Assessing density–damage relationships between water hyacinth and its grasshopper herbivore

Plants are variable in their responses to insect herbivory. Experimental increases in densities of phytophagous insects can reveal the type of plant response to herbivory in terms of impact and compensatory ability. The relationship between insect density and plant damage of a grasshopper, Cornops aquaticum Brüner (Orthoptera: Acrididae: Tetrataeniini), a candidate biological control agent, and an invasive aquatic plant, water hyacinth, Eichhornia crassipes Mart. Solms‐Laubach (Pontederiaceae), was investigated to assess potential damage to the weed. The impact of different densities of male and female grasshoppers on E. crassipes growth parameters was determined in a quarantine glasshouse experiment. Damage curves indicated that the relationship between plant biomass reduction and insect density was curvilinear whereas leaf production was linear. Female C. aquaticum were more damaging than males, causing high rates of plant mortality before the end of the trial at densities of three and four per plant. Feeding by C. aquaticum significantly reduced the total plant biomass and the number of leaves produced, and female grasshoppers caused a greater reduction in the number of leaves produced by water hyacinth plants than males. Grasshopper herbivory suppressed vegetative reproduction in E. crassipes, suggesting C. aquaticum could contribute to a reduction in the density and spread of E. crassipes infestations. The results showed that E. crassipes vigour and productivity decreases with an increase in feeding intensity by the grasshopper. Cornops aquaticum should therefore be considered for release in South Africa based on its host specificity and potential impact on E. crassipes.     

Influence of a large late summer precipitation event on food limitation and grasshopper population dynamics in a northern Great Plains grassland

The complex interplay between grasshoppers, weather conditions, and plants that cause fluctuations in grasshopper populations remains poorly understood, and little is known about the ecological processes that generate grasshopper outbreaks. Grasshopper populations respond to interacting extrinsic and intrinsic factors, with yearly and decadal weather patterns and the timing of precipitation all potentially important. The effects of initial and increasing grasshopper densities on grasshopper survival and reproductive correlates were examined at a northern mixed-grass prairie site through manipulations of grasshopper densities inside 10-m2 cages. High-quality grass growth occurred after a 9.1-cm mid-August rain. Reduced proportional survival was apparent in the two higher density treatments before the rain, indicative of food-limited density-dependent mortality. However, the large late summer rainfall event mediated the effects of exploitative competition on demographic characteristics because of the high-quality vegetation growth. This led to weaker effects of food limitation on survival and reproduction at the end of the experiment. The results indicate a direct link between weather variation, resource quality and grasshopper population dynamics led to a severe grasshopper outbreak and show that infrequent large precipitation events can have significant effects on population dynamics. Additional research is needed to examine the importance of infrequent large precipitation events on grasshopper population dynamics in grassland ecosystems.     

Herbivore phenology can predict response to changes in plant quality by livestock grazing

Livestock grazing can have a strong impact on herbivore abundance, distribution and community. However, not all species of herbivores respond the same way to livestock grazing, and we still have a poor understanding of the underlying mechanisms driving these differential responses. Here, we investigate the effect of light intensity cattle grazing on the abundance of two grasshoppers (Euchorthippus cheui and E. unicolor) that co-occur in the same grasslands and feed on the same food plant (the dominant grass Leymus chinensis). The two grasshopper species differ in phenology so that their peak abundances are separated into early- and late-growing seasons. We used an exclosure experiment to monitor grasshopper abundance and food quality in the field under grazed and ungrazed conditions, and performed feeding trials to examine grasshopper preference for grazed or ungrazed food plants in the laboratory. We found that the nitrogen content of L. chinensis leaves continuously declined in the ungrazed areas, but was significantly enhanced by cattle grazing over the growing season. Cattle grazing facilitated the early-season grasshopper E. cheui, whereas it suppressed the late-season grasshopper E. unicolor. Moreover, feeding trials showed that E. cheui preferred L. chinensis from grazed plots, while E. unicolor preferred the leaves from ungrazed plots. We conclude that livestock grazing has opposite effects on the two grasshopper species, and that these effects may be driven by grazing-induced changes in plant nutrient content and the unique nutritional niches of the grasshoppers. These results suggest that insects that belong to the same guild can have opposite nutrient requirements, related to their distinct phenologies, and that this can ultimately affect their response to cattle grazing. Our results show that phenology may link insect physiological needs to local resource availabilities, and should be given more attention in future work on interactions between large herbivores and insects.     

