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Species distributions are driven by abiotic and biotic factors, but the importance of variation in the availability and quality of critical resources is poorly understood. Disentangling the relative importance of these factors—abiotic environment, availability of critical resources, and resource quality—will be important to modeling species current distributions and responses to projected climate change. We address these questions using species distribution models (SDMs) for the western monarch butterfly population (Danaus plexippus), whose larvae feed exclusively on Asclepias species known for their heterogeneous distribution and variation in host quality. We modeled the distribution of 24 Asclepias species to compare three monarch distribution models with increasing levels of complexity: (1) a null model using only environmental factors (climate envelope model), (2) a model using environmental factors and Asclepias availability estimated as species richness, (3) and a model using environmental factors and Asclepias’ availability weighted by host plant quality as assessed through a greenhouse bioassay of larval performance. Asclepias models predicted that half of the Asclepias species will expand their ranges and shift toward higher latitudes, while half will contract. These patterns were uncorrelated with host plant quality. Among the three monarch models, the climate envelope model was the poorest performing. Models accounting for host plant availability performed best, while accounting for host plant quality did not improve model performance. The climate envelope model estimated more restrictive contemporary and future monarch ranges compared to both host plant models. Although all three models predicted future monarch range expansions, the projected future distributions varied among models. The climate envelope model predicted range expansions along the Pacific coast and contractions inland. In contrast, the host plant availability and quality models predicted range expansions in both of these regions and, as a result, 14% and 19% increases in distribution (respectively) relative to the climate envelope model. These models do not include other factors affecting monarch persistence. Nevertheless, our findings suggest that accounting for information on host plant availability and response to climate change is necessary to predict future species distributions, but that variation in the quality of those critical resources may be of secondary importance.

The enemy‐free space hypothesis (EFSH) contends that generalist predators select for dietary specialization in insect herbivores. At a community level, the EFSH predicts that dietary specialization reduces predation risk, and this pattern has been found in several studies addressing the impact of individual predator taxa or guilds. However, predation at a community level is also subject to combinatorial effects of multiple‐predator types, raising the question of how so‐called multiple‐predator effects relate to dietary specialization in insect herbivores. Here, we test the EFSH with a field experiment quantifying ant predation risk to insect herbivores (caterpillars) with and without the combined predation effects of birds. Assessing a community of 20 caterpillar species, we use model selection in a phylogenetic comparative framework to identify the caterpillar traits that best predict the risk of ant predation. A caterpillar species' abundance, dietary specialization, and behavioral defenses were important predictors of its ant predation risk. Abundant caterpillar species had increased risk of ant predation irrespective of bird predation. Caterpillar species with broad diet breadth and behavioral responsiveness to attack had reduced ant predation risk, but these ant effects only occurred when birds also had access to the caterpillar community. These findings suggest that ant predation of caterpillar species is density‐ or frequency‐dependent, that ants and birds may impose countervailing selection on dietary specialization within the same herbivore community, and that contingent effects of multiple predators may generate behaviorally mediated life‐history trade‐offs associated with herbivore diet breadth., This study is an experimental test of the hypothesis that dietary specialization of insect herbivores reduces their predation risk. Unexpectedly, ant predation of caterpillars increased with dietary specialization and depended on bird predation. In this community, dietary specialization was associated with reduced behavioral defenses, which predicted ant predation risk in a way that was also contingent upon bird predation.

Classic research on elevational gradients in plant–herbivore interactions holds that insect herbivore pressure is stronger under warmer, less seasonal climates characteristic of low elevations, and that this in turn selects for increased defence in low‐ (relative to high‐) elevation plants. However, recent work has questioned this paradigm, arguing that it overly simplifies the ecological complexity in which plant–insect herbivore interactions are embedded along elevational gradients. Numerous biotic and abiotic factors vary with elevation, and their simultaneous influences are the focus of current work on elevational gradients in insect herbivory and plant defences. The present review 1) synthesizes current knowledge on elevational gradients in plant–insect herbivore interactions; 2) critically analyses research gaps and highlights recent advances that contribute to filling these gaps; and 3) outlines new research opportunities to uncover underlying mechanisms and build towards a unified theory on elevational gradients. We conclude that the next generation of studies should embrace community complexity – including multi‐trophic dynamics and the multivariate nature of plant defence – and to do so by combining observational data, manipulative experiments and emerging analytical tools.

The effects of predator assemblages on herbivores are predicted to depend critically on predator-predator interactions and the extent to which predators partition prey resources. The role of prey heterogeneity in generating such multiple predator effects has received limited attention. Vertebrate and arthropod insectivores constitute two co-dominant predatory taxa in many ecosystems, and the emergent properties of their joint effects on insect herbivores inform theory on multiple predator effects as well as biological control of insect herbivores. Here we use a large-scale factorial manipulation to assess the extent to which birds and ants engage in antagonistic predator-predator interactions and the consequences of heterogeneity in herbivore body size and diet breadth (i.e. the diversity of host plants used) for prey partitioning. We excluded birds and reduced ant density (by 60%) in the canopies of eight northeastern USA deciduous tree species during two consecutive years and measured the community composition and traits of lepidopteran larvae (caterpillars). Birds did not affect ant density, implying limited intraguild predation between these taxa in this system. Birds preyed selectively upon large-bodied caterpillars (reducing mean caterpillar length by 12%) and ants preyed selectively upon small-bodied caterpillars (increasing mean caterpillar length by 6%). Birds and ants also partitioned caterpillar prey by diet breadth. Birds reduced the frequency dietary generalist caterpillars by 24%, while ants had no effect. In contrast, ants reduced the frequency of dietary specialists by 20%, while birds had no effect, but these effects were non-additive; under bird exclusion, ants had no detectable effect, while in the presence of birds, they reduced the frequency of specialists by 40%. As a likely result of prey partitioning by body size and diet breadth, the combined effects of birds and ants on total caterpillar density were additive, with birds and ants reducing caterpillar density by 44% and 20% respectively. These results show evidence for the role of prey heterogeneity in driving functional complementarity among predators and enhanced top-down control. Heterogeneity in herbivore body size and diet breadth, as well as other prey traits, may represent key predictors of the strength of top-down control from predator communities.

Indirect defence, the adaptive top-down control of herbivores by plant traits that enhance predation, is a central component of plant-herbivore interactions. However, the scope of interactions that comprise indirect defence and associated ecological and evolutionary processes has not been clearly defined. We argue that the range of plant traits that mediate indirect defence is much greater than previously thought, and we further organise major concepts surrounding their ecological functioning. Despite the wide range of plant traits and interacting organisms involved, indirect defences show commonalities when grouped. These categories are based on whether indirect defences boost natural enemy abundance via food or shelter resources, or, alternatively, increase natural enemy foraging efficiency via information or alteration of habitat complexity. The benefits of indirect defences to natural enemies should be further explored to establish the conditions in which indirect defence generates a plant-natural enemy mutualism. By considering the broader scope of plant-herbivore-natural enemy interactions that comprise indirect defence, we can better understand plant-based food webs, as well as the evolutionary processes that have shaped them.

Direct and indirect defenses are predicted to trade-off due to costs associated with redundancy in plant defense, but the factors mediating a plant's position along this trade-off axis are unknown. We conducted a bird exclusion experiment of nine sympatric shrub species to assess convergent associations among direct defense, indirect defense from birds, and shrub structural complexity, a trait predicted to influence bird foraging. We found high variation in defense; direct resistance varied four-fold, with indirect defense ranging from a 59% reduction to a 32% increase in herbivore density. These resistance strategies traded off and were mediated by plant structure; high complexity was associated with weaker indirect defense from birds, strong direct defense, and more predatory arthropods. Our findings suggest that species with growth forms that inhibit bird foraging invest more in direct defense and may provide refuge for arthropod predators. Accordingly, we provide evidence for a potentially widespread mechanism underlying the evolution of plant defenses.

It is well known that plant damage by leaf-chewing herbivores can induce resistance in neighbouring plants. It is unknown whether such communication occurs in response to sap-feeding herbivores, whether communication is specific to herbivore identity, and the chemical basis of communication, including specificity. We carried out glasshouse experiments using the California-native shrub Baccharis salicifolia and two ecologically distinct aphid species (one a dietary generalist and the other a specialist) to test for specificity of plant-plant communication and to document the underlying volatile organic compounds (VOCs). We show specificity of plant-plant communication to herbivore identity, as each aphid-damaged plant only induced resistance in neighbours against the same aphid species. The amount and composition of induced VOCs were markedly different between plants attacked by the two aphid species, providing a putative chemical mechanism for this specificity. Furthermore, a synthetic blend of the five major aphid-induced VOCs (ethanone, limonene, methyl salicylate, myrcene, ocimene) triggered resistance in receiving plants of comparable magnitude to aphid damage of neighbours, and the effects of the blend exceeded those of individual compounds. This study significantly advances our understanding of plant-plant communication by demonstrating the importance of sap-feeding herbivores and herbivore identity, as well as the chemical basis for such effects.

