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.     

Latitudinal variation in herbivory: influences of climatic drivers,herbivore identity and natural enemies

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 differing life‐history traits. Additionally, the strength of top–down effects from natural enemies on herbivores might also vary geographically and influence latitude–herbivory patterns. We carried out a field study where we investigated the effects 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 nudiflora (Acanthaceae). These 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 difference appeared to be mediated by contrasting effects of climate on each guild. Specifically, 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 significant effects 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 findings underscore that the factors driving latitudinal clines in herbivory might vary even among herbivore species coexisting on the same host plant.     

Can genetically based clines in plant defence explain greater herbivory at higher latitudes?

Greater plant defence is predicted to evolve at lower latitudes in response to increased herbivore pressure. However, recent studies question the generality of this pattern. In this study, we tested for genetically based latitudinal clines in resistance to herbivores and underlying defence traits of Oenothera biennis. We grew plants from 137 populations from across the entire native range of O. biennis. Populations from lower latitudes showed greater resistance to multiple specialist and generalist herbivores. These patterns were associated with an increase in total phenolics at lower latitudes. A significant proportion of the phenolics were driven by the concentrations of two major ellagitannins, which exhibited opposing latitudinal clines. Our analyses suggest that these findings are unlikely to be explained by local adaptation of herbivore populations or genetic variation in phenology. Rather greater herbivory at high latitudes can be explained by latitudinal clines in the evolution of plant defences.     

Does ontogeny cause changes in the defensive strategies of the myrmecophyte Cecropia peltata?

Cecropia peltata L. is a myrmecophyte, with a wide distribution in the neotropics, predominantly associated with ants of the genus Azteca. It has been shown that Azteca ants defend Cecropia plants against herbivores, and that the plants provide housing (hollow stems) and food (Müllerian bodies) for the ant colony. In the field, occupation by ants does not take place until plants have reached a minimum colonisable size (ca 1 m height), and defensive ants do not occupy small plants. Therefore, juvenile individuals lack such biotic defence. This constitutes an ontogenetic constraint to biotic defence in these plants. We tested the hypothesis that in the stage previous to colonisation plants of Cecropia peltata in a Mexican tropical forest may exhibit some alternative or complementary defensive mechanism against herbivores. We compared, in pre-colonised and colonised plants: rates of herbivory, concentrations of potentially defensive secondary metabolites (total phenolics and condensed tannins), and trichome density. We also conducted acceptability bioassays with a generalist herbivore (Spodoptera fugiperda). In addition, we measured plant growth to investigate if, by using alternative defence mechanisms, pre-colonised plants experience a reduced performance. Rates of herbivory were higher in pre-colonised individuals. Accordingly, leaf phenolics and tannin concentrations, as well as trichome density, were higher in colonised plants. In addition, acceptability bioassays showed that S. fugiperda preferred the leaves of pre-colonised plants. Relative growth rate was not statistically different between both types of plants. Contrary to our expectation, colonised plants, besides biotic defence by ants, also had higher concentrations of secondary metabolites, higher trichome density and lower herbivory and palatability than pre-colonised plants. This suggests that pre-colonised plants may deal with herbivores by other means and that older, larger plants invest more in all defences rather than shifting defensive mechanisms with ontogeny. Since growth rate of pre-colonised plants was comparable to that of colonised plants (despite the higher levels of herbivory of the former), we suggest that plant tolerance leading to compensation may be used by pre-colonised juveniles of C. peltata. This revised version was published online in August 2006 with corrections to the Cover Date.     

Explaining differential herbivory in sun and shade: the case of Aristotelia chilensis saplings

Differential herbivory in contrasting environments is commonly explained by differences in plant traits. When several plant traits are considered, separate correlation analyses between herbivory and candidate traits are typically conducted. This makes it difficult to discern which trait best explain the herbivory patterns, or to avoid spurious inferences due to correlated characters. Aristotelia chilensis saplings sustain greater herbivory in shaded environments than in open habitats. We measured alkaloids, phenolics, trichomes, leaf thickness and water content in the same plants sampled for herbivory. We conducted a multiple regression analysis to estimate the relationship between herbivory and each plant trait accounting for the effect of correlated traits, thus identifying which trait(s) better explain(s) the differential herbivory on A. chilensis. We also estimated insect abundance in both light environments. Palatability bioassays tested whether leaf consumption by the main herbivore on A. chilensis was consistent with field herbivory patterns. Overall insect abundance was similar in open and shaded environments. While saplings in open environments had thicker leaves, lower leaf water content, and higher concentration of alkaloids and phenolics, no difference in trichome density was detected. The multiple regression analysis showed that leaf thickness was the only trait significantly associated with herbivory. Thicker leaves received less damage by herbivores. Sawfly larvae consumed more leaf tissue when fed on shade leaves. This result is consistent with field herbivory and, together with results of insect abundance, renders unlikely that the differential herbivory in A. chilensis was due to greater herbivory pressure in open habitats.     