Grasshoppers (Orthoptera: Acrididae) select vegetation patches in local‐scale responses to foliar nitrogen but not phosphorus in native grassland

Abstract Key elements such as nitrogen (N) and phosphorus (P) are often limiting relative to the nutritional needs of herbivores that feed on them. While N often limits insect herbivores in natural terrestrial ecosystems, the effect of P is poorly studied in the field, even though compelling hypotheses from the ecological stoichiometry literature predict its importance. We evaluated small‐scale spatial distributions of, and herbivory by, grasshoppers among neighboring plots that vary in foliar‐N and ‐P in tallgrass prairie. Grasshopper densities were 67% greater in N‐fertilized plots but detected no effect to grasshopper densities from P‐fertilizer. Leaf damage to the dominant grass Andropogon gerardii was 32% greater in N‐fertilized plots, but no response to foliar‐P was detected. Herbivore damage to a common forb, goldenrod (Solidago missouriensis), was not strongly linked by fertilizer treatments, although there was increased leaf damage in N‐fertilizer treatments when no P was applied (a significant N × P interaction). Under field conditions at local scales, we conclude that spatially heterogeneous distributions of grasshoppers are primarily affected by foliar‐N in host plants with little evidence that P‐levels contribute to the spatial patterns.     

Effects of large herbivore grazing on grasshopper behaviour and abundance in a meadow steppe

1. Adaptive phenotypic plasticity has been a major subject in evolutionary ecology, but how a species' behaviour may respond to certain environmental change is still not clear. In grasslands worldwide, large herbivores are increasingly used as a tool for grazing management, and occur to interact with grasshoppers that dominate grassland insect communities. Previous studies have been well-documented about grazing effects on diversity and abundance of grasshoppers. Yet, how grazing may alter grasshopper behaviour, and potential effects on their abundance remains elusive. 2. We conducted a field experiment by manipulating grazing using sheep, cattle, and their mix to examine the behavioural responses and abundance of the grasshoppers (Euchorthippus unicolor) to grazing in a Leymus chinensis-dominated grassland. 3. Results showed that the grasshoppers spent less time on feeding and resting on grasses, but more time on switching and resting on forbs under cattle grazing and mixed grazing with cattle and sheep. In contrast, the grasshoppers spent more time on feeding but less time on switching and resting on forbs under sheep grazing. The behavioural changes were also potentially linked to grasshopper abundance in the context of grazing management. 4. The responses of grasshopper behaviour and abundance to grazing may be largely triggered by altered vegetation and microclimates. Such behavioural flexibility of grasshoppers must be considered when large herbivores are recognised as a management tool for influencing grasshopper abundance, and grazer species should be paid more attention both individually and jointly for better grassland conservation.     

Arthropod abundance in tall fescue,Lolium arundinaceum,pastures containing novel ‘safe’ endophytes

Poor livestock performance on tall fescue is linked to infection by a fungal endophyte that enhances grass resistance to stress, including herbivory, while producing ergot alkaloids toxic to vertebrate grazers. Novel ‘safe’ endophyte/grass associations produce no ergot alkaloids yet retain stand persistence, but they could be more susceptible to insect outbreaks. We tested the hypothesis that grass‐feeding insects are more abundant in novel endophyte pastures compared with those containing common endophyte. Above‐ and below‐ground herbivores were sampled across two growing seasons in pastures containing common strain endophyte (KY31), novel endophytes (MaxQ or AR584) or endophyte free. We also sampled natural enemies as an indicator of possible tritrophic effects. With a few exceptions, numbers of predatory, chewing (grasshoppers, crickets, caterpillars) or sucking insects (leafhoppers, planthoppers) were similar in common and novel endophyte pastures. Contrary to our hypothesis, Aphrodes spp. leafhoppers were more abundant in KY31 than other pasture types in 2008; their nymphs also were more abundant in KY31 than in MaxQ in 2009. Adults (but not total numbers or nymphs) of another leafhopper, Psammotettix striatus, were less abundant in KY31 than other pasture types in 2009. Popillia japonica and Cyclocephala spp. grubs did not differ in density, weight or instar among the endophyte‐containing associations. In feeding assays, armyworm (Mythimna unipuncta) survival was higher on MaxQ and NE9301, and weight gain was higher on NE9301, suggesting armyworm outbreaks could be more severe on such grasses, but that result may not be attributable solely to alkaloids because common strain and endophyte‐free grasses did not differ in either parameter. Caterpillar abundance did not differ among pasture types in the field. Our results suggest that re‐seeding common strain endophytic pastures with livestock‐safe novel endophyte/grass associations to alleviate fescue toxicosis is unlikely to promote markedly higher populations of plant‐feeding insects.     

