Elevational gradients in constitutive and induced oak defences based on individual traits and their correlated expression patterns

Elevational gradients are useful ecological settings for revealing the biotic and abiotic drivers of plant trait variation and plant–insect interactions. However, most work focusing on plant defences has looked at individual traits and few studies have assessed multiple traits simultaneously, their correlated expression patterns, and abiotic factors associated with such patterns across elevations. To address this knowledge gap, we studied elevational variation in direct (phenolic compounds) and indirect (volatile organic compounds) constitutive defences and their inducibility after feeding by a specialist beetle Altica quercetorum in saplings of 18 wild populations of Quercus pyrenaica. We tested for: 1) clines in each defensive trait individually, 2) their patterns of correlated expression and 3) associations between any such clines and climatic factors. We found that constitutive direct defences (lignins and hydrolysable tannins) decreased with increasing elevation. We observed no elevational gradient for constitutive indirect defences (volatile organic compounds) or the inducibility of direct or indirect defensive traits when looking at groups of compounds. However, at individual tree-level, increased induction of two monoterpenes (α-fenchene and camphene) at higher elevation was shown. Furthermore, we show a significant pattern of co-expression of constitutive and induced phenolics across populations, which weakened with increasing elevation. Finally, we found no evidence that climatic factors were associated with either individual or correlated trait expression patterns across elevations. Overall, these findings call for moving beyond elevational clines in individual plant defences, and argue that assessing elevational shifts in trait correlated expression patterns and their underlying mechanisms can increase our understanding of plant defence evolution and plant–herbivore interactions along environmental gradients.     

Plant stages with biotic,indirect defences are more palatable and suffer less herbivory than their undefended counterparts

Plants have evolved several anti‐herbivory strategies, including direct defences, such as mechanical and chemical defences, and indirect or biotic defences, such as the recruitment of defending animals. We examined whether the investment plants make in direct defences differs between those which do and do not invest in biotic defences, by comparing standing herbivory and palatability of congeneric species with and without indirect defences at two ontogenetic stages: before and after the onset of indirect defences. We used Cordia alliodora and Croton suberosus as the species with indirect defences and Cordia elaeagnoides and Croton pseudoniveus as the species without indirect defences. We predicted that herbivores would prefer to eat species and stages with indirect defences to those without them. As predicted, we found that herbivores preferred species and ontogenetic stages with indirect defences in all cases. Overall, however, natural levels of herbivory were lower in species with indirect defences. We conclude that indirect defences offer effective protection against herbivores and posit that their recruitment allows plants to reduce investment in other defence mechanisms. Our results support the notion that plants trade‐off between direct and indirect defensive strategies. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 536–543.     

Evolution of phenotypic plasticity: Genetic differentiation and additive genetic variation for induced plant defence in wild arugula Eruca sativa

Phenotypic plasticity is the primary mechanism of organismal resilience to abiotic and biotic stress, and genetic differentiation in plasticity can evolve if stresses differ among populations. Inducible defence is a common form of adaptive phenotypic plasticity, and long‐standing theory predicts that its evolution is shaped by costs of the defensive traits, costs of plasticity and a trade‐off in allocation to constitutive versus induced traits. We used a common garden to study the evolution of defence in two native populations of wild arugula Eruca sativa (Brassicaceae) from contrasting desert and Mediterranean habitats that differ in attack by caterpillars and aphids. We report genetic differentiation and additive genetic variance for phenology, growth and three defensive traits (toxic glucosinolates, anti‐nutritive protease inhibitors and physical trichome barriers) as well their inducibility in response to the plant hormone jasmonic acid. The two populations were strongly differentiated for plasticity in nearly all traits. There was little evidence for costs of defence or plasticity, but constitutive and induced traits showed a consistent additive genetic trade‐off within each population for the three defensive traits. We conclude that these populations have evolutionarily diverged in inducible defence and retain ample potential for the future evolution of phenotypic plasticity in defence.     