Compensation and resistance to herbivory in seagrasses: induced responses to simulated consumption by fish

Herbivory can induce changes in plant traits that may involve both tolerance mechanisms that compensate for biomass loss and resistance traits that reduce herbivore preference. Seagrasses are marine vascular plants that possess many attributes that may favour tolerance and compensatory growth, and they are also defended with mechanisms of resistance such as toughness and secondary metabolites. We quantified phenotypic changes induced by herbivore damage on the temperate seagrass Posidonia oceanica in order to identify specific compensatory and resistance mechanisms in this plant, and to assess any potential trade-offs between these two strategies of defence. We simulated three natural levels of fish herbivory by repeatedly clipping seagrass leaves during the summer period of maximum herbivory. Compensatory responses were determined by measuring shoot-specific growth, photosynthetic rate, and the concentration of nitrogen and carbon resources in leaves and rhizomes. Induced resistance was determined by measuring the concentration of phenolic secondary metabolites and by assessing the long-term effects of continued clipping on herbivore feeding preferences using bioassays. Plants showed a significant ability to compensate for low and moderate losses of leaf biomass by increasing aboveground growth of damaged shoots, but this was not supported by an increase in photosynthetic capacity. Low levels of herbivory induced compensatory growth without any measurable effects on stored resources. In contrast, nitrogen reserves in the rhizomes played a crucial role in the plant’s ability to compensate and survive herbivore damage under moderate and high levels of herbivory, respectively. We found no evidence of inducibility of long-term resistance traits in response to herbivory. The concentration of phenolics decreased with increasing compensatory growth despite all treatments having similar carbon leaf content, suggesting reallocation of these compounds towards primary functions such as cell-wall construction.     

Urbanization drives unique latitudinal patterns of insect herbivory and tree condition

Urban landscapes are characterized by high proportions of impervious surface resulting in higher temperatures than adjacent natural landscapes. In some cities, like those at cooler latitudes, trees may benefit from warmer urban temperatures, but trees in many cities are beset with problems like drought stress and increased herbivory. What drives patterns of urban tree health across urbanization and latitudinal temperature gradients? In natural systems, latitude–herbivory relationships are well‐studied, and recent temperate studies have shown that herbivory generally increases with decreasing latitudes (warmer temperatures). However, the applicability of this latitude–herbivory theory in already‐warmed urban systems is unknown. In this study, we investigated how the interaction of urbanization, latitudinal warming and scale insect abundance affected urban tree health. We predicted that trees in warmer, lower latitude cities would be in poorer health at lower levels of urbanization than trees at cooler, higher latitudes due to the interaction of urbanization, latitudinal temperature and herbivory. To evaluate our predictions, we surveyed the abundance of scale insect herbivores on a single, common tree species Acer rubrum in eight US cities spanning 10° of latitude. We estimated urbanization at two extents, a local one that accounted for the direct effects on an individual tree, and a larger one that captured the surrounding urban landscape. We found that urban tree health did not vary with latitudinal temperature but was best predicted by local urbanization and herbivore abundance. We did not observe increased herbivore abundance in warmer, lower latitudes cities, but instead herbivore abundance peaked in the mid latitudes of our study. This study demonstrates that urban landscapes may deviate from classical theory developed in natural systems and reinforces the need for research reconciling ecological patterns in urban landscapes.     

