Mobile Herbivores Research Articles

Movement drives population dynamics of one of the most mobile ungulates on Earth: Insights from a mechanistic model.

Long-distance movements are hypothesized to positively influence population size and stability of mobile species. We tested this hypothesis with a novel modeling approach in which moving herbivores interact with the environment created by a dynamic global vegetation model using highly mobile Mongolian gazelles in the eastern Mongolian grasslands as a case study. Gazelle population dynamics were modeled from 1901 to 2018 under two scenarios, one allowing free movement and one restricting movement. Gazelles were 2.2 times more abundant when they could move freely and were extirpated in 71% of the study area when mobility was restricted. Mobility resulted in greater population increases during times of abundant forage and smaller population decreases during drought. Reduced thermoregulatory costs associated with climate change, combined with an increase in vegetation biomass, increased gazelle abundance. Since high abundances often resulted in overgrazing and, thus, extirpation when movement was restricted, mobility had an important role in maintaining higher densities. The novel modeling approach shows how accounting for not just herbivore but also plant ecophysiology can improve our understanding of the population dynamics of highly mobile herbivores, in particular when examining the effects of habitat and climate change. Since the model simulates herbivores based on general physiological mechanisms that apply across large herbivores and the vegetation model can be applied globally, it is possible to adapt the model to other large-herbivore systems.

Exploring the metabolic basis of growth/defense trade-offs in complex environments with Nicotiana attenuata plants cosilenced in NaMYC2a/b expression.

In response to challenges from herbivores and competitors, plants use fitness-limiting resources to produce (auto)toxic defenses. Jasmonate signaling, mediated by MYC2 transcription factors (TF), is thought to reconfigure metabolism to minimize these formal costs of defense and optimize fitness in complex environments. To study the context-dependence of this metabolic reconfiguration, we cosilenced NaMYC2a/b by RNAi in Nicotiana attenuata and phenotyped plants in the field and increasingly realistic glasshouse setups with competitors and mobile herbivores. NaMYC2a/b had normal phytohormonal responses, and higher growth and fitness in herbivore-reduced environments, but were devastated in high herbivore-load environments in the field due to diminished accumulations of specialized metabolites. In setups with competitors and mobile herbivores, irMYC2a/b plants had lower fitness than empty vector (EV) in single-genotype setups but increased fitness in mixed-genotype setups. Correlational analyses of metabolic, resistance, and growth traits revealed the expected defense/growth associations for most sectors of primary and specialized metabolism. Notable exceptions were some HGL-DTGs and phenolamides that differed between single-genotype and mixed-genotype setups, consistent with expectations of a blurred functional trichotomy of metabolites. MYC2 TFs mediate the reconfiguration of primary and specialized metabolic sectors to allow plants to optimize their fitness in complex environments.

Spatial structure of natural boxwood and the invasive box tree moth can promote coexistence

In the absence of top-down and bottom-up controls, herbivores eventually drive themselves to extinction by exhausting their host plants. Poorly mobile herbivores may experiment only local disappearance, because they can recolonize intact plant patches elsewhere, leaving time to previously over-exploited patches to regrow. However most herbivores such as winged insects are highly mobile, which may prevent the formation of spatial heterogeneity. We test if long-distance dispersal can preclude coexistence using the invasion of box tree moth (Cydalima perspectalis) in Europe as a model system. We build a lattice model and estimate the parameters with a combination of field measurements, experimental data and literature sources. Space corresponds either to a realistic boxwood landscape in the Alps, or to theoretical landscapes of various sizes. We find that both species persist under a large range of realistic parameter values, despite a severe reduction in boxwood biomass, with an alternation of outbreaks and near-to-extinction moth densities. Large landscapes are necessary for coexistence, allowing the formation of spatial structure. Slow plant regrowth combined with long-distance dispersal could drive moths to extinction, because of resources depletion at the global scale even without a complete synchronization of the local dynamics. The spatial dynamics leads to formation of small plant patches evenly distributed in the landscape, because of a combination of local plant dispersal and global indirect competition between plants through their positive effect on moth population size. Coexistence is favoured by such heterogeneous landscapes, because empty patches increase moth mortality during dispersal: the system thus creates its own stability conditions.

Physiological status is a stronger predictor of nutrient selection than ambient plant nutrient content for a wild herbivore.

