Herbivory and plant tolerance: experimental tests of alternative hypotheses involving non‐substitutable resources

A mechanistic understanding of the highly variable effects of herbivores on plant production in different ecosystems remains a major challenge. To explain these patterns, the compensatory continuum hypothesis (CCH) predicts plants to compensate for defoliation when resources are abundant, whereas the growth rate hypothesis (GRH) makes the opposite claim of high herbivory tolerance under resource‐poor conditions. The limiting resource model (LRM) tries to reconcile this dichotomy by incorporating the indirect effects of herbivores on plant resources and predicts that the potential for plant compensation is dependent upon whether, and how, herbivory influences limiting resources. Although extensively evaluated in laboratory monocultures, it remains uncertain whether these models can also explain the response of heterogeneous and multi‐species natural plant communities to defoliation. Here we investigate community‐wide plant response to defoliation and report data from a field experiment in the arid and primarily water‐limited Trans‐Himalayan grazing ecosystem in northern India involving clipping, irrigation and nutrient‐feedback with herbivore dung. Without nutrient‐feedback, plants compensated for defoliation in absence of irrigation but failed to compensate under irrigation. Whereas, in the presence of nutrient‐feedback plants compensated for defoliation when irrigated. This divergent pattern is not consistent with the CCH and GRH, and is only partially explained by the LRM. Instead, these pluralistic results are consistent with the hypothesis that herbivory may alter the relative strengths of water and nutrient limitation since irrigation increased root:shoot ratio in absence of fertilization in unclipped plots, but not in the corresponding clipped plots. So, herbivory appears to increase relative strength of nutrient‐limitation for plants that otherwise seem to be primarily water‐limited. Extending the LRM framework to include herbivore‐mediated transitions between water and nutrient‐limitation may clarify the underlying mechanisms that modulate herbivory‐tolerance under different environmental conditions.     

Interactive effects of clipping and nutrient availability on the compensatory growth of a grass species

Resource availability is an important factor affecting the capacity of compensatory growth after grazing. We performed a greenhouse experiment with Poa bulbosa, a small perennial grass of the Mediterranean and Central Asian grasslands, to test the importance of nutrient availability for compensatory growth after clipping. We also compared the results with predictions of the limited resource model (LRM). Plants were grown at low and high fertilization levels and subjected to a clipping treatment. Contrary to the LMR, we found that in Poa plants compensatory growth occurred under the high fertilization level, while it did not occur under the low level. The LMR predicts a higher tolerance for grazing in the stressful environment. Our plants showed a significant decrease in their relative growth rates (RGR) after clipping. Although the plants allocated a 32–188% greater fraction of the mass to lamina growth after clipping, this greater allocation to the leaves did not fully compensate for the initial reduction in leaf area ratio (LAR). A sensitivity analysis showed for the clipped plants under the high fertilization treatment, that changes in leaf allocation (f _lam) enabled the plants to compensate for a part of the potential loss caused by defoliation. Probably, the increased biomass allocation comes largely from the bulbs. We conclude that the inconsistency of the LRM with our results originates in the lack of compensatory mechanisms in the model. To better understand how environmental conditions affect tolerance to herbivory, the effects of compensatory growth should be taken into account.     

Effects of clipping and N fertilization on insect herbivory and infestation by pathogenic fungi on bilberry

Interactions among herbivores or between herbivores and other plant natural enemies, such as fungal pathogens, range from competition to facilitation. Moreover, the outcome of these interactions depends on the ecological context where they occur. In this study we examined the effects of clipping, as a surrogate of herbivory by ungulates, on the damage caused by two types of natural enemies (herbivorous insects and foliar fungal pathogens) on bilberry, Vaccinium myrtillus, in combination with nitrogen (N) fertilization representing current N atmospheric deposition. To examine whether the responses of both these natural enemies were mediated by changes in the plant, we estimated the effects of the treatments on bilberry growth and branching and on chlorophyll content as proxy of N content in leaves. Clipping increased the proportion of leaves damaged by herbivorous insects regardless of whether it was combined with N fertilization or not in 2008. In 2007 and 2009 repeated damage to the shrub also facilitated insect herbivory but only under N applications. Regarding fungal infestation incidence, clipping decreased the proportion of infected leaves in all the years considered but only in fertilized plots. Our results suggest that vertebrate herbivores facilitate insect herbivory and reduce fungal infestation but that these effects are dependent on nutritional conditions. Moreover, we found a negative residual correlation between insect herbivory and fungal infestation on bilberry leaves. Therefore, interactions between insect herbivores and fungal pathogens could be implicated in the final outcome of interactions between browsing ungulates and both bilberry natural enemies.     

