Insects affect relationships between plant species richness and ecosystem processes

This study examined whether insects can alter relationships between plant species diversity and ecosystem function in grassland communities, by (i) altering biomass across a plant diversity gradient, (ii) altering relative abundances of plant species, or (iii) altering ecosystem function directly. We measured herbivore damage on seminatural grassland plots planted with 1, 2, 4, 8, or 12 plant species, and compared plant biomass in a subset of these plots with replicates in which insect levels were reduced. Plant biomass and herbivore damage increased with species richness. Reducing insect populations resulted in greater evenness of relative plant species abundances and revealed a strong positive relationship between plant species richness and above-ground biomass. Reducing insects also changed the relationship between plant species richness and decomposition. Plant species mixtures and their relative abundances partially explained plant biomass results, but not decomposition results. These results suggest that insects can alter relationships between plant diversity and ecosystem processes through all three mechanisms.     

Invariant antagonistic network structure despite high spatial and temporal turnover of interactions

Recent work has suggested that emergent ecological network structure exhibits very little spatial or temporal variance despite changes in community composition. However, the changes in network interactions associated with turnover in community composition have seldom been assessed. Here we examine whether changes in ecological networks are best detected by standard emergent network metrics or by assessing internal network changes (i.e. interaction and composition turnover). To eliminate possible spatial or phylogenetic effects, that in large‐scale studies may obscure mechanisms structuring networks and interactions, we sampled multiple antagonistic (plant–herbivore) networks for a single diverse plant family (the Restionaceae) in the hyperdiverse Cape Floristic Region. These are the first plant–herbivore networks constructed for this global biodiversity hotspot. We found invariant emergent network structure despite considerable changes in insect and plant composition across communities over time and space. In contrast, there was high interaction turnover between networks. Seasonally, this was driven by turnover in insect species and insect host switching. Spatially, this was driven by simultaneous turnover in plant and insect species, suggesting that many insects are host specific or that both groups exhibit parallel responses to environmental gradients. Spatial interaction turnover was also driven by turnover in plants, showing that many insects can utilise multiple (possibly closely related) hosts and this may create divergent selection gradients that promote insect speciation. Thus we show highly variable interaction fidelity, despite invariant emergent network structure. We suggest that evaluating internal network changes may be more effective at elucidating the processes structuring networks, and many fine‐scale changes may be obscured when only calculating emergent network metrics.     

Ecological interactions among insect herbivores,ants and the host plant Baccharis dracunculifolia in a Brazilian mountain ecosystem

Insect–plant interactions occur in several ways and have considerable environmental and ecological importance. Many feeding strategies have evolved among herbivorous insects, with host–herbivore systems likely being influenced by trophobionts with ants. We investigated how these interactions vary across elevation gradients by evaluating the structure of the herbivorous insect community and ants associated with Baccharis dracunculifolia at three distinct elevations (800, 1100, and 1400 m a.s.l.) on a mountain in southeastern Brazil. Moreover, we evaluated the diversity and specialisation of interactions between herbivores and host plants along the elevational gradient. We sampled herbivores and ants on 60 plants at each elevation (totalling 180 plant individuals). Herbivore species composition differed among elevations, as did interaction diversity and specialisation. Richness and abundance of chewing insects increased with elevation, while β‐diversity among patches of the host plant was higher at the lowest elevation, probably due to the patchy occurrence of B. dracunculifolia. Richness and abundance of sap‐sucking insects were higher at the intermediate elevation, possibly due to local environmental conditions. We observed a positive relationship between ant and herbivore trophobiont richness on B. dracunculifolia. We found that interactions were more specialised and less diverse at higher elevations compared to the lowest elevation. Changes in vegetation and environmental variables shaped species distributions and their ecological interactions along the elevation gradient. Our study demonstrates that increased elevation changes the structure and patterns of interactions of the herbivore insect guilds associated with the host plant B. dracunculifolia. Ant effects depend on the context, the environment, and the species of ants involved, and are essential for the presence of insect trophobionts.     