Interactive effects of insects and ungulates on root growth in a native grassland

Insects and ungulates co‐occur in grasslands, often feeding on the same plants at the same time and potentially having interactive effects on plant growth. Further, ungulate–insect interactions may differ between native ungulate guilds and domesticated cattle. Despite the prevalence of insects and ungulates in native grasslands, experiments simultaneously manipulating the densities of both these groups are rare. Using large, replicated paddocks, as well as insecticide application, we restricted access to vegetation by each group of herbivores. We also manipulated the species identity of the ungulate assemblage, allowing us to determine whether there are differential effects between native ungulate guilds (bison, elk and deer) and cattle on plant biomass. We found interactive effects of insect and ungulate herbivores on root growth. When insects were suppressed, both native ungulates and cattle caused an approximate doubling of root biomass. However, this stimulatory effect of ungulate grazing was eliminated when insects were also present. In contrast, neither insects nor ungulates had significant effects on shoot biomass at these densities. As a result, the dominant effects of above‐ground herbivory was on belowground plant growth. We suggest the effects of insect and ungulate assemblages on root biomass appear important in regulating primary production in this grassland and may account for some of the contradictory plant responses to ungulate herbivory in the literature.     

Herbivorous insects reduce growth and reproduction of big sagebrush (<Emphasis Type="Italic">Artemisia tridentata</Emphasis>)

Insect herbivores can reduce growth, seed production, and population dynamics of host plants, but do not always do so. Big sagebrush (Artemisia tridentata) has one of the largest ranges of any shrub in North America, and is the dominant and characteristic shrub of the extensive sagebrush steppe ecosystem of the western United States. Nevertheless, the impact of insect herbivores on big sagebrush, its dominant and characteristic shrub, is largely unknown. Occasional large effects of insect herbivore outbreaks are documented, but there is little knowledge of the impact of the more typical, nominal herbivory that is produced by the diverse community of insects associated with big sagebrush in natural communities. In 2008, we removed insects from big sagebrush plants with insecticide to evaluate whether insect herbivores reduced growth and seed production of big sagebrush. Removal of herbivores led to significant and substantial increases in inflorescence growth (22%), flower production (325%), and seed production (1053%) of big sagebrush. Our results showed the impact of insect herbivory in the current growing season on the growth and reproduction of big sagebrush and revealed an unrecognized, significant role of non-outbreak herbivores on big sagebrush.     

Heterogeneity in vertebrate and invertebrate herbivory and its consequences for New Zealand mistletoes

Vertebrate herbivores generally have greater effects than invertebrates on plants. However, few studies have investigated the effects of both invertebrate and vertebrate herbivores on a single plant species. In New Zealand, nationwide declines in mistletoe populations have often been attributed to possum herbivory, but never to insect herbivory. The main goal of the present study was to document levels of vertebrate and invertebrate herbivory on endemic New Zealand mistletoe plants to suggest whether herbivory is leading to mistletoe decline. In the present study, the annual amount of leaf loss from herbivory by the brushtail possum (Trichosurus vulpecula), insect herbivory and leaf abscission were measured in two populations each of three mistletoe species (Alepis flavida, Peraxilla colensoi, and Peraxilla tetrapetala, Loranthaceae). In two populations of each species from February 1997 to February 1998, abscission accounted for the most leaf loss (range 10–84% of total mean leaf area, mean 33%), whereas insects and possums usually removed small and similar amounts (less than 3%). Possum browse caused large amounts of abscission in only one population (A. flavida at Eglinton). Observed possum browse was more heterogeneous than insect browse among branches within a plant (possum coefficient of variation = 2.63, insect CV = 1.98, P < 0.001), among plants in a population (possum CV = 2.15, insect CV = 0.69, P < 0.001), and between populations (possum CV = 1.36, insect CV = 1.09). Moreover, insects damaged 100% of the study plants but never removed more than 16% of leaf area on a single plant, whereas possums only browsed 32% of the study plants but severely defoliated some plants. Thus, while the mean amount of biomass removed across a population may have important consequences for mistletoe survival, the effect of possums on mistletoe populations may also depend on the heterogeneity of browse among individuals in the population.     