Plants are able to adjust their anti-herbivore defenses in response to the volatile organic compounds (VOCs) emitted by herbivore-damaged neighbors, and some of these changes increase resistance against subsequent herbivory. This phenomenon of plant-plant communication is thought to be widespread, but recent investigations have cautioned that it can be context dependent, including variation in the strength of communication based on the identity of plants and their associated herbivores. Here, we performed three greenhouse experiments using multiple male and female genotypes of the dioecious woody shrub Baccharis salicifolia and its specialist aphid Uroleucon macolai to test for specificity of plant-plant communication with respect to plant sex and genotype. Moreover, we evaluated plant sexual dimorphism and genotypic variation in VOC emissions (i.e., the "speaking" side of the interaction) and response of plants to VOC exposure (i.e., the "listening" side of the interaction) in order to identify the chemical mechanisms underlying such specificity. We did not find genotypic specificity of communication; emitter plants damaged by U. macolai significantly reduced subsequent U. macolai performance on receivers, but these effects were indistinguishable for communication within vs. among genotypes. In contrast, we found sex specificity of communication; male emitter plants reduced subsequent U. macolai performance on male and female receiver plants equally, while female emitter plants only did so for female receivers. We found sexual (but not genotypic) dimorphism in speaking but not listening; of the seven compounds induced by U. macolai feeding (speaking), pinocarvone was approximately fivefold greater in female than in male plants, while exposure of plants to pinocarvone emissions (listening) reduced U. macolai performance equally in both male and female plants. Together, our study demonstrates novel evidence for sexually dimorphic specificity of plant-plant communication and the chemical mechanism underlying this effect.

Intraspecific plant trait variation can have cascading effects on plant-associated biotic communities. Sexual dimorphism is an important axis of genetic variation in dioecious plants, but the strength of such effects and the underlying mechanisms relative to genetic variation are unknown. We established a common garden with 39 genotypes of Baccharis salicifolia sampled from a single population that included male and female genotypes and measured plant traits and quantified associated arthropod communities. Genetic variation sensu lato (genotypic variation) had strong effects on most plant traits (flower number, relative growth rate, specific leaf area, percent water content, carbon-nitrogen ratio, monoterpene but not sesquiterpene concentrations) and on herbivore and predator density, and on arthropod community composition (relative abundance of 14 orders). In contrast, sexual dimorphism had weaker effects on only a few plant traits (flower number and relative growth rate), on predator density, and on arthropod community composition, but had no effect on herbivore density. Variation in flower number drove genetic variation sensu lato and sex dimorphism in predator density and arthropod community composition. There was unique genetic variation sensu lato in herbivore density (positively) associated with monoterpene concentration and in arthropod community composition associated with specific leaf area and carbon-nitrogen ratio. There was unique sexual dimorphism in arthropod community composition associated with plant relative growth rate. Together, these results demonstrate that genetic variation sensu lato and sexual dimorphism can shape plant-associated arthropod communities via both parallel and unique mechanisms, with greater overall effects of the former.

Summary Geographic variation in abiotic factors and species interactions is widespread and is hypothesized to generate concomitant patterns of species trait variation. For example, higher rates of herbivory at lower latitudes are thought to select for increased plant defences, although latitudinal variation in defences may also be influenced directly by abiotic factors and indirectly by predators and parasitoids reducing herbivore pressure. We measured defences of the herb Ruellia nudiflora among 30 populations spanning a latitudinal gradient from northern Yucatan to southern Belize that vary substantially in leaf herbivory (fourfold), seed herbivory (25-fold) and seed herbivore parasitism (14-fold). These surveyed populations span one-third of the species’ latitudinal distribution (5° of latitude), the entire precipitation gradient of its distribution, and one-third of the temperature gradient of its distribution. Our prior work showed that leaf herbivory decreased with latitude and that seed herbivory increased with latitude. Here, we measured leaf trichome density and leaf and seed phenolics and tested whether latitudinal variation in climate, herbivory and parasitism explained latitudinal variation in these defensive traits. Patterns of variation in leaf trichomes fully supported predictions, with trichome density increasing with a parallel increase in herbivory towards lower latitudes. While seed phenolics were positively associated with herbivory, and seed herbivory tended to increase with latitude, the predicted (positive) association between latitude and defence was not detectable. There was no detectable association between parasitoids and seed defences. In addition, the association between leaf herbivory and phenolics was weak, and leaf phenolics were not associated with latitude. Importantly, variation in the abiotic environment was associated with plant defence, indicating that abiotic factors can play a major role in shaping plant defences, independently of herbivory. Synthesis. Latitudinal variation in abiotic factors may drive concomitant patterns of variation in plant defences, independently of herbivory. Collectively, these findings highlight the need for assessing geographic variation in plant defences from a multi-factorial perspective, testing for the simultaneous influence of biotic and abiotic factors.

The effects of tree diversity on herbivore-enemy interactions have received relatively little attention and even fewer studies have compared the relative influence of tree intra- versus interspecific diversity on such dynamics. 2 We evaluated the effects of mahogany ( Swietenia macrophylla) genotypic diversity and tree species diversity on parasitoid attack and species richness associated with Phyllocnistis meliacella, a specialist herbivore on mahogany, in a forest diversity experiment consisting of 74 plots (21 × 21 m 2 ; 64 plants/plot). We sampled 34 of such plots classified as: mahogany monocultures of one maternal family (i.e. genotype), mahogany monocultures of four families and polycultures of four species (including mahogany). We surveyed leafminer abundance and collected mined leaves to estimate parasitism and parasitoid species richness. 3 Leafminer abundance was not influenced by either type of diversity. Similarly, there were no effects of genotypic diversity or species diversity on parasitism or parasitoid species richness. Plant diversity effects on parasitoids were probably absent because the species attackingP. meliacella are dietary generalists that likely recruited to multiple host species (in addition to P. meliacella) and their responses to diversity could have cancelled each other out. 4 Future work should explicitly investigate how predator and parasitoid traits mediate the effects of plant diversity on tritrophic interactions.

The abiotic environment drives species abundances and distributions both directly and indirectly through effects on multi-trophic species interactions. However, few studies have documented the individual and combined consequences of these direct and indirect effects. We studied an ant-tended aphid along an elevational gradient, where lower elevations were more arid. Hypotheses of stronger species interactions at lower elevations and a greater sensitivity of higher trophic levels to climate led us to predict increased top-down control of aphids by natural enemies (third trophic level) but even stronger protection from mutualist ants (fourth trophic level) with increasing aridity. As a result, we predicted that mutualism strength and aphid abundance would increase with aridity. We documented patterns of aphid abundance and tested for both the direct and multi-trophic indirect effects of aridity on aphid performance. To do so, we used both observational and manipulative methods across two years in replicate high- and low-elevation valleys, where summer temperatures decreased by 3.7°C and precipitation increased by 27 mm/mo from low to high elevations. Aphid colonies were 75% larger in the most (vs. least) arid sites, and this was best explained by changes in interactions with predators and ants. Aphids were unaffected by the direct effects of the abiotic environment or its indirect effects via host plant quality. In contrast, natural enemy effects increased with aridity; under ant exclusion, natural enemies had no effect on aphids in the least arid sites but depressed colony growth by 252% in the most arid sites. Ant activity also increased with aridity, with ants discovering more aphid colonies and experimental baits and allocating more foragers per aphid, although there was no effect of aridity on ant abundance or community composition. Correspondingly, the mutualist services provided by ants increased with aridity; ants provided no benefits to aphids in the least arid sites but doubled colony growth in the most arid sites. In summary, an elevational cline in herbivore abundance was driven by a monotonic increase in trophic-level sensitivity to aridity. These findings illustrate that predicting species responses to climate change will require a multi-trophic perspective.