Phytophagous insect fauna tracks host plant responses to exotic grass invasion

The high dependence of herbivorous insects on their host plants implies that plant invaders can affect these insects directly, by not providing a suitable habitat, or indirectly, by altering host plant availability. In this study, we sampled Asteraceae flower heads in cerrado remnants with varying levels of exotic grass invasion to evaluate whether invasive grasses have a direct effect on herbivore richness independent of the current disturbance level and host plant richness. By classifying herbivores according to the degree of host plant specialization, we also investigated whether invasive grasses reduce the uniqueness of the herbivorous assemblages. Herbivorous insect richness showed a unimodal relationship with invasive grass cover that was significantly explained only by way of the variation in host plant richness. The same result was found for polyphagous and oligophagous insects, but monophages showed a significant negative response to the intensity of the grass invasion that was independent of host plant richness. Our findings lend support to the hypothesis that the aggregate effect of invasive plants on herbivores tends to mirror the effects of invasive plants on host plants. In addition, exotic plants affect specialist insects differently from generalist insects; thus exotic plants affect not only the size but also the structural profile of herbivorous insect assemblages.     

Insect-damaged fossil leaves record food web response to ancient climate change and extinction

Plants and herbivorous insects have dominated terrestrial ecosystems for over 300 million years. Uniquely in the fossil record, foliage with well-preserved insect damage offers abundant and diverse information both about producers and about ecological and sometimes taxonomic groups of consumers. These data are ideally suited to investigate food web response to environmental perturbations, and they represent an invaluable deep-time complement to neoecological studies of global change. Correlations between feeding diversity and temperature, between herbivory and leaf traits that are modulated by climate, and between insect diversity and plant diversity can all be investigated in deep time. To illustrate, I emphasize recent work on the time interval from the latest Cretaceous through the middle Eocene (67-47 million years ago (Ma)), including two significant events that affected life: the end-Cretaceous mass extinction (65.5 Ma) and its ensuing recovery; and globally warming temperatures across the Paleocene-Eocene boundary (55.8 Ma). Climatic effects predicted from neoecology generally hold true in these deep-time settings. Rising temperature is associated with increased herbivory in multiple studies, a result with major predictive importance for current global warming. Diverse floras are usually associated with diverse insect damage; however, recovery from the end-Cretaceous extinction reveals uncorrelated plant and insect diversity as food webs rebuilt chaotically from a drastically simplified state. Calibration studies from living forests are needed to improve interpretation of the fossil data.     

On the elusiveness of enemy-free space: spatial, temporal, and host-plant-related variation in parasitoid attack rates on three gallmakers of goldenrods

Host shifting by phytophagous insects may play an important role in generating insect diversity by initiating host-race formation and speciation. Models of the host shifting process often invoke reduced rates of natural enemy attack on a novel host in order to balance the maladaptation expected following the shift. Such "enemy-free space" has been documented for some insects, at some times and places, but few studies have assessed the occurrence of enemy-free space across years, among sites, or among insect species. We measured parasitoid attack rates on three insect herbivores of two goldenrods (Solidago altissima L. and Solidago gigantea Ait.), with data from multiple sites and multiple years for each herbivore. For each insect herbivore, there were times and sites at which parasitoid attack rates differed strongly and significantly between host plants (that is, enemy-free space existed on one host plant or the other). However, the extent and even the direction of the attack-rate difference varied strongly among sites and even among years at the same site. There was no evidence of consistent enemy-free space for any herbivore on either host plant. Our data suggest that enemy-free space, like many ecological and evolutionary forces, is likely to operate as a geographic and temporal mosaic, and that conceptual models of host shifting that include enemy-free space as a consequence of host novelty are likely too simple.     

