Effects of novel, non-native detritus on decomposition and invertebrate community assemblage

When non-native primary producers become successful, the structure and function of native detrital food webs can be fundamentally altered. Salt marsh estuaries of the southeastern USA are in part detritus-based ecosystems and rely on the annual production of detritus from a single native species, the smooth cordgrass Spartina alterniflora. Over the last several decades, the success of a novel primary producer, the red macroalga Agarophyton vermiculophyllum (formerly Gracilaria vermiculophylla), in a system historically devoid of macroalgae provides the opportunity to measure the effect of non-native basal resources on native detrital pathways. We conducted 2 in situ experiments to compare (1) decomposition rates of A. vermiculophyllum and S. alterniflora and (2) invertebrate colonization rates onto dead A. vermiculophyllum and S. alterniflora. Relative to S. alterniflora, we found that A. vermiculophyllum decomposes more rapidly, losing 80% or more of its biomass within 3 wk, while S. alterniflora lost ~50%. Experimental litterbags with decomposed A. vermiculophyllum and S. alterniflora harbored similar highly abundant invertebrate communities that differed greatly from denuded areas. Our results demonstrate that A. vermiculophyllum provides a complementary source of labile organic matter relative to S. alterniflora, boosting the amount of food and available habitat for small invertebrates of intertidal salt marshes and mudflats. Thus, non-native macrophytes may differentially affect community and ecosystem properties just as much when dead as alive, especially when they are biologically distinct from native species.