Determining the factors affecting the structure of insect herbivore communities is a major challenge in ecology. Previous research demonstrated that plant defenses determine plant-herbivore associations. However, non-defensive variables may also explain why some plant species are associated with more diverse insect herbivore assemblages than others. Neotropical rolled-leaf beetles (Cephaloleia and Chelobasis) complete their life cycle inside the young rolled leaves of their host plants in the order Zingiberales. The diet breadth of each species in this assemblage is particularly well-known at our study site, La Selva Biological Station in Costa Rica. This study focused on the following non-defensive variables: host plant elevational and geographic range size, soil type, habitat, local abundance, plant size, and leaf size. Because plant characteristics among closely related plants are not independent, we analyzed these variables in a phylogenetic context. We detected a positive effect of leaf width on rolled-leaf beetle species richness (explaining 55% of the variation), abundance (28% of the variation and 57% when habitat is included in the model), diversity (37% of the variation), and community structure (6% of the variation, and 21%-26% when taxonomic family is included in the model). Our study demonstrates that Zingiberales leaf width influences positively rolled-leaf beetle species richness, abundance, and diversity. This effect varies among plant families. Our study shows that plant architecture plays an important role in structuring insect herbivore assemblages in Zingiberales. Our results highlight the importance of including variables beyond plant defenses to understand the ecology and evolution of plant-herbivore interactions.

Global change is causing unprecedented degradation of the Earth’s biological systems and thus undermining human prosperity. Past practices have focused either on monitoring biodiversity decline or mitigating ecosystem services degradation. Missing, but critically needed, are management approaches that monitor and restore species interaction networks, thus bridging existing practices. Our overall aim here is to lay the foundations of a framework for developing network management, defined here as the study, monitoring, and management of species interaction networks. We review theory and empirical evidence demonstrating the importance of species interaction networks for the provisioning of ecosystem services, how human impacts on those networks lead to network rewiring that underlies ecosystem service degradation, and then turn to case studies showing how network management has effectively mitigated such effects or aided in network restoration. We also examine how emerging technologies for data acquisition and analysis are providing new opportunities for monitoring species interactions and discuss the opportunities and challenges of developing effective network management. In summary, we propose that network management provides key mechanistic knowledge on ecosystem degradation that links species- to ecosystem-level responses to global change, and that emerging technological tools offer the opportunity to accelerate its widespread adoption.

Insectivorous bird populations are declining globally, as are the insects upon which they depend. Furthermore, many of the plants on which those herbivorous insects depend are being displaced by the spread of agriculture and invasion by exotic species. We discuss the consequences of these declines for changes in trophic control of herbivorous insects by insectivorous birds, and the indirect effects on host plants. We first briefly review the evidence for and causes of bird and insect decline, and the current evidence for trophic control by insectivorous birds. We then hypothesize how trophic control may change under three scenarios: reduced bird populations alone, invasion by exotic insect species and conversion of native habitat to agriculture. We hypothesize that trophic control will decrease under all three scenarios, resulting in higher abundance of herbivorous insects and more frequent outbreaks, higher chronic levels of herbivory and reduced primary productivity. Because birds often specialize to some degree on certain insect species and forage preferentially in certain plant species, reduced trophic control may in turn reduce plant diversity in more native vegetation. Similarly, reduced trophic control in agriculture will require greater reliance on pesticides and, with it, the negative consequences of increased pesticide use.

The net outcomes of mutualisms are mediated by the trade-offs between the costs and benefits provided by both partners. Our review proposes the existence of a trade-off in ant protection mutualisms between the benefits generated by the ants’ protection against the attack of herbivores and the losses caused by the disruption of pollination processes, which are commonly not quantified. This trade-off has important implications for understanding the evolution of extrafloral nectaries (EFNs), an adaptation that has repeatedly evolved throughout the flowering plant clade. We propose that the outcome of this trade-off is contingent on the specific traits of the organisms involved. We provide evidence that the protective mutualisms between ants and plants mediated by EFNs have optimal protective ant partners, represented by the optimum point of the balance between positive effects on plant protection and negative effects on pollination process. Our review also provides important details about a potential synergism of EFN functionality; that is, these structures can attract ants to protect against herbivores and/or distract them from flowers so as not to disrupt pollination processes. Finally, we argue that generalizations regarding how ants impact plants should be made with caution since ants’ effects on plants vary with the identity of the ant species in their overall net outcome.