Oikos 000: 001–009, 2015 doi: 10.1111/oik.02033 © 2015 Th e Authors. Oikos © 2015 Nordic Society Oikos Subject Editor: Martijn Bezemer. Editor-in-Chief: Dries Bonte. Accepted 13 January 2015 Comparison of tree genotypic diversity and species diversity effects on different guilds of insect herbivores Luis Abdala-Roberts , Kailen A. Mooney , Teresa Quijano-Medina , Mar i a Jos e Campos-Navarrete , Alejandra Gonz a lez-Moreno and V i ctor Parra-Tabla L. Abdala-Roberts (labdala@uci.edu) and K. A. Mooney, Dept of Ecology and Evolutionary Biology, Univ. of California, Irvine, 321 Steinhaus Hall, Irvine, CA 92697-2525, USA. – LAR, M. J. Campos-Navarrete and V. Parra-Tabla, Depto de Ecolog i a Tropical, Campus de Ciencias Biol o gicas y Agropecuarias, Univ. Aut o noma de Yucat a n, Apartado Postal 4-116, Itzimn a , MX-97000 M e rida, Yucat a n, M e xico. – T. Quijano-Medina, Wageningen Univ., Droevendaalsesteeg 2, NL-6708 PB Wageningen, the Netherlands. – A. Gonz a lez-Moreno, Inst. Tecnol o gico de Conkal, Km. 16.3 Antigua Carretera M e rida-Motul, MX-97345 Conkal, Yucat a n, M e xico. Although the eff ects of plant diversity on herbivores are contingent upon herbivore traits and the source of plant diversity (e.g. intra- and interspecifi c), most studies have analyzed these eff ects separately. We compared the eff ects of genotypic diversity of big-leaf mahogany Swietenia macrophylla with that of tree species diversity on two specialist caterpillars (Hypsipyla grandella stem borers and Phyllocnistis meliacella leaf miners) and three generalist leafhoppers (Cicadellidae) feeding on mahogany in a large-scale (7.2 ha) forest diversity experiment in southern Mexico. Th e experiment consisted of fi fty-nine 21 ! 21-m plots, with 64 tree saplings each (3-m spacing between plants). Plots were either mahogany monocul- tures or species polycultures of four species (including mahogany) and – within each of these two plot types – mahogany was represented by either one or four genotypes. Th roughout a fi ve-month period, beginning six months after planting, we measured mahogany growth and monitored herbivore and predator (spider) abundance. We found no eff ect of mahogany genotypic diversity on either specialist caterpillars or generalist leafhoppers, and this result was consistent across levels of tree species diversity. In contrast, species diversity had signifi cant eff ects on both specialists but neither of the generalist herbivores. Specifi cally, species diversity lowered H. grandella attack at the middle of the sampling season, but increased attack at the end of the season, whereas P. meliacella abundance was consistently reduced. Such eff ects were not mediated by eff ects of species diversity on plant growth (of which there were none), but rather through resource heterogeneity. Diversity did not infl uence spider abundance. Th is study is one of few to directly compare sources of plant diversity, and uniquely compares such eff ects among herbivores with contrasting life histories (e.g. diet breadths). Overall, we demonstrate that plant species diversity eff ects outweigh those of genotypes, and our results suggest that such eff ects are stronger on specialist than generalist herbivores. Evidence has mounted for the eff ects of intra- (reviewed by Bailey et al. 2009) and inter-specifi c (Siemann 1998, Haddad et al. 2009) plant diversity on higher trophic levels. Numerous studies have found eff ects of plant diversity on arthropod species richness and abundance (Koricheva et al. 2000, Crutsinger et al. 2006, Haddad et al. 2009) and on consumptive interactions at higher trophic levels (Moreira and Mooney 2013, Abdala-Roberts and Mooney 2014), with the basis of such eff ects being variation in ecologically important traits among plant species or genotypes within species (Hare 2002, Mooney and Singer 2012). Within this context, a widely documented pattern is that greater plant diversity frequently leads to reductions in herbivory (reviewed by Andow 1991, Barbosa et al. 2009). Two hypotheses have been off ered to explain this phenomenon and invoke the infl u- ence of resource heterogeneity on consumers. According to the ‘ enemies hypothesis ’ (hereafter EH; Root 1973), greater habitat complexity at high plant diversity favors increased predator recruitment (e.g. because of greater availability of shelters or prey) resulting in stronger top – down suppression of herbivore populations and thus lower herbivory (reviewed by Russell 1989). Alternatively, the ‘ resource concentra- tion hypothesis ’ (RCH) (Root 1973) holds that herbivore foraging is density-dependent and increasing plant species diversity at a constant plant density reduces the probabil- ity of fi nding a preferred host plant, which lowers herbivore recruitment and damage on individual plants. While both hypotheses have received considerable attention, evi- dence from natural systems has generated mixed support (Bommarco and Banks 2003, Underwood et al. 2014). Th e inconsistent support for the EH and RCH could be due to unaccounted variation in herbivores traits (Vehvil a inen et al. 2007, Plath et al. 2012). For example, the dynamics predicted by the EH should be more likely to occur for generalist herbivores because they are more sus- ceptible to natural enemies as they frequently lack defense mechanisms found in specialists (e.g. crypsis, sequestration of plant toxic compounds; Mooney et al. 2012, Singer et al. EV-1

9 páginas.-- 4 figuras.-- 2 tablas.-- 44 referencias, The predator satiation hypothesis poses that synchronous and variable seed production during masting events increases seed escape through seed predator satiation. The success of this strategy depends upon the type of consumer functional response, in this case defined as the change in seed consumption rate by a predator as a function of change in seed density. Type II (where the proportion of seed consumed is highest at low levels of seed availability) and type III (where the proportion of seed consumed is highest at some intermediate level of seed availability and then declines towards zero) functional responses describe negative density-dependence and indicate predator satiation. The type of function response should be contingent upon herbivore traits: type II responses are predicted for dietary specialist predators with low mobility, and type III responses are predicted for highly mobile, dietary generalist predators. Surprisingly, most studies have not evaluated whether functional responses vary among seed predator guilds. Here we describe the functional responses at population and individual tree level of highly mobile generalist (birds and rodents) and less mobile specialist (insects) pre-dispersal seed predators attacking acorns of two sympatric oaks (Quercus suber and Q. canariensis) over a 10-year period. Our results showed that in most cases specialist seed predators exhibited the predicted type II functional response at both the individual tree and population level for both oak species. However, generalist seed predators did not exhibit the predicted type III response; instead, they also exhibited a type II response at the individual tree and population level for both oak species. By independently assessing the effects of multiple seed predators associated with the same host tree species, our work highlights the influence of herbivore traits on the outcome of plant–seed predator interactions in masting species, and thus furthers our understanding of the ecological and evolutionary mechanisms underlying masting behaviour., This study was supported by the Ramón y Cajal Research Programme to XM (RYC-2013-13230) and to IMPR (RYC-2013- 13937), by a grant from the Regional Government of Galicia (IN607D 2016/001) to XM, by a Spanish National Research Grant (AGL2015-70748-R) to XM, by the CONACyT Repatriation Programme (no. 250934) to LAR, by the Spanish MEC projects Heteromed (REN2002-4041-C02-02), Dinamed (CGL2005-5830-C03-01) and Interbos (CGL2008-04503-C03-01), and by the Andalusian Anasinque-PE2010-RNM-5782 project. Statement of authorship – XM and IMPR contributed equally to this paper and share the first authorship

A plant's performance and interactions with other trophic levels are recorgnized to be contingent upon plant diversity and underlying associational dynamics, but far less is known about the plant traits driving such phenomena. We manipulated diversity in plant traits using pairs of plant and a substitutive design to elucidate the mechanisms underlying diversity effects operating at a fine spatial scale. Specifically, we measured the effects of diversity in sex (sexual monocultures vs. male and female genotypes together) and growth rate (growth rate monocultures vs. fast- and slow-growing genotypes together) on growth of the shrub Baccharis salicifolia and on above- and belowground consumers associated with this plant. We compared effects on associate abundance (# associates per plant) vs. density (# associates per kg plant biomass) to elucidate the mechanisms underlying diversity effects; effects on abundance but not density suggest diversity effects are mediated by resource abundance (i.e. plant biomass) alone, whereas effects on density suggest diversity effects are mediated by plant-based heterogeneity or quality. Sexual diversity increased root growth but reduced the density (but not abundance) of the dietary generalist aphid Aphis gossypii and its associated aphid-tending ants, suggesting sex mixtures were of lower quality to this herbivore (e.g. via reduced plant quality), and that this effect indirectly influenced ants. Sexual diversity had no effect on the abundance or density of parasitoids attacking A. gossypii, the dietary specialist aphid Uroleucon macolai, or mycorrhizae. In contrast, growth rate diversity did not influence plant growth or any associates except for the dietary specialist aphid U. macolai, which increased in both abundance and density at high diversity, suggesting growth rate mixtures were of higher quality to this herbivore. These results highlight that plant associational and diversity effects on consumers are contingent upon the source of plant trait variation, and that the nature of such dynamics may vary both within and among trophic levels.