Scale and system dependencies of indirect effects of large herbivores on phytophagous insects: a meta-analysis

An increasing number of studies are being conducted to examine the density- or trait-mediated indirect effects of large herbivores on phytophagous insects. However, these effects are highly context dependent and no general trends have been made clear. We conducted a meta-analysis focusing on three factors capable of affecting detection of the indirect effects of large herbivores on phytophagous insects: type of response variable, experimental scale, and characteristics of study organisms. Overall, large herbivores exerted a negative effect on insects, a trend that was prominent in studies using insect abundance as a response variable. No particular trends were observed in studies using herbivory rate as a variable, and these studies often focused on plant trait-mediated effects more than density-mediated ones. Experimental scale affected the strength of indirect effects: within-year or individual tree level experiments did not follow any trends, whereas 1–10 year experiments or 0–10 ha scale experiments show a negative impact on insects. Characteristics of large herbivores and growth forms of transmitter plants also contributed to variations in the observed effect size; negative effects were reported in livestock-grassland ecosystems and neutral effects in tree-dominated systems. There was a close association between response variable, experimental scale, and characteristics of study organisms, and these effects jointly contributed to the apparent trends. To predict the impacts of large herbivores at ecosystem level, it is necessary to eliminate these biases arising from study design and to evaluate the effect on insect densities at large spatial and temporal scales.     

Impact of grasshoppers and an invasive grass on establishment and initial growth of restoration plant species

Exotic plant invasion can have dramatic impacts on native plants making restoration of native vegetation at invaded sites challenging. Though invasives may be superior competitors, it is possible their dominance could be enhanced by insect herbivores if native plants are preferred food sources. Insect herbivory can regulate plant populations, but little is known of its effects in restoration settings. There is a need to better understand relationships between insect herbivores and invasive plants with regard to their combined potential for impacting native plant establishment and restoration success. The objective of this study was to assess impacts of grasshopper herbivory and the invasive grass Bromus tectorum (cheatgrass) on mortality and growth of 17 native plant species used in restoration of critical sagebrush steppe ecosystems. Field and greenhouse experiments were conducted using moderate densities of a common, generalist pest grasshopper (Melanoplus bivittatus). Grasshoppers had stronger and more consistent impacts on native restoration plants in field and greenhouse studies than cheatgrass. After 6 weeks in the greenhouse, grasshoppers were associated with 36% mortality over all native restoration species compared to 2% when grasshoppers were absent. Herbivory was also associated with an approximately 50% decrease in native plant biomass. However, effects varied among species. Artemisia tridentata, Chrysothamnus viscidiflorus, and Coreopsis tinctoria were among the most negatively impacted, while Oenothera pallida, Pascopyrum smithii, and Leymus cinerus were unaffected. These findings suggest restoration species could be selected to more effectively establish and persist within cheatgrass infestations, particularly when grasshopper populations are forecasted to be high.     

Prey change behaviour with predation threat, but demographic effects vary with prey density: experiments with grasshoppers and birds

Increasingly, ecologists emphasize that prey frequently change behaviour in the presence of predators and these behavioural changes can reduce prey survival and reproduction as much or more than predation itself. However, the effects of behavioural changes on survival and reproduction may vary with prey density due to intraspecific competition. In field experiments, we varied grasshopper density and threat of avian predation and measured grasshopper behaviour, survival and reproduction. Grasshopper behaviour changed with the threat of predation and these behavioural changes were invariant with grasshopper density. Behavioural changes with the threat of predation decreased per capita reproduction over all grasshopper densities; whereas the behavioural changes increased survival at low grasshopper densities and then decreased survival at high densities. At low grasshopper densities, the total reproductive output of the grasshopper population remained unchanged with predation threat, but declined at higher densities. The effects of behavioural changes with predation threat varied with grasshopper density because of a trade-off between survival and reproduction as intraspecific competition increased with density. Therefore, resource availability may need to be considered when assessing how prey behavioural changes with predation threat affect population and food web dynamics.     