Costs of predator‐induced morphological defences in Daphnia

1. Inducible defences are advantageous because they protect the prey while limiting associated fitness costs. The presence of these costs is an essential component of this conditional strategy, since their absence would favour constitutive (fixed) defences. In some cases, however, these costs have been difficult to measure because of complex interactions between the defences themselves, resultant life history changes and the organism’s environment. 2. The pond‐dwelling water flea, Daphnia pulex, forms defensive neck spines in response to kairomones released by predatory larvae of the phantom midge, Chaoborus. This predator–prey interaction and the formation of these inducible defences have been well studied, but costs associated with the development of neck spines remain unclear. In this study, I address this problem by analysing the effect of Chaoborus kairomones on the life history responses (and fitness costs associated with these responses) of two clones of D. pulex that are from the same pond population, but differ greatly in their degree of neck spine development. 3. Both D. pulex clones exhibited the same predator‐induced shifts in life history: larger size at birth, reduced juvenile growth rate (producing a smaller size at maturity), delayed reproduction and a reduction in the number of neonates produced after the first clutch. Relative fitness decreased significantly and to the same degree (c. 10% reduction in r) in each clone. This observed fitness cost was not directly related to the neck spines per se since the cost was the same in both clones, despite their considerable differences in neck spine development. Rather, it appears to be indirectly related to this antipredator morphology via a combination of delayed reproduction and a set of life history trade‐offs (decreased growth rate, decreased reproduction after the first clutch) for increased neonate body size, which is necessary for neck spines to be effective defences. This suite of induced responses is probably a result of local adaptation of these two D. pulex clones to their common pond environment. 4. Costs of inducible defences do not always entail direct allocation costs associated with forming and maintaining a defence, but may also involve indirect life history responses that are specific to particular environmental situations. This local adaptation would explain the highly variable life history responses observed among D. pulex clones from different pond environments.     

Inducible chemical defences in animals

Phenotypic plasticity is extremely widespread in the behaviour, morphology and life‐history of animals. However, inducible changes in the production of defensive chemicals are described mostly in plants and surprisingly little is known about similar plasticity in chemical defences of animals. Inducible chemical defences may be common in animals because many are known to produce toxins, the synthesis of toxins is likely to be costly, and there are a few known cases of animals adjusting their toxin production to changes in environmental conditions. We outline what is known about the occurrence of inducible chemical defences in animals and argue that there is immense potential for progress in this field. Possible directions include surveying diverse taxa to explore how general its occurrence may be and testing for selection acting on inducible chemical defences. Data on inducible chemical defences would provide insight into life‐history tradeoffs by enabling novel tests of how time‐costs and resource‐costs affect life‐history. If the synthesis of toxic compounds by animals proves accessible to manipulation, as it is in plants and fungi, this will open the way to refined estimates of the fitness costs of defence, ultimately providing a clearer picture of how plasticity evolves and is maintained in nature. Synthesis Inducible changes in the behaviour, morphology, and life‐history of animals are extremely widespread, but surprisingly little is known about similar changes in the production of defensive chemicals. We outline what is known about the occurrence of inducible chemical defences in animals and argue that there is immense potential for progress in this field. Possible directions include surveying diverse taxa to explore how general its occurrence may be and testing for selection acting on inducible chemical defences. Data on inducible chemical defences would provide insight into life‐history tradeoffs by enabling novel tests of how time‐costs and resource‐costs affect life‐history. If the synthesis of toxic compounds by animals proves accessible to manipulation, we will be able to estimate the fitness costs of defence more precisely, and ultimately provide a clearer picture of how plasticity evolves and is maintained in nature.     

Trade-offs in non-native plant herbivore defences enhance performance

Non-native plants are typically released from specialist enemies but continue to be attacked by generalists, albeit at lower intensities. This reduced herbivory may lead to less investment in constitutive defences and greater investment in induced defences, potentially reducing defence costs. We compared herbivory on 27 non-native and 59 native species in the field and conducted bioassays and chemical analyses on 12 pairs of non-native and native congeners. Non-natives suffered less damage and had weaker constitutive defences, but stronger induced defences than natives. For non-natives, the strength of constitutive defences was correlated with the intensity of herbivory experienced, whereas induced defences showed the reverse. Investment in induced defences correlated positively with growth, suggesting a novel mechanism for the evolution of increased competitive ability. To our knowledge, these are the first linkages reported among trade-offs in plant defences related to the intensity of herbivory, allocation to constitutive versus induced defences, and growth.     

