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1. Arthropods represent most of global biodiversity, with the highest diversity found in tropical rain forests. Nevertheless, we have a very incomplete understanding of how tropical arthropod communities are assembled. 2. We conducted a comprehensive mass sampling of arthropod communities within three major habitat types of lowland Amazonian rain forest, including terra firme clay, white-sand and seasonally flooded forests in Peru and French Guiana. We examined how taxonomic and functional composition (at the family level) differed across these habitat types in the two regions. 3. The overall arthropod community composition exhibited strong turnover among habitats and between regions. In particular, seasonally flooded forest habitats of both regions comprised unique assemblages. Overall, 17.7% (26 of 147) of arthropod families showed significant preferences for a particular habitat type. 4. We present a first reproducible arthropod functional classification among the 147 taxa based on similarity among 21 functional traits describing feeding source, major mouthparts and microhabitats inhabited by each taxon. We identified seven distinct functional groups whose relative abundance contrasted strongly across the three habitats, with sap and leaf feeders showing higher abundances in terra firme clay forest. 5. Our novel arthropod functional classification provides an important complement to link these contrasting patterns of composition to differences in forest functioning across geographical and environmental gradients. This study underlines that both environment and biogeographical processes are responsible for driving arthropod taxonomic composition while environmental filtering is the main driver of the variance in functional composition.

Arthropods in the leaf litter layer of forest soils influence ecosystem processes such as decomposition. Climate‐change models predict both increases and decreases in average rainfall. Increased drought may have greater impacts on the litter arthropod community. In addition to affecting survival or behaviour of desiccation‐sensitive species, lower rainfall may indirectly lower abundances of consumers that graze drought‐stressed fungi, with repercussions for higher trophic levels. We tested the hypothesis that trophic structure will differ between the two rainfall scenarios. In particular, we hypothesized that densities of several broadly defined trophic groupings of arthropods would be lower under reduced rainfall. To test this hypothesis, we used sprinklers to impose two rainfall treatments during three growing seasons in roofed, fenced 14‐m2 plots and documented changes in abundance from initial, pre‐treatment densities of 39 arthropod taxa. Experimental plots were subjected to either LOW (fortnightly) or HIGH (weekly) average rainfall based upon climate models and the previous 100 years of regional weekly averages. Unroofed open plots, our reference treatment (REF), experienced higher than average rainfall during the experiment. The two rainfall extremes produced clear negative effects of lowered rainfall on major trophic groups. Broad categories of fungivores, detritivores and predators were more abundant in HIGH than LOW plots by the final year. Springtails (Collembola), which graze fungal hyphae, were 3× more abundant in the HIGH rainfall treatment. Taxa of larger‐bodied fungivores and detritivores, spiders (Araneae), and non‐spider predators were 2× more abundant under HIGH rainfall. Densities of mites (Acari), which include fungivores, detritivores and predators, were 1.5× greater in HIGH rainfall plots. Abundances and community structure of arthropods were similar in REF and experimental plots, showing that effects of rainfall uncovered in the experiment are applicable to nature. This pattern suggests that changes in rainfall will alter bottom‐up control processes in a critical detritus‐based food web of deciduous forests. Our results, in conjunction with other findings on the impact of desiccation on arthropods and fungal growth, suggest that drier conditions will depress densities of fungal consumers, causing declines in higher trophic levels, with possible impacts on soil processes and the larger forest food web. This experiment revealed how rainfall extremes predicted by climate‐change models may impact bottom‐up control processes in the leaf litter food web. Comparing results with reference plots demonstrated that effects uncovered in experimental mesocosms can be extrapolated to nature, and incorporating different sites in the design uncovered spatial variation in treatment effects.