Endophyte-grass-herbivore interactions: the case of Neotyphodium endophytes in Arizona fescue populations

Neotyphodium endophytes in introduced agronomic grasses are well known to increase resistance to herbivores, but little is known of interactions between Neotyphodium endophytes and herbivores in native grass populations. We investigated whether endophytes mediate plant-herbivore interactions in a native grass species, Festuca arizonica in the southwestern United States, in two ways. First, to test the prediction that the presence and frequency of endophyte-infected (E+) plants should increase with increasing herbivory, we determined endophyte frequencies over a 4-year period in six natural Arizona fescue populations. We compared Neotyphodium frequency among plants growing inside and outside long-term vertebrate grazing exclosures. Second, we experimentally tested the effects of Neotyphodium infection, plant clone, and soil nutrients on plant resistance to the native grasshopper Xanthippus corallipes. Contrary to predictions based upon the hypothesis that endophytes increase herbivore resistance, levels of infection did not increase in plants subjected to grazing outside of exclosures relative to ungrazed plants within exclosures. Instead, endophyte frequencies tended to be greater inside the exclosures, where long-term vertebrate grazing was reduced. The grasshopper bioassay experiment corroborated these long-term patterns. Survival of grasshoppers did not differ between infected (E+) and uninfected (E–) plants. Instead, mean relative growth rate of grasshoppers was higher on E+ grasses than on E– ones. Growth performance of newly hatched grasshopper nymphs varied among host plant clones, although two of six clones accounted for most of this variation. Our results suggest that Neotyphodium-grass-herbivore interactions may be much more variable in natural communities than predicted by studies of agronomically important Neotyphodium-grass associations, and herbivory is not always the driving selective force in endophyte-grass ecology and evolution. Thus, alternative hypotheses are necessary to explain the wide distribution and variable frequencies of endophytes in natural plant populations. Received: 15 February 1999 / Accepted: 19 July 1999     

Conservation Refugium Value of a Large Mesa for Grasshoppers in South Africa

A 3-year study was undertaken on and around a prominent South African mesa to determine its role for grasshoppers in a sea of grazed flatlands. The number of grasshopper species and individuals on the summit, slopes and flatlands varied significantly in relation to measured environmental variables. The summit, through inaccessibility to livestock grazing, was effectively a conservation refugium for one highly responsive grasshopper species, Orthochtha dasycnemis. There was no significant difference in species richness between years of sampling, although there were significant variations in grasshopper abundance between years. The difference in rainfall between years was significant and appeared to be the key factor influencing grasshopper population dynamics. This study clearly shows that a mesa can act as a conservation island and refugium supporting an insect assemblage that would be otherwise altered by heavy livestock grazing on the surrounding flatlands. This summit assemblage was strongly linked with those on the slopes, and is determined by low grazing intensity and associated soil and vegetation structure.     

An experimental analysis of grasshopper community responses to fire and livestock grazing in a northern mixed-grass prairie

The outcomes of grasshopper responses to both vertebrate grazing and fire vary across grassland ecosystems, and are strongly influenced by local climactic factors. Thus, the possible application of grazing and fire as components of an ecologically based grasshopper management strategy must be investigated in regional studies. In this study, we examined the effects of grazing and fire on grasshopper population density and community composition in a northern Great Plains mixed-grass prairie. We employed a large-scale, replicated, and fully-factorial manipulative experimental design across 4 yr to examine the separate and interactive effects of three grazing systems in burned and unburned habitats. Grasshopper densities were low throughout the 4-yr study and 1 yr of pretreatment sampling. There was a significant fire by grazing interaction effect on cumulative density and community composition, resulting from burned season long grazing pastures having higher densities than unburned pastures. Shannon diversity and grasshopper species richness were significantly higher with twice-over rotational livestock grazing. The ability to draw strong conclusions regarding the nature of species composition shifts and population changes in the presence of fire and grazing is complicated by the large site differences and low grasshopper densities. The results reinforce the importance of long-term research to examine the effects of habitat manipulation on grasshopper population dynamics.     

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.     

