Local Species Richness of Leaf-Chewing Insects Feeding on Woody Plants from One Hectare of a Lowland Rainforest

Abstract:  Local species diversity of insect herbivores feeding on rainforest vegetation remains poorly known. This ignorance limits evaluation of species extinction patterns following various deforestation scenarios. We studied leaf-chewing insects feeding on 59 species of woody plants from 39 genera and 18 families in a lowland rainforest in Papua New Guinea and surveyed all plants with a stem diameter at breast height of ≥5 cm in a 1-ha plot within the same area. We used two extrapolation methods, based on randomized species-accumulation curves, to combine these two data sets and estimate the number of species of leaf-chewing herbivores feeding on woody plants from the 1-ha area. We recorded 58,483 feeding individuals from 940 species of leaf-chewing insects. The extrapolation estimated that there were 1567–2559 species of leaf-chewing herbivores feeding on the 152 plant species from 97 genera and 45 families found in 1 ha of the forest. Most of the herbivore diversity was associated with plant diversity on the familial and generic levels. We predicted that, on average, the selection of 45 plant species each representing a different family supported 39% of all herbivore species, the 52 plant species each representing a different additional genus from these families supported another 39% of herbivore species, and the remaining 55 plant species from these genera supported 22% of herbivore species. Lepidoptera was the most speciose taxon in the local fauna, followed by Coleoptera and orthopteroids (Orthoptera and Phasmatodea). The ratio of herbivore to plant species and the estimated relative species richness of the Lepidoptera, Coleoptera, and orthopteroids remained constant on the spatial scale from 0.25 to 1 ha. However, the utility of local taxon-to-taxon species ratios for extrapolations to geographic scales requires further study.     

When there is no escape: the effects of natural enemies on native, invasive, and noninvasive plants

An important question in the study of biological invasions is the degree to which successful invasion can be explained by release from control by natural enemies. Natural enemies dominate explanations of two alternate phenomena: that most introduced plants fail to establish viable populations (biotic resistance hypothesis) and that some introduced plants become noxious invaders (natural enemies hypothesis). We used a suite of 18 phylogenetically related native and nonnative clovers (Trifolium and Medicago) and the foliar pathogens and invertebrate herbivores that attack them to answer two questions. Do native species suffer greater attack by natural enemies relative to introduced species at the same site? Are some introduced species excluded from native plant communities because they are susceptible to local natural enemies? We address these questions using three lines of evidence: (1) the frequency of attack and composition of fungal pathogens and herbivores for each clover species in four years of common garden experiments, as well as susceptibility to inoculation with a common pathogen; (2) the degree of leaf damage suffered by each species in common garden experiments; and (3) fitness effects estimated using correlative approaches and pathogen removal experiments. Introduced species showed no evidence of escape from pathogens, being equivalent to native species as a group in terms of infection levels, susceptibility, disease prevalence, disease severity (with more severe damage on introduced species in one year), the influence of disease on mortality, and the effect of fungicide treatment on mortality and biomass. In contrast, invertebrate herbivores caused more damage on native species in two years, although the influence of herbivore attack on mortality did not differ between native and introduced species. Within introduced species, the predictions of the biotic resistance hypothesis were not supported: the most invasive species showed greater infection, greater prevalence and severity of disease, greater prevalence of herbivory, and greater effects of fungicide on biomass and were indistinguishable from noninvasive introduced species in all other respects. Therefore, although herbivores preferred native over introduced species, escape from pest pressure cannot be used to explain why some introduced clovers are common invaders in coastal prairie while others are not.     

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.     

