Underestimation of Spartina productivity in western Atlantic marshes: marsh invertebrates eat more than just detritus

Abstract

Quantifying primary productivity and understanding the factors that control plant growth are primary goals of ecosystem ecology. Although some methods have been very successful in providing accurate measurements of plant growth and elucidating the importance of both physical and biological factors in regulating primary production (e.g. terrestrial systems: Smith 1996; marine communities: Bertness et al. 2001), many techniques are still hampered by methodological biases that greatly affect productivity estimates and overall experimental results. For example, a recent review of tropical seagrass systems suggests that past experiments investigating seagrass productivity may have been compromised because they did not control for the confounding effects of grazing (i.e. monitoring and/or excluding fish and invertebrate grazers – Valentine and Heck 1999). The authors argue that the probable consequences of not accounting for herbivore effects are: (1) inaccurate estimates of both net and gross primary production and (2) an intellectual bias concerning the relative roles of bottom-up (i.e. nutrients and nutrient regulating factors) and top-down (e.g. herbivory) forces in controlling seagrass growth. In this paper, we argue that these conclusions also apply broadly to salt marshes along the East Coast of both North and South America. Specifically, we suggest that past studies of plant productivity in Western Atlantic marshes, by not accounting for the confounding effects of grazing (i.e. excluding herbivorous crabs and snails), are likely to have significantly underestimated plant growth and overestimated the relative importance of bottom-up factors in regulating marsh primary production. We contend that current methods of measuring marsh grass growth must be amended to include grazer exclusions in carefully controlled field experiments. Only then will we be able to obtain accurate measurements of marsh plant productivity and assess the relative importance of top-down effects in controlling marsh grass growth.