Grasshopper Herbivory Affects Native Plant Diversity and Abundance in a Grassland Dominated by the Exotic Grass Agropyron cristatum

The indirect effects of native generalist insect herbivores on interactions between exotic and native grassland plants have received limited attention. Crested wheatgrass ( Agropyron cristatum ) is the most common exotic rangeland grass in western North America. Crested wheatgrass communities are resistant to colonization by native plant species and have strong competitive effects on native species, imposing problems for the restoration of native grasslands. Grasshoppers are generalist herbivores that are often abundant in Crested wheatgrass–dominated sites in the northern Great Plains. We conducted two experiments in a Crested wheatgrass–dominated grassland in western North Dakota to test the hypothesis that grasshopper herbivory influences local Crested wheatgrass community composition by impeding native seedlings. Grasshopper herbivory negatively affected the species richness, abundance, and Shannon diversity of native plants in 3 of 4 years. Although additional research is needed to determine if grasshoppers actively select native plants, the effects of grasshopper herbivory may be an important consideration in the restoration of Crested wheatgrass areas. Our findings illustrate the importance of understanding the impact of native generalist invertebrate herbivores on the relationships between exotic and native plants.     

Response of galling invertebrates on Salix lanata to reindeer herbivory

Browsing and defoliation often increase the densities of insect herbivores on woody plants. Densities of herbivorous invertebrates were estimated in a long-term grazing manipulation experiment. More then 30-yr-old fences allow us to compare densities of invertebrate herbivores on Salix lanata in areas heavily grazed and areas lightly grazed by reindeer. The number of gall-forming insects ( Pontania glabrifons) and gall-forming mites were higher on the heavily grazed shrubs than on lightly grazed shrubs. In contrast to most short-term studies, the heavily grazed S. lanata had shorter current annual shoots. No difference in leaf size, leaf nitrogen content, or C:N ratio between grazing intensities were detected. However, the enhanced natural δ15N value indicates that heavily grazed shrubs get a higher proportion of their N directly from reindeer faeces. Leaf weight per unit area and relative fluctuating asymmetry of leaf shape increased in heavily grazed S. lanata . Enhanced relative fluctuating asymmetry might indicate higher susceptibility to herbivores. Long-term grazing seems to increase the density of invertebrate herbivory in the same way as short-term grazing, even if the plant responses differ substantially.     

Ontogenetic shifts within the selfish herd: predation risk and foraging trade-offs change with age in colonial web-building spiders

We tested for the existence of density dependence in annual adult rangeland grasshopper (Orthoptera: Acrididae) data from Montana, USA (1951–1991). Statistical density dependence was, in the sense of a stochastic equilibrium or return tendency, detected in all of the grasshopper mean density time-series from the three major physiographic regions of the state, Northern Glaciated Plains, Southern Unglaciated Plains, and Western Mountains. Parameters were estimated for a model that described the stochastic equilibrium characteristics of regional mean densities. The analyses showed that rangeland grasshopper regional densities fluctuate according to gamma distribution with a mean of 6.1–6.3 grasshoppers per m². Further, when regions exhibit outbreaks, the resulting infestation period (duration of outbreak) is short, spanning only a few generations.     

The Invasive Plant Alternanthera philoxeroides Was Suppressed More Intensively than Its Native Congener by a Native Generalist: Implications for the Biotic Resistance Hypothesis

Prior studies on preferences of native herbivores for native or exotic plants have tested both the enemy release hypothesis and the biotic resistance hypothesis and have reported inconsistent results. The different levels of resistance of native and exotic plants to native herbivores could resolve this controversy, but little attention has been paid to this issue. In this study, we investigated population performance, photosynthesis, leaf nitrogen concentration, and the constitutive and induced resistances of the successful invasive plant, Alternanthera philoxeroides, and its native congener, Alternanthera sessilis, in the presence of three population densities of the grasshopper, Atractomorpha sinensis. When the grasshopper was absent, leaf biomass, total biomass, photosynthesis, and leaf nitrogen concentration of A. philoxeroides were higher than those of A. sessilis. However, the morphological and physiological performances of A. philoxeroides were all decreased more intensively than A. sessilis after herbivory by grasshoppers. Especially as the concentrations of constitutive lignin and cellulose in leaf of A. philoxeroides were higher than A. sessilis, A. philoxeroides exhibited increased leaf lignin concentration to reduce its palatability only at severe herbivore load, whereas, leaf lignin, cellulose, and polyphenolic concentrations of A. sessilis all increased with increasing herbivory pressure, and cellulose and polyphenolic concentrations were higher in A. sessilis than in A. philoxeroides after herbivory. Our study indicated that the capability of the invasive plant to respond to native insect damage was lower than the native plant, and the invasive plant was suppressed more intensively than its native congener by the native insect. Our results support the biotic resistance hypothesis and suggest that native herbivores can constrain the abundance and reduce the adverse effects of invasive species.