There is generally a close relationship between a consumer's food and its optimal nutrients. When there is a mismatch, it is hypothesized that mobile herbivores switch between food items to balance nutrients, however, there are limited data for field populations. In this study, we measured ambient plant nutrient content at two time points and contrasted our results with the nutrient ratio selected by wild female and male grasshoppers (Oedaleus senegalensis). Few plants were near O. senegalensis' optimal protein:carbohydrate ratio (P:C), nor were plants complementary. Grasshoppers collected earlier all regulated for a carbohydrate-biased ratio but females ate slightly more protein. We hypothesized that the long migration undertaken by this species may explain its carbohydrate needs. In contrast to most laboratory studies, grasshoppers collected later did not tightly regulate their P:C. These results suggest that field populations are not shifting their P:C to match seasonal plant nutrient shifts and that mobile herbivores rely on post-ingestive mechanisms in the face of environmental variation. Because this is among the first studies to examine the relationship between ambient nutrient landscape and physiological state our data are a key step in bridging knowledge acquired from lab studies to hypotheses regarding the role ecological factors play in foraging strategies.

Regional variation in green-up timing along a caribou migratory corridor: Spatial associations with snowmelt and temperature

ABSTRACT Spring green-up in arctic and alpine systems is predominantly controlled by temperature and snowmelt timing preceding and during the growing season. Variation in the timing of green-up across space is an important aspect of resource variability with which mobile herbivores must contend. Here, we measure the explanatory power of abiotic drivers of green-up in a Low Arctic region of west Greenland, host to a migratory caribou population. We identify inconsistent relationships between green-up and abiotic drivers across space. Whereas green-up timing is most closely related to snowmelt in some areas, in others it is most closely related to spring temperature. The negative correlation between the explanatory power of snowmelt and temperature suggests that at broad scales, where green-up is more constrained by snow cover, such as moist, mountainous coastal areas, it is less constrained by temperature. Where snow is less persistent through winter, such as cold, dry inland areas, temperature becomes the predominant factor driving green-up. If the principal driver of spring plant growth is inconsistent across a region, long-term trends in resource phenology could vary spatially. For seasonal migrants like caribou, synchronizing migration timing with resource phenology may be complicated by discordant interannual change across drivers of green-up timing.

Investigating the Movement Components of Host Preference in a Highly Mobile Insect Herbivore, Nephotettix cincticeps (Hemiptera: Cicadellidae).

Effective insect management strategies require a firm understanding of the factors determining host preference, particularly in highly mobile insect herbivores. Host preference studies commonly employ average or first position as a proxy for preference. Yet few studies have explored host preference in relation to transitory attraction and leaving rates, yet these are both components of host plant selection. We investigated the transitory dynamics of preference by the green rice leafhopper, Nephotettix cincticeps (Uhler) (Hemiptera: Cicadellidae) by conducting experiments on groups of females, males, or mixed-sex leafhoppers, and recording their position over time between low-N and normal-N rice plants. Utilizing a log-linear model and variants of a biostatistical model we used these positional data to extract attraction, leaving and tenure rates to better understand the process of host-plant selection. We found a general preference for normal-N over low-N plants at equilibrium. However, between sexes there was variation in the relative significance of attraction or leaving rates on that preference. Female leafhoppers were more attracted to host plants with higher nitrogen content. Male leafhoppers were less discriminate in their initial attraction to hosts but left low-N hosts at a faster rate. Whereas estimated tenure times on both normal- and low-N plants exceeded transmission times for the leafhopper-transmitted rice dwarf virus, longer tenure on normal-N plants likely increases the likelihood of virus acquisition from these plants. Our findings support previous recommendations that growers can mitigate the risks of leafhopper damage and pathogen transmission by optimizing their application of nitrogenous fertilizers.