Preference and Performance in Plant–Herbivore Interactions across Latitude–A Study in U.S. Atlantic Salt Marshes

High-latitude plants are often more palatable to herbivores than low-latitude conspecifics. Does increased plant palatability lead to better herbivore performance? Our field and laboratory work investigated (A) whether high-latitude plants have traits indicating that they should be higher-quality foods for herbivores; (B) whether geographic differences in plant quality are more important than local adaptation of herbivores. We studied 3 plant species and 6 invertebrate herbivores in U.S. Atlantic Coast. Past studies had shown high-latitude individuals of these plants are more palatable than low-latitude conspecifics. We documented plant traits and herbivore performance (body size) in the field across latitude. We collected individuals from different latitudes for factorial (plant region x herbivore region) laboratory experiments, examining how herbivore performance was affected by plant region, herbivore region, and their interaction (i.e., local adaptation). Field surveys suggested high-latitude plants were likely of higher quality to herbivores. Leaf nitrogen content in all plant species increased toward high latitudes, consistent with lower leaf C/N and higher leaf chlorophyll content at high latitudes. Furthermore, leaf toughness decreased toward higher latitudes in 1 species. The body size of 4 herbivore species increased with latitude, consistent with high-latitude leaves being of higher quality, while 2 grasshopper species showed the opposite pattern, likely due to life-history constraints. In the laboratory, high-latitude plants supported better performance in 4 herbivore species (marginal in the 5th). The geographic region where herbivores were collected affected herbivore performance in all 6 species; however, the pattern was mixed, indicating a lack of local adaptation by herbivores to plants from their own geographic region. Our results suggest that more-palatable plants at high latitudes support better herbivore growth. Given that geographic origin of either plants or herbivores can affect herbivore performance, the nature of plant-herbivore interactions is likely to change if climate change “reshuffles” plant and herbivore populations across latitude.     

Latitudinal variation in resistance and tolerance to herbivory of a salt marsh shrub

Interactions between plants and herbivores often vary on a geographic scale. Although theory about plant defenses and tolerance is predicated on temporal or spatial variation in herbivore damage, no single study has compared the pattern of herbivory, plant defenses and tolerance to herbivory of a single species across a latitudinal gradient. In 2002–2005 we surveyed replicate salt marshes along the Atlantic coast of the United States from Florida to Maine. At each field site we scored leaves of Iva frutescens for herbivore damage. In laboratory experiments we measured constitutive resistance and induced resistance in I. frutescens from high and low latitude sites along the Atlantic Coast. In another common garden experiment we studied tolerance to herbivory of I. frutescens from various sites. Theory predicts that constitutive resistance should matter more when damage is high, and induced resistance when herbivory is high but variable. In the field, average levels of herbivore damage, and spatial and temporal variation in herbivore damage were all greater at low versus high latitudes, indicating that constitutive as well as induced resistance should be stronger at low latitudes. Consistent with this prediction, constitutive resistance to herbivory was stronger at low latitudes. Induced resistance to herbivores was also stronger at low latitudes: it was deployed faster and lasted longer. Theory also predicts that tolerance to herbivory should be greater where average herbivory damage is greater; however, tolerance to herbivory in Iva did not depend on geographic origin. Our results emphasize the value of considering multiple ways in which plants respond to herbivores when examining geographic variation in plant–herbivore interactions.     

The Herbivore Assemblage, Herbivory and Leaf Chemistry of the Mangrove Kandelia obovata in Two Contrasting Forests in Hong Kong

The herbivore assemblage, intensity of herbivory and factors determining herbivory levels on the mangrove Kandelia obovata (previously K. candel, Rhizophoraceae) were studied over a 13-month period at two forests with contrasting growing conditions in Hong Kong. Mai Po was part of an eutrophic embayment in the Pearl River estuary and generally offered more favourable conditions for mangrove growth, whereas Ting Kok had a rocky substratum and oceanic salinity. Twenty-four insect herbivore species were recorded on K. obovata, with lepidopteran larvae that consume leaf lamina being the dominant species. While leaf litter production was similar at the two forests, herbivory level at Mai Po (mean = 3.9% in terms of leaf area loss) was more severe than that at Ting Kok (mean = 2.3%). Peak herbivory levels were found in summer at both locations (6.5% for Mai Po and 3.8% for Ting Kok). Young leaves of K. obovata at both locations were generally preferred by the herbivores from the period of late spring to summer. Concentrations of most feeding deterrents (ash, crude fibre, and total soluble tannins) were significantly higher in both young and mature leaves at Ting Kok, whereas leaf nutrients (total nitrogen and water) were the same at the two sites. Young leaves at Ting Kok contained about 30% more tannins than their counterparts at Mai Po. Significant differences in leaf chemistry also existed between young and mature leaves at either site. The differences were concomitant with the observed patterns of leaf herbivory on K. obovata, and suggest a potential relationship between environmental quality and plant defence against herbivory.     