The mechanisms by which plant diversity influences consumers are largely unexplored. Plant diversity reduces among-plant competition, and as a result, may have cascading bottom-up effects through altered resource quantity and quality. Less understood are parallel effects of resource heterogeneity on consumer foraging behaviors, and the consequences of such effects for trophic interactions and feedbacks on plant performance. Here, we asked whether genotypic diversity in the herb Ruellia nudiflora influences seed predator (SP) and parasitoid functional responses, and if such effects in turn influence selection on the plant. We established plots with plants of one or five genetic families (pool = 14) and measured fruit, seed predator, and parasitoid abundance. By eliminating direct plant–plant interactions, this experiment explicitly tested for plant diversity effects occurring through altered consumer behaviors (vs. effects through changes in plant quantity or quality). We compared observed plant fitness (under three trophic levels) to projected fitness in the absence of parasitoids (two trophic levels) and in the absence of seed predation (one trophic level) by computing the number of seeds consumed by the SP and the number of seeds rescued from consumption by parasitoids. We then compared the strength and mode of selection on fruit number between levels of diversity, separately under each trophic scenario. Plant diversity did not influence fruit, seed predator, or parasitoid abundance, but did alter plant–SP interactions. SP recruitment resembled a Type II functional response but saturated weakly in polycultures, with fewer SPs at intermediate fruit abundance and more SPs at high fruit abundance relative to monoculture. Parasitoid recruitment was weakly positively density dependent and unaffected by diversity. Importantly, we found that under the bi-trophic scenario, the effect of diversity on SP recruitment altered the mode of selection on fruit number, from directional selection at low diversity to nonlinear (stabilizing) selection at high diversity. In contrast, diversity did not alter selection under mono- or tri-trophic scenarios. Therefore, diversity effects on SP functional responses fed back to alter selection on fruit number, but parasitoids eliminated this linkage by weakening herbivore selection. Collectively, these findings provide novel evidence for the mechanistic basis of eco-evolutionary feedbacks between plant diversity and consumers.

The magnitude of plant intra-specific variation for indirect defence and the underlying plant traits influencing predators remain relatively unstudied, particularly in cultivated plants. 2. We tested whether differences in flower number, pollen production, and leaf trichome density among 17 pepper ( Capsicum annuum Linnaeus) varieties influenced the abundance and predation intensity by the omnivorous mite Amblyseius swirskii Athias-Henriot. 3. A greenhouse experiment was conducted where pepper plants were infested with thrips (Frankliniella cephalica Crawford DL) and subsequently exposed to A. swirskii. We estimated thrips and mite density based on arthropod counts conducted over a 4-week period, and also performed flower and trichome counts, and estimated pollen production per anther. 4. Significant differences were found among varieties for all three traits, as well as mite and thrips density. After accounting for all traits in a multiple regression model, we found that flower and trichome number had significant positive effects on mite density (by providing food and shelter, respectively). Increased mite density was in turn associated with a decrease in thrips density, presumably as a result of mite predation. Moreover, we found that flower number (but not trichome density) increased the strength of thrips suppression and that such an effect was mediated by mite density. 5. These findings suggest that genetic variation for plant traits may indirectly influence herbivore suppression in peppers (although traits may vary in the strength or direction of their effects), and underscore the evolutionary potential and importance of selection not only for direct but also indirect resistance in crops.

Steep climatic gradients may select for clinal adaptation in plant functional traits with implications for interspecifi c interactions and response to future climate change. Terpenes are common in Mediterranean environments and mediate plant interactions with both the abiotic and biotic environment, including herbivores. Clines in traits such as terpenes have received much attention because they are linked to plant fi tness and experience strong selection from the abiotic and biotic environment. In this study, we tested for intraspecifi c variation in Artemisia californica terpene chemistry in a common garden of plants sourced from populations spanning a large precipitation gradient (6 ° latitude) and grown in treatments of high and low precipitation. We found genetic variation in terpene richness, diversity, concentration and composition among A. californica populations spanning this species ’ range. Of these traits, terpene composition and monoterpene concentration varied clinally with respect to source site latitude. Regarding terpene composition, pairwise dissimilarity among populations increased in parallel with geographic distance between source sites. At the same time, monoterpene concentration decreased monotonically from plants of southern origin (source sites with high temperature, aridity, and precipitation variability) to plants of northern origin. Our precipitation manipulation suggests that phenotypic selection by precipitation may underlie this clinal variation in monoterpene concentration, and that monoterpene concentration and other aspects of terpene chemistry are not phenotypically plastic. In summary, this study provides novel evidence for a genetically based latitudinal cline in plant secondary chemistry and suggests that adaptation to a key aspect of the abiotic environment may contribute to this intraspecifi c variation. Accordingly, changes in terpene chemistry under projected future climates will likely occur solely through the relatively slow process of adaptation, with important consequences for plant interactions with the abiotic environment and a diverse community of associates.

While invasive plants are widely studied for their effects on native plants, we questioned how plant invasions affect higher trophic levels. We investigated the effects of the invasive plant, Brassica nigra on the multi-trophic arthropod community residing on the native California annual Deinandra fasciculata. In a common garden experiment, we planted D. fasciculata without B. nigra or with one of 45 B. nigra half-sib genetic families. We in turn crossed this B. nigra treatment with the suppression of soil fungi to both test for effects of arbuscular mycorrhizal fungi (AMF) on arthropods, and elucidate the mechanisms of B. nigra’s below ground effects. B. nigra had no effect on D. fasciculata traits (biomass, inflorescence number, root colonization by AMF), arthropod community composition or predator density, but increased herbivore density. While B. nigra families varied 18-fold in size, there was no genetic variation for effects on D. fasciculata or its arthropods. Soil fungi suppression had no effect on D. fasciculata traits, herbivore density or herbivore community composition, but increased predator density and altered predator community composition. While the exact mechanisms of B. nigra effects are unclear, they do not appear to have been mediated by altered plant performance or reductions in root colonization by AMF. Our experiment shows that invasive plants such as B. nigra may affect higher trophic levels even when they do not measurably affect native plant performance.

There is growing evidence for the influence of plant intraspecific variation on associated multi-trophic communities, but the traits driving such effects are largely unknown. We conducted a field experiment with selected genetic lines of the dioecious shrub Baceharis salicifolia to investigate the effects of plant growth rate (two-fold variation) and gender (males vs. females of the same growth rate) on above- and belowground insect and fungal associates. We documented variation in associate density to test for effects occurring through plant-based habitat quality (controlling for effects of plant size) as well as variation in associate abundance to test for effects occurring through both habitat quality and abundance (including effects of plant size). Whereas the dietary specialist aphid Uroleucon macaolai was unaffected by plant sex and growth rate, the generalist aphid Aphis gossypii and its tending ants (Linepithema humile) had higher abundances and densities on male (vs. female) plants, suggesting males provide greater habitat quality. In contrast, Aphis and ant abundance and density were unaffected by plant growth rate, while Aphis parasitoids were unaffected by either plant sex or growth rate. Arbuscular mycorrhizal fungi had higher abundance and density (both marginally significant) on females (vs. males), suggesting females provide greater habitat quality, but lower abundances (marginally significant) and higher densities on slow- (vs. fast-) growing genotypes, suggesting slow-growing genotypes provided lower resource abundance but greater habitat quality. Overall, plant sex and growth rate effects on associates acted independently (i.e., no interactive effects), and these effects were of a greater magnitude than those coming from other axes of plant genetic variation. These findings thus demonstrate that plant genetic effects on associated communities may be driven by a small number of trait-specific mechanisms.