Herbivore regulation of plant abundance in aquatic ecosystems

Herbivory is a fundamental process that controls primary producer abundance and regulates energy and nutrient flows to higher trophic levels. Despite the recent proliferation of small‐scale studies on herbivore effects on aquatic plants, there remains limited understanding of the factors that control consumer regulation of vascular plants in aquatic ecosystems. Our current knowledge of the regulation of primary producers has hindered efforts to understand the structure and functioning of aquatic ecosystems, and to manage such ecosystems effectively. We conducted a global meta‐analysis of the outcomes of plant–herbivore interactions using a data set comprised of 326 values from 163 studies, in order to test two mechanistic hypotheses: first, that greater negative changes in plant abundance would be associated with higher herbivore biomass densities; second, that the magnitude of changes in plant abundance would vary with herbivore taxonomic identity. We found evidence that plant abundance declined with increased herbivore density, with plants eliminated at high densities. Significant between‐taxa differences in impact were detected, with insects associated with smaller reductions in plant abundance than all other taxa. Similarly, birds caused smaller reductions in plant abundance than echinoderms, fish, or molluscs. Furthermore, larger reductions in plant abundance were detected for fish relative to crustaceans. We found a positive relationship between herbivore species richness and change in plant abundance, with the strongest reductions in plant abundance reported for low herbivore species richness, suggesting that greater herbivore diversity may protect against large reductions in plant abundance. Finally, we found that herbivore–plant nativeness was a key factor affecting the magnitude of herbivore impacts on plant abundance across a wide range of species assemblages. Assemblages comprised of invasive herbivores and native plant assemblages were associated with greater reductions in plant abundance compared with invasive herbivores and invasive plants, native herbivores and invasive plants, native herbivores and mixed‐nativeness plants, and native herbivores and native plants. By contrast, assemblages comprised of native herbivores and invasive plants were associated with lower reductions in plant abundance compared with both mixed‐nativeness herbivores and native plants, and native herbivores and native plants. However, the effects of herbivore–plant nativeness on changes in plant abundance were reduced at high herbivore densities. Our mean reductions in aquatic plant abundance are greater than those reported in the literature for terrestrial plants, but lower than aquatic algae. Our findings highlight the need for a substantial shift in how biologists incorporate plant–herbivore interactions into theories of aquatic ecosystem structure and functioning. Currently, the failure to incorporate top‐down effects continues to hinder our capacity to understand and manage the ecological dynamics of habitats that contain aquatic plants.     

The population ecology of novel plant–herbivore interactions

Many native herbivores are known to attack exotic plants, and we can expect these interactions to occur with increasing frequency in coming years as invasive plants become naturalized and new invaders arrive in native communities. In some cases, evolutionary biologists and ecologists have learned a great deal from insects adapting to novel hosts. However, there is more to be learned and we suggest that the ecological study of exotic host colonization by native insects has been impeded by a lack of focus in the questions being asked, and also from overlap with other areas of plant–insect ecology, including the study of specialization. In the present paper, a conceptual model is described for the colonization of a novel host‐plant, which focuses on the relationship between occupancy and availability. Occupancy is the fraction of patches of novel hosts that are utilized by an herbivore, and availability is the abundance or presence of a novel host on the landscape. Considering the slope of that relationship (between occupancy and availability), hypotheses are suggested that involve dispersal and, most important, population growth rate of an insect herbivore associated with an exotic host. A focus on the occupancy–availability relationship highlights the strengths and weaknesses of common experimental approaches, such as preference–performance experiments. Suggestions for future work are offered that include integration with evolutionary theory and exploration of more complex demographic and ecological scenarios.     