The evolutionary processes that underlie variation in plant genome size have been much debated. Abiotic factors are thought to have played an important role, with negative and positive correlations between genome size and seasonal or stressful climatic conditions being reported in several systems. In turn, variation in genome size may influence plant traits which affect interactions with other organisms, such as herbivores. The mechanisms underlying evolutionary linkages between plant genome size and biotic and abiotic factors nonetheless remain poorly understod. To address this gap, we conducted phylogenetically controlled analyses testing for associations between genome size, climatic variables, plant traits (defenses and nutrients), and herbivory across 29 oak (Quercus) species. Genome size is significantly associated with both temperature and precipitation seasonality, whereby oak species growing in climates with lower and less variable temperatures but more variable rainfall had larger genomes. In addition, we found a negative association between genome size and leaf nutrient concentration (found to be the main predictor of herbivory), which in turn led to an indirect effect on herbivory. A follow-up test suggested that the association between genome size and leaf nutrients influencing herbivory was mediated by variation in plant growth, whereby species with larger genomes have slower growth rates, which in turn are correlated with lower nutrients. Collectively, these findings reveal novel associations between plant genome size and biotic and abiotic factors that may influence life history evolution and ecological dynamics in this widespread tree genus.

ABSTRACTAimLong‐standing theory predicts that the intensity of biotic interactions increases from high to low latitudes. Studies addressing geographic variation in predation on insect prey have often relied on prey models, which lack many characteristics of live prey. Our goals were to explore global latitudinal patterns of predator attack rates on standardised live insect prey and to compare the patterns in predation on live insects with those on plasticine prey models.LocationGlobal forested areas.Time Period2021–2023.Major TaxaArthropods, birds.MethodsWe measured predation rates in 43 forested locations distributed across five continents from 34.1° S to 69.5° N latitude. At each location, we exposed 20 sets of three bait types, one set per tree. Each set included three live fly larvae (maggots), three live fly puparia and three plasticine models of the puparia. We used glue rings to isolate half of the sets from non‐flying predators.ResultsArthropod attack rates on plasticine prey decreased linearly from low to high latitudes, whereas attack rates on maggots had a U shaped distribution, with the lowest predation rates at temperate latitudes and the highest rates at tropical and boreal latitudes. This difference emerged from intensive predator attacks on live maggots, but not on plasticine models, in boreal sites. Site‐specific attack rates of arthropod predators on live and plasticine prey were not correlated. In contrast, bird attack rates on live maggots and plasticine models were positively correlated, but did not show significant latitudinal changes.Main ConclusionsLatitudinal patterns in predation differ between major groups of predators and between types of prey. Poleward decreases in both arthropod and combined arthropod and bird predation on plasticine models do not mirror patterns of predation on our live prey, the latter likely reflecting real patterns of predation risk better than do patterns of attack on artificial prey.

Investment of plants in defensive and leaf nutritional quality traits may vary with plant ontogeny and the availability of soil nutrients. In this study, we evaluated the effect of ontogeny and soil fertilization on leaf traits of the Brazilian cerrado tree Stryphnodendron adstringens (Fabaceae). Leaf defense traits (total phenolics, condensed and hydrolysable tannins, the number of extrafloral nectaries (EFNs) and the concentration of glucose, sucrose, fructose and total sugar in the extrafloral nectar) and leaf nutritional quality traits (water content, macronutrients, micronutrients and N:P and C:N ratios) were measured in 60 seedlings and 13 reproductive adults that grew in areas with previous P and Ca fertilization over eight years and in neighboring control plots in a Cerrado area in Distrito Federal, Brazil. Sampling was carried out three years after the last application of fertilizer. We found a strong effect of ontogeny on leaf traits: leaves of adult plants were more chemically defended and lower in nutritional quality than those of seedlings. Fertilization modified leaf nutritional quality traits in adults and seedlings, and defense traits in seedlings, increasing the number of EFNs on leaves, total phenolics and condensed tannins. In summary, adult plants of S. adstringens were poor in nutrients and invested more in phenolics and tannins than seedlings, all consistent with plant apparency theory. In contrast, seedlings were much more flexible in their defensive phenotype, with fertilization increasing allocation to total phenolics, condensed tannins and EFN production. This latter result suggests an ability of seedlings to take advantage of very local variation in resources that might increase the likelihood of surviving this vulnerable stage in the life cycle.