Plant-mediated and nonadditive effects of two global change drivers on an insect herbivore community

Warmer temperatures can alter the phenology and distribution of individual species. However, differences across species may blur community-level phenological responses to climate or cause biotic homogenization by consistently favoring certain taxa. Additionally, the response of insect communities to climate will be subject to plant-mediated effects, which may or may not overshadow the direct effect of rising temperatures on insects. Finally, recent evidence for the importance of interaction effects between global change drivers suggests that phenological responses of communities to climate may be altered by other drivers. We used a natural temperature gradient (generated by elevation and topology), combined with experimental nitrogen fertilization, to investigate the effects of elevated temperature and globally increasing anthropogenic nitrogen deposition on the structure and phenology of a seminatural grassland herbivore assemblage (lepidopteran insects). We found that both drivers, alone and in combination, severely altered how the relative abundance and composition of species changed through time. Importantly, warmer temperatures were associated with biotic homogenization, such that herbivore assemblages in the warmest plots had more similar species composition than those in intermediate or cool plots. Changes in herbivore composition and abundance were largely mediated by changes in the plant community, with increased nonnative grass cover under high treatment levels being the strongest determinant of herbivore abundance. In addition to compositional changes, total herbivore biomass more than doubled under elevated nitrogen and increased more than fourfold with temperature, bearing important functional implications for herbivores as consumers and as a prey resource. The crucial role of nonnative plant dominance in mediating responses of herbivores to change, combined with the frequent nonadditive (positive and negative) effects of the two drivers, and the differential responses of species, highlight that understanding complex ecosystem responses will benefit from multifactor, multitrophic experiments at community scales or larger.     

Spillover of Agriculturally Subsidized Predators as a Potential Threat to Native Insect Herbivores in Fragmented Landscapes

Abstract:  Habitat loss and fragmentation can have strong negative impacts on populations of some native species. Spillover of generalist natural enemies from the surrounding landscape matrix is one mechanism potentially generating such effects, yet this has been rarely studied in insects. We examined the influence of habitat conversion to agriculture on the abundance and potential effects of predatory coccinellid beetles on native insect herbivores within 12 grassland remnants in central Nebraska (U.S.A.). Results of sweep sampling revealed that coccinellids were three to six times more abundant at native grassland sites embedded within cropland-dominated landscapes compared with control sites in grassland-dominated landscapes over the 3 years of the study. Exclusion experiments further demonstrated that predation intensity was strongly related to coccinellid abundances across sites and that coccinellids can dramatically reduce densities of a native aphid herbivore. In contrast to studies of specialized insect parasitoids, which have generally found reduced enemy pressure in fragmented landscapes, our results suggest that native herbivores may in some cases experience increased consumer pressure in landscapes with increasing habitat loss because of spillover of generalist predators from surrounding cropland habitats.     

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.     

Assessing the Potential to Restore Historic Grazing Ecosystems with Tortoise Ecological Replacements

The extinction of large herbivores, often keystone species, can dramatically modify plant communities and impose key biotic thresholds that may prevent an ecosystem returning to its previous state and threaten native biodiversity. A potentially innovative, yet controversial, landscape‐based long‐term restoration approach is to replace missing plant‐herbivore interactions with non‐native herbivores. Aldabran giant (Aldabrachelys gigantea) and Madagascan radiated (Astrochelys radiata) tortoises, taxonomically and functionally similar to the extinct Mauritian giant tortoises (Cylindraspis spp.), were introduced to Round Island, Mauritius, in 2007 to control the non‐native plants that were threatening persistence of native species. We monitored the response of the plant community to tortoise grazing for 11 months in enclosures before the tortoises were released and, compared the cost of using tortoises as weeders with the cost of using manual labor. At the end of this period, plant biomass; vegetation height and cover; and adult, seedling, flower, and seed abundance were 3–136 times greater in adjacent control plots than in the tortoise enclosures. After their release, the free‐roaming tortoises grazed on most non‐native plants and significantly reduced vegetation cover, height, and seed production, reflecting findings from the enclosure study. The tortoises generally did not eat native species, although they consumed those native species that increased in abundance following the eradication of mammalian herbivores. Our results suggest that introduced non‐native tortoises are a more cost‐effective approach to control non‐native vegetation than manual weeding. Numerous long‐term outcomes (e.g., change in species composition and soil seed bank) are possible following tortoise releases. Monitoring and adaptive management are needed to ensure that the replacement herbivores promote the recovery of native plants. Estudiando el Potencial para Restaurar Ecosistemas Históricos de Forrajeo con Reemplazos Ecológicos de Tortugas Terrestres     

