Effects of environmental stress cascade up through four trophic levels in a Florida salt marsh study system

Although in recent years there have been a number of studies demonstrating trophic cascades in terrestrial systems, it is still unclear what environmental conditions enable or enhance such cascades.   The majority of studies on trophic cascades have focused on the effects of nutrients in three trophic level study systems.  In this study, we examined the influence of environmental stress (increased soil pore water salinity) on a four trophic level study system in a Florida salt marsh.  Soil pore water salinity was increased experimentally through the addition of salt pellets to 2m diameter circular plots of the host plant Batis maritima.  We examined the effects of increased salinity on the quality of the host plant, and assessed the degree to which this influenced the lepidopteran herbivore Ascia monuste, and the primary and hyperparasitoid of its caterpillars.  Increased salinity altered host plant quality (nitrogen and salt content of plant tissues), which subsequently affected the consumer species.  These effects of altered plant quality cascaded up through the herbivore and primary parasitoid to the hyperparasitoid Hypopteromalus sp., influencing its density, fecundity, and sex ratio.  These results demonstrate how heterogeneity in environmental stress can result in effects that cascade up through four trophic levels.