International audience; A rich body of theory has been developed to predict the effects of plant diversity on communities at higher trophic levels and the mechanisms underpinning such effects. However, there are currently a number of key gaps in knowledge that have hindered the development of a predictive framework of plant diversity effects on consumers. For instance, we still know very little about how the magnitude of plant trait variation (e.g. intra-specific vs. inter-specific), as well as the identity and combined effects of plant, herbivore and natural enemy traits, mediate plant diversity effects on consumers. Moreover, the fine-scale mechanisms (e.g. changes in consumer behaviour or recruitment responses) underlying such diversity effects in many cases remain elusive or have been overlooked. In addition, most studies of plant diversity effects on associated consumers have been developed under a static, unidirectional (bottom-up) framework of effects on herbivores and predators without taking into account the potential for dynamic feedbacks across trophic levels. Here we seek to address these key gaps in knowledge as well as to capitalize on recent advances and emerging frameworks in plant biodiversity research. In doing so, we provide new insights as well as recommendations which will stimulate new research and advance this field of study.

Sex is an ecologically important form of genetic variation in dioecious plants, with males and females generally differing in constitutive resistance to herbivores. Yet little is known about sexual dimorphism with respect to induced or indirect defense, or whether sex-based differences are underlain by trade-offs among modes of defense. We compared male and female Valeriana edulis plants for constitutive and induced direct resistance to two herbivores, an early-season caterpillar and a late-season aphid, and for constitutive and induced indirect resistance in terms of abundance of natural enemies and aphid-tending ants. No sexual dimorphism was found in constitutive direct plant resistance, yet the sexes differed for constitutive indirect resistance, with 78 % more natural enemies and 117 % more ants present on females than males. Past feeding damage by caterpillars induced direct and indirect resistance in both males and females, increasing caterpillar development time by 26 % and the abundance of natural enemies by 147 %. Caterpillar feeding did not induce direct resistance with respect to caterpillar final mass or aphid performance. In all cases, there were no interactions between the effects of caterpillar damage and plant sex. In summary, plant sexual dimorphism and induced responses to herbivore damage independently influenced herbivore performance and the composition of arthropod communities at higher trophic levels.

The means by which plant genotypes influence species interactions and arthropod community structure remain poorly understood. One potential, but largely unstudied mechanism is that occurring through plant genetic variation in induced responses to herbivory. Here we test whether induced responses to leaf damage and genotypic variation for induction in Asclepias syriaca influence interactions among Formica podzolica ants, the ant-tended aphid Aphis asclepiadis, and the untended aphid Myzocallis asclepiadis. In so doing, we assess genetic variation in plant-mediated interactions among different herbivore guilds. We conducted a three-way factorial field experiment manipulating plant genotype, leaf damage by specialist monarch caterpillars Danaus plexippus, and ant presence, and documented effects on aphid and ant abundances. Leaf damage increased Aphis abundance in both the presence and absence of ants and Myzocallis abundance under ant exclusion. In the presence of ants, leaf damage decreased Myzocallis abundance, likely due to effects on ant–Myzocallis interactions; ants showed a positive association with Myzocallis, leaf damage increased the strength of this association (425% more ants per aphid), and this in turn fed back to suppress Myzocallis abundance. Yet, these aggregate effects of leaf damage on Myzocallis and ants were underlain by substantial variation among milkweed genotypes, with leaf damage inducing lower aphid and ant abundances on some genotypes, but higher abundances on others. As a consequence, a substantial fraction of the variation in leaf damage effects on ants (R 2  0.42) was explained by milkweed genetic variation in the strength and sign of leaf damage effects on Myzocallis. Although plant genetic variation influenced Aphis abundance, this did not translate into genetic variation in ant abundance, and leaf damage did not influence Aphis–ant interactions. Overall, we show that variation in induced responses to herbivory is a relevant condition by which plant genotype influences interactions in plant-centered arthropod communities and provide novel results of effects on the third trophic level.

Effects of plant traits on herbivore-carnivore interactions are well documented in component communities but are not well understood at the level of large, complex communities. We report on a 2-year field experiment testing mechanisms by which variation in food quality among eight temperate forest tree species alters avian suppression of an assemblage of dietary generalist caterpillars. Plant quality and bird effects varied dramatically among tree species; high-quality plants yielded herbivores of 50% greater mass than those on low-quality plants, and bird effects ranged from near 0% to 97% reductions in caterpillar density. We also find evidence for two mechanisms linking host plant quality to bird effects. If caterpillar density was statistically controlled for, birds had relatively strong effects on the herbivores of low-quality plants, as predicted by the slow-growth/high-mortality hypothesis. At the same time, caterpillar density increased with plant quality, and bird effects were density dependent. Consequently, the net effect of birds was strongest on the herbivores of high-quality plants, a dynamic we call the high-performance/high-mortality hypothesis. Host plant quality thus changes highly generalized herbivore-carnivore interactions by two complementary but opposing mechanisms. These results highlight the interrelatedness of plant-herbivore and herbivore-carnivore interactions and thus the importance of a tritrophic perspective.

Theory on trophic interactions predicts that predators increase plant biomass by feeding on herbivores, an indirect interaction called a trophic cascade. Theory also predicts that predators feeding on predators, or intraguild predation, will weaken trophic cascades. Although past syntheses have confirmed cascading effects of terrestrial arthropod predators, we lack a comprehensive analysis for vertebrate insectivores—which by virtue of their body size and feeding habits are often top predators in these systems—and of how intraguild predation mediates trophic cascade strength. We report here on a meta-analysis of 113 experiments documenting the effects of insectivorous birds, bats, or lizards on predaceous arthropods, herbivorous arthropods, and plants. Although vertebrate insectivores fed as intraguild predators, strongly reducing predaceous arthropods (38%), they nevertheless suppressed herbivores (39%), indirectly reduced plant damage (40%), and increased plant biomass (14%). Furthermore, effects of vertebrate insectivores on predatory and herbivorous arthropods were positively correlated. Effects were strongest on arthropods and plants in communities with abundant predaceous arthropods and strong intraguild predation, but weak in communities depauperate in arthropod predators and intraguild predation. The naturally occurring ratio of arthropod predators relative to herbivores varied tremendously among the studied communities, and the skew to predators increased with site primary productivity and in trees relative to shrubs. Although intraguild predation among arthropod predators has been shown to weaken herbivore suppression, we find this paradigm does not extend to vertebrate insectivores in these communities. Instead, vertebrate intraguild preda-tion is associated with strengthened trophic cascades, and insectivores function as dominant predators in terrestrial plant-arthropod communities.

Host plant defenses are known to cascade up food chains to influence herbivores and their natural enemies, but how herbivore and predator traits and identity mediate such tri-trophic dynamics is largely unknown. We assessed the influence of plant defense on aphid and coccinellid performance in laboratory trials with low- vs. high-glucosinolate varieties of Brassica napus, a dietary specialist (Brevicoryne brassicae) and generalist (Myzus persicae) aphid, and five species of aphidophagous coccinellids. The performance of the specialist and generalist aphids was similar and unaffected by variation in plant defense. Aphid glucosinolate concentration and resistance to predators differed by aphid species and host plant defense, and these effects acted independently. With respect to aphid species, the dietary generalist aphid (vs. specialist) had 14% lower glucosinolate concentration and coccinellid predators ate three-fold more aphids. With respect to host plant variety, the high-glucosinolate plants (vs. low) increased aphid glucosinolate concentration by 21%, but had relatively weak effects on predation by coccinellids and these effects varied among coccinellid species. In turn, coccinellid performance was influenced by the interactive effects of plant defense and aphid species, as the cascading, indirect effect of plant defense was greater when feeding upon the specialist than generalist aphid. When feeding upon specialist aphids, low- (vs. high-) glucosinolate plants increased coccinellid mass gain by 78% and accelerated development by 14%. In contrast, when feeding upon generalist aphids, low- (vs. high-) glucosinolate plants increased coccinellid mass gain by only 11% and had no detectable effect on development time. These interactive effects of plant defense and aphid diet breadth on predator performance also varied among coccinellid species; the indirect negative effects of plant defenses on predator performance was consistent among the five predators when transmitted via the dietary specialist aphid, but these effects varied substantially among predators—in both the magnitude and direction—when transmitted via the dietary generalist aphid. Accordingly, the cascading effect of plant defense on predators was stronger in magnitude and more consistent among predator taxa when transmitted by the specialist than generalist herbivore. Overall, these findings support a central role of herbivore diet breadth in mediating both the strength and contingency of tri-trophic interactions.