Modelling tritrophic interactions mediated by induced defence volatiles

Many plant species defend themselves against herbivorous insects indirectly by producing and releasing induced volatiles to attract natural enemies of the herbivores. In this paper, we consider the recruitment of natural enemies attracted by plant-induced volatiles and introduce the An–Liu–Johnson–Lovett model into the Lotka–Volterra model in an attempt to add this missing vital link in tritrophic interaction. Increase in attraction strength of plant-induced volatiles to the natural enemy leads to high fluctuation amplitude of plant biomass and herbivore population. When the attack strength of natural enemies reaches a certain level, fluctuation amplitude of plant biomass and herbivore population will decrease and plant biomass will approach to its environmental carrying capacity. The simulation demonstrates that plant volatile compounds induced by insects have led to the introduction of a third tritrophic level, e.g., natural enemies, into the plant–herbivore system, resulting in the coexistence of plants, insects, and natural enemies during the evolution process.     

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.     

Insect herbivores, density dependence, and the performance of the perennial herb Solanum carolinense

How insect herbivores affect plant performance is of central importance to basic and applied ecology. A full understanding of herbivore effects on plant performance requires understanding interactions (if any) of herbivore effects with plant density and size because these interactions will be critical for determining how herbivores influence plant population size. However, few studies have considered these interactions, particularly over a wide enough range of densities to detect nonlinear effects. Here we ask whether plant density and herbivores influence plant performance linearly or nonlinearly, how plant density affects herbivore damage, and how herbivores alter density dependence in transitions between plant size classes. In a large field experiment, we manipulated the density of the herbaceous perennial plant Solanum carolinense and herbivore presence in a fully crossed design. We measured plant size, sexual reproduction, and damage to plants in two consecutive years, and asexual reproduction of new stems in the second year, allowing us to characterize both plant performance and rates of transition between plant size classes across years. We found nonlinear effects of plant density on damage. Damage by herbivores and plant density both influenced sexual and asexual reproduction of S. carolinense; these effects were mostly mediated via effects on plant size. Importantly, we found that herbivores altered the pattern of linear density dependence in some transition rates (including survival and asexual reproduction) between plant size classes. These results suggest that understanding the ecological or evolutionary effects of herbivores on plant populations requires consideration of plant density and plant size, because feedbacks between density, herbivores, and plant size may complicate longer-term dynamics.     

Leaf quality, predators, and stochastic processes in the assembly of a diverse herbivore community

Ecological communities are structured by both deterministic, niche-based processes and stochastic processes such as dispersal. A pressing issue in ecology is to determine when and for which organisms each of these types of processes is important in community assembly. The roles of deterministic and stochastic processes have been studied for a variety of communities, but very few researchers have addressed their contribution to insect herbivore community structure. Insect herbivore niches are often described as largely shaped by the antagonistic pressures of predation and host plant defenses. However host plants are frequently discrete patches of habitat, and their spatial arrangement can affect herbivore dispersal patterns. We studied the roles of predation, host plant quality, and host spatial proximity for the assembly of a diverse insect herbivore community on Quercus alba (white oak) across two growing seasons. We examined abundances of feeding guilds to determine if ecologically similar species responded similarly to variation in niches. Most guilds responded similarly to leaf quality, preferring high-nitrogen, low-tannin host plants, particularly late in the growing season, while bird predation had little impact on herbivore abundance. The communities on the high-quality plants tended to be larger and, in some cases, have greater species richness. We analyzed community composition by correlating indices of community similarity with predator presence, leaf quality similarity, and host plant proximity. Birds did not affect community composition. Community similarity was significantly associated with distance between host plants and uncorrelated with leaf quality similarity. Thus although leaf quality significantly affected the total abundance of herbivores on a host plant, in some cases leading to increased species richness, dispersal limitation may weaken this relationship. The species composition of these communities may be driven by stochastic processes rather than variation in host plant characteristics or differential predation by insectivorous birds.     