The evolution and ecology of multiple antipredator defences

Prey seldom rely on a single type of antipredator defence, often using multiple defences to avoid predation. In many cases, selection in different contexts may favour the evolution of multiple defences in a prey. However, a prey may use multiple defences to protect itself during a single predator encounter. Such “defence portfolios” that defend prey against a single instance of predation are distributed across and within successive stages of the predation sequence (encounter, detection, identification, approach (attack), subjugation and consumption). We contend that at present, our understanding of defence portfolio evolution is incomplete, and seen from the fragmentary perspective of specific sensory systems (e.g., visual) or specific types of defences (especially aposematism). In this review, we aim to build a comprehensive framework for conceptualizing the evolution of multiple prey defences, beginning with hypotheses for the evolution of multiple defences in general, and defence portfolios in particular. We then examine idealized models of resource trade-offs and functional interactions between traits, along with evidence supporting them. We find that defence portfolios are constrained by resource allocation to other aspects of life history, as well as functional incompatibilities between different defences. We also find that selection is likely to favour combinations of defences that have synergistic effects on predator behaviour and prey survival. Next, we examine specific aspects of prey ecology, genetics and development, and predator cognition that modify the predictions of current hypotheses or introduce competing hypotheses. We outline schema for gathering data on the distribution of prey defences across species and geography, determining how multiple defences are produced, and testing the proximate mechanisms by which multiple prey defences impact predator behaviour. Adopting these approaches will strengthen our understanding of multiple defensive strategies.     

Testing the optimal defence hypothesis for two indirect defences: extrafloral nectar and volatile organic compounds

Many plants respond to herbivory with an increased production of extrafloral nectar (EFN) and/or volatile organic compounds (VOCs) to attract predatory arthropods as an indirect defensive strategy. In this study, we tested whether these two indirect defences fit the optimal defence hypothesis (ODH), which predicts the within-plant allocation of anti-herbivore defences according to trade-offs between growth and defence. Using jasmonic acid-induced plants of Phaseolus lunatus and Ricinus communis, we tested whether the within-plant distribution pattern of these two indirect defences reflects the fitness value of the respective plant parts. Furthermore, we quantified photosynthetic rates and followed the within-plant transport of assimilates with (13)C labelling experiments. EFN secretion and VOC emission were highest in younger leaves. Moreover, the photosynthetic rate increased with leaf age, and pulse-labelling experiments suggested transport of carbon to younger leaves. Our results demonstrate that the ODH can explain the within-plant allocation pattern of both indirect defences studied.     

The unfolding of plant growth form‐defence syndromes along elevation gradients

Understanding the functional economics that drives plant investment of resources requires investigating the interface between plant phenotypes and the variation in ecological conditions. While allocation to defence represents a large portion of the carbon budget, this axis is usually neglected in the study of plant economic spectrum. Using a novel geometrical approach, we analysed the co‐variation in a comprehensive set of functional traits related to plant growth strategies, as well as chemical defences against herbivores on all 15 Cardamine species present in the Swiss Alps. By extracting geometrical information of the functional space, we observed clustering of plants into three main syndromes. Those different strategies of growth form and defence were also distributed within distinct elevational bands demonstrating an association between the functional space and the ecological conditions. We conclude that plant strategies converge into clear syndromes that trade off abiotic tolerance, growth and defence within each elevation zone.     

Anatomical defences against bark beetles relate to degree of historical exposure between species and are allocated independently of chemical defences within trees

Conifers possess chemical and anatomical defences against tree‐killing bark beetles that feed in their phloem. Resins accumulating at attack sites can delay and entomb beetles while toxins reach lethal levels. Trees with high concentrations of metabolites active against bark beetle‐microbial complexes, and more extensive resin ducts, achieve greater survival. It is unknown if and how conifers integrate chemical and anatomical components of defence or how these capabilities vary with historical exposure. We compared linkages between phloem chemistry and tree ring anatomy of two mountain pine beetle hosts. Lodgepole pine, a mid‐elevation species, has had extensive, continual contact with this herbivore, whereas high‐elevation whitebark pines have historically had intermittent exposure that is increasing with warming climate. Lodgepole pine had more and larger resin ducts. In both species, anatomical defences were positively related to tree growth and nutrients. Within‐tree constitutive and induced concentrations of compounds bioactive against bark beetles and symbionts were largely unrelated to resin duct abundance and size. Fewer anatomical defences in the semi‐naïve compared with the continually exposed host concurs with directional differences in chemical defences. Partially uncoupling chemical and morphological antiherbivore traits may enable trees to confront beetles with more diverse defence permutations that interact to resist attack.     

