Asymmetric competition via induced resistance: specialist herbivores indirectly suppress generalist preference and populations

Species may compete indirectly by altering the traits of a shared resource. For example, herbivore-induced responses in plants may make plants more resistant or susceptible to additional herbivorous insect species. Herbivore-induced plant responses can significantly affect interspecific competition and herbivore population dynamics. These herbivore-herbivore indirect interactions have been overlooked in aquatic ecosystems where previous studies used the same herbivore species to induce changes and to assess the effects of these changes. We asked whether seaweed grazing by one of two herbivorous, congeneric snail species (Littorina obtusata or Littorina littorea) with different feeding strategies and preferences would affect subsequent feeding preferences of three herbivore species (both snails and the isopod Idotea baltica) and population densities of three herbivore species (both snails and a third periwinkle snail, Lacuna vincta). In addition, we measured phlorotannin concentrations to test the hypothesis that these metabolites function as induced defenses in the Phaeophyceae. Snail herbivory induced cue-specific responses in apical tissues of the seaweed Fucus vesiculosus that affected the three herbivore species similarly. When compared to ungrazed controls, direct grazing by Littorina obtusata reduced seaweed palatability by at least 52% for both snail species and the isopod species. In contrast, direct grazing by L. littorea did not decrease seaweed palatability for any herbivore, indicating herbivore-specific responses. Previous grazing by L. obtusata reduced populations of L. littorea on outplanted seaweeds by 46% but had no effect on L. obtusata populations. Phlorotannins, a potential class of inducible chemicals in brown algae, were not more concentrated in grazed seaweed tissues, suggesting that some other trait was responsible for the induced resistance. Our results indicate that marine herbivores may compete via inducible responses in shared seaweeds. These plant-mediated interactions were asymmetric with a specialist (L. obtusata) competitively superior to a generalist (L. littorea).     

Lack of phlorotannin induction in the kelp Laminaria hyperborea in response to grazing by two gastropod herbivores

Phlorotannins (polyphenolic compounds) are found exclusively in brown algae and are commonly regarded as an important component of algal defense against herbivores. The variation in phlorotannin concentration within and among individual seaweed specimens of the same species can be substantial, and seaweeds may respond to herbivore damage by an increased production of phlorotannins in order to prevent further herbivory. In the present study, we investigated the natural variation of phlorotannins in different parts of the kelp Laminaria hyperborea in an observational field survey. We also performed a manipulative induction experiment in which we studied the effect of natural grazing by the patellid limpet Ansates (=Helcion) pellucida and the littorinid snail Lacuna vincta, as well as artificial damage on the production of phlorotannins in L. hyperborea. There was a large variation in phlorotannin content both among and within L. hyperborea individuals and populations. The control plants in the induction experiment had a higher phlorotannin content compared to plants in natural populations, but grazing by A. pellucida or L. vincta, or artificial damage did not induce a higher phlorotannin concentration in the kelps. Instead, the phlorotannin content in wounded kelps was significantly lower than in the control plants, possibly due to removal of phlorotannin-rich outer layers of the kelp thallus, or to an induced compensatory growth response in the L. hyperborea plants. These results imply that L. hyperborea phlorotannins probably do not function as an inducible chemical defense against A. pellucida and L. vincta, instead grazing by these herbivores decreases the phlorotannin content of the kelps.     

Screening for induced herbivore resistance in Swedish intertidal seaweeds

Terrestrial plants have long been known to induce resistance towards herbivores in response to direct grazing, and strong evidence of inter-plant information transfer through volatile signals has been reported recently. Still, little is known about information exchange in aquatic plant–herbivore interactions. In this study, 12 Swedish seaweed species were exposed either to direct grazing by a generalist crustacean herbivore (Idotea granulosa), or to waterborne signals produced by actively feeding herbivores for 1 week. In order to test for the presence of induced chemical resistance in the different seaweed species, the dried and homogenized seaweed tissues were incorporated into an agar matrix, and herbivores were allowed to choose between the resulting control and induced artificial diets in two different two-choice feeding trials. The herbivores were actively feeding from all seaweed species in the induction experiments, although the amount of seaweed tissue consumed differed significantly between species. A chemically based induced herbivore resistance was found in response to direct grazing in four of the tested seaweed species (two red, one brown, and one green seaweed species). Furthermore, four seaweeds (one red, two brown, and one green seaweed species) induced resistance towards further grazing in response to waterborne chemical signals. Several seaweed species responded differently when exposed to different herbivore-related cues, indicating that both cues and responses can be highly specific. The results show that herbivore-induced resistance is present in 7 of 12 of the tested Swedish seaweed species, but that different signals (i.e., direct grazing and waterborne cues) elicit complex responses in the seaweeds.     