Leaf shelter construction by herbivorous insects can improve leaf quality, sometimes changing resultant herbivory. In two experiments in a Missouri (USA) deciduous forest we quantified the impact of leaf tie construction and changes to leaf quality on subsequent leaf damage. First, using eight Quercus species, we compared damage to single leaves versus experimental leaf ties that had been stocked with either Pseudotelphusa quercinigracella (Gelechiidae) or Psilocorsis cryptolechiella (Depressariidae) to determine how initial leaf quality (total phenolics) influenced damage caused by shelter inhabitants. Skeletonization by leaf tying caterpillars and leaf edge chewing by free feeding species were 12.2× and 1.3× greater on tied than on non‐tied leaves, respectively. July and September leaf phenolic content had a slight positive effect on the probability of skeletonization, none on the probability of edge damage, and a weakly positive or negative effect on the intensity of skeletonization and edge damage, depending on leaf position. Second, we created experimental leaf ties, protected from herbivores, on the same Quercus species to determine whether tie formation changes leaf quality (total phenolics, nitrogen, water, toughness). Tie formation decreased phenolics, but this change was predicted to add only 0.8% leaf area loss. Synthesis. Herbivory increased dramatically when leaves were in ties, with the effect mostly due to the tie itself rather than a change in leaf quality. We predict that the advantages of building and using leaf ties in this system are more likely to be escape from natural enemies and changes in abiotic factors.

Shelter building caterpillars act as ecosystem engineers by creating and maintaining leaf shelters, which are then colonized by other arthropods. Foliage quality has been shown to influence initial colonization by shelter-building caterpillars. However, the effects of plant quality on the interactions between ecosystem engineers and their communities have yet to be studied at the whole plant level. We examined how leaf tying caterpillars, as ecosystem engineers, impact arthropod communities on Quercus alba (white oak), and the modifying effect of foliage quality on these interactions. We removed all leaf tying caterpillars and leaf ties on 35 Q. alba saplings during the season when leaf tying caterpillars were active (June–September), and compared these leaf tie removal trees to 35 control trees whose leaf ties were left intact. Removal of these ecosystem engineers had no impact on overall arthropod species richness, but reduced species diversity, and overall arthropod abundance and that of most guilds, and changed the structure of the arthropod community as the season progressed. There was an increase in plant-level species richness with increasing number of leaf ties, consistent with Habitat Diversity Hypothesis. In turn, total arthropod density, and that of both leaf tying caterpillars and free-feeding caterpillars were affected by foliar tannin and nitrogen concentrations, and leaf water content. The engineering effect was greatest on low quality plants, consistent with the Stress-Gradient Hypothesis. Our results demonstrate that interactions between ecosystem engineering and plant quality together determine community structure of arthropods on Q. alba in Missouri.

Accumulating evidence suggests that herbivorous insects influence the local composition and richness of Neotropical plant species, particularly in species-rich genera. Species richness, phylogenetic diversity, and chemical diversity all influence the ability of insect herbivores to find and utilize their hosts. The relative impact of these components of diversity on species coexistence and plant-herbivore interactions is not well understood. We constructed 60 local communities of up to 13 species of Piper (Piperaceae) in native, mature forest at a lowland wet forest location in Costa Rica. The species composition of each community was chosen such that species richness, phylogenetic diversity, and GCMS-based chemical diversity were varied independently among communities. We predicted that chemical diversity would most strongly affect the communities across time, with smaller effects of taxonomic and phylogenetic diversity. At 13 months after the experimental planting, we assessed survivorship of each cutting, measured total leaf area loss of the survivors, leaf area loss to generalist and specialist herbivorous insect species, and local extinction of species. Generalist and specialist herbivory decreased with increasing levels of species richness and phylogenetic diversity, respectively. Surprisingly, there was no independent effect of chemical diversity on any of the three measures of herbivore damage. Nevertheless, plots with a higher chemical and phylogenetic diversity showed decreased plant mortality and local species extinction. Overall, our results suggested that both chemical and phylogenetic similarity are important factors in the assembly and maintenance of tropical plant communities. The fact that chemical diversity influences plant mortality suggests that leaf herbivores, and possibly other plant natural enemies, could increase plant diversity via the selective mortality of similar chemotypes.

