102 field experiments in six different ecosystems: lentic (lake and pond), marine, and stream benthos, lentic and marine plankton, and terrestrial (grasslands and agricultural fields). Predator effects varied considerably among systems and were strongest in lentic and marine benthos and weakest in marine plankton and terrestrial food webs. Predator effects on herbivores were generally larger and more variable than on plants, suggesting that cascades often become attenuated at the plant–herbivore interface. Top‐down control of plant biomass was stronger in water than on land; however, the differences among the five aquatic food webs were as great as those between wet and dry systems.     

Bottom-up trophic cascades and material transfer in terrestrial food webs

In contrast to top-down trophic cascades, few reviews have appeared of bottom-up trophic cascades. We review the recent development of research on bottom-up cascades in terrestrial food webs, focusing on tritrophic systems consisting of plants, herbivorous insects, and natural enemies, and attempt to integrate bottom-up cascade and material transfer among trophic levels. Bottom-up cascades are frequently reported in various tritrophic systems, and are important to determine community structure, population dynamics, and individual performance of higher trophic levels. In addition, we highlight several features of bottom-up cascades. Accumulation or dilution of plant nutritional and defensive materials by herbivorous insects provides a mechanistic base for several bottom-up cascades. Such a stoichiometric approach has the potential to improve our understanding of bottom-up cascading effects in terrestrial food webs. We suggest a future direction for research by integration of bottom-up cascades and material transfer among trophic levels.     

Estimating co-extinction threats in terrestrial ecosystems

The biosphere is changing rapidly due to human endeavour. Because ecological communities underlie networks of interacting species, changes that directly affect some species can have indirect effects on others. Accurate tools to predict these direct and indirect effects are therefore required to guide conservation strategies. However, most extinction-risk studies only consider the direct effects of global change—such as predicting which species will breach their thermal limits under different warming scenarios—with predictions of trophic cascades and co-extinction risks remaining mostly speculative. To predict the potential indirect effects of primary extinctions, data describing community interactions and network modelling can estimate how extinctions cascade through communities. While theoretical studies have demonstrated the usefulness of models in predicting how communities react to threats like climate change, few have applied such methods to real-world communities. This gap partly reflects challenges in constructing trophic network models of real-world food webs, highlighting the need to develop approaches for quantifying co-extinction risk more accurately. We propose a framework for constructing ecological network models representing real-world food webs in terrestrial ecosystems and subjecting these models to co-extinction scenarios triggered by probable future environmental perturbations. Adopting our framework will improve estimates of how environmental perturbations affect whole ecological communities. Identifying species at risk of co-extinction (or those that might trigger co-extinctions) will also guide conservation interventions aiming to reduce the probability of co-extinction cascades and additional species losses.     

Breaking down Complex Saproxylic Communities: Understanding Sub-Networks Structure and Implications to Network Robustness

Saproxylic insect communities inhabiting tree hollow microhabitats correspond with large food webs which simultaneously are constituted by multiple types of plant-animal and animal-animal interactions, according to the use of trophic resources (wood- and insect-dependent sub-networks), or to trophic habits or interaction types (xylophagous, saprophagous, xylomycetophagous, predators and commensals). We quantitatively assessed which properties of specialised networks were present in a complex networks involving different interacting types such as saproxylic community, and how they can be organised in trophic food webs. The architecture, interacting patterns and food web composition were evaluated along sub-networks, analysing their implications to network robustness from random and directed extinction simulations. A structure of large and cohesive modules with weakly connected nodes was observed throughout saproxylic sub-networks, composing the main food webs constituting this community. Insect-dependent sub-networks were more modular than wood-dependent sub-networks. Wood-dependent sub-networks presented higher species degree, connectance, links, linkage density, interaction strength, and were less specialised and more aggregated than insect-dependent sub-networks. These attributes defined high network robustness in wood-dependent sub-networks. Finally, our results emphasise the relevance of modularity, differences among interacting types and interrelations among them in modelling the structure of saproxylic communities and in determining their stability.     

Identifying important interaction modifications in ecological systems

Trophic interaction modifications, where a consumer–resource link is affected by additional species, are widespread and significant causes of non-trophic effects in ecological networks. The sheer number of potential interaction modifications in ecological systems poses a considerable challenge, making prioritisation for empirical study essential. Here, we introduce measures to quantify the topological relationship of individual interaction modifications relative to the underlying network. We use these, together with measures for the strength of trophic interaction modifications, to identify features of modifications that are most likely to exert significant effects on the dynamics of whole systems. Using a set of simulated food webs and randomly distributed interaction modifications, we test whether a subset of interaction modifications important for the local stability and direction of species responses to perturbation of complex networks can be identified. We show that trophic interaction modifications have particular importance for dynamics when they affect interactions with a high biomass flux, connect species otherwise distantly linked, and where high trophic-level species modify interactions lower in the food web. In contrast, the centrality of modifications in the network provided little information. This work demonstrates that analyses of interaction modifications can be tractable at the network scale and highlights the importance of understanding the relationship between the distributions of trophic and non-trophic effects.     

Trophic consequences of a biological invasion: do plant invasions increase predator abundance?

As invasive species become integrated into existing communities, they engage in a wide variety of trophic interactions with other community members. Many of these interactions are direct (e.g. predator–prey interactions or interference competition), but invasive species also can affect native community members indirectly, by influencing the abundances of intermediary species in trophic webs. Observational studies suggest that invasive plant species affect herbivorous arthropod communities and that these effects may flow up trophic webs to influence the abundance of predators. However, few studies have experimentally manipulated the presence of invasive plants to quantify the effects of plant invasion on higher trophic levels. Here, I use comparisons across sites that have or have not been invaded by the invasive plant Medicago polymorpha, combined with experimental removals of Medicago and insect herbivores, to investigate how a plant invasion affects the abundance of predators. Both manipulative and observational experiments showed that Medicago increased the abundance of the exotic herbivore Hypera and predatory spiders, suggesting positive bottom–up effects of plant invasions on higher trophic levels. Path analyses conducted on data from natural habitats revealed that Medicago primarily increased spider abundance through herbivore‐mediated indirect pathways. Specifically, Medicago density was positively correlated with the abundance of the dominant herbivore Hypera, and increased Hypera densities were correlated with increased spider abundance. Smaller‐scale experimental studies confirmed that Medicago may increase spider abundance through herbivore‐mediated indirect pathways, but also showed that the effects of Medicago varied across sites, including having no effect or having direct effects on spider abundance. If effects of invasive species commonly flow through trophic webs, then invasive species have the potential to affect numerous species throughout the community, especially those species whose dynamics are tightly connected to highly‐impacted community members through trophic linkages.