Geographic covariation of chemical quality of the host alga<Emphasis Type="Italic"> Fucus vesiculosus</Emphasis> with fitness of the herbivorous isopod<Emphasis Type="Italic"> Idotea baltica</Emphasis>

Environmental and/or genetic among-site variation in plant quality may influence growth and fecundity of specialized herbivores inhabiting a particular site. Such variation is important as it generates spatial variation in selection for traits related to plant–herbivore interaction. Littoral macroalgae are known to respond plastically to environmental variation by modifying their chemistry or morphology. We studied geographic variation in phlorotannin, nitrogen, protein, and sugar (fucose, mannitol, and melibiose) concentrations of the brown alga Fucus vesiculosus at 12 sites separated by 0.5 to 40 km in the naturally fragmented Archipelago Sea in the northern Baltic Sea. By this regional variation in algal chemistry we attempted to explain among-population variation in size and fecundity of the crustacean herbivore Idotea baltica. We observed high spatial variation in all the measured chemical characteristics of F. vesiculosus, as well as in female size and the number of eggs produced by the herbivores. Spatial variation in nitrogen or protein contents of the alga did not explain the variation of herbivore traits. However, egg size positively covaried with spatial variation in the concentration of mannitol, the major storage carbohydrate of the alga. Such a positive relationship may arise if I. baltica can utilize the nutritive value of a mannitol-rich diet thereby being better able to provision the developing eggs with energy-rich metabolites. Unexpectedly, the concentration of phlorotannins, secondary metabolites having a putative role in defense against herbivory, positively covaried with the size of the herbivore. Among-population variation in host plant chemistry and covariation of that with herbivore growth and reproduction imply that herbivores respond to the local quality of their host plants, and that geographical structuring of populations has to be taken into account in studies of plant–herbivore interactions.Communicated by M. Kühl, Helsingør     

Sources of variation in herbivore preference: among-individual and past diet effects on amphipod host choice

Understanding which factors affect the feeding preferences of herbivores is essential for predicting the effects of herbivores on plant assemblages and the evolution of plant–herbivore interactions. Most studies of marine herbivory have focussed on the plant traits that determine preferences (especially secondary metabolites), while few studies have considered how preferences may vary among individual herbivores due to genetic or environmental sources of variation. Such intraspecific variation is essential for evolutionary change in preference behaviour and may alter the outcome of plant–herbivore interactions. In an abundant marine herbivore, we determined the relative importance of among-individual and environmental effects on preferences for three host algae of varying quality. Repeated preference assays were conducted with the amphipod Peramphithoe parmerong and three of its brown algal hosts: Sargassum linearifolium, S. vestitum and Padina crassa. We found no evidence that preference varied among individuals, thus constraining the ability of natural selection to promote increased specialisation on high-quality S. linearifolium. Most of the variation in preference occurred within individuals, with amphipod preferences strongly influenced by past diet. The increased tendency for amphipods to select alternate hosts to that on which they had been recently feeding indicates that amphipods are actively seeking mixed diets. Such a feeding strategy provides an explanation for the persistence of this herbivore on hosts in the field that support poor growth and survival if consumed alone. The effects of past diet indicate that herbivore preferences are a function of herbivore history in addition to plant traits and are likely to vary with the availability of algae in space and time.     

Why do plants ‘talk’?

Summary Plant defence can be induced by herbivory. This is true for both direct and indirect plant defence. Induced direct defence has been the most studied of the two. However, in most cases induced direct defence does not appear to be a water-tight defence option. In contrast, induced indirect defence through the production of herbivore-induced carnivore attractants can be a decisive factor in the extermination of herbivore populations. In this paper the main characteristics of induced attraction of carnivores by plants are reviewed. This includes the similarities and dissimilarities among tritrophic systems. There are two main patterns of induced carnivore attraction. (1) Through the emission of the same bouquet as that emitted by mechanically damaged plants, but in larger quantities and for a longer period of time after damage. (2) Through emission of large amounts of new volatiles that are synthesizedde novo in response to herbivore feeding andnot in response to mechanical wounding.Herbivore populations may be decimated by carnivores. Therefore it should be realized that herbivoreinduced carnivore attractants are essential in an important step in carnivore foraging,i.e. long-distance herbivore location. Once herbivores have started feeding on a plant and direct defence is not effective, induced indirect defence may be decisive for plant survival. Therefore, it is concluded that indirect defence is an essential aspect of induced plant defence directed at herbivorous arthropods.     