Variation in multiple traits of vegetative and reproductive seagrass tissues influences plant–herbivore interactions

Plant–herbivore interactions have strong ecological and evolutionary consequences, but have been traditionally overlooked in marine higher plants. Despite recent advances in seagrass ecology that highlight the importance of herbivory, the mechanisms that regulate the feeding behaviour of seagrass consumers remain largely unknown. Herbivores have been shown to reduce the sexual reproductive success of seagrasses through direct consumption of inflorescences and seeds, but we know little about intraspecific variation in susceptibility to grazing of different seagrass tissues. We contrasted the relative palatability of reproductive and vegetative tissues of the temperate seagrass Posidonia oceanica in the field, and we assessed the feeding preferences among these tissues of the main consumers of the plant, the fish Sarpa salpa and the urchin Paracentrotus lividus. Moreover, we identified the plant traits that explained the observed feeding behaviour. We provide strong evidence for herbivore selectivity among seagrass tissues. In the field, 70–90% of inflorescences were damaged by herbivores compared to 3–60% of leaves of similar age. In feeding assays, the urchin P. lividus showed over a twofold preference for reproductive tissue at various stages of development. By contrast, we detected no feeding activity on either leaves or inflorescences from the fish S. salpa, which is known to migrate to deeper waters soon after flowering starts and during the period of fruit maturation. Despite being the preferred food of urchins, inflorescences were chemically defended, had higher levels of phenolics and lower nutrient and calorific content than leaves. We experimentally demonstrated that leaf structural defences are the primary factor in determining urchin feeding preferences. Removal of plant structure results in a drastic shift in urchin selectivity towards the most nutritious and less chemically defended leaf tissue, indicating that multiple mechanisms of defence to herbivory may coexist in seagrasses.     

Assessing the latitudinal gradient in herbivory

Plant–herbivore interactions occur in all ecosystems and provide a major avenue for energy flow to higher trophic levels. A long‐standing hypothesis to explain the latitudinal gradient in species diversity proposes that the relatively stable and frost‐free climate of the tropics should lead to more intense biotic interactions in tropical compared with temperate environments, giving rise to a greater diversity of plants and herbivores. Herbivory rates have been compared across latitudes to test this biotic interactions hypothesis, with herbivory typically being measured from observable leaf damage. However, we argue that a measure of percentage leaf damage alone does not straightforwardly reflect the cost of herbivory to the plant, and on its own does not constitute an appropriate test of the biotic interactions hypothesis. For a given amount of herbivory, the impact of herbivory is dependent upon many factors, such as the construction cost of the leaf, the growth and replacement rates and leaf life span. We investigate the latitudinal gradient in herbivory by analysing a large dataset of herbivory rates for 452 tree species and separating the species into those with short and long leaf life spans. We show that annual herbivory rates tend to be greater at lower latitudes for evergreen species (which have long‐lived leaves), but no trend in herbivory rate with latitude was found for species with short leaf life spans. Phylogenetic least squares regression assuming Ornstein‐Uhlenbeck processes also showed a negative effect of latitude on herbivory rate for evergreen trees, but we caution that viewing herbivory as a species trait is problematic. An integrative approach that incorporates leaf life span, as well as the costs of investment in growth and potential costs of losing leaf tissue, is needed to further our understanding of the ecological and evolutionary dynamics of herbivory.     

Seedling Mortality and Herbivory Damage in Subtropical and Temperate Populations: Testing the Hypothesis of Higher Herbivore Pressure Toward the Tropics

Herbivory rates are generally thought to be higher in tropical than in temperate forests. Nevertheless, tests of this biogeographic prediction by comparing a single plant species across a tropical-temperate range are scarce. Here, we compare herbivore damage between subtropical and temperate populations of the evergreen tree Aextoxicon punctatum (Olivillo), distributed between 30° and 43° S along the Pacific margin of Chile. To assess the impact of herbivory on Olivillo seedlings, we set up 29 experimental plots, 1.5 × 3 m: 16 in forests of Fray Jorge National Park (subtropical latitude), and 13 in Guabún, Chiloé Island (temperate latitude). Half of each plot was fenced around with chicken wire, to exclude small mammals, and the other half was left unfenced. In each half of the plots we planted 16 seedlings of Olivillo in December 2003, with a total of 928 plants. Seedling survival, leaf production and herbivory by invertebrates were monitored over the next 16 mo. Small mammal herbivores killed ca 30 percent of seedlings in both sites. Nevertheless, invertebrate herbivory was greater in the temperate forest, thus contradicting the expected trend of increasing herbivore impact toward the tropics. Seedling growth was greater in subtropical forest suggesting better conditions for tree growth or that higher invertebrate herbivory depressed seedling growth in the temperate forest. Invertebrate herbivory increased toward temperate latitudes while small mammal herbivory was similar in both sites. We suggest that comparison of single species can be useful to test generalizations about latitudinal patterns and allow disentangling factors controlling herbivory patterns across communities.     