Water stress in grasslands: dynamic responses of plants and insect herbivores

Global climate change is altering precipitation patterns. The effect of water stress on plant–herbivore interactions is poorly understood even though this is a primary ecological interaction that will be altered by climate change. This is especially true for grasslands where water is often limiting. In this study we manipulated water inputs in open grassland plots (1 m²) during a severe drought and assessed plant and insect herbivore responses. There were two watering treatments: ambient and supplemented. Supplemented plots received water weekly in amounts that mimicked average seasonal rainfall. For plants, we were interested in how water input affected protein and digestible carbohydrate content; previous studies predicted water stress would increase the concentration of these two nutrients. Grasshoppers are the dominant insect herbivores in grasslands and we assessed their responses to water inputs by measuring abundance and diversity. Previous studies suggested grasshoppers would prefer water‐stressed plots. Protein and carbohydrate content in bulk grass and forb samples, plus plant biomass and diversity, were measured monthly (May–August). Immediately prior to harvesting plant tissue, we counted and identified individual grasshoppers in each plot. Grass biomass was reduced with water stress, but macronutrient content and species diversity were unaffected. After three months water‐stressed forbs were less protein biased, and diverse, relative to watered forbs; forb biomass was indistinguishable between treatments. Grasshopper abundance and diversity were lower in water‐stressed plots as the season progressed. However, grasshopper‐feeding biology mattered: densities of mixed‐feeders and grass‐feeders, but not forb‐specialists, decreased over time in water‐stressed plots, but not in water supplemented plots. Our results demonstrate the importance of focusing on plant and insect herbivore functional groups and provide valuable new data that can be incorporated into models to explore the effects of global climate change in greater detail.     

Impacts of climate warming on hybrid zone movement: Geographically diffuse and biologically porous “species borders”

Abstract The ecology and evolutionary biology of insect–plant associations has realized extensive attention, especially during the past 60 years. The classifications (categorical designations) of continuous variation in biodiversity, ranging from global patterns (e.g., latitudinal gradients in species richness/diversity and degree of herbivore feeding specialization) to localized insect–plant associations that span the biospectrum from polyphenisms, polymorphisms, biotypes, demes, host races, to cryptic species, remain academically contentious. Semantic and biosystematic (taxonomical) disagreements sometimes detract from more important ecological and evolutionary processes that drive diversification, the dynamics of gene flow and local extinctions. This review addresses several aspects of insect specialization, host‐associated divergence and ecological (including “hybrid”) speciation, with special reference to the climate warming impacts on species borders of hybridizing swallowtail butterflies (Papilionidae). Interspecific hybrid introgression may result in collapse of multi‐species communities or increase species numbers via homoploid hybrid speciation. We may see diverging, merging, or emerging genotypes across hybrid zones, all part of the ongoing processes of evolution. Molecular analyses of genetic mosaics and genomic dynamics with “divergence hitchhiking”, combined with ecological, ethological and physiological studies of “species porosity”, have already begun to unveil some answers for some important ecological/evolutionary questions. (i) How rapidly can host‐associated divergence lead to new species (and why doesn't it always do so, e.g., resulting in “incomplete” speciation)? (ii) How might “speciation genes” function, and how/where would we find them? (iii) Can oscillations from specialists to generalists and back to specialists help explain global diversity in herbivorous insects? (iv) How could recombinant interspecific hybridization lead to divergence and speciation? From ancient phytochemically defined angiosperm affiliations to recent and very local geographical mosaics, the Papilionidae (swallowtail butterflies) have provided a model for enhanced understanding of ecological patterns and evolutionary processes, including host‐associated genetic divergence, genomic mosaics, genetic hitchhiking and sex‐linked speciation genes. Apparent homoploid hybrid speciation in Papilio appears to have been catalyzed by climate warming‐induced interspecific introgression of some, but not all, species diagnostic traits, reflecting strong divergent selection (discordant), especially on the Z (= X) chromosome. Reproductive isolation of these novel recombinant hybrid genotypes appears to be accomplished via a delayed post‐diapause emergence or temporal isolation, and is perhaps aided by the thermal landscape. Changing thermal landscapes appear to have created (and may destroy) novel recombinant hybrid genotypes and hybrid species.     

Intraspecific variation in plant size,secondary plant compounds,herbivory and parasitoid assemblages during secondary succession