Historically, where forest habitats are deemed as the pristine landscape state, anthropogenic habitats such as managed grasslands or open spaces are often perceived to be antagonistic and of secondary conservation priority. Traditionally, studies on biodiversity responses to ecological variation, i.e. edge effect, have mostly focused on forest habitats. Yet recently there has been increased attention on communities beyond the forest edge in an effort to better understand how interactions between forests and adjacent habitats may potentially affect regional biodiversity. However, in Europe and the Mediterranean basin (a biodiversity hotspot), areas with high landscape heterogeneity and high edge density, there is a paucity of studies analysing the community responses across forest and “beyond edge” habitats across ecotones. In a protected area of central Italy, we investigated the responses of ground-dwelling arthropods [Araneae (spiders), Chilopoda (centipedes) and Carabidae (ground beetles)], which were differentiated into habitat-specific guilds (forest, edge and grassland species) across a forest–grassland ecotone. We investigated the extent to which a habitat edge influenced communities of arthropods associated with either the forest or grassland, and how far from the edge this effect penetrated into each habitat. Twelve 150 m-transects perpendicular to a forest–grassland edge were established and arthropods were sampled at nine progressive distances across the ecotone. An indicator species analysis was used to detect species significantly associated with forest, edge-belt or grassland habitats, which were assumed representative of the respective communities. Logistic models of indicator species richness and abundances were used to describe responses of grassland and forest communities across the ecological boundaries. We found that grassland and edge habitats had habitat specialists and higher species richness compared to the forest habitat. Moreover, the occurrence of grassland-specific species was influenced by the presence of an edge up to 15 m from the habitat border. In contrast forest-associated indicator species were not affected by proximity to the habitat edge, rather individuals typical of forest habitats tended to “spill over” into grassland habitats. These findings support the hypothesis that in a forest–grassland mosaic, forest species are less sensitive to an edge and influence the community beyond the forest edge and into the grassland more than the reverse, i.e. the effect was asymmetric. From these data, we estimated that a minimum grassland habitat width of 600 m is necessary for grassland species to maintain a core area that is relatively unaffected by the spillover of species from adjacent forest habitats. Incorporating the directional influences of adjacent communities on each other allows for an empirical assessment of habitat vulnerability that doesn’t a priori value the conservation of one habitat over another.

Recent studies have shown that genetically based traits of plants can structure associated arthropod and microbial communities, but whether the effects are consistent and repeatable across years is unknown. If communities are both heritable (i.e., related individuals tend to support similar communities) and repeatable (i.e., the same patterns observed over multiple years), then plant genetics may also affect community properties previously thought to be emergent, such as \"stability.\" Using replicated clones of narrowleaf cottonwood ( Populus angustifolia ) and examining an arthropod community of 103 species, we found that (1) individual tree genotypes supported significantly different arthropod communities, which exhibited broad-sense heritability; (2) these findings were highly repeatable over three consecutive years (repeatability = 0.91) indicating that community responses to individual tree genotypes are consistent from year to year; (3) differences among tree genotypes in community stability (i.e., changes in community composition over multiple years) exhibited broad-sense heritability ( = 0.32). In combination, these findings suggest that an emergent property such as stability can be genetically based and thus subject to natural selection.