Construct (“shelter”)-building caterpillars are those that build structures on or in plants. When these constructs are colonized by other arthropods, the caterpillars act as ecosystem engineers. Here we describe the known and predicted impacts of caterpillars on their associated arthropod faunas, and how such impacts might change with increasing influence of humans on the global ecosystem. First, we provide evidence that the presence of constructs built by caterpillars influences community composition of arthropods on plants, encourages the initial invasion and abundance of exotic arthropod species, and influences the amount of damage inflicted on the plant. Second, we describe a qualitative model of the influence of construct traits (openness, presence of frass, and volume) on colonization by other arthropods. We predict that constructs with one or more openings, those that contain frass, and those with larger internal volumes will attract more colonists. Third, available data suggest increasing drought and temperature will reduce the abundance of constructs and increase the use of those constructs by predators, while increasing rainfall and higher predation will increase the use of constructs by herbivorous insects. Altogether, studies are needed that quantify current environmental impacts on both construct building by caterpillars and the use of those constructs by other arthropods. Such studies will necessitate a tritrophic approach, i.e., understanding the role of host plant variation, the responses of caterpillars and associated arthropods to each other and to the host plant, and the role of natural enemies in shaping the use of caterpillar constructs.

The construction of shelters on plants by arthropods might influence other organisms via changes in colonization, community richness, species composition, and functionality. Arthropods, including beetles, caterpillars, sawflies, spiders, and wasps often interact with host plants via the construction of shelters, building a variety of structures such as leaf ties, tents, rolls, and bags; leaf and stem galls, and hollowed out stems. Such constructs might have both an adaptive value in terms of protection (i.e., serve as shelters) but may also exert a strong influence on terrestrial community diversity in the engineered and neighboring hosts via colonization by secondary occupants. Although different traits of the host plant (e.g., physical, chemical, and architectural features) may affect the potential for ecosystem engineering by insects, such effects have been, to a certain degree, overlooked. Further analyses of how plant traits affect the occurrence of shelters may therefore enrich our understanding of the organizing principles of plant-based communities. This data set includes more than 1000 unique records of ecosystem engineering by arthropods, in the form of structures built on plants. All records have been published in the literature, and span both natural structures (91% of the records) and structures artificially created by researchers (9% of the records). The data were gathered between 1932 and 2021, across more than 50 countries and several ecosystems, ranging from polar to tropical zones. In addition to data on host plants and engineers, we aggregated data on the type of constructs and the identity of inquilines using these structures. This data set highlights the importance of these subtle structures for the organization of terrestrial arthropod communities, enabling hypotheses testing in ecological studies addressing ecosystem engineering and facilitation mediated by constructs. There are no copyright restrictions and please cite this paper when using the data in publications.

Current climate change is disrupting biotic interactions and eroding biodiversity worldwide. However, species sensitive to aridity, high temperatures, and climate variability might find shelter in microclimatic refuges, such as leaf rolls built by arthropods. To explore how the importance of leaf shelters for terrestrial arthropods changes with latitude, elevation, and climate, we conducted a distributed experiment comparing arthropods in leaf rolls versus control leaves across 52 sites along an 11,790 km latitudinal gradient. We then probed the impact of short- versus long-term climatic impacts on roll use, by comparing the relative impact of conditions during the experiment versus average, baseline conditions at the site. Leaf shelters supported larger organisms and higher arthropod biomass and species diversity than non-rolled control leaves. However, the magnitude of the leaf rolls' effect differed between long- and short-term climate conditions, metrics (species richness, biomass, and body size), and trophic groups (predators vs. herbivores). The effect of leaf rolls on predator richness was influenced only by baseline climate, increasing in magnitude in regions experiencing increased long-term aridity, regardless of latitude, elevation, and weather during the experiment. This suggests that shelter use by predators may be innate, and thus, driven by natural selection. In contrast, the effect of leaf rolls on predator biomass and predator body size decreased with increasing temperature, and increased with increasing precipitation, respectively, during the experiment. The magnitude of shelter usage by herbivores increased with the abundance of predators and decreased with increasing temperature during the experiment. Taken together, these results highlight that leaf roll use may have both proximal and ultimate causes. Projected increases in climate variability and aridity are, therefore, likely to increase the importance of biotic refugia in mitigating the effects of climate change on species persistence.