During secondary succession on abandoned agricultural fields the diversity and abundance of insect communities often increases, whereas the performance and nutritional quality of early successional plants often declines. As the diversity and abundance of insects on a single plant are determined by characteristics of the environment as well as of the host plant, it is difficult to predict how insects associated with a single plant species will change during succession. We examined how plant characteristics of the early successional plant species ragwort (Jacobaea vulgaris), and the herbivores and parasitoids associated with these plants change during secondary succession. In ten grasslands that differed in time since abandonment (3–26 years), we measured the size and primary and secondary chemistry of individual ragwort plants. For each plant we also recorded the presence of herbivores in flowers, leaves and stems, and reared parasitoids from these plant parts. Ragwort plants were significantly larger but had lower nitrogen concentrations in recently abandoned sites than in older sites. Pyrrolizidine alkaloid (PA) composition varied among plants within sites but also differed significantly among sites. However, there was no relationship between the age of a site and PA composition. Even though plant size decreased with time since abandonment, the abundance of stem-boring insects and parasitoids emerging from stems significantly increased with site age. The proportion of plants with flower and leaf herbivory and the number of parasitoids emerging from flowers and leaves was not related to site age. Parasitoid diversity significantly increased with site age. The results of our study show that ragwort and insect characteristics both change during secondary succession, but that insect herbivore and parasitoid abundances are not directly related to plant size or nutritional quality.     

Comparing galling insect richness among Neotropical savannas: effects of plant richness, vegetation structure and super-host presence

Plant species diversity maintains the stability of ecosystems and the diversity of consumer species such as insect herbivores. Considering that gall-inducing insects are highly specialized on their host plants and dependent on the occurrence, abundance and distribution of plants, we evaluated the diversity patterns of gall-inducing insect along Brazilian Neotropical savannas and the potential role of plant species richness, vegetation structure and super-host presence on determining these patterns. We found 1,882 individual plants that belonged to 64 different host plant species grouped in 31 families, associated to 112 galling insect species. The galling richness was positively influenced by plant species richness and the presence of the super-host genus Qualea (Vochysiaceae). Plant species richness explained 48 % of the galling richness and areas with presence of super-hosts had more than twice of galling species than areas where they were absent. On the other hand, we found no evidence that larger plants hosted more species of galling insects. We observed that for the diversity of galling insects in the Brazilian Cerrado, vegetation structure explained almost the same portion as plant richness, because structural variables did not have an effect on residuals of galling richness and plant richness regression. Our findings suggests that plant richness has a more important role on the mitigation of natural enemies and adaptive radiation of galling species, while structural aspects of the vegetation does not seem to have that effect. Furthermore, we show that the super-host taxa provide an increment in local galling richness because they present a great diversity of local number of gall morphospecies (i.e. alpha diversity) and the high turnover of morphospecies among different localities (i.e. beta diversity). Therefore we argue that the quality of resources (richness and super host presence) appears to be a most important factor for the diversity of galling insects in Neotropical systems, than the amount of resources.     

Context dependency of insect and mammalian herbivore effects on tall thistle (Cirsium altissimum) populations

Aims Identifying factors that drive variation in herbivore effects on plant populations can provide insight for explaining plant distributions and for limiting weeds. Abiotic resource availability to plants is a key explanation for variation in herbivore effects on individual plants, but the role of resources in determining herbivore effects on plant populations is largely unexplored. We tested the hypothesis that soil nutrient availability drives variation in insect and mammal herbivore effects on tall thistle (Cirsium altissimum) population growth.     

To speciate,or not to speciate? Resource heterogeneity,the subjectivity of similarity,and the macroevolutionary consequences of niche‐width shifts in plant‐feeding insects

Coevolutionary studies on plants and plant‐feeding insects have significantly improved our understanding of the role of niche shifts in the generation of new species. Evolving plant lineages essentially constitute moving islands and archipelagoes in resource space, and host shifts by insects are usually preceded by colonizations of novel resources. Critical to hypotheses concerning ecological speciation is what happens immediately before and after colonization attempts: if an available plant is too similar to the current host(s), it simply will be incorporated into the existing diet, but if it is too different, it will not be colonized in the first place. It thus seems that the probability of speciation is maximized when alternative hosts are at an ‘intermediate’ distance in resource space. In this review, I wish to highlight the possibility that resource similarity and, thus, the definition of ‘intermediate’, are subjective concepts that depend on the herbivore lineage's tolerance to dietary variation. This subjectivity of similarity means that changes in tolerance can either decrease or increase speciation probabilities depending on the distribution of plants in resource space: insect lineages with narrow tolerances are likely to speciate by ‘island‐hopping’ on young, species‐rich plant groups, whereas more generalized lineages could speciate by shifting among resource archipelagoes formed by higher plant taxa. Repeated and convergent origins of traits known to broaden or to restrict host‐plant use in multiple different insect groups provide opportunities for studying how tolerance and resource heterogeneity may interact to determine speciation rates.     