Differential responses to defoliation frequency in little bluestem (<Emphasis Type="Italic">Schizachyrium scoparium</Emphasis>) in tallgrass prairie: implications for herbivory tolerance and avoidance

Plant responses to herbivory are complex. In grasses, relative growth rate (RGR), seed, and vegetative reproduction, resource allocation, and architecture vary differentially and often nonlinearly with grazing intensity. High grazing tolerance may be achieved through compensatory photosynthesis and leaf growth, or through demographic mechanisms such as activation of a belowground dormant bud bank. This study assessed the relationship between grazing frequency and responses of Schizachyrium scoparium (little bluestem) in a tallgrass prairie, and examined the roles of tiller growth, reproduction, and bud (meristem) populations in its persistence under grazing. Genets were subjected to varying simulated grazing frequencies for a period of 2 years. Strong differential responses were observed among plant traits. RGR, biomass, and flowering showed strong nonlinear reductions in response to increasing clipping frequency, with no evidence of threshold effects. However, meristem density was unaffected, and plants maintained a large bud bank across all clipping treatments. Tiller natality decreased initially, but increased with >4 clippings, suggesting that declines in tiller RGR are partially offset by increasing tiller natality, and that variation in genet size is driven more by demography than by variation in individual tiller growth. Increased grazing frequency also resulted in differential activation of buds at different positions (emerging within vs. outside the subtending leaf sheath), explaining the shift to a more prostrate growth form observed in many caespitose grasses under persistent grazing. Thus, although this grass species lacks the capacity for compensatory foliage re-growth, the maintenance of a large dormant bud bank and the differential activation of buds in different positions contribute to its grazing tolerance and avoidance, respectively, and its long-term persistence in grazed grasslands.     

Effect of Herbivory and Plant Species Replacement on Primary Production

Grazing optimization occurs when herbivory increases primary production at low grazing intensities. In the case of simple plant-herbivore interactions, such an effect can result from recycling of a limiting nutrient. However, in more complex cases, herbivory can also lead to species replacement in plant communities, which in turn alters how primary production is affected by herbivory. Here we explore this issue using a model of a limiting nutrient cycle in an ecosystem with two plant species. We show that two major plant traits determine primary production at equilibrium: plant recycling efficiency (i.e., the fraction of the plant nutrient stock that stays within the ecosystem until it is returned to the nutrient pool in mineral form) and plant ability to deplete the soil mineral nutrient pool through consumption of this resource. In cases where sufficient time has occurred, grazing optimization requires that herbivory improve nutrient conservation in the system sufficiently. This condition sets a minimum threshold for herbivore nutrient recycling efficiency, the fraction of nutrient consumed by herbivores that is recycled within the ecosystem to the mineral nutrient pool. This threshold changes with plant community composition and herbivore preference and is, therefore, strongly affected by plant species replacement. The quantitative effects of these processes on grazing optimization are determined by both the recycling efficiencies and depletion abilities of the plant species. However, grazing optimization remains qualitatively possible even with plant species replacement.     

Effects of resource availability on tolerance of herbivory: a review and assessment of three opposing models

Although it is widely acknowledged that a plant's tolerance of herbivore damage depends on resource availability in the plant's environment, there is no consensus on whether higher resource levels lead to greater or to lower tolerance. The prevailing model, the compensatory continuum hypothesis (CCH), predicts that tolerance of herbivory should be greater in high-resource or low-competition conditions. The main rival hypothesis, the growth rate model (GRM), makes the opposite prediction: tolerance of herbivory should be greater in more stressful conditions. The tolerance predictions of a recently introduced model, the limiting resource model (LRM), are more flexible and depend on the type of resource and herbivore under consideration. We reviewed 48 studies (from 40 published articles) of plant tolerance of leaf damage in conditions differing in levels of light, inorganic nutrients, water stress, or competition. The results of 31%, 48%, and 95% of the studies were consistent with the predictions of the CCH, GRM, and LRM, respectively. Thus, by considering which resource is primarily affected by herbivory and which resource is limiting a plant's fitness, the LRM offers a substantial advance in predicting how tolerance will be affected by environmental differences in resource availability.     