Induced defences in marine and freshwater phytoplankton: a review

Many organisms have developed defences to avoid predation by species at higher trophic levels. The capability of primary producers to defend themselves against herbivores affects their own survival, can modulate the strength of trophic cascades and changes rates of competitive exclusion in aquatic communities. Algal species are highly flexible in their morphology, growth form, biochemical composition and production of toxic and deterrent compounds. Several of these variable traits in phytoplankton have been interpreted as defence mechanisms against grazing. Zooplankton feed with differing success on various phytoplankton species, depending primarily on size, shape, cell wall structure and the production of toxins and deterrents. Chemical cues associated with (i) mechanical damage, (ii) herbivore presence and (iii) grazing are the main factors triggering induced defences in both marine and freshwater phytoplankton, but most studies have failed to disentangle the exact mechanism(s) governing defence induction in any particular species. Induced defences in phytoplankton include changes in morphology (e.g. the formation of spines, colonies and thicker cell walls), biochemistry (such as production of toxins, repellents) and in life history characteristics (formation of cysts, reduced recruitment rate). Our categorization of inducible defences in terms of the responsible induction mechanism provides guidance for future work, as hardly any of the available studies on marine or freshwater plankton have performed all the treatments that are required to pinpoint the actual cue(s) for induction. We discuss the ecology of inducible defences in marine and freshwater phytoplankton with a special focus on the mechanisms of induction, the types of defences, their costs and benefits, and their consequences at the community level.     

Constitutive immune defences correlate with life-history variables in tropical birds

1. It has been suggested that immune defences are shaped by life history and ecology, but few general patterns have been described across species. We hypothesized that 'fast' life-history traits (e.g. short development times, large clutch sizes) would be associated with developmentally inexpensive immune defences, minimizing the resource demands of young animals' immune systems during periods of rapid growth. Conversely, 'slow' life histories should be associated with well developed antibody-mediated defences, which are developmentally costly. 2. We therefore predicted that 'fast-living' species would exhibit higher levels of complement proteins, a component of non-specific innate defence, but lower levels of constitutive ('natural') antibodies. Additionally, we predicted that constitutive immune defences in general would be higher in species with ecological characteristics that might increase exposure to pathogens, such as open nests, omnivorous diets, gregariousness, and closed forested habitat. 3. Across 70 Neotropical bird species, we found a strongly positive relationship between incubation period and natural antibody levels in adult birds, suggesting that longer developmental times might allow the production of a more diverse and/or more reactive adaptive immune system. Complement activity was positively, although weakly, correlated with clutch size, providing some support for the hypothesis that faster-living species rely more on innate defences, such as complement. Unexpectedly, solitary species had higher natural antibody titres than species that frequently join flocks. 4. Our results suggest that, despite probably widespread differences in the intensity and diversity of pathogen exposure, species-level variation in constitutive immune defences is understandable within the context of life-history theory.     

Linking herbivore-induced defences to population dynamics

1. Theoretical studies have shown that inducible defences have the potential to affect population stability and persistence in bi‐ and tritrophic food chains. Experimental studies on such effects of prey defence strategies on the dynamics of predator–prey systems are still rare. We performed replicated population dynamics experiments using the herbivorous rotifer Brachionus calyciflorus and four strains of closely related algae that show different defence responses to this herbivore. 2. We observed herbivore populations to fluctuate at a higher frequency when feeding on small undefended algae. During these fluctuations minimum rotifer densities remained sufficiently high to ensure population persistence in all the replicates. The initial growth of rotifer populations in this treatment coincided with a sharp drop in algal density. Such a suppression of algae by herbivores was not observed in the other treatments, where algae were larger due to induced or permanent defences. In these treatments we observed rotifer population densities to first rise and then decline. The herbivore went extinct in all replicates with large permanently defended algae. The frequency of herbivore extinctions was intermediate when algae had inducible defences. 3. A variety of alternative mechanisms could explain differential herbivore persistence in the different defence treatments. Our analysis showed the density and fraction of highly edible algal particles to better explain herbivore persistence and extinctions than total algal density, the fraction of highly inedible food particles or the accumulation of herbivore waste products or autotoxins. 4. We argue that the rotifers require a minimum fraction and density of edible food particles for maintenance and reproduction. We conjecture that induced defences in algae may thus favour larger zooplankton species such as Daphnia spp. that are less sensitive to shifts in their food size spectrum, relative to smaller zooplankton species, such as rotifers and in this way contributes to the structuring of planktonic communities.     