Summary Ecological communities consist of antagonistic and mutualistic interactions that can vary in their strength. Indirect effects act among species within or across trophic levels through multiple pathways in an interaction network. Although there are many studies showing indirect effects in ecological communities, we know little about how indirect effects impact the wider community by linking other direct and indirect interactions. Herbivore‐induced indirect effects are ubiquitous and powerful forces in structuring ecological communities. In a plant‐associated network, aphids have the potential to connect multiple interactions through ant‐ and plant‐mediated indirect effects on co‐occurring and/or temporally separated species. We examined how aphids affect the interaction network on tall goldenrod, Solidago altissima, based on data of arthropod species on it with and without the aphid, Uroleucon nigrotuberculatum, using structural equation modelling (SEM) analysis. The presence of aphids greatly changed the strength of several species interactions throughout the season. In the early season, aphids had negative indirect effects on leafhoppers and moth caterpillars through ants. On the other hand, aphid‐induced leaf regrowth decreased scale insects but increased grasshoppers in the late season, when the aphid was no longer present. Moreover, the aphid‐generated interactions increased seed production of tall goldenrods, due to indirect pathways through the leaf regrowth. The aphid thus played a critical role as a network creator in determining the interaction network by generating ant‐ and plant‐mediated indirect effects. In particular, the aphid‐induced leaf regrowth contributed to interaction diversity (i.e. total number of positive/negative interactions and link density per each species) by connecting arthropod species in the early and late seasons. We disentangled the complexity of direct and indirect pathways generated by the aphid (i.e. keystone herbivore), which largely determines community structure of associated arthropods and plant reproductive success. Our study combining ant‐ and plant‐mediated indirect interactions mediated by the aphid provides a basis of better understanding of the underlying mechanisms of how the impact of the keystone herbivore can spread through plant‐based insect networks via direct and indirect pathways. A lay summary is available for this article. Lay Summary

Predators can indirectly enhance plant performance via herbivore suppression, with both prey consumption and changes in prey traits (e.g. changes in foraging behaviour) contributing to the reduction in herbivory. We performed a field experiment to determine the extent of such non-consumptive effects which consisted of repeatedly placing spiders (Pisaura mirabilis) on enclosed plants (Urtica dioica) for cue deposition. Control plants were enclosed in the same way but without spiders. After cue deposition, the enclosures were removed to allow arthropods to colonize the plants and feed on them. Arthropods were removed from the plants before the subsequent spider deposition or control enclosure. During six cycles of enclosure, we quantified leaf damage on the plants. After a seventh cycle, the colonizing arthropods were sampled to determine community composition in relation to the presence/absence of spider cues. We found that the presence of chemotactile spider cues reduced leaf damage by 50 %. In addition, spider cues led to changes in the arthropod community: smaller spiders avoided plants with spider cues. In contrast, the aphid-tending ant Myrmica rubra showed higher recruitment of workers on cue-bearing plants, possibly to protect aphids. Our results show that the risk of spider predation can reduce herbivory on wild plants and also demonstrate that non-consumptive effects can be particularly strong within the predator guild.

Ecosystem engineers have important effects on abundance and diversity of organisms and are vital for conservation efforts. Some large mammalian grazers are engineers because their grazing activity radically changes plant community structure, an effect which then cascades to other consumers. Many large grazers also behaviorally modify the physical environment. American bison (Bison bison) are known for their wallowing, a behavior that creates distinct areas of high disturbance with modified biological and physical characteristics, but how this behavior affects other consumers is poorly understood. In this study, we investigated arthropod abundance and diversity patterns in active and abandoned wallows compared to those in surrounding tallgrass prairie. We found that active wallows contained lower arthropod abundance and diversity compared to surrounding prairie. Herbivorous arthropods were particularly affected and only about 50% as abundant in wallows, while carnivores and detritivores were affected similarly, but to a lesser degree. In contrast, abandoned wallows had higher arthropod abundance seasonally and higher species richness in several feeding groups. Because of arthropod differences in active and abandoned wallows compared to adjacent prairie, it appears that the impact of wallowing is dependent on time since occurrence, with long‐term effects creating patches of higher arthropod abundance and richness. These patches are likely important, at least seasonally, for other consumers higher on the food web. Together with publications documenting positive effects of bison grazing on arthropod abundance and diversity, our results indicate that wallowing effects are potentially additive. These results suggest that physical changes caused by bison behavior are important for maintaining arthropod biodiversity of tallgrass prairies, and bison may therefore be valuable conservation tools. Bison have been proposed as important candidates for rewilding portions of North America, and our results suggest that they could indeed be valuable toward this end.