Pyric Herbivory and the Nexus Between Forage,Fire and Native and Introduced Large Grazing Herbivores in Australian Tropical Savannas

Earth’s tropical savannas typically support high biomass of diverse grazing herbivores that depend on a highly fluctuating resource: high-quality forage. An annual wet–dry cycle, fire and herbivory combine to influence forage quality and availability throughout the year. In the savannas of northern Australia, a depauperate suite of large native (marsupial) herbivores (wallaroos [Osphranter spp.] and the agile wallaby [Notamacropus agilis]) compete for resources with non-native large herbivores introduced in the late nineteenth century, particularly bovines (feral and managed cattle [Bos spp.] and feral water buffalo [Bubalus bubalis]) that now dominate the landscape. Anecdotal reports of recent population declines of large macropods and negative impacts of bovines highlight the need to better understand the complex relationship between forage, fire and abundance of native and introduced large herbivores. The pyric herbivory conceptual model, which posits complex feedbacks between fire and herbivory and was developed outside Australia, predicts that native and introduced large herbivores will both respond positively to post-fire forage production in Australian savannas where they co-occur. We used grazing exclosures, forage biomass and nutrient analyses and motion-sensor camera-trapping to evaluate the overall robustness of the pyric herbivory model in the Australian context, specifically whether forage quantity and quality are impacted by herbivory, season and fire activity, and which forage attributes most influence large grazing herbivore abundance. Forage quantity, as measured by live, dead and total herbaceous biomass and proportion of biomass alive, was higher inside herbivore exclosures, even at relatively low densities of herbivores. Forage quality, as measured by fibre content, was not affected by herbivory, however, crude protein content of live herbaceous biomass was greater outside herbivore exclosures. Recent fire was an important predictor of all measures of forage quantity and quality. Recent fire occurrence decreased overall quantity (biomass) but increased quality (decreased fibre content and increased crude protein content); late dry season fires resulted in forage with the highest crude protein content. The predictions of the pyric herbivory conceptual model are consistent with observations of the feeding behaviour of introduced bovines and some large macropods in northern Australian savannas, lending support to the global generality of pyric herbivory in fire-prone grassy biomes.

     

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.     

Herbivory promotes plant production and reproduction in nutrient-poor conditions: effects of plant adaptive phenology

Many studies have demonstrated positive effects of herbivory on plant performance, and these encompass two categories of effects: enhancement of primary production and enhancement of reproductive success. These positive responses of plants to herbivory have been called "grazing optimization." One possible mechanism of these paradoxical phenomena is the nutrient cycling promoted by herbivory. This article models the nutrient cycling hypothesis and analyzes the evolution of plant production and reproduction enhanced by herbivores, using dynamic optimization of plant phenology. Especially when there is nutrient competition among plant individuals or nutrient transportation by herbivores, we can apply the concept of evolutionary stability for the dynamic optimization. Two types of plant responses, long-term and short-term, are examined. Long-term response is an adaptive response for a given level of herbivory pressure, while short-term response is a nonadaptive one to various levels of herbivory, different from the level to which the plant is adapted. The analysis shows that both long-term and short-term grazing optimizations in primary production can occur under poor nutrient conditions and high nutrient recycling rates. However, grazing optimization in reproduction occurs under the same conditions but requires further conditions. In particular, long-term reproductive grazing optimization occurs only when nutrient competition exists among plant individuals. Accordingly, the present analysis revealed the following points concerning grazing optimization: poor nutrient condition is necessary, nutrient competition between plant individuals can promote optimization, and the native condition of the plant is important in the short-term response.     

Influence of root herbivory on plant communities in heterogeneous nutrient environments

While plant species respond differently to nutrient patches, the forces that drive this variability have not been extensively examined. In particular, the role of herbivory in modifying plant-resource interactions has been largely overlooked. We conducted a glasshouse study in which nutrient heterogeneity and root herbivory were manipulated, and used differences in foraging among plant species to predict the influence of root herbivores on these species in competition. We also tracked the influence of neighborhood composition, heterogeneity, and herbivory on whole-pot plant biomass. When herbivores were added to mixed-species neighborhoods, Eupatorium compositifolium, the most precise forager, was the only plant species to display a reduction in shoot biomass. Neighborhood composition had the greatest influence on whole-pot biomass, followed by nutrient heterogeneity; root herbivory had the smallest influence. These results suggest that root herbivory is a potential cost of morphological foraging in roots. Root herbivores reduced standing biomass and influenced the relative growth of species in mixed communities, but their effect was not strong enough at the density examined to overwhelm the bottom-up effects of resource distribution.     

Putting the ‘upstairs–downstairs’ into ecosystem service: What can aboveground–belowground ecology tell us?