Effects of genetic diversity of grass on insect species diversity at higher trophic levels are not due to cascading diversity effects

Genetic diversity in plant populations has been shown to affect the species diversity of insects. In grasses, infection with fungal endophytes can also have strong effects on insects, potentially modifying the effects of plant genetic diversity. We manipulated the genetic diversity and endophyte infection of a grass in a field experiment. We show that diversity of primary parasitoids (3rd trophic level) and, especially, secondary parasitoids (4th trophic level) increases with grass genetic diversity while there was no effect of endophyte infection. The increase in insect diversity appeared to be due to a complementarity effect rather than a sampling effect. The higher parasitoid diversity could not be explained by a cascading diversity effect because herbivore diversity was not affected and the same herbivore species were present in all treatments. The effects on the higher trophic levels must therefore be due to a direct response to plant traits or mediated by effects on traits at intermediate trophic levels.     

Human impacts, energy availability and invasion across Southern Ocean Islands

Aim Ongoing biological invasions will enhance the impacts of humans on biodiversity. Nonetheless, the effects of exotic species on diversity are idiosyncratic. Increases in diversity might be a consequence of similar responses by species to available energy, or because of positive relationships between human density, energy and propagule pressure. Here we use data from the Southern Ocean island plants and insects to investigate these issues. Location The Southern Ocean Islands ranging from Tristan da Cunha to Heard Island and South Georgia. Methods Generalized linear models are used to explore the relationships between indigenous and exotic species richness for plants and insects on two different islands. Similar models are used to examine interactions between indigenous and exotic species richness, energy availability and propagule pressure at the regional scale. Results Positive relationships were found between indigenous and exotic species richness at local scales, although for plants, the relationship was partially triangular. Across the Southern Ocean Islands, there was strong positive covariation between indigenous and exotic plant species richness and insect species richness, even taking spatial autocorrelation into account. Both exotic and indigenous plant and insect species richness covaried with energy availability, as did human visitor frequency. When two islands with almost identical numbers of human visits were contrasted, it was clear that energy availability, or perhaps differences in climate‐matching, were responsible for differences in the extent of invasion. Conclusion In plants and insects, there are positive relationships between indigenous and exotic diversity at local and regional scales across the Southern Ocean islands. These relationships are apparently a consequence of similar responses by both groups and by human occupants to available energy. When visitor frequency is held constant, energy availability is the major correlate of exotic species richness, though the exact mechanistic cause of this relationship requires clarification.     

Detecting local diversity‐dependence in diversification

Whether there are ecological limits to species diversification is a hotly debated topic. Molecular phylogenies show slowdowns in lineage accumulation, suggesting that speciation rates decline with increasing diversity. A maximum‐likelihood (ML) method to detect diversity‐dependent (DD) diversification from phylogenetic branching times exists, but it assumes that diversity‐dependence is a global phenomenon and therefore ignores that the underlying species interactions are mostly local, and not all species in the phylogeny co‐occur locally. Here, we explore whether this ML method based on the nonspatial diversity‐dependence model can detect local diversity‐dependence, by applying it to phylogenies, simulated with a spatial stochastic model of local DD speciation, extinction, and dispersal between two local communities. We find that type I errors (falsely detecting diversity‐dependence) are low, and the power to detect diversity‐dependence is high when dispersal rates are not too low. Interestingly, when dispersal is high the power to detect diversity‐dependence is even higher than in the nonspatial model. Moreover, estimates of intrinsic speciation rate, extinction rate, and ecological limit strongly depend on dispersal rate. We conclude that the nonspatial DD approach can be used to detect diversity‐dependence in clades of species that live in not too disconnected areas, but parameter estimates must be interpreted cautiously.     