Antagonistic interactions between plant competition and insect herbivory

Interspecific competition between plants and herbivory by specialized insects can have synergistic effects on the growth and performance of the attacked host plant. We tested the hypothesis that competition between plants may also negatively affect the performance of herbivores as well as their top-down effect on the host plant. In such a case, the combined effects of competition and herbivory may be less than expected from a simple multiplicative response. In other words, competition and herbivory may interact antagonistically. In a greenhouse experiment, Poa annua was grown in the presence or absence of a competitor (either Plantago lanceolata or Trifolium repens), as well as with or without a Poa-specialist aphid herbivore. Both competition and herbivory negatively affected Poa growth. Competition also reduced aphid density on Poa. This effect could in part be explained by changes in the biomass and the nitrogen content of Poa shoots. In treatments with competitors, reduced aphid densities alleviated the negative effect of herbivory on above- and belowground Poa biomass. Hence, we were able to demonstrate an antagonistic interaction between plant-plant interspecific competition and herbivory. However, response indices suggested that antagonistic interactions between competition and herbivory were contingent on the identity of the competitor. We found the antagonistic effect only in treatments with T. repens as the competitor. We conclude that both competitor identity and the herbivore's ability to respond with changes in its density or activity to plant competition affect the magnitude and direction (synergistic vs. antagonistic) of the interaction between competition and herbivory on plant growth.     

Eutrophication and Consumer Control of New England Salt Marsh Primary Productivity

Abstract:  Although primary productivity in salt marshes is thought to be controlled by physical forces, recent evidence suggests that human disturbances can drive a switch to consumer control in these ecologically valuable ecosystems. We tested the hypothesis that nitrogen enrichment can trigger consumer control in salt marshes in Narragansett Bay, Rhode Island, with (1) a field experiment in which we manipulated nutrient availability (with nutrient additions) and insect herbivory (with insecticide application), (2) a survey of 20 salt marshes that examined the relationship between marsh nutrient status and herbivore pressure, and (3) insect herbivore removal at high and low nutrient input sites to directly test the hypothesis that nutrient enrichment is increasing insect herbivory in these marshes. Experimental nitrogen eutrophication initially increased plant productivity but eventually led to reduced plant biomass due to insect herbivory, and our surveys revealed that marsh nitrogen supply was a good predictor of herbivore damage to plants. Insects had minimal impacts on primary productivity in pristine marshes, but suppressed primary productivity in eutrophic salt marshes by 50–75%. Thus, eutrophication is currently triggering consumer suppression of primary productivity in New England salt marshes and may ultimately jeopardize the ecological and societal services these systems provide.     

Seed availability and insect herbivory limit recruitment and adult density of native tall thistle

Understanding spatial and temporal variation in factors influencing plant regeneration is critical to predicting plant population growth. We experimentally evaluated seed limitation, insect herbivory, and their interaction in the regeneration and density of tall thistle (Cirsium altissimum) across a topographic ecosystem productivity gradient in tallgrass prairie over two years. On ridges and in valleys, we used a factorial experiment manipulating seed availability and insect herbivory to quantify effects of: seed input on seedling density, insect herbivory on juvenile density, and cumulative impacts of both seed input and herbivory on reproductive adult density. Seed addition increased seedling densities at three of five sites in 2006 and all five sites in 2007. Insect herbivory reduced seedling survival across all sites in both years, as well as rosette survival from the previous year's seedlings. In both years, insecticide treatment of seed addition plots led to greater adult tall thistle densities in the following year, reflecting the increase in juvenile thistle densities in the experimental year. Seedling survival was not density dependent. Our analytical projection model predicts a significant long-term increase in adult densities from seed input, with a greater increase under experimentally reduced insect herbivory. While plant community biomass and water stress varied significantly between ridges and valleys, the effects of seed addition and insect herbivory did not vary with gradient position. These results support conceptual models that predict seedling and adult densities of short-lived monocarpic perennial plants should be seed limited. Further, the experiment demonstrates that even at high juvenile plant densities, at which density dependence potentially could have overridden herbivore effects on plant survival, insect herbivory strongly affected juvenile thistle performance and adult densities of this native prairie species.     