Interactions between spatially-separated aboveground and belowground biota exert important influences on the functioning of terrestrial ecosystems. Plant root exudates and litter inputs affect root-associated and decomposer sub-communities, which, in turn, regulate nutrient availability and plant growth. Ecosystem services theoretically attributed to specific functional components of aboveground or belowground biota are, therefore, influenced by indirect (plant-mediated) interactions with the wider community. Some recent studies have considered aboveground–belowground interactions in a climate change context, with implications for altered ecosystem service provision. This review is a conceptual discussion of the mechanisms by which aboveground–belowground interactions affect specific ecosystem services: control of herbivores by natural enemies, insect pollination and nutrient mineralization by soil decomposers. While some mechanisms are well-characterized, others are poorly understood. Reducing root and shoot herbivory, in addition to the direct plant benefit, indirectly promotes antagonism of the spatially-separate herbivore by its natural enemies. Soil decomposers and mycorrhizal fungi can increase shoot herbivore performance such that control by natural enemies is weakened, or initiate bottom-up trophic cascades which strengthen antagonism of shoot herbivores. Aboveground herbivory generally stimulates nutrient cycling by decomposers. Root herbivory and mycorrhizal association both appear to increase floral attractiveness to insect pollinators. Mechanisms reflect alterations to plant growth, nutritional quality and chemical defenses. Climate change has considerable potential to alter aboveground–belowground interactions, with largely unexplored implications for biological control, pollination and soil nutrient cycling.     

Differential herbivory tolerance of dominant and subordinate plant species along gradients of nutrient availability and competition

We tested whether differences in the herbivory tolerance of plant species is related to their abundance in grassland communities and how herbivory and nutrient availability affect competitive balances among plant species through changes in their tolerance. The experimental approach involved a simulated grazing treatment (clipping) of two competitive grass species (Arrhenatherum elatius and Holcus lanatus) and two subordinate forb species (Prunella vulgaris and Lotus corniculatus) along a gradient of nutrient availability and under conditions of competition. Total standing, aboveground, root, and regrowth biomass were evaluated at the end of the experiment as an estimate of the capacity to compensate for twice removing aboveground biomass at different nutrient levels (NPK). Although clipping had a more pronounced negative effect on dominant plant species (Arrhenatherum and Holcus) than on subordinate species, the negative effects on dominant species were offset by the application of fertilizer. The combined effect of fertilizer and competition had more negative effects on the performance of Lotus and Prunella than on the dominant species. In terms of competition, the regrowth ability of Arrhenatherum and Holcus increased with the application of fertilizer, while the opposite pattern was observed for Lotus and Prunella. The addition of fertilizer has a positive effect on both grass species in terms of growth in clipped pots and competition, while subordinate species did not respond to the addition of fertilizer to the clipped pots and were negatively affected by competition with both grass species. The results suggest (1) that species replacement towards subordinate species as a function of herbivory is partially dependent on the herbivory tolerance of that species, (2) competitive relations between competitive grass species and subordinate forb species change under different environmental conditions, and (3) although grazing disturbance significantly influences competitive relations in favor of less competitive species, increasing nutrient levels counteract the negative effect of grazing on dominant competitive plant species.     

Effects of mycorrhizal symbiosis on tallgrass prairie plant–herbivore interactions

Complex relationships occur among plants, mycorrhizal fungi, and herbivores. By altering plant nutrient status, mycorrhizas may alter herbivory or plant tolerance to herbivory via compensatory regrowth. We examined these interactions by assessing grasshopper preference and plant growth and fungal colonization responses to herbivory under mycorrhizal and non‐mycorrhizal conditions within tallgrass prairie microcosms. Mycorrhizal symbiosis increased plant regrowth following defoliation, and some strongly mycotrophic plant species showed overcompensation in response to herbivory when they were mycorrhizal. Although grasshoppers spent more time on mycorrhizal plants, herbivory intensity did not differ between mycorrhizal and non‐mycorrhizal plants. Aboveground herbivory by grasshoppers significantly increased mycorrhizal fungal colonization of plant roots. Thus mycorrhizas may greatly benefit plants subjected to herbivory by stimulating compensatory growth, and herbivores, in turn, may increase the development of the symbiosis. Our results also indicate strong interspecific differences among tallgrass prairie plant species in their responses to the interaction of aboveground herbivores and mycorrhizal symbionts.     