Here, we introduce the concept of “caterpillars in the middle”, a theme echoed throughout the chapters of this book. Caterpillars must deal with plant defenses while at the same escape the life threatening impact of their natural enemies, predators, parasitoids, and disease. We foreshadow the multitude of topics to be covered in this compendium, including adaptation, behavior, defenses, evolution, morphology, physiology, and conservation. We reflect on how the work of early natural historians led to our greater understanding of Lepidoptera life histories. We finish by paying homage to many published works in this field that have given rise to the work of scientists contributing to this volume and explain how this book came to be.

We place the information entailed within this book in the larger context of caterpillar studies, beginning with the early naturalists and ranging through various new approaches made available by advancing technology and statistics. We suggest two major research areas that could profitably become the foci of research in the near future: forces driving the diversity of caterpillar morphology, color, and behavior, with a focus on larval ontogeny; and anthropogenic influences on declining Lepidoptera diversity, especially impacts via the caterpillar stage of the lepidopteran life cycle. The research results covered in this volume provide overwhelming evidence that both the generation and maintenance of Lepidoptera diversity can only be understood in the context of tritrophic interactions.

The majority of tropical arboreal ant species nest in tree cavities. These cavities, often produced initially by wood-boring beetles, can be in live or dead wood and represent long-lasting and highly defensible nesting resources. Yet the size of cavity entrances can constrain their use. Active entrance modification may be an effective way to overcome this constraint. Here, we conduct the first systematic study of nest-entrance modification in an arboreal ant community. Using field experiments deployed across a number of tree species, we show that 14% of 2631 experimental cavities were modified by either enlargement, or reducing entrance size by construction. Entrance modifications, which were made by a majority (18/29 species) of the species that occupied experimental nests, used a variety of construction techniques and materials. Combined, these modifications were context-dependent with respect to available entrance sizes: Enlargement was more common when the diversity of available entrance sizes was limited, whereas reduction was more prevalent when the diversity of entrance sizes was higher. Nevertheless, the context of tree species identity did not significantly influence the number of modified cavities or the construction materials. Overall, we show that nest-entrance modification is a widespread, active, and context-dependent strategy in the nesting ecology of arboreal ants.

Current climate change is disrupting biotic interactions and eroding biodiversity worldwide. However, species sensitive to drought, high temperatures and climate variability might persist in microclimatic refuges, such as leaf shelters built by arthropods. We conducted a distributed experiment across an 11,790 km latitudinal gradient to explore how the importance of leaf shelters for terrestrial arthropods changes with latitude, elevation and underlying climate. Our analyses revealed leaf shelters to be key facilitative elements for the diversity of arthropods. Predator diversity and overall biomass within shelters increased with local drought and temperature variability, regardless of latitude and elevation. In contrast, shelter usage by herbivores increased with abundance of predators on those same plants and in wetter climates. Projected increase in climatic variability and drought in certain geographic regions is therefore likely to enhance the importance of biotic refuges, especially for predators, in mitigating the impact of climate change on species persistence.

Wood-boring beetle larvae act as ecosystem engineers by creating stem cavities that are used secondarily as nests by many arboreal ant species. Understanding the heterogeneity and distribution of available cavities and their use by ants is therefore key to understanding arboreal ant community assembly and diversity. Our goals were to quantify the abundance and diversity of beetle-produced cavity resources in a tropical canopy, reveal how ants use these resources, and determine which characteristics of the cavity resource contribute to ant use. We dissected branches from six common tree species in the Brazilian Cerrado savanna, measuring cavity characteristics and identifying the occupants. We sampled 2310 individual cavities, 576 of which were used as nests by 25 arboreal ant species. We found significant differences among tree species in the proportion of stem length bored by beetles, the number of cavities per stem length, average entrance-hole size, and the distribution of cavity volumes. The likelihood that a cavity was occupied was greater for cavities with larger entrance-hole sizes and larger volumes. In particular, there was a strong positive correlation between mean head diameters of ant species and the mean entrance-hole diameter of the cavities occupied by those ant species. Wood-boring beetles contribute to the structuring of the Cerrado ant community by differentially attacking the available tree species. In so doing, the beetles provide a wide range of entrance-hole sizes which ant species partition based on their body size, and large volume cavities that ants appear to prefer.