Herbivores enable plant survival under nutrient limited conditions in a model grazing system

We make a theoretical study of nitrogen cycling in a model of a grazing system with five compartments. The rates of uptake of nutrient by plants and herbivores are allowed nonlinear forms which involve no a priori assumptions about whether the system is subject to top-down or bottom-up control. We derive a method of piecewise linear approximation which allows analytical study of the system. We then use this method to investigate the properties of the equilibrium states of the system, and in particular whether the system favours donor- or recipient-control, the grazing optimization problem, and the potential benefits of herbivory to plant growth. We are able to generalise our results to all uptake functions of the same qualitative class as those considered, and to show that in general the system will tend to a stable equilibrium state of donor-controlled herbivory. In this model, the presence of the ‘right’ class of herbivore is not only beneficial to plant growth in certain circumstances, but can be essential to their survival, allowing plants to co-exist with herbivores under conditions in which they would be unable to survive alone.     

Macro- and mesoherbivores prefer native seaweeds over the invasive brown seaweed <Emphasis Type="Italic">Sargassum muticum</Emphasis>: a potential regulating role on invasions

Herbivory has a strong impact on algal distribution, abundance and community structure and may influence the establishment and spread of introduced seaweed species. In this study, we assess the potential regulating role of herbivory on one of the most invasive brown seaweeds: Sargassum muticum. Multiple choice feeding experiments were conducted with 13 native seaweeds, S. muticum and 5 herbivore species from the Northwest, Southwest and South of Portugal. S. muticum was always the least or among the least preferred seaweeds and attained one of the highest growth rates of the tested seaweeds, with and without herbivores. The addition of herbivores increased the number of cases by 40% in which the invader had higher growth rates. Our results suggest that low grazing pressure on S. muticum by the recipient herbivore community may give the invader a competitive advantage over at least part of the native seaweed community, thereby contributing to the invasiveness of S. muticum along the Portuguese coast.     

Effects of nutrients, herbivory, and depth on the macroalgal community in the rocky sublittoral

We studied the interacting roles of nutrient availability and herbivory in determining the macroalgal community in a rocky littoral environment. We conducted a factorial field experiment where we manipulated nutrient levels and herbivory at two sublittoral depths and measured macroalgal colonization and the following young assemblage during the growing season. At the community level, grazing reduced algal colonization, though the effect varied with depth and its interaction with nutrient availability varied in time. In shallow water, the total density of macroalgae increased in response to nutrient enrichment, but the ability of grazers to reduce macroalgal density also increased with the nutrient enrichment, and thus, the community could not escape from the top-down control. In deep water, the algal density was lower, except in July when nutrient enrichment caused a very dense algal growth. Grazing at the greater depth, though effective, was generally of smaller magnitude, and in July it could not limit algal recruitment and growth. Species richness peaked at the intermediate nutrient level in deep but not in shallow water during most of the growing season. Grazing had no effect on diversity of the algal community at either depth and only a minor effect on species richness at the greater depth. Opportunistic and ephemeral algae benefited from the nutrient enrichment but were also grazed to very low densities. Slowly growing and/or perennial species colonized poorly in the nutrient enriched treatments, and depending on the species, either suffered or indirectly benefited from herbivory. For all species, effects of nutrients on colonization depended on depth; usually both nutrient and herbivory effects were more pronounced at the shallow depth. We conclude that grazers are able to reduce macroalgae over a large range of nutrient availabilities, up to 12-fold nutrient enrichment in the current experiment, and that the sublittoral depth gradient generates variation in the algal community control exerted by both herbivory and nutrient availability. Thus temporal and spatial variability in both top-down and bottom-up control and in their interaction, especially along the depth gradient, may be crucially important for producer diversity and for the successional dynamic in a rocky sublittoral environment.     