Mistletoe generates non‐trophic and trait‐mediated indirect interactions through a shared host of herbivore consumers

AbstractIndirect interactions emerge among a wide range of herbivores sharing the same plant resource. Consumers usually belong to different trophic guilds, from folivores and sapsuckers to parasitic plants. We propose that mistletoes parasitizing pines could play a key role acting as herbivores on host pines and coming indirectly into competition with other herbivores feeding on the same host. Changes caused by mistletoes on its host have been well studied, but its effects running across trophic webs remain unrevealed. In this study, we investigate the effect of European mistletoe (Viscum album subsp. austriacum) on the host‐feeding herbivores via trait‐mediated indirect interactions (TMIIs) across their shared pine host (Pinus nigra subsp. salzmannii). We performed field and laboratory experiments, and analyzed the net effect of different mistletoe parasite loads on three host‐phytophagous species: the sapsucker Cinara pini (Aphididae), the winter folivore Thaumetopoea pityocampa (Thaumetopoeidae), and the summer folivore Brachyderes sp. (Curculionidae), all being members of different functional feeding groups (FFGs). We summarize the mistletoe–host–herbivore interactions by means of a TMII, where mistletoe parasitism causes non‐trophic links and detrimental indirect interactions on pine‐feeding herbivores across its shared host, suggesting a worsening of host quality as food. These indirect interactions vary according to three parameters. First, the intensity has a non‐proportional relation with parasite load, showing an impact threshold on highly parasitized pines. Second, the movement capacity of insect herbivores determines their response, by decreasing the abundance of herbivores with low movement ability (aphids and pine processionary caterpillars) while altering the behavior (plant selection) of more mobile herbivores (pine weevils). Finally, FFG determines the intensity of mistletoe parasitism effects, folivores being more responsive than sapsuckers. Overall, mistletoe generates non‐trophic interaction linkages in the forest able to modify community structure by becoming a nexus of the entire herbivore community of the pine canopy.

Roe deer prefer mixed-sex willow stands over monosexual stands but do not discriminate between male and female plants

Male and female plants of dioecious species often experience differential herbivory, possibly due to differences in defences such as secondary metabolite composition or nutritional quality. These plant sex effects on herbivory have been extensively studied for plant individuals, but not for stands/populations. For mobile herbivores, such as deer, stands may be a more relevant scale to study than individual plants. We predicted that male Salix viminalis plants should be subject to more extensive roe deer (Capreolus capreolus) browsing than female plants due to weaker defence in male plants. Furthermore, we expected that mixed-sex stands should experience more damage than monosexual stands due to positive effects of diet mixing on browsing by generalists. We tested for differences in roe deer browsing in plots that were either monosexual male or female, or a mix of male and female plants in a replicated field experiment. Roe deer browsing was estimated after one growth season with heavy herbivory. We also measured plant secondary metabolite concentrations and nitrogen content in leaves from all experimental clones to test the assumption that the sexes differed in defence or nutrients. Mixed-sex plots were more extensively browsed than monosexual plots. However, there was no difference in browsing between male and female plant individuals within mixed-sex plots or between monosexual plots. Plant secondary metabolite profiles differed between male and female plants, while nitrogen content did not. Our findings suggest that the diversified plant secondary metabolite contents of mixed-sex plots may have led to more extensive herbivory. Higher browsing of plant sex mixes may impact both natural and commercial S. viminalis stands with different sex ratios.

A test of trophic cascade theory: fish and benthic assemblages across a predator density gradient on coral reefs

Removal of predators is often hypothesized to alter community structure through trophic cascades. However, despite recent advances in our understanding of trophic cascades, evidence is often circumstantial on coral reefs because fishing pressure frequently co-varies with other anthropogenic effects, such as fishing for herbivorous fishes and changes in water quality due to pollution. Australia's outer Great Barrier Reef (GBR) has experienced fishing-induced declines of apex predators and mesopredators, but pollution and targeting of herbivorous fishes are minimal. Here, we quantify fish and benthic assemblages across a fishing-induced predator density gradient on the outer GBR, including apex predators and mesopredators to herbivores and benthic assemblages, to test for evidence of trophic cascades and alternative hypotheses to trophic cascade theory. Using structural equation models, we found no cascading effects from apex predators to lower trophic levels: a loss of apex predators did not lead to higher levels of mesopredators, and this did not suppress mobile herbivores and drive algal proliferation. Likewise, we found no effects of mesopredators on lower trophic levels: a decline of mesopredators was not associated with higher abundances of algae-farming damselfishes and algae-dominated reefs. These findings indicate that top-down forces on coral reefs are weak, at least on the outer GBR. We conclude that predator-mediated trophic cascades are probably the exception rather than the rule in complex ecosystems such as the outer GBR.

