Community impacts of a tussock sedge: Is ecosystem engineering important in benign habitats?

Ecosystem engineers impact other organisms through environmentally mediated indirect interactions, but the potential for the mechanisms and importance of engineering to vary predictably across environmental gradients has not been investigated. Previously, we investigated how hummock formation by Triglochin maritimum in physically stressful salt marsh pannes influences the plant community that grows only on hummock tops and found that species diversity was dependent on indirect positive effects of T. maritimum engineering. Here we examined how Carex stricta, the tussock sedge, drives superficially similar spatial patterning of plants in relatively benign tidal freshwater marshes at the opposite extreme of an estuarine salinity gradient. In C. stricta marshes, the entire vegetative community is located almost exclusively on top of tussocks while inter-tussock spaces are bare mud buried in C. stricta wrack. We manipulated tussock height, belowground substrate, and aboveground competition for light to investigate the mechanisms by which C. stricta influences the wetland plant community. We also conducted wrack-removal experiments and phytometer assays to examine the importance of wrack deposition in this community and to investigate the spatial distribution of herbivore pressure, respectively. Transplants of four common species performed well in all treatments, except natural intertussock spaces buried in wrack. Removal of wrack from inter-tussock spaces enabled substantial seedling emergence and vegetative cover within one growing season. C. stricta therefore drives plant distribution primarily through indirect negative impacts of wrack accumulation in inter-tussock spaces. Once tussocks are established, plants obtain a secondary benefit when growing on tussocks, as the inter-tussock spaces become runways for small mammalian herbivores that concentrate feeding in low protected areas. These results differ dramatically from nearby salt marshes where T. maritintum facilitates community diversity by alleviating physical stresses. The mechanisms and outcomes of hummock-forming engineers may thus vary predictably across estuarine salinity gradients where hummock formation has facilitative and important impacts in physically stressful environments, but negative and less essential impacts in physically benign environments.