Environmental effects on constitutive and inducible resin defences of Pinus taeda

The ecological literature abounds with studies of environmental effects on plant antiherbivore defences. While various models have been proposed (e.g. plant stress, optimal allocation, growth-differentiation balance), each has met with mixed support. One possible explanation for the mixed results is that constitutive and induced defences are differentially affected by environmental conditions. In this study, constitutive oleoresin flow from Pinus tadea was least during periods of rapid tree growth and most when drought conditions limited growth; this is as expected if constitutive secondary metabolism is a function of the carbohydrate pool size after growth has been maximised. Induced increases in resin flow, however, were greatest in the fastest growing trees during the season of greatest growth. Apparently, resin production becomes an allocation priority after wounding but not before. Understanding environmental effects on plant antiherbivore defences requires physiological and evolutionary models that account for the differences between constitutive and induced secondary metabolism.     

Growth cost and ontogenetic expression patterns of defence in cyanogenic Eucalyptus spp.

Plant defences can incur allocation costs and such costs incurred early in ontogeny may result in opportunity costs with effects evident later in life. A unified understanding of the growth cost of defence requires the identification of plants with varying ontogenetic trajectories of preferably resource demanding defences and an appropriate measurement of the growth cost of these defences. To develop such tools, we first compared nitrogen-based chemical defence (cyanogenic glycosides) in juvenile and adult foliage of three species of Eucalyptus (Myrtaceae). We found marked differences between the species, with two having much lower concentrations of foliar cyanogenic glycosides in seedlings compared to adults. We next used seedlings of two species to measure the resource (nitrogen) and growth cost of deploying cyanogenic glycosides. We found evidence that for every 1.0 nitrogen invested in cyanogenic glycosides, 1.49 additional nitrogens were effectively added to the leaves. We also found that deployment of cyanogenic glycosides was associated with a reduction in net assimilation rate (NAR) at constant leaf nitrogen. We did not, however, detect an overall growth cost associated with cyanogenic glycoside deployment because the rise in leaf nitrogen associated with this deployment apparently counteracted the reduction in NAR.     

Elevated carbon dioxide and warming impact silicon and phenolic‐based defences differently in native and exotic grasses

Global climate change may increase invasions of exotic plant species by directly promoting the success of invasive/exotic species or by reducing the competitive abilities of native species. Changes in plant chemistry, leading to altered susceptibility to stress, could mediate these effects. Grasses are hyper‐accumulators of silicon, which play a crucial function in the alleviation of diverse biotic and abiotic stresses. It is unknown how predicted increases in atmospheric carbon dioxide (CO₂) and air temperature affect silicon accumulation in grasses, especially in relation to primary and secondary metabolites. We tested how elevated CO₂ (eCO₂) (+240 ppm) and temperature (eT) (+4°C) affected chemical composition (silicon, phenolics, carbon and nitrogen) and plant growth in eight grass species, either native or exotic to Australia. eCO₂ increased phenolic concentrations by 11%, but caused silicon accumulation to decline by 12%. Moreover, declines in silicon occurred mainly in native species (?19%), but remained largely unchanged in exotic species. Conversely, eT increased silicon accumulation in native species (+19%) but decreased silicon accumulation in exotic species (?10%). Silicon and phenolic concentrations were negatively correlated with each other, potentially reflecting a defensive trade‐off. Moreover, both defences were negatively correlated with plant mass, compatible with a growth‐defence trade‐off. Grasses responded in a species‐specific manner, suggesting that the relative susceptibility of different species may differ under future climates compared to current species rankings of resource quality. For example, the native Microlaena stipoides was less well defended under eCO₂ in terms of both phenolics and silicon, and thus could suffer greater vulnerability to herbivores. To our knowledge, this is the first demonstration of the impacts of eCO₂ and eT on silicon accumulation in grasses. We speculate that the greater plasticity in silicon uptake shown by Australian native grasses may be partly a consequence of evolving in a low nutrient and seasonally arid environment.     

Intraspecific trait cospecialization of constitutive and inducible morphological defences in a marine snail from habitats with different predation risk

1.?Studies examining the integration of constitutive and inducible aspects of multivariate defensive phenotypes are rare. 2.?I asked whether marine snails (Nucella lamellosa) from habitats with and without abundant predatory crabs differed in constitutive and inducible aspects of defensive shell morphology. 3.?I examined multivariate shell shape development of snails from each habitat in the presence and absence of waterborne cues from feeding crabs (Cancer productus). I also examined the influence of constitutive and inducible shell morphology on resistance to crushing. 4.?Regardless of the presence of crabs, snails from high-risk (HR) habitats developed rotund, short-spired shells, while snails from low-risk habitats developed elongate shells, tall-spired shells, indicating among-habitat divergence in constitutive shell shape. Moreover, allometry analyses indicated that constitutive developmental patterns underlying this variation also differed between habitats. However, snails from HR habitats showed greater plasticity for apertural lip thickness and apertural area in the presence of crab cues, indicating among-habitat variation in defence inducibility. 5.?Both shell shape and apertural lip thickness contributed to shell strength suggesting that constitutive shell shape development and inducible lip thickening have evolved jointly to form an effective defence in habitats where predation risk is high.     