Effect of water availability on tolerance of leaf damage in tall morning glory,Ipomoea purpurea

Resource availability may limit plant tolerance of herbivory. To predict the effect of differential resource availability on plant tolerance, the limiting resource model (LRM) considers which resource limits plant fitness and which resource is mostly affected by herbivore damage. We tested the effect of experimental drought on tolerance of leaf damage in Ipomoea purpurea, which is naturally exposed to both leaf damage and summer drought. To seek mechanistic explanations, we also measured several morphological, allocation and gas exchange traits. In this case, LRM predicts that tolerance would be the same in both water treatments. Plants were assigned to a combination of two water treatments (control and low water) and two damage treatments (50% defoliation and undamaged). Plants showed tolerance of leaf damage, i.e., a similar number of fruits were produced by damaged and undamaged plants, only in control water. Whereas experimental drought affected all plant traits, leaf damage caused plants to show a greater leaf trichome density and reduced shoot biomass, but only in low water. It is suggested that the reduced fitness (number of fruits) of damaged plants in low water was mediated by the differential reduction of shoot biomass, because the number of fruits per shoot biomass was similar in damaged and undamaged plants. Alternative but less likely explanations include the opposing direction of functional responses to drought and defoliation, and resource costs of the damage-induced leaf trichome density. Our results somewhat challenge the LRM predictions, but further research including field experiments is needed to validate some of the preliminary conclusions drawn.     

Generalities in grazing and browsing ecology: using across-guild comparisons to control contingencies

In community ecology, broad-scale spatial replication can accommodate contingencies in patterns within species groups, but contingencies in processes across species groups remain problematic. Here, based on a focused review of grazing and browsing by large mammals, we use one trophic guild as a “control” for the other to identify generalities that are not contingent upon specific consumer-resource interactions. An example of such a generality is the Jarman–Bell principle, which explains how allometries of metabolism and digestion influence dietary tolerance and thereby enable resource partitioning within both guilds at multiple scales. By comparing the grazing succession with browsing stratification we show how competition from smaller herbivores, rather than facilitation from larger ones, is the underlying process structuring ungulate assemblages when shared resources become limiting. Also, grazing lawns and browsing hedges are functionally similar. In each case, plants expressing tolerance traits can withstand chronic grazing or browsing in sites where the nutritive value of the local food resource is enhanced in positive feedback to the actions of its consumers. The debate over whether ungulates accelerate or decelerate nutrient cycling can be resolved by comparing grazing and browsing effects in the same ecosystem type. Evidence from African savannas points to the rate of nutrient cycling being controlled by the mix of tolerance and resistance traits in plants; not the relative dominance of grazing or browsing by local herbivores. We recommend this across-guild comparative approach as a novel solution with widespread utility for resolving contingencies in community processes.     

季节放牧下内蒙古温带草原羊草根茎叶功能性状的权衡

放牧是草地主要利用方式之一,不同季节放牧通过影响草地功能性状间的权衡从而影响牧后再生及补偿性生长。通过测定内蒙古温带草原优势种羊草(Leymus chinensis)的株高、节间距和分蘖数等软性状及气体交换、抗氧化酶系统和根茎叶渗透调节物质的含量等硬性状,分析了不同季节放牧处理下羊草功能性状的变化及其权衡关系。结果表明,3年短期放牧处理下,类连续放牧(T1)比春季放牧样地(T2)羊草表现出更强的避牧性与耐牧性。类连续放牧与春季放牧样地羊草软性状及光合特性表现出一致性,6月放牧干扰降低了羊草的净光合速率(P_n),8月放牧干扰通过增加电子传递速率(ETR)及光系统Ⅱ(PSⅡ)分配于光化学反应(P)的比值等增大P_n。但春季放牧样地羊草株高较高,且光合产物较多分配于叶片,导致大量有机物质被啃食,不利于牧草再生。而类连续放牧羊草将较多的有机物质分配于根茎,有利于牧草根系吸水及牧后再生。因此,3年短期放牧处理下,类连续放牧更有利于牧草再生及草原的可持续利用。     