Beyond the ecological: biological invasions alter natural selection on a native plant species

Biological invasions can have strong ecological effects on native communities by altering ecosystem functions, species interactions, and community composition. Even though these ecological effects frequently impact the population dynamics and fitness of native species, the evolutionary consequences of biological invasions have received relatively little attention. Here, I show that invasions impose novel selective pressures on a native plant species. By experimentally manipulating community composition, I found that the exotic plant Medicago polymorpha and the exotic herbivore Hypera brunneipennis alter the strength and, in some instances, the direction of natural selection on the competitive ability and anti-herbivore defenses of the native plant Lotus wrangelianus. Furthermore, the community composition of exotics influenced which traits were favored. For example, high densities of the exotic herbivore Hypera selected for increased resistance to herbivores in the native Lotus; however, when Medicago also was present, selection on this defense was eliminated. In contrast, selection on tolerance, another plant defense trait, was highest when both Hypera and Medicago were present at high densities. Thus, multiple exotic species may interact to influence the evolutionary trajectories of native plant populations, and patterns of selection may change as additional exotic species invade the community.     

Herbivory by an introduced Asian weevil negatively affects population growth of an invasive Brazilian shrub in Florida

The enemy release hypothesis (ERH) is often cited to explain why some plants successfully invade natural communities while others do not. This hypothesis maintains that plant populations are regulated by coevolved enemies in their native range but are relieved of this pressure where their enemies have not been co-introduced. Some studies have shown that invasive plants sustain lower levels of herbivore damage when compared to native species, but how damage affects fitness and population dynamics remains unclear. We used a system of co-occurring native and invasive Eugenia congeners in south Florida (USA) to experimentally test the ERH, addressing deficiencies in our understanding of the role of natural enemies in plant invasion at the population level. Insecticide was used to experimentally exclude insect herbivores from invasive Eugenia uniflora and its native co-occurring congeners in the field for two years. Herbivore damage, plant growth, survival, and population growth rates for the three species were then compared for control and insecticide-treated plants. Our results contradict the ERH, indicating that E. uniflora sustains more herbivore damage than its native congeners and that this damage negatively impacts stem height, survival, and population growth. In addition, most damage to E. uniflora, a native of Brazil, is carried out by Myllocerus undatus, a recently introduced weevil from Sri Lanka, and M. undatus attacks a significantly greater proportion of E. uniflora leaves than those of its native congeners. This interaction is particularly interesting because M. undatus and E. uniflora share no coevolutionary history, having arisen on two separate continents and come into contact on a third. Our study is the first to document negative population-level effects for an invasive plant as a result of the introduction of a novel herbivore. Such inhibitory interactions are likely to become more prevalent as suites of previously noninteracting species continue to accumulate and new communities assemble worldwide.     

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.     

Differential responses of vertebrate and invertebrate herbivores to traits of New Zealand subalpine shrubs

Plant traits are influenced by herbivore diet selection, but little is known about how traits are affected by different types of herbivores. We related eight traits of 27 subalpine shrub species in South Island, New Zealand, to damage of these shrubs by introduced red deer (Cervus elaphus) and native invertebrate herbivores using phylogenetically explicit modeling. Deer preferentially consumed species that grew quickly, were low in foliar tannins, or had high leaf area per unit mass. However, these traits did not trade off against each other; rather, they could be related to different multivariate defense strategies. Although the proportion of leaves damaged by leaf-chewing invertebrates also increased with stem growth, invertebrates did not damage the same fast growing species as those preferred by deer. Other traits may also be important in determining herbivore preferences, as suggested by the high proportion of variation in herbivory explained by phylogeny. Last, we found that the composition of invertebrate herbivore communities was more similar among closely related shrubs, and consequently, the range of invertebrate-plant associations may change if introduced deer shift plant composition toward slow-growing species. Overall, our results demonstrate the importance of herbivore type and coevolved interactions for the adaptive significance of plant traits.     

