The effects of a nutrient pulse versus nutrient press on the structure of an arthropod food-web

Natural and anthropogenic inputs of nutrients into natural ecosystems promote fluctuations in the availability of resources that normally limit plant growth; these inputs encourage changes in primary productivity that in turn can have important, community-wide consequences. Natural wetlands are increasingly threatened by nutrient pollution, particularly nitrogen and phosphorus runoff from agriculture and coastal development. We are investigating the effects of a short-term increase in nutrients (pulse) versus a long-term increase in nutrients (press) on the productivity of the cordgrass Spartina alterniflora, an important component of the vegetation of Atlantic coastal marshes, and to follow how these treatment effects change over time. Treatments have been specifically designed to mimic nutrient-runoff scenarios. We are experimentally testing not only the direct effects of nutrient addition on Spartina growth and how these effects differ between pulse and press treatments, but also how the effects of nutrient pulses may differ from year to year. We are currently in our third field season and have two complete years of data to present; for the first time we are able to compare interannual variation between the press and pulse treatments. Our results indicate that natural enemies continue to rapidly increase their densities in the press treatment and have maintained relatively high densities in the pulse treatment. As a result, herbivore and detritivore populations have declined in both experimental treatments. In addition, we are monitoring the response of Spartina herbivores, detritivores and their respective predators to test our prediction that pulse and press treatments will generate significantly different foodweb dynamics, and thus cause the trophic cascades associated with these disparate arthropod communities to vary significantly in strength.