The plant stress and plant vigor hypotheses (PVH) are two of the most widely recognized hypothesis invoked to explain differential distribution of insect herbivores among their host plants. In both cases, the emphasis is on bottomÁup processes (i.e. host-plant quality), but a recent meta-analytical review of the literature has shown that the plant stress hypothesis might have limited support among insect herbivores. In this study, we conducted a meta-analysis of the effects of plant vigor on insect herbivore abundance and survivorship by reviewing 71 published articles that explicitly tested the PVH and enabled 161 independent comparisons. Z-transform was used as the metric to standardize the results of all independent comparisons. Our quantitative results have shown that Hymenoptera (sawflies) was the most abundant group in the reviewed studies, representing 28.1% of the independent comparisons, followed by Diptera (25.1%) and Homoptera (22.6%). Amongst all the guilds studied, gall-formers were the most representative group (68.0%), whereas leaf-miners and stem-borers were underrepresented (less than 4.0% of the available comparisons). Insect herbivores were significantly more abundant on more vigorous plants (E'' 00.6432, CI 00.7558Á0.7280), but no significant effect was detected on herbivore survivorship. When herbivores were categorized into feeding guilds, effects of plant vigor on herbivore abundance were stronger for sap-suckers, leaf-miners and gall-formers. Our results have shown a strong herbivore preference for more vigorous plants, although our results do not support a preferenceÁperformance linkage.An increased number of studies in plantÁherbivore interactions have recognized that intra-plant heterogeneity affects both the distribution and the performance of herbivorous insects (Schultz 1983, Strong et al. 1984, Crawley and Akhteruzzaman 1988. The plant stress and plant vigor hypotheses are two of the most widely recognized hypotheses invoked to explain heterogeneous distributions of herbivores among their host plants (Williams and Cronin 2004). The plant stress hypothesis (PSH) proposed by White (1984) argues that herbivore abundance is higher on stressed host plants due to an increased availability of nutrients, a decreased concentration of defensive compounds and/or changes in the ratio of nutrients to chemical defenses. Evidence supporting the prediction that moderate stress benefits herbivores due to increased nutritional quality are abundant (McClure 1980, Lewis 1984, Mattson and Haack 1987 and positive relationships between insect herbivory and plant stress have been found for some tree species, crops and herbaceous plants (Mattson and Haack 1987, Heinrichs 1988). Nevertheless, some authors claim that experimental tests of the PSH have generated conflicting results (Bultman and Faeth 1987, Louda and Collinge 1992, Koricheva et al. 1998, De Bruyn et al. 2002, and many authors (Larson 1989, Koricheva et al. 1998) have championed the abandonment of the PSH and the search for alternative hypothes...

Although specific associations between spiders and particular types of plants have been reported for several taxonomic groups, their consequences for spiders and plants are still poorly understood. The most common South American lynx spiders, Peucetia flava and P. rubrolineata, live strictly associated with various plant species that have glandular trichomes.To understand more about these spider-plant relationships, we investigated the influence of the spiders on the fitness of a neotropical glandular shrub (Trichogoniopsis adenantha) and on the arthropod community structure on the plant. We also tested whether glandular hairs provided any benefit to the spiders. Spiders reduced the abundance of several species and guilds of herbivores on the leaves and inflorescences. Consequently, damage to the leaves, capitula, ovaries, corollas, and stigmas caused by leaf-mining and chewing insects, as well as endophagous insects, were strongly reduced in the presence of Peucetia spp. Although the spiders fed on flower visitors, their negative influence on ovary fertilization was only marginally nonsignificant (P ¼ 0.065). Spiders on plants of Trichogoniopsis adenantha that fed on common fruit flies that had died before adhering to the glandular trichomes did not lose body mass. However, those living on plants without stalked glandular trichomes (Melissa officinalis) did not feed on dead flies and lost 13-20% of their biomass. These results indicate that Peucetia spiders are effective plant bodyguards and that when there is limited live prey they may feed on insect carcasses adhered to glandular trichomes. Since several spider species of the genus Peucetia live strictly associated with glandular trichome-bearing plants in neotropical, Neartic, Paleartic, and Afrotropical regions, this type of facultative mutualism involving Peucetia and glandular plants may be common worldwide.

Abstract. Although bromeliads are believed to obtain nutrients from debris deposited by animals in their rosettes, there is little evidence to support this assumption. Using stable isotope methods, we found that the Neotropical jumping spider Psecas chapoda (Salticidae), which lives strictly associated with the terrestrial bromeliad Bromelia balansae, contributed 18% of the total nitrogen of its host plant in a greenhouse experiment. In a one-year field experiment, plants with spiders produced leaves 15% longer than plants from which the spiders were excluded. This is the first study to show nutrient provisioning in a spider-plant system. Because several animal species live strictly associated with bromeliad rosettes, this type of facultative mutualism involving the Bromeliaceae may be more common than previously thought.

We examined the effects of the sit-and-wait spider Misumenops argenteus (Thomisidae) on the herbivore assemblage and fitness of the perennial woody shrub Trichogoniopsis adenantha (Asteraceae). Because crab spiders prey on both pollinators and phytophagous insects, they can have potentially negative and positive effects on plants. In a manipulative experiment using paired plants, spiders decreased the density of sucking and some endophagous herbivores on the leaves and capitula and reduced the number of damaged achenes produced by the plants. Damaged capitula had a higher proportion of fertilized achenes in plants with spiders than without spiders, but not undamaged capitula. These results indicate that M. argenteus exerted a double positive effect on seed production in T. adenantha. The effect of M. argenteus on herbivores may be taxon specific and vary among years with different herbivore abundances.

Summary 1.In several regions of South America, the neotropical jumping spider Psecas chapoda inhabits and reproduces strictly on the bromeliad Bromelia balansae . Previous studies reported that this spider is more frequent on bromeliads in grasslands than on those growing in forests, and on larger plants, but only when the bromeliads are without inflorescence. Upon blooming, B. balansae fold their leaves back, thereby changing the plant architecture from a tri-dimensional to a bi-dimensional flattened shape, and our hypothesis is that this alteration affects the spider's habitat-selection decisions. 2. In the present study, we examined experimentally the effects of inflorescence, plant size and blockade of the axil of the leaves (spider shelters) of forest bromeliads on the colonization of a bromeliad by P. chapoda . By using sticky traps, we also compared prey availability in grassland and forest. 3. Plants with simulated inflorescence were colonized at a lower frequency than those without inflorescence simulation. Grassland bromeliads in which the rosettes were blocked with dry leaves were colonized less frequently than open bromeliads, whereas forest bromeliads from which dry leaves had been removed were not colonized. Spiders generally abandoned bromeliads in which three-quarters of the length of the leaves had been removed, although females with eggsacs remained on these plants. Prey availability (biomass and number) was up to 18 fold higher in the grassland than in the forest. These results suggest that microhabitat structure and prey availability shape the spatial distribution of P. chapoda populations. 4. Our findings suggest that Psecas chapoda can evaluate, in fine detail, the physical state of its microhabitat, and this unusual spider-plant association is readily destabilized by changes in the microhabitat (i.e., it is strictly dependent on the size and morphology of the host plant). This study is one of the few to report a strict association between a spider species and its host plant, and also one of the few to examine the effects of habitat and microhabitat structure on the spatial distribution of active hunters on plants.