Potential impacts of climate change on patterns of insect herbivory on understorey plant species: A transplant experiment

Species-specific responses to climate change will lead to changes in species interactions across multiple trophic levels. Interactions between plants and their insect herbivores, in particular, may become increasingly disrupted if mobile herbivores respond more rapidly to climatic change than their associated host plants. We present a multispecies transplant experiment aimed at assessing potential climatic impacts on patterns of leaf herbivory. Four shrubby understorey plant species were transplanted outside their native range into a climate 2.5°C warmer in annual mean temperature. After 12 months, we assessed the types and amount of herbivore leaf damage, compared with plants transplanted to a control site within their native range. The overall amount of foliage loss to herbivores ranged from approximately 3–10% across species and sites, a range consistent with most estimates of leaf loss in other studies. The most common types of leaf damage were sucking and chewing and this pattern was consistent for all four plant species at all sites. There were no significant differences in levels and patterns of herbivory between control and warm sites for three out of four plant species. This suggests that with moderate climate warming, most herbivory will continue to be dominated by chewers and suckers, and that the overall level of foliage loss will be similar to that experienced presently.

Climate‐driven change in plant–insect interactions along elevation gradients

Summary Global warming is predicted to dramatically alter communities' composition through differential colonization abilities, such as between sessile plants and their mobile herbivores. Novel interactions between previously non‐overlapping species may, however, also be mediated by altered plants' responses to herbivore attack. Syndromes of plant defences and tolerance are driven by inherited functional traits, biotic and abiotic conditions, and the geographical and historical contingencies affecting the community. Therefore, understanding climate change‐driven herbivore responses and evolution towards a particular plant defence syndrome is key to forecasting species interactions in the near future. In this paper, we first document variations in herbivory, and plant defences along altitudinal gradients that act as ‘natural experiments’. We then use an empirical model to predict how specialist herbivore abundance may shift with respect to elevation in the near future. Our field surveys and field experiment showed a decrease in herbivory with elevation. However, contrary to expectations, our meta‐regression analyses showed that plant defences, particularly leaf toughness and flavonoid compounds, tend to be higher at high elevations, while secondary metabolites showed no clear trend with elevation. Based on those results, we discuss how plant communities and species‐specific plant defence syndromes will change in response to the climate‐driven herbivore colonization of higher altitudes. Particularly, plant from high elevation, due to high protection against abiotic stress may be already ecologically fitted to resist the sudden increase in herbivory pressure that they will likely experience during global change.

Proximate weather patterns and spring green‐up phenology effect Eurasian beaver (Castor fiber) body mass and reproductive success: the implications of climate change and topography

Low spring temperatures have been found to benefit mobile herbivores by reducing the rate of spring-flush, whereas high rainfall increases forage availability. Cold winters prove detrimental, by increasing herbivore thermoregulatory burdens. Here we examine the effects of temperature and rainfall variability on a temperate sedentary herbivore, the Eurasian beaver, Castor fiber, in terms of inter-annual variation in mean body weight and per territory offspring production. Data pertain to 198 individuals, over 11 years, using capture-mark-recapture. We use plant growth (tree cores) and fAPAR (a satellite-derived plant productivity index) to examine potential mechanisms through which weather conditions affect the availability and the seasonal phenology of beaver forage. Juvenile body weights were lighter after colder winters, whereas warmer spring temperatures were associated with lighter adult body weights, mediated by enhanced green-up phenology rates. Counter-intuitively, we observed a negative association between rainfall and body weight in juveniles and adults, and also with reproductive success. Alder, Alnus incana, (n = 68) growth rings (principal beaver food in the study area) exhibited a positive relationship with rainfall for trees growing at elevations >2 m above water level, but a negative relationship for trees growing <0.5 m. We deduce that temperature influences beavers at the landscape scale via effects on spring green-up phenology and winter thermoregulation. Rainfall influences beavers at finer spatial scales through topographical interactions with plant growth, where trees near water level, prone to water logging, producing poorer forage in wetter years. Unlike most other herbivores, beavers are an obligate aquatic species that utilize a restricted 'central-place' foraging range, limiting their ability to take advantage of better forage growth further from water during wetter years. With respect to anthropogenic climate change, interactions between weather variables, plant phenology and topography on forage growth are instructive, and consequently warrant examination when developing conservation management strategies for populations of medium to large herbivores.