Lianas as a food resource for herbivorous insects: a comparison with trees

Woody climbers or, ‘lianas’, are one of the features that characterise rainforests. They contribute substantially to plant diversity and leaf biomass which makes them a potentially important food source for herbivores. Here, we focus on insect herbivores, folivores in particular, to show how disparities in the quantitative and qualitative availability of leaves between lianas and trees may differentially influence insect folivory and the herbivore communities themselves. We develop a conceptual model and show that lianas in general have lower structural and chemical defences, a greater nutritional profile and a preferable phenology in comparison with trees, which, contrary to our expectations, has led to assemblages of more‐specialised insects. The impacts this has on higher trophic levels and broader ecological networks, however, are poorly known. We show through a study of four tropical floras from different biogeographic realms that lianas are likely to be a target for a wide range of insect herbivore taxa as they are a phylogenetically diverse group and increase diversity of higher taxa at local scales. This, in combination with their highly palatable leaves, may also make them a suitable temporary food source for insects during times when preferred host plants are scarce. This phenomenon has been observed in mammalian herbivores but awaits investigation in insects as does the effects this may have on survival and fitness. Apparent recent increases in liana abundances in some forests, likely due to climate change, makes understanding their role in supporting and maintaining biodiversity an increasingly important and necessary challenge. Since trees or saplings have usually been the subject of studies on insect herbivory, major knowledge gaps remain about the ways in which lianas contribute to, support and maintain the ecosystems in which they exist. We use our conceptual model to guide future research directions and express the necessity for caution when extrapolating explanations of herbivory derived from data on trees to growth forms with fundamentally different ecologies.     

Herbivore‐induced plant responses in Brassica oleracea prevail over effects of constitutive resistance and result in enhanced herbivore attack

1. Plant responses to herbivore attack may have community‐wide effects on the composition of the plant‐associated insect community. Thereby, plant responses to an early‐season herbivore may have profound consequences for the amount and type of future attack. 2. Here we studied the effect of early‐season herbivory by caterpillars of Pieris rapae on the composition of the insect herbivore community on domesticated Brassica oleracea plants. We compared the effect of herbivory on two cultivars that differ in the degree of susceptibility to herbivores to analyse whether induced plant responses supersede differences caused by constitutive resistance. 3. Early‐season herbivory affected the herbivore community, having contrasting effects on different herbivore species, while these effects were similar on the two cultivars. Generalist insect herbivores avoided plants that had been induced, whereas these plants were colonised preferentially by specialist herbivores belonging to both leaf‐chewing and sap‐sucking guilds. 4. Our results show that community‐wide effects of early‐season herbivory may prevail over effects of constitutive plant resistance. Induced responses triggered by prior herbivory may lead to an increase in susceptibility to the dominant specialists in the herbivorous insect community. The outcome of the balance between contrasting responses of herbivorous community members to induced plants therefore determines whether induced plant responses result in enhanced plant resistance.     

Proportion of exotics and relatedness of host species mediate the positive effect of plant richness on the species richness of fruit flies

1. All else being equal, the greater the local species richness of plants, the greater the number of associated herbivore species. Because most herbivore insects feed on a subset of closely related plant species, plant phylogenetic diversity is expected to play a key role in determining the number of herbivore species. What is not well known, however, is how an increase in the species richness of exotic plants affects the species richness of herbivores. 2. In this study, we used plant–fruit fly interactions to investigate the influence of the proportion and species richness of exotic host plants on the species richness of herbivorous insects. We also tested whether the phylogenetic diversity of host plants increases when the number of exotic plant species increases. 3. We found that the species richness of fruit flies is more accurately predicted by the richness of native host plants than by total plant species richness (including both native and exotic species). The proportion of exotic host species and the phylogenetic diversity of host plants had negative and positive effects, respectively, on the species richness of fruit flies. 4. Our findings suggest that a positive effect of plant richness on herbivore richness occurs only when an increase in plant diversity involves plant species with which native herbivores share some evolutionary history.