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Ants can have important, but sometimes unexpected, effects on the plants they associate with. For carnivorous plants, associating with ants may provide defensive benefits in addition to nutritional ones. We examined the effects of increased ant visitation and exclusion of insect prey from pitchers of the hooded pitcher plant Sarracenia minor, which has been hypothesized to be an ant specialist. Visitation by ants was increased by placing PVC pipes in the ground immediately adjacent to 16 of 32 pitcher plants, which created nesting/refuge sites. Insects were excluded from all pitchers of 16 of the plants by occluding the pitchers with cotton. Treatments were applied in a 2 x 2 factorial design in order to isolate the hypothesized defensive benefits from nutritional ones. We recorded visitation by ants, the mean number of ants captured, foliar nitrogen content, plant growth and size, and levels of herbivory by the pitcher plant mining moth Exyra semicrocea. Changes in ant visitation and prey capture significantly affected nitrogen content, plant height, and the number of pitchers per plant. Increased ant visitation independent of prey capture reduced herbivory and pitcher mortality, and increased the number of pitchers per plant. Results from this study show that the hooded pitcher plant derives a double benefit from attracting potential prey that are also capable of providing defense against herbivory.

Resource quality (plant nitrogen) and resource quantity (plant density) have often been argued to be among the most important factors influencing herbivore densities. A difficulty inherent in the studies that manipulate resource quality, by changing nutrient levels, is that resource quantity can be influenced simultaneously, i.e. fertilized plants grow more. In this study we disentangled the potentially confounding effects of plant quality and quantity on herbivore trophic dynamics by separately manipulating nutrients and plant density, while simultaneously reducing pressure from natural enemies (parasitoids) in a fully factorial design. Plant quality of the sea oxeye daisy, Borrichia frutescens, a common coastal species in Florida, was manipulated by adding nitrogen fertilizer to increase and sugar to decrease available nitrogen. Plant density was manipulated by pulling by hand 25 or 50% of Borrichia stems on each plot. Because our main focal herbivore was a gall making fly, Asphondylia borrichiae, which attacks only the apical meristems of plants, manipulating plant nitrogen levels was a convenient and reliable way to change plant quality without impacting quantity because fertilizer and sugar altered plant nitrogen content but not plant density. Our other focal herbivore was a sap-sucker, Pissonotus quadripustulatus, which taps the main veins of leaves. Parasitism of both herbivores was reduced via yellow sticky traps that caught hymenopteran parasitoids. Plant quality significantly affected the per stem density of both herbivores, with fertilization increasing, and sugar decreasing the densities of the two species but stem density manipulations had no significant effects. Parasitoid removal significantly increased the densities of both herbivores. Top-down manipulations resulted in a trophic cascade, as the density of Borrichia stems decreased significantly on parasitoid removal plots. This is because reduced parasitism increases gall density and galls can kill plant stems. In this system, plant quality and natural enemies impact per stem herbivore population densities but plant density does not.

In this study we investigated the potential importance of species identity and herbivore feeding mode in determining the strengths of top-down and bottom-up effects on phytophagous insect densities. In 1998, we conducted two factorial field experiments in which we manipulated host plant quality and intensity of parasitoid attack on three salt marsh herbivores, the planthoppers Prokelisia marginata and Pissonotus quadripustulatus (Homoptera: Delphacidae), which feed only on Spartina alterniflora and Borrichia frutescens, respectively, and the gall fly Asphondylia borrichiae (Diptera: Cecidomyiidae), which feeds only on B. frutescens. We increased plant quality through addition of nitrogen fertilizer, and decreased parasitism by trapping hymenopteran parasitoids continuously throughout the study. Herbivore densities were censused biweekly. Increasing plant quality through fertilization increased the density of all three herbivores within 2 weeks of treatment application, and higher densities were maintained for the duration of the study. Reduction of top-down pressure had no effect on either planthopper species, possibly because of compensatory mortality affecting the two species. In contrast, reduction of parasitism significantly increased the density of A. borrichiae galls, perhaps because development within gall tissue reduces the sources of compensatory mortality affecting this species. The results of this study show that the bottom-up effects of plant quality were strong and consistent for all three species, but the strength of top-down effects differed between the two feeding guilds. Thus, even for herbivores feeding on the same host plant, conclusions drawn regarding the relative importance of top-down and bottom-up effects may vary depending upon the feeding mode of the herbivore.

The relative importance of bottom-up versus top-down forces, and the effect of productivity on community dynamics continue to be of much interest to ecologists. Trophic dynamic theories are difficult to test, as they require explicit knowledge of the many organisms involved, as well as the nature of the interactions between them. The Oksanen-Fretwell (OF) theory, which suggests that the relative roles of top-down and bottom-up factors vary with primary productivity, is well known in the literature, but is difficult to test rigorously. Recently, two experimental studies have tried to test OF theory. In this paper we discuss methodological problems associated with these studies that may weaken the conclusions drawn by the authors.