Mycorrhizal species differentially alter plant growth and response to herbivory

Plants simultaneously interact with multiple organisms which can both positively and negatively affect their growth. Herbivores can reduce plant growth through loss of plant biomass and photosynthetic area, while plant mutualists, such as mycorrhizal fungi, can increase plant growth through uptake of essential nutrients. This is the first study examining whether species-specific associations with mycorrhizal fungi alter plant tolerance to herbivory. We grew Plantago lanceolata plants with three species of mycorrhizal fungi previously shown to have differential impacts on plant growth and subjected them to herbivory by the specialist lepidopteran herbivore, Junonia coenia. Association with mycorrhizal fungus Glomus white provided the greatest growth benefit but did not alter plant response to herbivory. Alternatively, association with Archaeospora trappei provided less growth promotion but did lead to tolerance to herbivory in the form of an increased growth rate. Finally, an association with the fungus Scutellospora calospora led to neither plant growth promotion nor tolerance to herbivory. In fact, an association with S. calospora appeared to reduce plant tolerance to herbivory. An association with all three species of mycorrhizae resulted in a pattern of growth similar to that of plants grown only with Glomus white, suggesting that growth promotion by multiple mycorrhizal species is driven by the inclusion of a "super fungus," in this case, Glomus white. This work illustrates that plant response to herbivory depends upon the mycorrhizal fungal mutualist with which a plant is associated.     

Dealing with double trouble: consequences of single and double herbivory in Brassica juncea

In their natural environment, plants are often attacked simultaneously by many insect species. The specificity of induced plant responses that is reported after single herbivore attacks may be compromised under double herbivory and this may influence later arriving herbivores. The present study focuses on the dynamics of induced plant responses induced by single and double herbivory, and their effects on successive herbivores. Morphological (leaf length, area and trichome density) and chemical changes (leaf alkenyl and indole glucosinolates) in Brassica juncea were evaluated 4, 10, 14 and 20 days after damage by the specialist Plutella xylostella alone, or together with the generalist Spodoptera litura. To assess the biological effect of the plant’s responses, the preference and performance of both herbivores on previously induced plants were measured. We found that alkenyl glucosinolates were induced 20 days after damage by P. xylostella alone, whereas their levels were elevated as early as 4 days after double herbivory. Trichome density was increased in both treatments, but was higher after double herbivory. Interestingly, there was an overall decrease in indole glucosinolates and an increase in leaf size due to damage by P. xylostella, which was not observed during double damage. S. litura preferred and performed better on undamaged plants, whereas P. xylostella preferred damaged plants and performed better on plants damaged 14 and 10 days after single and double herbivory, respectively. Our results suggest that temporal studies involving single versus multiple attacker situations are necessary to comprehend the role of induced plant responses in plant–herbivore interactions.     

Herbivore species, infestation time, and herbivore density affect induced volatiles in tea plants

In response to insect herbivory, plants emit volatiles that are used by the herbivores’ natural enemies to locate their host or prey. Herbivore attack also enhances tea aroma. Herbivore-induced plant volatiles (HIPVs) vary both quantitatively and qualitatively with infestation duration and herbivore density. Thus, whether HIPVs can reliably communicate the identities of herbivores is of interest. Here, we studied the tea plant volatiles induced by the tea leafhopper (Empoasca vitis, a piercing–sucking insect), the tea geometrid (Ectropis oblique, a chewing insect), and methyl jasmonate (MeJA, a plant hormone). Geometrid feeding induced more complex volatile blends than did leafhopper infestation. The volatiles induced by both herbivores significantly increased in quality and quantity with time during the first 16 h of infestation, after which the profiles of induced volatile blends and the emission of induced compounds varied diurnally. (E)-β-Ocimene displayed a unique rhythm in which emission peaked at night. The amount of HIPVs significantly increased, while their profiles changed little, with herbivore density. Overall, the leafhoppers and geometrids induced significantly different volatiles from tea plants, while the HIPV profiles varied with a circadian rhythm and were similar at different herbivore densities. Our findings also suggest a new method of enhancing tea flavor using exogenously applied plant hormones, because the volatiles induced by leafhoppers and MeJA were similar in general composition.     