Insectivorous birds and ants co-occur in most terrestrial communities, and theory predicts that emergent properties (i.e., nonadditive effects) can determine their combined influence on arthropods and plants. In a three-year factorial experiment, I investigated whether the effects of birds on pine and its arthropods differed based on the presence of ants that were predators of most arthropods, but mutualists with tended aphid species. Birds and ants reduced the abundance of most herbivorous and carnivorous arthropods in an additive fashion, with the effects of ants being stronger than those of birds. In sharp contrast, the opposing influences of birds and ants on tended aphid species interacted strongly; ants only increased tended aphid abundance in the absence of birds, while birds only reduced their abundance in the presence of ants. This interaction was mirrored in total herbivore abundance because tended aphids dominated the herbivore community. I develop a novel lexicon to discuss the emergent properties from these effects of opposing sign (predation, mutualism). Despite having emergent effects on herbivores, birds indirectly increased pine wood and foliage growth to a similar extent whether or not ants were present, while ants had no detectable effects. Birds also indirectly increased the abundance of some pine phloem monoterpenes, but these effects differed based on the presence or absence of ants. Thus, I report on a novel yet possibly widespread indirect interaction between intraguild predators, herbivore mutualists, and plant traits (growth, secondary chemistry) mediated through a species-rich community of arthropods.

Although a number of investigations have concluded that lower latitudes are associated with increases in herbivore abundance and plant damage, the generality of this pattern is still under debate. Multiple factors may explain the lack of consistency in latitude – herbivory relationships. For instance, latitudinal variation in herbivore pressure may be shaped entirely or not by climatic variables, or vary among herbivore guilds with diff ering life-history traits. Additionally, the strength of top – down eff ects from natural enemies on herbivores might also vary geographically and infl uence latitude – herbivory patterns. We carried out a fi eld study where we investigated the eff ects of latitude and climate on herbivory by a seed-eating caterpillar and leaf chewers, as well as parasitism associated to the former across 30 populations of the perennial herb Ruellia nudifl ora (Acanthaceae). Th ese populations were distributed along a 5 ° latitudinal gradient from northern Yucatan (Mexico) to southern Belize, representing one-third of the species ’ latitudinal distribution and the entirety and one-third of the precipitation and temperature gradient of this species ’ distribution (respectively). We found opposing latitudinal gradients of seed herbivory and leaf herbivory, and this diff erence appeared to be mediated by contrasting eff ects of climate on each guild. Specifi cally, univariate regressions showed that seed herbivory increased at higher latitudes and with colder temperatures, while leaf herbivory increased toward the equator and with wetter conditions. Multiple regressions including temperature, precipitation and latitude only found signifi cant eff ects of temperature for seed herbivory and latitude for leaf herbivory. Accordingly, that latitudinal variation in seed herbivory appears to be driven predominantly by variation in temperature whereas latitudinal variation in leaf herbivory was apparently driven by other unexplored correlates of latitude. Parasitism did not exhibit variation with latitude or climatic factors. Overall, these fi ndings underscore that the factors driving latitudinal clines in herbivory might vary even among herbivore species coexisting on the same host plant.

Species abundance is typically determined by the abiotic environment, but the extent to which such effects occur through the mediation of biotic interactions, including mutualisms, is unknown. We explored how light environment (open meadow vs. shaded understory) mediates the abundance and ant tending of the aphid Aphis helianthi feeding on the herb Ligusticum porteri. Yearly surveys consistently found aphids to be more than 17-fold more abundant on open meadow plants than on shaded understory plants. Manipulations demonstrated that this abundance pattern was not due to the direct effects of light environment on aphid performance, or indirectly through host plant quality or the effects of predators. Instead, open meadows had higher ant abundance and per capita rates of aphid tending and, accordingly, ants increased aphid population growth in meadow but not understory environments. The abiotic environment thus drives the abundance of this herbivore exclusively through the mediation of a protection mutualism.

Introduction Understanding the relative roles of resource availability and natural enemies (i.e., predators, parasites, pathogens) as determinants of species abundance and trait variation has been a research area of fundamental interest to ecologists and evolutionary biologists for decades. Essentially every organism copes with the dual concerns of bottom-up and top-down trophic pressure. Primary producers are positioned between the acquisition of nutrients, water, space, and light versus herbivory and disease; herbivores between plants versus predators, parasitoids, and disease; and predators between prey acquisition versus other predators, parasites, and disease. Consequently, the conceptual framework of bottom-up versus top-down control can be applied uniformly across all trophic levels. In this chapter, we consider how species evolve in response to pressures imposed by resources and consumers, with a focus on species responses to the so-called “trophic sandwich,” where selective pressures are imposed simultaneously from trophic levels both above and below. A consideration of evolutionary processes within the context of trophic dynamics is critical, as it is the evolved traits of the species consuming and being consumed that determine the nature of those interactions (Mooney et al., 2010). We first provide background on the concepts and theories pertaining to the ecological and evolutionary consequences of top-down and bottom-up dynamics. Second, we review work that has addressed (implicitly or explicitly) evolution in the context of bottom-up and top-down trophic dynamics. Third, we explicitly compare aquatic and terrestrial systems. Fourth, we present a framework outlining the mechanisms that determine the combined selective effects of resources and consumers and, based on this framework, consider how common such dynamics might be. Fifth, we present a case study from our research on the interactions between the perennial herb Ruellia nudiflora Engelm. and Gray Urban (Acanthaceae), a seed predator (caterpillars of a noctuid moth), and parasitic wasps attacking the latter. Finally, we outline our perspective on future directions. Because of the long history of studies of plant–herbivore interactions, we center our review of the literature within this setting.

We investigated the tritrophic interactions among southwestern dwarf mistletoe [Arceuthobium vaginatum (Willd.) Presl subsp. cryptopodum], mistletoe herbivores, and host pine (Pinus ponderosa Dougl. ex Laws. and C. Laws. variety scopulorum Engelm.)-associated predators. In an observational study, we characterized differences in pine-associated arthropods and pine branch morphology between branches either parasitized by mistletoe (brooms) or not visibly infected. Compared with noninfected branches, brooms had a more reticulate branching structure, collected 36 times more dead needles and supported 1.7 times more arthropod predators. In a manipulative field experiment, we investigated whether pine-associated predators fed upon lepidopteran herbivores of mistletoe and thereby reduced herbivore damage to the parasite. Over a 30-d trial, herbivores fed upon approximately two-thirds of available mistletoe shoots. Predator removal increased herbivore survival by 56% but had no detectable effect on the level of herbivory damage. We speculate that herbivores compete for mistletoe shoots and that increased per-capita feeding compensated for predator reduction of herbivore abundance. In summary, our results demonstrate that mistletoe parasitism altered the pine arthropod community, including an increase in the density of predators that likely feed upon mistletoe herbivores.

Predators affect herbivores directly and indirectly, by consumptive and nonconsumptive effects, and the combined influence of multiple predators is shaped by interactions among predators. I documented the individual and combined effects of birds (chickadees, nuthatches, warblers) and ants (Formica podzolica) on arthropods residing in pine (Pinus ponderosa) canopies in a factorial field experiment. Birds and ants removed herbivores but simultaneously benefited them by removing predatory arthropods. Birds and ants had net negative and positive effects, respectively, on the abundance of herbivore prey, supporting the notion that vertebrate predators have stronger negative effects on herbivores than do arthropod predators. Aphids (ant-tended and untended species) constituted three-quarters of herbivore biomass. The effect of birds on ant-tended aphids was twice that on untended aphid species or tended aphid species without ants. This was not due to there being more ant-tended aphids for birds to prey on; tended and untended aphid species were in similar abundances in the absence of birds. Instead, the effects of birds were strengthened by attributes of the mutualism that rendered tended aphids susceptible to predation. These dynamics led to nonadditive effects of birds and ants: birds only reduced tended aphid species and total herbivore abundances on trees with ants, while ants only increased tended aphid species and total herbivore abundances in the absence of birds. Consequently, top predators in this system only influenced total herbivore abundance when they disrupted an ant-aphid mutualism.