Detecting herbivory in two mosses from an Atlantic Forest, Brazil

We recorded evidence for herbivory in the mosses Hypopterygium tamarisci and Lopidium concinnum in a tropical rainforest (São Paulo State, Brazil). We described this herbivory quantitatively, measured the protein and phenol abundance, and identified potential herbivores in the laboratory. We sampled plants from 10 colonies of each species in the years 2007 and 2008. To quantify herbivory, we used an index of damage (ID), with six categories of injury from 0 (0%) to 5 (50–100%). Air-dried moss samples were weighed (100 mg) and used for protein and phenol detection by the Bradford and Folin-Ciocalteau methods, respectively. The plants were cultivated under controlled laboratory conditions and the identification of herbivores was performed from photographs. Injury was higher in H. tamarisci than in L. concinnum (68% and 38% in 2007 and 35% and 23% in 2008, respectively). H. tamarisci had higher mean ID (1·70 in 2007 and 1·09 in 2008); however, the ID values were low compared with ID values detected in angiosperms from tropical forests. Proteins were more abundant than phenol compounds for both species. H. tamarisci had the highest levels of protein and phenols. The phenol/protein ratios (ph/pt ratio) from H. tamarisci (0·42) and L. concinnum (0·40) were similar to those of other terrestrial plants. No correlation between ID and ph/pt ratio, proteins or phenols was detected. Lepidoptera larvae (Geometridae) and micro-snails (Charopidae) were found feeding mainly on the leaves of these mosses. Thus, H. tamarisci and L. concinnum are food sources for herbivores in the field, mainly as alternative food sources for some small and less mobile herbivores.

Understanding the effects of rainfall on elephant–vegetation interactions around waterholes

The distribution of surface water affects herbivore–vegetation interactions in arid and semi-arid regions. Limited access to surface water typically results in the emergence of vegetation gradients around natural and artificial water sources. In particular, African elephants can create large-scale gradients of woody vegetation. Understanding the dynamics of these gradients is of particular importance for the conservation of other, less mobile herbivores that depend on woody vegetation in areas close to water. While rainfall is known to be a key determinant of herbivore–vegetation interactions in dry areas, we only have limited understanding on how it impacts woody vegetation gradients around waterholes. To address this problem, we developed a deterministic simulation model that describes the interplay of rainfall, elephants and woody vegetation in the vicinity of waterholes. The model is based on elephant telemetry data and the ecological conditions in Etosha National Park (ENP), Namibia. We found that decreasing amounts of rainfall led to an increased degradation of woody vegetation, which was particularly severe in areas close to water. Based on this result we conclude that low rainfall was an important driver of recently observed patterns of vegetation degradation in ENP. More generally, rainfall appears to be a key factor that determines elephant–vegetation interactions and thus dynamics of woody vegetation gradients around waterholes. Using long-term rainfall data from ENP, we also demonstrate that an increase in the number of water sources during periods of low rainfall can mitigate the destructive impact of elephants in areas close to water. However, more research is required to assess the sustainability and effectiveness of rainfall-adapted strategies of artificial water provisioning in more detail. In particular it is important to investigate potential effects on elephant population dynamics.

Plant volatiles

Plant volatiles are the metabolites that plants release into the air. The quantities released are not trivial. Almost one-fifth of the atmospheric CO2 fixed by land plants is released back into the air each day as volatiles. Plants are champion synthetic chemists; they take advantage of their anabolic prowess to produce volatiles, which they use to protect themselves against biotic and abiotic stresses and to provide information - and potentially disinformation - to mutualists and competitors alike. As transferors of information, volatiles have provided plants with solutions to the challenges associated with being rooted in the ground and immobile.

Spatial heterogeneity of primary production as both cause and consequence of foraging patterns of an expanding Greater Snow Goose colony