Acceleration of Exotic Plant Invasion in a Forested Ecosystem by a Generalist Herbivore

Abstract: The successful invasion of exotic plants is often attributed to the absence of coevolved enemies in the introduced range (i.e., the enemy release hypothesis). Nevertheless, several components of this hypothesis, including the role of generalist herbivores, remain relatively unexplored. We used repeated censuses of exclosures and paired controls to investigate the role of a generalist herbivore, white‐tailed deer (Odocoileus virginianus), in the invasion of 3 exotic plant species (Microstegium vimineum, Alliaria petiolata, and Berberis thunbergii) in eastern hemlock (Tsuga canadensis) forests in New Jersey and Pennsylvania (U.S.A.). This work was conducted in 10 eastern hemlock (T. canadensis) forests that spanned gradients in deer density and in the severity of canopy disturbance caused by an introduced insect pest, the hemlock woolly adelgid (Adelges tsugae). We used maximum likelihood estimation and information theoretics to quantify the strength of evidence for alternative models of the influence of deer density and its interaction with the severity of canopy disturbance on exotic plant abundance. Our results were consistent with the enemy release hypothesis in that exotic plants gained a competitive advantage in the presence of generalist herbivores in the introduced range. The abundance of all 3 exotic plants increased significantly more in the control plots than in the paired exclosures. For all species, the inclusion of canopy disturbance parameters resulted in models with substantially greater support than the deer density only models. Our results suggest that white‐tailed deer herbivory can accelerate the invasion of exotic plants and that canopy disturbance can interact with herbivory to magnify the impact. In addition, our results provide compelling evidence of nonlinear relationships between deer density and the impact of herbivory on exotic species abundance. These findings highlight the important role of herbivore density in determining impacts on plant abundance and provide evidence of the operation of multiple mechanisms in exotic plant invasion.     

Not just the usual suspects: insect herbivore populations and communities are associated with multiple plant nutrients

The relationship between plant nutrient content and insect herbivore populations and community structure has long interested ecologists. Insect herbivores require multiple nutrients, but ecologists have focused mostly on nitrogen (an estimate of plant protein content), and more recently phosphorus (P); other nutrients have received little attention. Here we document nutrient variation in grass and forb samples from grassland habitats in central Nebraska using an elemental approach; in total we measured foliar concentrations of 12 elements (N and P, plus S, B, Ca, Mg, Na, K, Zn, Fe, Mn, and Cu). We detected significant variability among sites for N, P, Mg, Na, K, and Cu. We next used a model selection approach to explore how this nutritional variation and plant biomass correlate with grasshopper densities (collectively and at the feeding-guild level), and principal component analysis to explore nutrient correlations with grasshopper community species composition. When all grasshoppers were pooled, densities varied among sites, but only P was associated with abundance of the elements shown to vary between sites. Different responses occurred at the feeding-guild level. For grass specialists, densities were associated with N, plus P, Mg, and Na. For forb specialists, N and P were often associated with density, but associations with Na and K were also observed. Finally, mixed-feeder abundance was strongly associated with biomass, and to a lesser extent P, Mg, Na, and Cu. At the community level, B, Ca, Zn, and Cu, plus biomass, explained > 30% of species composition variation. Our results confirm the positive association of N and P with insect herbivore populations, while suggesting a potential role for Mg, Na, and K. They also demonstrate the importance of exploring effects at the feeding-guild level. We hope our data motivate ecologists to think beyond N and P when considering plant nutrient effects on insect herbivores, and make a call for studies to examine functional responses of insect herbivores to dietary manipulation of Mg, Na, and K. Finally, our results demonstrate correlations between variation in nutrients and species assemblages, but factors not linked to plant nutrient quality or biomass likely explain most of the observed variation.     

Herbivore vs. nutrient control of marine primary producers: context-dependent effects