1. Intraguild predation occurs when top predators feed upon both intermediate predators and herbivores. Intraguild predators may thus have little net impact on herbivore abundance. Variation among communities in the strength of trophic cascades (the indirect effects of predators on plants) may be due to differing frequencies of intraguild predation. Less is known about the influence of variation within communities in predator-predator interactions upon trophic cascade strength. 2. We compared the effects of a single predator community between two sympatric plants and two herbivore guilds. We excluded insectivorous birds with cages from ponderosa pine Pinus ponderosa trees parasitized by dwarf mistletoe Arceuthobium vaginatum. For 3 years we monitored caged and control trees for predatory arthropods that moved between the two plants, foliage-feeding caterpillars and sap-feeding hemipterans that were host-specific, and plant damage and growth. 3. Excluding birds increased the abundance of ant-tended aphids on pine and resulted in an 11% reduction in pine woody growth. Mutualist ants protected pine-feeding aphids from predatory arthropods, allowing aphid populations to burgeon in cages even though predatory arthropods also increased in cages. By protecting pine-feeding aphids from predatory arthropods but not birds, mutualist ants created a three-tiered linear food chain where bird effects cascaded to pine growth via aphids. 4. In contrast to the results for tended aphids on pine, bird exclusion had no net effects on untended pine herbivores, the proportion of pine foliage damaged by pine-feeding caterpillars, or the proportion of mistletoe plants damaged by mistletoe-feeding caterpillars. These results suggest that arthropod predators, which were more abundant in cages as compared with control trees, compensated for bird predation of untended pine and mistletoe herbivores. 5. These contrasting effects of bird exclusion support food web theory: where birds were connected to pine by a linear food chain, a trophic cascade occurred. Where birds fed as intraguild predators, the reticulate food webs linking birds to pine and mistletoe resulted in no net effects on herbivores or plant biomass. Our study shows that this variation in food web structure occurred between sympatric plants and within plants between differing herbivore guilds.

We investigated the effects of chemical variation in thyme (Thymus vulgaris L.) on its interactions with competitors, herbivores, and herbivore predators. Four different thyme monoterpene phe- notypes (chemotypes) were grown in a factorial of chemo- 4 # 2 # 2 type, caging (sham half-cages vs. full cages), and competition (control vs. the grass Bromus madritensis L.). Cages reduced numbers of ar- thropod predators. Thyme-feeding aphids Aphis serpylli Koch passed through full cage walls to increase more than fourfold. As a result, freed from their predators, aphids had a large negative effect on thyme size and flowering. Similarly, competition from Bromus had a negative effect on thyme size and flowering. Individual effects of aphids and competition were nonadditive, however, and their com- bined effect was significantly less than that predicted by a multipli- cative null model. Differential thyme sizes among chemotypes were not mediated by herbivores or competitors, but differential flowering was due to the effects of chemotype on aphids. We thus document differential selection by aphids among thyme chemotypes and the influence of Bromus on the strength of these negative effects of aphids.

The response of plant traits to global change is of fundamental importance to understanding anthropogenic impacts on natural systems. Nevertheless, little is known about plant genetic variation in such responses or the indirect effect of environmental change on higher trophic levels. In a three-year common garden experiment, we grew the shrub Artemisia californica from five populations sourced along a 700 km latitudinal gradient under ambient and nitrogen (N) addition (20 kg N ha-1) and measured plant traits and associated arthropods. N addition increased plant biomass to a similar extent among all populations. In contrast, N addition effects on most other plant traits varied among plant populations; N addition reduced specific leaf area and leaf percent N and increased carbon to nitrogen ratios in the two northern populations, but had the opposite or no effect on the three southern populations. N addition increased arthropod abundance to a similar extent among all populations in parallel with an increase in plant biomass, suggesting that N addition did not alter plant resistance to herbivores. N addition had no effect on arthropod diversity, richness, or evenness. In summary, genetic variation among A. californica populations mediated leaf-trait responses to N addition, but positive direct effects of N addition on plant biomass and indirect effects on arthropod abundance were consistent among all populations.

Predicting the impact of carnivores on plants has challenged community and food web ecologists for decades. At the same time, the role of predators in the evolution of herbivore dietary specialization has been an unresolved issue in evolutionary ecology. Here, we integrate these perspectives by testing the role of herbivore diet breadth as a predictor of top-down effects of avian predators on herbivores and plants in a forest food web. Using experimental bird exclosures to study a complex community of trees, caterpillars, and birds, we found a robust positive association between caterpillar diet breadth (phylodiversity of host plants used) and the strength of bird predation across 41 caterpillar and eight tree species. Dietary specialization was associated with increased enemy-free space for both camouflaged (n = 33) and warningly signaled (n = 8) caterpillar species. Furthermore, dietary specialization was associated with increased crypsis (camouflaged species only) and more stereotyped resting poses (camouflaged and warningly signaled species), but was unrelated to caterpillar body size. These dynamics in turn cascaded down to plants: a metaanalysis (n = 15 tree species) showed the beneficial effect of birds on trees (i.e., reduced leaf damage) decreased with the proportion of dietary specialist taxa composing a tree species' herbivore fauna. We conclude that herbivore diet breadth is a key functional trait underlying the trophic effects of carnivores on both herbivores and plants.

The contents of mineral elements in phloem vary among trees, and this variation is often associated with host choice by diverse parasites and herbivores. The relative importance of genotype and environment was analyzed in order to determine the broad‐sense heritability (H2) of accumulation of 15 elements (Al, B, Ca, Cu, Fe, K, Mg, Mn, Mo, N, Na, P, Si, Ti, Zn) in phloem of ponderosa pine (Pinus ponderosa var. ponderosa Laws.). The phloem was collected in 1997 from trees planted between 1979 and 1983 at two locations in the Sierra Nevada of California. Each plantation consists of multiple copies, called ramets, of specific genotypes grafted onto root stocks of very heterogeneous genotypes. Within each plantation, we sampled five ramets of each of 17 different genotypes. The same 17 genotypes were analyzed at two locations. There are many differences in heritabilities and other features of elemental contents between the two plantations. The H2 values range from 0.0 to 0.65 and vary between the plantations, ...

Trophic Trade-Offs There have been many attempts to document and explain the effects of predators on plant biomass in so-called “trophic cascades.” Theory suggests that fast-growing plants are relatively undefended and suffer more from herbivory, which implies a functional trade-off between investment in traits relating to growth and defensive strategies. Mooney et al. (p. 1642 ; see the Perspective by Hambäck ) compared responses to fertilization and aphid predators in 16 milkweed species. As predicted, interspecific variation in the strength of top-down control in terms of a tradeoff with growth was observed.

Mast seeding is a reproductive strategy in some perennial plants defined as synchronous production of large seed crops at irregular intervals. One widely accepted theory to explain this behaviour is the predator satiation hypothesis, which states that the synchronous and variable production of seeds within a population will maximize the probability of seed survival through satiation of seed predators. Although some short-term studies have documented the influence of variable and synchronized production of seeds on herbivore attack rate during one or few mast years, long-term data including multiple mast seeding years and patterns of cone production and herbivore attack on individual trees are needed to assess (i) how cone production, variability and synchrony affect individual plant fitness and (ii) the functional responses of seed predators to mast seeding events. We tested these objectives, collecting long-term (29 years) data on female seed cone production and rates of seed predator attack from 217 individual contiguous trees within a Pinus ponderosa population. Our results support the predator satiation hypothesis. First, we found high interannual synchrony and variability in seed cone production and a type II functional response of seed predators to available cones. Secondly, years with high cone production (mast years) had markedly lower rates of seed predator attack than years of low production (i.e. a population-level satiation effect). Thirdly, within mast years, individuals with high cone production had markedly lower rates of attack than individuals with low cone production (i.e. an individual-level satiation effect). Finally, individual trees with greater synchrony and more variable cone production suffered lower rates of attack. Synthesis: Our long-term data on individual trees lend strong support to the hypothesis that mast seeding in Pinus ponderosa has evolved in response to natural selection from insect seed predators. Pinus ponderosa escapes its seed predators in time by reproducing at irregular intervals (masting). In years when many cones are available, trees suffer markedly lower rates of seed predator attack than years of low production: white dots = non-mast years, grey dots = intermediate years, black dots = mast years. These data provide evidence that mast seeding in this species evolved in response to natural selection from insect seed predators. © 2014 British Ecological Society., National Foundation Science, Ministry of Education, This research was supported by National Foundation Science grants BMS 75-14050, DEB78-16798, BSR 8918478, and BSR 912065. XM received financial support from Postdoctoral Fulbright/Ministry of Education grant program.