The latitudinal herbivory hypothesis revisited: To be part is to be whole

As the big data accumulation in ecology picks up pace, we now have the opportunity to test several macroecological hypotheses, such as the latitudinal herbivory hypothesis (LHH) dated from the 1990s. The LHH proposes that plant–herbivore interactions decrease as latitude increases, that is, from lower latitudinal areas (i.e., the equator) to higher latitudinal areas (i.e., the poles). This hypothesis has been challenged in recent years. In this study, we used the greatest volume dataset of leaf herbivory from the study of Zhang et al. (Journal of Ecology, 104, 2016, 1089) to test the LHH at a global scale, based on a quantile regression model. We found that the mean annual temperature, mean annual precipitation, and potential net primary production were heterogeneously correlated with herbivory at different quantiles or variable intervals. Although the Northern Hemisphere (NH) and the global‐scale trends are in accordance with the expected latitudinal variation, the Southern Hemisphere (SH) was found to exhibit inverse trends. The latitude has a negative effect on plant–herbivore interactions in the NH and on a global scale; leaf herbivory decreased more at a given latitude in higher latitudinal areas, which is attributed to harsher survival conditions in these areas. The uniformity of leaf herbivory variability along the climate and latitude gradient in the NH and on a global scale motivates that the loosening of this herbivory variability in the SH is not significant enough to dismiss the prevalence of the LHH, a testable macroecology hypothesis.     

Palatability, decomposition and insect herbivory: patterns in a successional old-field plant community

We tested the hypothesis that selective feeding by insect herbivores in an old‐field plant community induces a shift of community structure towards less palatable plant species with lower leaf and litter tissue quality and may therefore affect nutrient cycling. Leaf palatability of 20 herbaceous plant species which are common during the early successional stages of an old‐field plant community was assayed using the generalist herbivores Deroceras reticulatum (Mollusca: Agriolomacidae) and Acheta domesticus (Ensifera: Gryllidae). Palatability was positively correlated with nitrogen content, specific leaf area and water content of leaves and negatively correlated with leaf carbon content and leaf C/N‐ratio. Specific decomposition rates were assessed in a litter bag experiment. Decomposition was positively correlated with nitrogen content of litter, specific leaf area and water content of living leaves and negatively correlated with leaf C/N‐ratio. When using phylogentically independent contrasts the correlations between palatability and decomposition versus leaf and litter traits remained significant (except for specific leaf area) and may therefore reflect functional relationships. As palatability and decomposition show similar correlations to leaf and litter traits, the correlation between leaf palatability and litter decomposition rate was also significant, and this held even in a phylogenetically controlled analysis. This correlation highlights the possible effects of invertebrate herbivory on resource dynamics. In a two‐year experiment we reduced the density of above‐ground and below‐ground insect herbivores in an early successional old‐field community in a two‐factorial design by insecticide application. The palatability ranking of plants showed no relationship with the specific change of cover abundance of plants due to the reduction of above‐ or below‐ground herbivory. Thus, changes in the dominance structure as well as potentially associated changes in the resource dynamics are not the result of differences in palatability between plant species. This highlights fundamental differences between the effects of insect herbivory on ecosystems and published results from vertebrate‐grazing systems.     

Abundance and diversity of birch‐feeding leafminers along latitudinal gradients in northern Europe

Latitudinal patterns in biotic interactions, including herbivory, have been widely debated during the past years. In particular, recent meta‐analysis questioned the hypothesis that herbivory increases from the poles towards the equator. Our study was designed to verify this hypothesis by exploring latitudinal patterns in abundance and diversity of birch‐feeding insect herbivores belonging to the leafminer guild in northern Europe, from 59° to 69°N. We collected branches from five mature trees of two birch species (Betula pendula and B. pubescens) at each study site (ten sites for each of five latitudinal gradients) twice per season (in early and late summer of 2008–2011) and attributed all mines found on leaves of these branches to a certain taxon of insects. Latitudinal patterns were quantified by calculating Spearman rank correlation coefficients between both abundance and diversity of leafmining taxa and latitudes of sampling sites. In general, both abundance and diversity of leafminers significantly decreased with latitude. However, we discovered pronounced variation in patterns of latitudinal changes among study years and leafminer taxa. Variation among study years was best explained by mean temperatures in July at the northern ends of our gradients. During cold years, abundance of leafminers significantly decreased with latitude, while during warm years the abundance was either independent of latitude or even increased towards the pole. In the northern boreal forests (66° to 69°N), herbivores demonstrated larger changes in densities in response to temperature variations than in the boreo‐nemoral forests (59° to 62°N). Our data suggest that climate warming will result in a stronger increase in herbivory at higher latitudes than at lower latitudes.     