Herbivory reduces plant interactions with above- and belowground antagonists and mutualists

Herbivores affect plants through direct effects, such as tissue damage, and through indirect effects that alter species interactions. Interactions may be positive or negative, so indirect effects have the potential to enhance or lessen the net impacts of herbivores. Despite the ubiquity of these interactions, the indirect pathways are considerably less understood than the direct effects of herbivores, and multiple indirect pathways are rarely studied simultaneously. We placed herbivore effects in a comprehensive community context by studying how herbivory influences plant interactions with antagonists and mutualists both aboveground and belowground. We manipulated early-season aboveground herbivore damage to Cucumis sativus (cucumber, Cucurbitaceae) and measured interactions with subsequent aboveground herbivores, root-feeding herbivores, pollinators, and arbuscular mycorrhizal fungi (AMF). We quantified plant growth and reproduction and used an enhanced pollination treatment to determine if plants were pollen limited. Increased herbivory reduced interactions with both antagonists and mutualists. Plants with high levels of early herbivory were significantly less likely to suffer leaf damage later in the summer and tended to be less attacked by root herbivores. Herbivory also reduced pollinator visitation, likely due to fewer and smaller flowers, and reduced AMF colonization. The net effect of herbivory on plant growth and reproduction was strongly negative, but lower fruit and seed production were not due to reduced pollinator visits, because reproduction was not pollen limited. Although herbivores influenced interactions between plants and other organisms, these effects appear to be weaker than the direct negative effects of early-season tissue loss.     

Forest age influences oak insect herbivore community structure, richness, and density.

Plant succession is one of many factors that may affect the composition and structure of herbivorous insect communities. However, few studies have examined the effect of forest age on the diversity and abundance of insect communities. If forest age influences insect diversity, then the schedule of timber harvest rotation may have consequent effects on biodiversity. The insect herbivore community on Quercus alba (white oak) in the Missouri Ozarks was sampled in a chronoseries, from recently harvested (2 yr) to old-growth (approximately 313 yr) forests. A total of nine sites and 39 stands within those sites were sampled in May and August 2003. Unique communities of plants and insects were found in the oldest forests (122-313 yr). Density and species richness of herbivores were positively correlated with increasing forest age in August but not in May. August insect density was negatively correlated with heat load index; in addition, insect density and richness increased over the chronoseries, but not on the sunniest slopes. Forest structural diversity (number of size classes) was positively correlated with forest age, but woody plant species richness was not. In sum, richness, density, and community structure of white oak insect herbivores are influenced by variation in forest age, forest structure, relative abundance of plant species, and abiotic conditions. These results suggest that time between harvests of large, long-lived, tree species such as white oak should be longer than current practice in order to maintain insect community diversity.     

Predator diversity and identity drive interaction strength and trophic cascades in a food web