Pervasive overharvesting of consumers and anthropogenic nutrient loading are changing the strengths of top-down and bottom-up forces in ecosystems worldwide. Thus, identifying the relative and synergistic roles of these forces and how they differ across habitats, ecosystems, or primary-producer types is increasingly important for understanding how communities are structured. We used factorial meta-analysis of 54 field experiments that orthogonally manipulated herbivore pressure and nutrient loading to quantify consumer and nutrient effects on primary producers in benthic marine habitats. Across all experiments and producer types, herbivory and nutrient enrichment both significantly affected primary-producer abundance. They also interacted to create greater nutrient enrichment effects in the absence of herbivores, suggesting that loss of herbivores produces more dramatic effects of nutrient loading. Herbivores consistently had stronger effects than did nutrient enrichment for both tropical macroalgae and seagrasses. The strong effects of herbivory but limited effects of nutrient enrichment on tropical macroalgae suggest that suppression of herbivore populations has played a larger role than eutrophication in driving the phase shift from coral- to macroalgal-dominated reefs in many areas, especially the Caribbean. For temperate macroalgae and benthic microalgae, the effects of top-down and bottom-up forces varied as a function of the inherent productivity of the ecosystem. For these algal groups, nutrient enrichment appeared to have stronger effects in high- vs. low-productivity systems, while herbivores exerted a stronger top-down effect in low-productivity systems. Effects of herbivores vs. nutrients also varied among algal functional groups (crustose algae, upright macroalgae, and filamentous algae), within a functional group between temperate and tropical systems, and according to the metric used to measure producer abundance. These analyses suggest that human alteration of food webs and nutrient availability have significant effects on primary producers but that the effects vary among latitudes and primary producers, and with the inherent productivity of ecosystems.     

Soil nitrogen availability and herbivore attack influence the chemical defenses of an invasive plant (Linaria dalmatica; Plantaginaceae)

Chemical defenses are thought to contribute to the invasion success and impacts of many introduced plants; however, for most of these species, little is known about these compounds and how they vary in natural environments. Plant allelochemical concentrations may be affected by a variety of abiotic and biotic factors, including soil nutrients and herbivores. Moreover, such quantitative variation is likely to play an important role in species interactions involving these invasive plants. The purpose of this study was to examine patterns of variation in iridoid glycoside concentrations of the invasive plant Linaria dalmatica (Plantaginaceae). We conducted a greenhouse experiment to investigate the effect of soil nitrogen availability on iridoid glycoside concentrations. Results from this experiment showed that plant iridoid glycoside concentrations decreased with increased nitrogen availability. Additionally, plants were collected from multiple field sites in order to characterize the influence of population, soil nitrogen availability, and herbivore attack on iridoid glycoside variation. Results from field studies indicated that plants demonstrated considerable seasonal variation, as well as variation within and among populations, with iridoid glycoside concentrations ranging from approximately 1 to 15% dry weight. The relationship between soil nitrogen and plant iridoid glycosides varied among populations, with a strong negative correlation in one population, a marginally significant negative relationship in a second population, and no relationship in the remaining two populations. Additionally, we found a negative relationship between iridoid glycoside concentrations and plant injury by an introduced biocontrol agent, the stem-mining weevil Mecinus janthinus (Cucurlionidae). These results show that plant allelochemical concentrations can vary widely in natural environments and suggest that levels of plant defense may be reduced by increased soil nitrogen availability and herbivore attack in this invasive plant species.     

Water stress alters aphid-induced glucosinolate response in Brassica oleracea var. italica differently

Glucosinolates (GS) are the main secondary metabolites in brassicaceous plants that play an important role in plant defense and plant–insect communication. GS biosynthesis in plants and their accumulation may be influenced by biotic and abiotic stressors from the environment. In the present study, GS levels in broccoli plants, Brassica oleracea var. italica Plenck, grown under different water status conditions of soil—well-watered, drought and waterlogged—were determined after two aphid species, the specialist Brevicoryne brassicae (L.) and the generalist Myzus persicae (Sulzer), had fed on them for 7 days. High-performance liquid chromatography analysis revealed that GS levels were significantly induced after B. brassicae had fed on B. italica plants grown for 2 weeks under the various water status conditions. No significant differences were determined between GS induction in the three treatments. In contrast, the induction of GS after M. persicae had fed on the plants depended greatly on water stress levels. GS content in well-watered plants increased more significantly than in plants grown under drought conditions. Feeding by M. persicae did not increase GS levels when plants were grown under waterlogged conditions. The lowest plant growth were found in M. persicae-infested plants for all three water treatments, and measurements of GS showed that the induction of GS by M. persicae decreased in line with the reduction in infested plant growth. GS induction by B. brassicae did not alter remarkably following change in plant growth under water stress. Only one of the five glucosinolates detected, the aliphatic GS 4-methylsulfinylbutyl, remained unchanged after feeding of both, M. persicae and B. brassicae, on plants under water stress. Variation in GS levels was mainly observed for the induction of indolyl GS due to aphid herbivory, but changes in indol-3-ylmethyl GS were not always consistent with the induction of other indolyl GS.