Density-dependent effects of prey defences

In this study, we show that the protective advantage of a defence depends on prey density. For our investigations, we used the predator-prey model system Chaoborus-Daphnia pulex. The prey, D. pulex, forms neckteeth as an inducible defence against chaoborid predators. This morphological response effectively reduces predator attack efficiency, i.e. number of successful attacks divided by total number of attacks. We found that neckteeth-defended prey suffered a distinctly lower predation rate (prey uptake per unit time) at low prey densities. The advantage of this defence decreased with increasing prey density. We expect this pattern to be general when a defence reduces predator success rate, i.e. when a defence reduces encounter rate, probability of detection, probability of attack, or efficiency of attack. In addition, we experimentally simulated the effects of defences which increase predator digestion time by using different sizes of Daphnia with equal vulnerabilities. This type of defence had opposite density-dependent effects: here, the relative advantage of defended prey increased with prey density. We expect this pattern to be general for defences which increase predator handling time, i.e. defences which increase attacking time, eating time, or digestion time. Many defences will have effects on both predator success rate and handling time. For these defences, the predator’s functional response should be decreased over the whole range of prey densities. Received: 15 September 1999 / Accepted: 23 December 1999     

Life‐history constraints in grassland plant species: a growth‐defence trade‐off is the norm

Plant growth can be limited by resource acquisition and defence against consumers, leading to contrasting trade‐off possibilities. The competition‐defence hypothesis posits a trade‐off between competitive ability and defence against enemies (e.g. herbivores and pathogens). The growth‐defence hypothesis suggests that strong competitors for nutrients are also defended against enemies, at a cost to growth rate. We tested these hypotheses using observations of 706 plant populations of over 500 species before and following identical fertilisation and fencing treatments at 39 grassland sites worldwide. Strong positive covariance in species responses to both treatments provided support for a growth‐defence trade‐off: populations that increased with the removal of nutrient limitation (poor competitors) also increased following removal of consumers. This result held globally across 4 years within plant life‐history groups and within the majority of individual sites. Thus, a growth‐defence trade‐off appears to be the norm, and mechanisms maintaining grassland biodiversity may operate within this constraint.     

Latitudinal variation in plant chemical defences drives latitudinal patterns of leaf herbivory

A long‐standing paradigm in ecology holds that herbivore pressure and thus plant defences increase towards lower latitudes. However, recent work has challenged this prediction where studies have found no relationship or opposite trends where herbivory or plant defences increase at higher latitudes. Here we tested for latitudinal variation in herbivory, chemical defences (phenolic compounds), and nutritional traits (phosphorus and nitrogen) in leaves of a long‐lived tree species, the English oak Quercus robur. We further investigated the underlying climatic and soil factors associated with such variation. Across 38 populations of Q. robur distributed along an 18° latitudinal gradient, covering almost the entire latitudinal and climatic range of this species, we observed strong but divergent latitudinal gradients in leaf herbivory and leaf chemical defences and nutrients. As expected, there was a negative relationship between latitude and leaf herbivory where oak populations from lower latitudes exhibited higher levels of leaf herbivory. However, counter to predictions there was a positive relationship between leaf chemical defences and latitude where populations at higher latitudes were better defended. Similarly, leaf phosphorus and nitrogen increased with latitude. Path analysis indicated a significant (negative) effect of plant chemical defences (condensed tannins) on leaf herbivory, suggesting that the latitudinal gradient in leaf herbivory was driven by an inverse gradient in defensive investment. Leaf nutrients had no independent influence on herbivory. Further, we found significant indirect effects of precipitation and soil porosity on leaf herbivory, which were mediated by plant chemical defences. These findings suggest that abiotic factors shape latitudinal variation in plant defences and that these defences in turn underlie latitudinal variation in leaf herbivory. Overall, this study contributes to a better understanding of latitudinal variation in plant–herbivore interactions by determining the identity and modus operandi of abiotic factors concurrently shaping plant defences and herbivory.