Declining predator diversity may drastically affect the biomass and productivity of herbivores and plants. Understanding how changes in predator diversity can propagate through food webs to alter ecosystem function is one of the most challenging ecological research topics today. We studied the effects of predator removal in a simple natural food web in the Sierra Nevada mountains of California (USA). By excluding the predators of the third trophic level of a food web in a full-factorial design, we monitored cascading effects of varying predator diversity and composition on the herbivorous beetle Chrysomela aeneicollis and the willow Salix orestera, which compose the first and second trophic levels of the food web. Decreasing predator diversity increased herbivore biomass and survivorship, and consequently increased the amount of plant biomass consumed via a trophic cascade. Despite this simple linear mean effect of diversity on the strength of the trophic cascade, we found additivity, compensation, and interference in the effects of multiple predators on herbivores and plants. Herbivore survivorship and predator-prey interaction strengths varied with predator diversity, predator identity, and the identity of coexisting predators. Additive effects of predators on herbivores and plants may have been driven by temporal niche separation, whereas compensatory effects and interference occurred among predators with a similar phenology. Together, these results suggest that while the general trends of diversity effects may appear linear and additive, other information about species identity was required to predict the effects of removing individual predators. In a community that is not temporally well-mixed, predator traits such as phenology may help predict impacts of species loss on other species. Information about predator natural history and food web structure may help explain variation in predator diversity effects on trophic cascades and ecosystem function.     

Integrating quality and quantity of mutualistic service to contrast ant species protecting Ferocactus wislizeni

Generalized, facultative mutualisms are often characterized by great variation in the benefits provided by different partner species. This variation may be due to differences among species in the quality and quantity of their interactions, as well as their phenology. Many plant species produce extrafloral nectar, a carbohydrate-rich resource, to attract ant species that can act as "bodyguards" against a plant's natural enemies. Here, we explore differences in the quality and quantity of protective service that ants can provide a plant by contrasting the four most common ant visitors to Ferocactus wislizeni, an extrafloral nectary-bearing cactus in southern Arizona. The four species differ in abundance when tending plants, and in the frequency at which they visit plants. By adding surrogate herbivores (Manduca sexta caterpillars) to plants, we demonstrate that all four species recruit to and attack potential herbivores. However, their per capita effectiveness in deterring herbivores (measured as the inverse of the number of workers needed to remove half of the experimentally added caterpillars) differs. Using these among-species differences in quality (per capita effectiveness) and quantity (number of workers that visit a plant and frequency of visitation), we accurately predicted the variation in fruit production among plants with different histories of ant tending. We found that plant benefits (herbivore removal and maturation of buds and fruits) typically saturated at high levels of ant protection, although plants could be "well defended" via different combinations of interaction frequency, numbers of ant workers per interaction, and per capita effects. Our study documents variation among prospective mutualists, distinguishes the components of this variation, and integrates these components into a predictive measure of protection benefit to the plant. The method we used to average saturating benefits over time could prove useful for quantifying overall service in other mutualisms.     

Glucosinolates and other metabolites in the leaves of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> from natural populations and their effects on a generalist and a specialist herbivore

Because many secondary metabolites in plants act as defense against herbivores it has been postulated that these compounds have evolved under selective pressure by insect herbivores. One explanation for the within-species variation in metabolite patterns in a particular species is that different populations are under selection by different herbivores. We tested this hypothesis, using Arabidopsis thaliana plants that originated from dune and inland areas. We analyzed Arabidopsis thaliana leaves using NMR spectroscopy and multivariate data analysis. Major differences in chemical composition were found in water-methanol fractions and were due to higher concentrations of sinigrin and fumaric acid in dune plants. Inland plants showed lower levels of glucose. Quantitative analysis of glucosinolates was performed with HPLC. Individual plants and populations demonstrated differences in glucosinolate composition and concentration. In growth chamber experiments, the generalist herbivore, Spodoptera exigua grew significantly better on the inland plants, while the specialist herbivore Plutella xylostella performed equally well on plants of both origins. Aliphatic glucosinolate as well as total glucosinolate concentrations negatively correlated with larval mass of Spodoptera exigua. No significant correlations, however, were found between larval mass of Plutella xylostella and glucosinolates in the leaves. A specialist and a generalist herbivore were responding differently to plant secondary chemistry, as was also found in several other studies. This is an important indication that differences in glucosinolate concentrations among populations may result from differential selection by different guilds of herbivores.