Bottom-up and top-down effects on insect herbivores do not vary among sites of different salinity

It has been suggested, but rarely tested, that the relative strength of top-down and bottom-up factors in communities varies along an environmental stress gradient. We compared the strength of bottom-up and top-down effects on the densities of insect herbivores along a range of sites of different salinities in west-central Florida. We used a 2 x 2 factorial design with plots divided into four treatments: (1) bottom-up manipulation, where fertilizer was applied to increase plant quality; (2) top-down manipulation, where sticky traps were used to reduce the effects of natural enemies (parasitoids); (3) bottom-up and top-down manipulation, where fertilizer was applied and sticky traps were used; and (4) control plots. These plots were established along a range of salinities among seven different sites containing the salt marsh plant Borrichia frutescens. In each plot, we determined the parasitism levels and abundances of the sap sucker Pissonotus quadripustulatus, the gall maker Asphondylia borrichiae, and the lepidopteran stem borer Argyresthia spp. Gall density, Pissonotus density, and stem borer density were significantly higher in lower salinity sites, suggesting a strong effect of environmental stress. There was a significant increase of galls and Pissonotus and a marginally significant increase of bored stems on fertilized plots but not on trapped plots. There was a significant interaction of site and fertilizer on gall parasitism. There were no interactions of either treatment with salinity on herbivore densities. The general lack of interaction between salinity level and other treatments on herbivore densities contrasts with our previous result where treatment effects did vary with salinity level on a large experimentally generated salinity gradient at one site. Thus, the results of the present paper suggest that, while environmental stress can modify top-down and bottom-up effects on herbivores at single sites, variation in site-to-site factors, possibly including clonal identity of plant, affects herbivore densities so much as to swamp out any observable interaction between environmental stress and top-down or bottom-up factors.