In response to spatial heterogeneity of resources, many herbivores move between discrete areas to enhance access to the best foraging areas. Local forage removal by mobile herd-foraging herbivores in turn is likely to produce spatial variability in both plant nutritive quality and quantity. On the tundra of Bylot island, Nunavut, owing to the recent demographic explosion of their population, most greater snow geese move out of their nesting area soon after hatch to rear their young at distant feeding sites. A previous study showed that goslings using these distant sites are generally heavier and larger than those that stay in the colony throughout the brood-rearing period. In this study, we examine the hypothesis that goslings' growth was reduced in the nesting area compared to distant brood-rearing areas because grazing pressure reduced standing crop. In light of the recent expansion in the distribution of geese during brood-rearing, we also investigated if the negative effect of chronic grazing on net above-ground primary production (NAPP) differed between the colony and distant brood-rearing sites. We monitored NAPP, grazing pressure, and intensity of use in the goose colony and in 2 distant brood-rearing areas over a 10-y period by sampling plant biomass inside and outside moveable goose exclosures erected annually and by counting goose feces along transects at the end of the summer. NAPP of graminoids in the nesting area was 40% lower than at the other brood-rearing areas, but the percentage of primary production consumed by geese (28%) did not differ among the 3 sites despite large annual variations. Cumulative feces density revealed that intensity of use of the 2 brood-rearing areas by geese was nearly 2 times higher than at the colony, but the timing of use differed as grazing on the brood-rearing areas occurred only after hatch, unlike the nesting area. We conclude that geese not only respond to spatial heterogeneity in resource availability but also create and sustain it through their foraging behaviour.

Fruit abscission by Physalis species as defense against frugivory

AbstractFruit abscission as a response to herbivory is well‐documented in many plant species, but its effect on further damage by mobile herbivores that survive fruit abscission is relatively unstudied. Physalis plants (Solanaceae) abscise fruit containing feeding larvae of their main frugivore, Heliothis subflexa Guenée (Lepidoptera: Noctuidae), a specialist moth. We tested the ability of H. subflexa larvae placed under the plant canopy to find and climb onto two architecturally different Physalis species. Physalis pubescens L., a low, shrub‐like, spreading plant, abscises its fruit at a higher rate than Physalis angulata L., a tall arborescent plant. As a result, small larvae are more often dropped from P. pubescens. Third and fifth instars located P. pubescens faster and with a higher probability than P. angulata. Although fifth instars outperformed third‐instar caterpillars at finding P. angulata, P. pubescens was located equally fast by the two instars. Heliothis subflexa located Physalis plants more successfully and more quickly than a close relative, the generalist Heliothis virescens Fabricius. The higher fruit abscission rates in P. pubescens may be an evolved response to its greater susceptibility to searching caterpillars.

Stream mosses as chemically‐defended refugia for freshwater macroinvertebrates

Marine and terrestrial studies show that small, sedentary herbivores that utilize plants as both food and habitat can gain enemy‐free space by living on hosts that are chemically defended from larger, generalist consumers. Although large herbivores are increasingly recognized as important consumers of macrophytes in freshwater communities, the potential indirect effects of herbivory on plant‐associated macroinvertebrates have rarely been studied. Here, we show that the large, generalist consumers in a riverine system, Canada geese, Branta canadensis, and crayfish, Procambarus spiculifer, both selectively consumed riverweed, Podostemum ceratophyllum, over an aquatic moss, Fontinalis novae‐angliae, even though moss comprised 89% of the total plant biomass on riverine rocky shoals. Moss supported twice as many plant‐associated macroinvertebrates as riverweed, suggesting that it might provide a spatial refuge from consumption by these larger consumers. Bioassay‐guided fractionation of moss extracts led to the isolation of a C18 acetylenic acid, octadeca‐9,12‐dien‐6‐ynoic acid, that deterred crayfish feeding. In contrast to results with Canada geese and crayfish, both the amphipod Crangonyx gracilis and the isopod Asellus aquaticus consumed significant amounts of moss but rejected riverweed in laboratory feeding assays. Moreover, neither amphipod nor isopod feeding was deterred by the crude organic extract of Fontinalis, suggesting that these mesograzers tolerate or circumvent the chemical defenses that deterred larger consumers. Thus, herbivory by large, generalist herbivores may drive freshwater plant community structure towards chemically defended plants and favor the ecological specialization of smaller, less mobile herbivores on unpalatable hosts that represent enemy‐free space.