A growing body of research documents the importance of plant genetic effects on arthropod community structure. However, the mechanisms underlying these effects are often unclear. Additionally, plant genetic effects have largely been quantified in common gardens, thus inflating the estimates of their importance by minimizing levels of natural variation. Using Valeriana edulis, a dioecious plant with genetically based sex determination, we conducted surveys and experiments on wild-grown individuals to document field patterns of arthropod association between the sexes and the mechanisms underlying these plant genetic effects. Three years of surveys revealed strong and consistent sex-biased arthropod association in wild-grown plants: female plants supported 4-fold, 1.5-fold, and 4-fold higher densities of aphids, aphid predators, and aphid-tending ants, respectively, compared to males. There was mixed evidence that the female bias for aphids was due to higher plant quality, while we found no difference between plant sexes in aphid preference or the top-down effects of predators and tending ants. Female bias for ants was due to both the greater attractiveness of female plants (direct effect mediated by floral nectar) and an independent, weaker effect of higher aphid abundance on females (density-mediated indirect effect). Conversely, the female bias for predators was driven solely by the greater attractiveness of female plants. We did not find interaction modification, i.e., ant-aphid and predator-aphid interactions were equivalent between plant sexes. Plant sex explained 0.24%, 2.28%, and 4.42% of the variance in aphids, predators, and ants, respectively, values comparable to but slightly weaker than those previously reported from common-garden studies. In contrast to the prediction of diminished plant genetic effects with increasing trophic level, we show how weak indirect effects on predators and parasitoids (via herbivores) can be complemented by strong direct effects via common plant traits (floral resources). In summary, we document direct and indirect effects of genetically based sex on a multi-trophic arthropod community that were expressed in wild- grown plants across multiple years.

While the ecological consequences of plant diversity have received much attention, the mechanisms by which intraspecific diversity affects associated communities remains understudied. We report on a field experiment documenting the effects of patch diversity in the plant Baccharis salicifolia (genotypic monocultures versus polycultures of four genotypes), ants (presence versus absence) and their interaction on ant-tended aphids, ants and parasitic wasps, and the mechanistic pathways by which diversity influences their multi-trophic interactions. Five months after planting, polycultures (versus monocultures) had increased abundances of aphids (threefold), ants (3.2-fold) and parasitoids (1.7-fold) owing to non-additive effects of genetic diversity. The effect on aphids was direct, as plant genetic diversity did not mediate ant-aphid, parasitoid-aphid or ant-parasitoid interactions. This increase in aphid abundance occurred even though plant growth (and thus aphid resources) was not higher in polycultures. The increase in ants and parasitoids was an indirect effect, due entirely to higher aphid abundance. Ants reduced parasitoid abundance by 60 per cent, but did not affect aphid abundance or plant growth, and these top-down effects were equivalent between monocultures and polycultures. In summary, intraspecific plant diversity did not increase primary productivity, but nevertheless had strong effects across multiple trophic levels, and effects on both herbivore mutualists and enemies could be predicted entirely as an extension of plant-herbivore interactions., National Science Foundation, This research was funded by National Science Foundation grants DEB-0919178 and DEB-1120794. XM received financial support from Postdoctoral Fulbright/Ministry of Education grant program., Spanish Ministry of Education

Insectivorous birds reduce arthropod abundances and their damage to plants in some, but not all, studies where predation by birds has been assessed. The variation in bird effects may be due to characteristics such as plant productivity or quality, habitat complexity, and/or species diversity of predator and prey assemblages. Since agroforestry systems vary in such characteristics, these systems provide a good starting point for understanding when and where we can expect predation by birds to be important. We analyze data from bird exclosure studies in forests and agroforestry systems to ask whether birds consistently reduce their arthropod prey base and whether bird predation differs between forests and agroforestry systems. Further, we focus on agroforestry systems to ask whether the magnitude of bird predation (1) differs between canopy trees and understory plants, (2) differs when migratory birds are present or absent, and (3) correlates with bird abundance and diversity. We found that, across all studies, birds reduce all arthropods, herbivores, carnivores, and plant damage. We observed no difference in the magnitude of bird effects between agroforestry systems and forests despite simplified habitat structure and plant diversity in agroforests. Within agroforestry systems, bird reduction of arthropods was greater in the canopy than the crop layer. Top-down effects of bird predation were especially strong during censuses when migratory birds were present in agroforestry systems. Importantly, the diversity of the predator assemblage correlated with the magnitude of predator effects; where the diversity of birds, especially migratory birds, was greater, birds reduced arthropod densities to a greater extent. We outline potential mechanisms for relationships between bird predator, insect prey, and habitat characteristics, and we suggest future studies using tropical agroforests as a model system to further test these areas of ecological theory.

Despite increasing evidence that plant intra- and inter-specific diversity increases primary productivity, and that such effect may in turn cascade up to influence herbivores, there is little information about plant diversity effects on plant antiherbivore defenses, the relative importance of different sources of plant diversity, and the mechanisms for such effects. For example, increased plant growth at high diversity may lead to reduced investment in defenses via growth-defense trade-offs. Alternatively, positive effects of plant diversity on plant growth may lead to increased herbivore abundance which in turn leads to a greater investment in plant defenses. The magnitude of trait variation underlying diversity effects is usually greater among species than among genotypes within a given species, so plant species diversity effects on resource use by producers as well as on higher trophic levels should be stronger than genotypic diversity effects. Here we compared the relative importance of plant genotypic and species diversity on anti-herbivore defenses and whether such effects are mediated indirectly via diversity effects on plant growth and/or herbivore damage. To this end, we performed a large-scale field experiment where we manipulated genotypic diversity of big-leaf mahogany (Swietenia macrophylla) and tree species diversity, and measured effects on mahogany growth, damage by the stem-boring specialist caterpillar Hypsipyla grandella, and defensive traits (polyphenolics and condensed tannins in stem and leaves). We found that both forms of plant diversity had positive effects on stem (but not leaf) defenses. However, neither source of diversity influenced mahogany growth, and diversity effects on defenses were not mediated by either growth-defense trade-offs or changes in stem-borer damage. Although the mechanism(s) of diversity effects on plant defenses are yet to be determined, our study is one of the few to test for and show producer diversity effects on plant chemical defenses. © 2014 Moreira et al.

Several influential hypotheses in plant-herbivore and herbivore-predator interactions consider the interactive effects of plant quality, herbivore diet breadth, and predation on herbivore performance. Yet individually and collectively, these hypotheses fail to address the simultaneous influence of all three factors. Here we review existing hypotheses, and propose the tri-trophic interactions (TTI) hypothesis to consolidate and integrate their predictions. The TTI hypothesis predicts that dietary specialist herbivores (as compared to generalists) should escape predators and be competitively dominant due to faster growth rates, and that such differences should be greater on low quality (as compared to high quality) host plants. To provide a preliminary test of these predictions, we conducted an empirical study comparing the effects of plant (Baccharis salicifolia) quality and predators between a specialist (Uroleucon macolai) and a generalist (Aphis gossypii) aphid herbivore. Consistent with predictions, these three factors interactively determine herbivore performance in ways not addressed by existing hypotheses. Compared to the specialist, the generalist was less fecund, competitively inferior, and more sensitive to low plant quality. Correspondingly, predator effects were contingent upon plant quality only for the generalist. Contrary to predictions, predator effects were weaker for the generalist and on low-quality plants, likely due to density-dependent benefits provided to the generalist by mutualist ants. Because the TTI hypothesis predicts the superior performance of specialists, mutualist ants may be critical to A. gossypii persistence under competition from U. macolai. In summary, the integrative nature of the TTI hypothesis offers novel insight into the determinants of plant-herbivore and herbivore-predator interactions and the coexistence of specialist and generalist herbivores.

Summary: Although plants and herbivores interact under varying soil resources and natural enemy effects, little is known about how these factors influence plant-herbivore interactions and shape the evolution of plant and herbivore traits. Here, we ask whether soil fertility and parasitoids shape selection on fruit number imposed by a seed predator (SP) on the perennial herb Ruellia nudiflora. We used a common garden where half the plants of 14 genetic families were fertilized and recorded the abundance of cleistogamous (CL) fruits and seeds, SPs and parasitoids. We calculated relative fitness per family based on CL seed number under the following three scenarios: three trophic levels (accounting for SP and parasitoid effects), two trophic levels (accounting for SP but not parasitoid effects), and one trophic level (fitness in absence of SPs), and compared selection strength on fruit number between trophic scenarios and fertility environments. In unfertilized conditions, SPs selected for increased CL fruit number, whereas parasitoids dampened (but did not eliminate) this selective impact. With fertilization, however, selection by SPs was reduced and unaffected by parasitoids. Synthesis. Overall, we show that parasitoids can shape herbivore selection on plants, but that both herbivore and parasitoid selective impacts depend upon the abiotic environment. These findings underscore how linkages between abiotic factors and trophic complexity influence the ecological and evolutionary outcomes of species interactions. © 2014 British Ecological Society.