Plants establish multitrophic interactions above- and belowground with arthropods and microorganisms. Trichoderma spp. are common soil fungi that colonize roots and promote plant growth and health. Under controlled conditions, Trichoderma spp. have been shown to induce plant resistance against the attack of foliar herbivore insects. Here, we investigated the effect of field inoculation with Trichoderma harzianum in the rhizosphere of maize plants during the vegetative plant growth phase on the community of pest and beneficial arthropods associated with maize foliage. Independent of T. harzianum inoculation, the arthropod community was complex and harbored chewing and piercing-sucking herbivores as well as natural enemies. Inoculation with T. harzianum increased the abundance of chewing herbivores, decreased the number of piercing-sucking herbivores and increased the abundance of sampled pest regulating arthropods. In addition, we provide a biochemical basis of shoot metabolites, which may be involved in the alterations of the foliage arthropod community mediated by T. harzianum . Inoculation with T. harzianum caused substantial changes in the levels of sucrose, jasmonic acid (an activator of defense responses against herbivory) and ( Z )-3-hexen-1-ol (a volatile compound that attracts natural enemies of herbivores). In conclusion, our results show that inoculation with T. harzianum can alter the arthropod community associated with maize foliage and reduce the abundance of specific pest insects under field conditions. Graphic abstract

Intraspecific variation in plant traits has been clearly shown to drive the structure of associated arthropod communities at the spatial scale of individual plant populations. Nevertheless, it is largely unknown whether plant trait variation among populations drives landscape-scale variation in arthropod communities, and how the strength of such plant genetic effects compares to, and interacts with, those of environmental variation. We documented the structure of arthropod communities on Artemisia californica for two consecutive years in a common garden of plants sourced from five populations along a 5° latitudinal gradient and grown under precipitation treatments approximating the four-fold difference between the north and south range margins for this species. Previous study of plant traits from this garden documented clinal genetic variation, suggesting local adaptation to this environmental gradient, as well as effects of precipitation manipulation that were consistent among populations (i.e., no genotype-by-environment interaction). Within the common garden, arthropod density, evenness, and diversity increased clinally with population source latitude, and arthropod community composition (i.e., species relative abundance) showed a north-south divide. The 2.6-fold cline of northward increase in arthropod density in the common garden was mirrored by a 6.4-fold increase in arthropod density on wild plants sampled along the species range. In contrast to the strong influence of plant genotype, the precipitation manipulation only influenced arthropod community composition, and plant genetic effects on arthropods operated independently of precipitation regime (no genotype-by-environment interaction). Accordingly, we conclude that the strongest driver of landscape-level variation in arthropod communities in this foundational plant species is not variation in the abiotic environment itself, but rather variation in plant traits underlain by the evolutionary process of plant local adaptation.

Aquatic insects are a common and important subsidy to terrestrial systems, yet little is known about how these inputs affect terrestrial food webs, especially around lakes. Mývatn, a lake in northern Iceland, has extraordinary midge (Chironomidae) emergences that result in large inputs of biomass and nutrients to terrestrial arthropod communities. We simulated this lake-to-land resource pulse by collecting midges from Mývatn and spreading their dried carcasses on 1-m 2 plots at a nearby site that receives very little midge deposition. We hypothesized a positive bottom-up response of detritivores that would be transmitted to their predators and would persist into the following year. We sampled the arthropod community once per month for two consecutive summers. Midge addition resulted in significantly different arthropod communities and increased densities of some taxa in both years. Detritivores, specifically Diptera larvae, Collembola, and Acari increased in midge-addition plots, and so did some predators and parasitoids. Arthropod densities were still elevated a year after midge addition, and two years of midge addition further increased the density of higher-order consumers (e.g., Coleoptera and Hymenoptera). Midge addition increased arthropod biomass by 68%% after one year and 108%% after two years. By manipulating the nutrient pulse delivered by midges we were able to elucidate food web consequences of midge deposition and spatial and temporal dynamics that are difficult to determine based on comparative approaches alone. Resources cross ecosystem boundaries and are assimilated over time because of life-history strategies that connect aquatic and terrestrial food webs and these systems cannot be fully understood in isolation from each other.