Seagrasses and eutrophication

This review summarizes the historic, correlative field evidence and experimental research that implicate cultural eutrophication as a major cause of seagrass disappearance. We summarize the underlying physiological responses of seagrass species, the potential utility of various parameters as indicators of nutrient enrichment in seagrasses, the relatively sparse available information about environmental conditions that exacerbate eutrophication effects, and the better known array of indirect stressors imposed by nutrient over-enrichment that influence seagrass growth and survival. Seagrass recovery following nutrient reductions is examined, as well as the status of modeling efforts to predict seagrass response to changing nutrient regimes.The most common mechanism invoked or demonstrated for seagrass decline under nutrient over-enrichment is light reduction through stimulation of high-biomass algal overgrowth as epiphytes and macroalgae in shallow coastal areas, and as phytoplankton in deeper coastal waters. Direct physiological responses such as ammonium toxicity and water-column nitrate inhibition through internal carbon limitation may also contribute. Seagrass decline under nutrient enrichment appears to involve indirect and feedback mechanisms, and is manifested as sudden shifts in seagrass abundance rather than continuous, gradual changes in parallel with rates of increased nutrient additions. Depending on the species, interactions of high salinity, high temperature, and low light have been shown to exacerbate the adverse effects of nutrient over-enrichment. An array of indirect effects of nutrient enrichment can accelerate seagrass disappearance, including sediment re-suspension from seagrass loss, increased system respiration and resulting oxygen stress, depressed advective water exchange from thick macroalgal growth, biogeochemical alterations such as sediment anoxia with increased hydrogen sulfide concentrations, and internal nutrient loading via enhanced nutrient fluxes from sediments to the overlying water. Indirect effects on trophic structure can also be critically important, for example, the loss of herbivores, through increased hypoxia/anoxia and other habitat shifts, that would have acted as “ecological engineers” in promoting seagrass survival by controlling algal overgrowth; and shifts favoring exotic grazers that out-compete seagrasses for space. Evidence suggests that natural seagrass population shifts are disrupted, slowed or indefinitely blocked by cultural eutrophication, and there are relatively few known examples of seagrass meadow recovery following nutrient reductions.Reliable biomarkers as early indicators of nutrient over-enriched seagrass meadows would benefit coastal resource managers in improving protective measures. Seagrasses can be considered as “long-term" integrators (days to weeks) of nutrient availability, especially through analyses of their tissue content, and of activities of enzymes such as nitrate reductase and alkaline phosphatase. The ratio of leaf nitrogen content to leaf mass has also shown promise as a “nutrient pollution indicator” for the seagrass Zostera marina, with potential application to other species. In modeling efforts, seagrass response to nutrient loading has proven difficult to quantify beyond localized areas because long-term data consistent in quality are generally lacking, and high inter-annual variability in abundance and productivity depending upon stochastic meteorological and hydrographic conditions.Efforts to protect remaining seagrass meadows from damage and loss under eutrophication, within countries and across regions, are generally lacking or weak and ineffective. Research needs to further understand about seagrasses and eutrophication should emphasize experimental studies to assess the response of a wider range of species to chronic, low-level as well as acute, pulsed nutrient enrichment. These experiments should be conducted in the field or in large-scale mesocosms following appropriate acclimation, and should emphasize factor interactions (N, P, C; turbidity; temperature; herbivory) to more closely simulate reality in seagrass ecosystems. They should scale up to address processes that occur over larger scales, including food-web dynamics that involve highly mobile predators and herbivores. Without any further research, however, one point is presently very clear: Concerted local and national actions, thus far mostly lacking, are needed worldwide to protect remaining seagrass meadows from accelerating cultural eutrophication in rapidly urbanizing coastal zones.     

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.     

Plants can benefit from herbivory: stimulatory effects of sheep saliva on growth of Leymus chinensis

Background

Plants and herbivores can evolve beneficial interactions. Growth factors found in animal saliva are probably key factors underlying plant compensatory responses to herbivory. However, there is still a lack of knowledge about how animal saliva interacts with herbivory intensities and how saliva can mobilize photosynthate reserves in damaged plants.

Methodology/Principal Findings

The study examined compensatory responses to herbivory and sheep saliva addition for the grass species Leymus chinensis in three experiments over three years. The first two experiments were conducted in a factorial design with clipping (four levels in 2006 and five in 2007) and two saliva treatment levels. The third experiment examined the mobilization and allocation of stored carbohydrates following clipping and saliva addition treatments. Animal saliva significantly increased tiller number, number of buds, and biomass, however, there was no effect on height. Furthermore, saliva effects were dependent on herbivory intensities, associated with meristem distribution within perennial grass. Animal saliva was found to accelerate hydrolyzation of fructans and accumulation of glucose and fructose.

Conclusions/Significance

The results demonstrated a link between saliva and the mobilization of carbohydrates following herbivory, which is an important advance in our understanding of the evolution of plant responses to herbivory. Herbivory intensity dependence of the effects of saliva stresses the significance of optimal grazing management.