High abscission rates of damaged expanding leaves: field evidence from seedlings of a Bornean rain forest tree

Herbivore damage is known to cause the premature loss of mature leaves. However, the effects of herbivory on abscission during the early stages of leaf development remain unexplored, even though herbivores frequently prefer unlignified, immature leaves. In a field experiment, we removed 50% of the tissue from leaves at various stages of development on seedlings of Shorea hopeifolia (Dipterocarpaceae), a dominant rain forest tree in Indonesian Borneo. Four weeks following simulated herbivory, >88% of unlignified expanding leaves had been abscised, compared to only 20% of fully expanded, unlignified leaves and 0% of recently lignified, mature leaves. In a separate experiment over 9 wk, simulated herbivory did not increase abscission rates of mature leaves, even when 75% of leaf tissue was removed. Because most (58%) of S. hopeifolia seedlings under natural conditions had lost 1% or less of the tissue from their mature leaves, herbivore damage probably has little effect on the abscission of mature leaves. In contrast, the tendency for damaged expanding leaves to abscise may explain why 49% of S. hopeifolia seedlings had already lost their youngest leaf. If similar patterns occur in other species, herbivore attacks on developing leaves may contribute substantially to both leaf loss and the cumulative impact of herbivory on the growth and survival of whole plants.     

Weak latitudinal gradients in insect herbivory for dominant rangeland grasses of North America

Patterns of insect herbivory may follow predictable geographical gradients, with greater herbivory at low latitudes. However, biogeographic studies of insect herbivory often do not account for multiple abiotic factors (e.g., precipitation and soil nutrients) that could underlie gradients. We tested for latitudinal clines in insect herbivory as well as climatic, edaphic, and trait‐based drivers of herbivory. We quantified herbivory on five dominant grass species over 23 sites across the Great Plains, USA. We examined the importance of climate, edaphic factors, and traits as correlates of herbivory. Herbivory increased at low latitudes when all grass species were analyzed together and for two grass species individually, while two other grasses trended in this direction. Higher precipitation was related to more herbivory for two species but less herbivory for a different species, while higher specific root length was related to more herbivory for one species and less herbivory for a different species. Taken together, results highlight that climate and trait‐based correlates of herbivory can be highly contextual and species‐specific. Patterns of insect herbivory on dominant grasses support the hypothesis that herbivory increases toward lower latitudes, though weakly, and indicates that climate change may have species‐specific effects on plant–herbivore interactions.     

Leaf Colour as a Signal of Chemical Defence to Insect Herbivores in Wild Cabbage (Brassica oleracea)

Leaf colour has been proposed to signal levels of host defence to insect herbivores, but we lack data on herbivory, leaf colour and levels of defence for wild host populations necessary to test this hypothesis. Such a test requires measurements of leaf spectra as they would be sensed by herbivore visual systems, as well as simultaneous measurements of chemical defences and herbivore responses to leaf colour in natural host-herbivore populations. In a large-scale field survey of wild cabbage (Brassica oleracea) populations, we show that variation in leaf colour and brightness, measured according to herbivore spectral sensitivities, predicts both levels of chemical defences (glucosinolates) and abundance of specialist lepidopteran (Pieris rapae) and hemipteran (Brevicoryne brassicae) herbivores. In subsequent experiments, P. rapae larvae achieved faster growth and greater pupal mass when feeding on plants with bluer leaves, which contained lower levels of aliphatic glucosinolates. Glucosinolate-mediated effects on larval performance may thus contribute to the association between P. rapae herbivory and leaf colour observed in the field. However, preference tests found no evidence that adult butterflies selected host plants based on leaf coloration. In the field, B. brassicae abundance varied with leaf brightness but greenhouse experiments were unable to identify any effects of brightness on aphid preference or performance. Our findings suggest that although leaf colour reflects both levels of host defences and herbivore abundance in the field, the ability of herbivores to respond to colour signals may be limited, even in species where performance is correlated with leaf colour.