INDUCIBLE CHEMICAL RESISTANCE TO HERBIVORY IN THE BROWN SEAWEEDASCOPHYLLUM NODOSUM

Physical damage caused by herbivores or artificial clipping is known to induce responses in secondary chemistry as well as increased resistance to further grazing in a large number of terrestrial vascular plants, but this has only rarely been described for marine algae. In the present study, it was found that a few weeks of grazing by the gastropod Littorina obtusata can induce substantially increased concentrations of phlorotannins in the intertidal brown seaweed Ascophyllum nodosum. In contrast, grazing by the isopod Idotea granulosa and simulations of herbivory through momentary and continuous clipping caused no significant changes in phlorotannin levels. This indicates a high degree of specificity in the elicitation of chemical responses to physical damage, something which previously has been shown for terrestrial vascular plants but not for marine algae. Ascophyllum plants that had been grazed by L. obtusata were also less susceptible than undamaged plants to further grazing by gastropods, but no such induced resistance was found in the experiments with I. granulosa. Feeding experiments with undamaged Ascophyllum plants and artificial food containing different levels of phlorotannins provided further support for the sensitivity of the feeding behavior of L. obtusata, and the insensitivity of I. granulosa, to intraspecific variation in the phlorotannin content of Ascophyllum. Since L. obtusata is specialized to live and feed on a few fucoid species, including A. nodosum, the results imply that phlorotannins have an important mediating role in the interactions between these macroalgae and L. obtusata. The experimentally induced increase of phlorotannins was consistent with results from phlorotannin analyses of Ascophyllum individuals from natural populations, where plants that had been heavily grazed by L. obtusata contained significantly higher levels of phlorotannins compared to undamaged plants. A field survey of the distribution and abundance of L. obtusata revealed that the density of the gastropod is highly variable at the same spatial scale as the phlorotannin content of Ascophyllum in the study area. These results suggest that grazing by L. obtusata can be an important factor in explaining natural phenotypic variation in the phlorotannin content of Ascophyllum. Together with the results from a few other studies on the interactions between mesoherbivores and marine algae, the results of this study support the previously proposed hypothesis that it is feeding by relatively small, less mobile herbivores that is most likely to cue for induced production of defense chemicals in seaweeds. More studies on such interactions may reveal that the apparent rarity of inducible chemical defenses in seaweeds is misleading.

Leaf Damage and Associated Cues Induce Aggressive Ant Recruitment in a Neotropical Ant-Plant

Induced chemical responses following herbivory are common in plants. Plant responses that change the level of physical or biotic defense are less well documented and poorly understood. Many Azteca spp. ants are obligate inhabitants of Cecropia spp. trees. In such ant–plant associations the ants are thought to be analogous to chemical defenses; previous experiments have demonstrated that ant occupation of C. obtusifolia reduced herbivory and plant competition and increased growth. Experiments, conducted over two years, on the dynamics of ant defense demonstrate that leaf damage caused a fivefold increase in the number of Azteca spp. ants on damaged leaves of C. obtusifolia compared to that on disturbed but undamaged control leaves. Ant activity peaked 8–12 min after damage, and differences between damaged and control leaves remained evident for 24 h. Such rapid induction of ant recruitment is likely to be particularly effective against unpredictable and mobile herbivores. The magnitude of the induced ant response to damage was strongly correlated with the number of ants patrolling the leaves before damage occurred. Ant responses to disturbance were not influenced by the presence of damage that had been applied 24 h previously. However, ant responses to subsequent damage, 24 h after initial damage, resulted in greater recruitment than to previously undamaged leaves. Ant recruitment to several other cues associated with herbivory was also tested. Presence of pyralid caterpillars that naturally feed on C. obtusifolia induced a low level of ant recruitment, and most larvae were removed from leaves by the ants within 10 min. Exposure to plant sap collected from damaged conspecifics and a commercially available green leaf volatile (hexanal) commonly released by plants after damage, both resulted in a doubling of ant numbers relative to controls. However, the levels of recruitment in response to these stimuli were insufficient to account for the high numbers of ants and persistence of recruitment observed on experimentally damaged leaves. Experimental wounding of leaves with minimal leaf tissue removal (using pin pricks) revealed that leaf wounds per se can only partially explain the induced ant recruitment following leaf damage. The type of herbivory and size of leaf wounds may be important cues for ant recruitment. Severed C. obtusifolia leaves that were freshly damaged failed to elicit an induced ant response when held adjacent to conspecific leaves with ants. However, induction of ant recruitment on damaged plants did significantly induce a low level of ant recruitment on neighboring conspecifics, providing evidence for interplant communication. Induced ant responses in the Cecropia–Azteca system are the result of multiple physical and chemical cues associated with herbivory. Ant responses to herbivory, although not previously studied in detail, are likely to be common among myrmecophytic plants and are likely to be an important component of antiherbivore defense in such systems.